Sunday, December 12, 2010

The Unspecified Drive


Deep space propulsion is, unsurprisingly, a major concern of this blog. I regularly specify the performance of interplanetary craft fitted with some form of high specific impulse drive. Sometimes I describe it as a nuclear electric or solar electric drive, sometimes simply as electric, often not even that much. Sometimes, especially when discussion takes us to the wide open spaces beyond Jupiter, I allude to fusion.

Since I got myself in a bit of hot water, or some more exotic (and much hotter) coolant, by some snide remarks about fission power plants, a few comments on deep space propulsion are in order.

First of all it is not the main barrier to widespread interplanetary travel. That would be the sheer amount of costly design engineering needed to build a fleet of prototype spacecraft, followed by the cost of getting them all into space.

But once we are up there, how we get around is an important concern. The current limit to human space missions is about six months, beyond which the health consequences of prolonged microgravity become severe. Longer missions require a spin hab, adding cost and complexity. Even with spin habs, radiation and ordinary human factors limit practical mission duration to a couple of years or so.

Within these constraints we could reach Mars with chemfuel (and a spin hab), but the Hohmann round trip to the main asteroid belt is two and a half years, without any stay time at the destination, while to Jupiter and back is five and a half years. This is too long for regular human travel.

So for a human interplanetary presence we need fast orbits. These are above my math pay grade to calculate, but a klugewerks of flat space modeling, sketching orbits, interpolation, and sheer guesswork indicates that reaching Mars in three months or Jupiter in a year calls for a mission delta v in the range of about 30-100 km/s, and therefore some form of high specific impulse drive. Even NERVA style nuclear thermal rockets - the classic Atomic Rockets that gave the website its name - fall short of this requirement.

The time honored high specific impulse drive in science fiction is ion propulsion, used in real life to send the Dawn mission to Ceres and Vesta, but not suited to much larger human-carrying spacecraft. To a great many people, however, 'ion drive' is more or less synonymous with electric drive in general.

The most likely such drive for human missions appears to be some form of plasma jet. Unlike ion drive this is a thermal drive: The plasma has a meaningful temperature - and it is extremely hot. But the thrust chamber is a magnetic field, so it won't melt. Only the gizmos that produce the field are exposed, and they don't get up close and personal with the plasma. They and their supporting struts must have heat shielding, forming a 'lantern' structure.

(The strictly technical term for this drive is electrothermal magneto-plasma propulsion - doesn't that sound exactly like classic Trek technobabble? "I've engaged the electrothermal magneto-plasma thrusters, Keptain - she canna take much more!")

So far plasma drive has gone no further than the laboratory bench, but there don't seem (yet) to be any serious problems in scaling it up to be suitable to large spacecraft. Like many forms of electric drive it has no inherent exhaust velocity and therefore no fixed specific impulse. At least in principle these drives can be configured either to expel a relatively large flow of relatively (very relatively!) cool plasma at lower velocity, or a smaller quantity of hotter plasma at higher velocity.

The effect is very closely analogous to gearing; these drives can be set for a higher acceleration and lower specific impulse or vice versa. VASIMR is supposed to achieve this not only in principle but in engineering practice, permitting clever tweaking of engine settings to get the optimum performance in each phase of flight.

For all of its advantages, electric drive has one essential drawback. It does not produce its own energy, as chemfuels do, or even use a reactor directly to heat the propellant, as nuke thermal does. It must be plugged into an external electric power supply. This is seriously inconvenient, because it takes a lot of electric power, tens to hundreds of megawatts, to drive a big, human carrying ship even at milligee acceleration.

For travel in the inner system I am partial to solar electric power. It hums along quietly with little fuss and practically no moving parts. But the butterfly's wings must be enormous, a hectare for every few megawatts, and extremely light. Even milligee forces may be problematic when the wing structure is that big and that light. And solar electric fades rapidly with distance from the Sun, unsuitable for travel beyond Mars.

For the asteroid belt and Jupiter the practical alternative is nuclear electric drive, which was the cause of my original grump. All vivid if misleading imagery of clanking steam engines aside, nuclear power plants are heavy, filled with complex plumbing that must operate for months under fiercely hostile conditions, and produce two or three times their useful output in waste heat, which must be got rid of through large radiators with their own demanding plumbing.

That eerie green glow is produced by the disintegration of money.

There is an upside to all this downside: Ships with nuclear or solar electric drive have plenty of juice at the main switchboard, making these drives, especially nuke electric, well suited to laser stars. All you need is the laser installation; the power supply is already provided, and you can zap away as long as you want to hold down the trigger.

But the general messy inconvenience of carrying around a naval-equivalent fission power reactor accounts for much of the appeal of fusion. In principle, and popular imagination, fusion is an ideal power source for a plasma drive, because the fusion plasma and the thrust plasma can be one and the same. VASIMR in fact is a byproduct of fusion research; in a conceptual sense it is a derated fusion drive.

Fusion in practice could turn out to be another matter. What else is new? The easiest fusion reactions to sustain (and we can't yet fully sustain any of them) release most of their energy as neutrons, useless for propulsion, but - irony alert - suitable for heating a steam boiler.

On the other hand, fusion propulsion is in some respects simpler than fusion power for earthly energy needs. It does not need to be an economical means of producing electric power. In fact to serve as a drive it need not produce any electric power at all, though any fusion drive would likely produce some 'bleed' power.

There are alternatives to fusion, all about as speculative as fusion itself. Orion is arguably the least speculative of the bunch, though the organ music and black cape factor has pretty much overshadowed the actual technical challenges of building a spacecraft that must nuke itself thousands of times, at close range, in the course of normal operation. (Those have to be some badass shock absorbers!)

But on the whole the specific technical details of a high specific impulse drive matter surprisingly little. What matters is how heavy the thing is, relative to the thrust power it puts out. The benchmark here is is a specific power output of roughly 1 kW/kg, or a megawatt per ton, for the full drive installation including thrusters, power supply, and waste heat radiators. (For a complete drive bus add propellant tankage and keel structure; mate it all to a payload to get a ship.)

Example: Suppose a 100 MW, 100 ton VASIMR style drive engine. With exhaust velocity tuned to 75 km/s, specific impulse near 7500 seconds, propellant mass flow is 36 grams/second, producing about 2.7 kN of thrust, enough to push a 500 ton ship at just over a half a milligee, gradually increasing as propellant is burned off. If half the departure mass is propellant (250 tons, plus 100 tons for the drive, leaving 150 tons for tankage, structures, and payload), mission delta v is just over 50 km/s. Full power burn duration is about 80 days. This broadly corresponds to the requirement for a fast, three month orbit to Mars.

Tune the same drive to an exhaust velocity of 150 km/s, specific impulse near 15,000 seconds. Propellant mass flow falls to about 9 grams/second, producing 1.3 kN of thrust, pushing the ship at a quarter of a milligee. With the same mass proportions our ship has a mission delta v of just over 100 km/s and full power burn duration of 11 months, approximating a one year trip to Jupiter.

Improving on this performance will not be easy. To reduce travel time on semi-brachistochrone orbits with prolonged burns you must reach a higher peak speed in less time, and must therefore increase both thrust and specific impulse. In the flat space approximation, drive power increases as the inverse cube of travel time - that is, you need eight times the drive power output to cut travel time in half.

The good news, such as it is, is that this also works the other way. An early generation drive with a more modest 250 W/kg power output can still take a relatively fast orbit to Mars. But you pretty much need fusion drive, or an equivalent array of oscillating hands, to reach Jupiter in a few months, or for practical travel to the outer planets.

Still, the Solar System as far as Jupiter should be a decent sized playground for a while.



The image comes from a NASA publication on VASIMR.

228 comments:

1 – 200 of 228   Newer›   Newest»
Jedidia said...

It might just be that I'm stupid, but I never quite got what the 1kw/1kg figure refers to... Is it 1 kw of usable (i.e. converted to electricity) energy output, or 1 kw total energy output (i.e. you'd be getting maybe about 0.2 kw of actually usable electricity out of it...)

Rick said...

You never got it because I have never clarified that important little detail. I mean 1 kw thrust power, which will be a bit less than the electric power fed into it, and a lot less than the total energy output of the power plant.

Anonymous said...

I would think that 3 to 1 total power vs electrical output would be a good approximation.

As far as the main post goes, I have two thoughts:
1) The fission-fragment drive might be a good choice for a deep space engine.
Pros; self contained, produces both thrust and power, relitively light, and while it doesn't need additional propellant you can use a 'booster' propellant to increase thrust (at the expense of ISP), exhast velocity is in the region of 3% c!
Cons; The unfiltered version resembles a 1950's-style Death Ray, the 'boosted' version has some of the same drawbacks of the open gas core nuclear rocket.

2) if you are writting a story about long, fast interplanetary travel, you might consider just naming your drive and not explaining it. Just be consistant with its performance and don't be too outragious and you'll be fine.

Ferrell

Elukka said...

This is a tad offtopic, but what I've always wondered about that picture is why they'd design the radiators so that they radiate right into each other. I know it's just an illustration, but it seems oddly careless considering the source. Or maybe there's a better reason?

Geoffrey S H said...

Auxiliaries? Retract four for when, say 1 is damaged, then wheel out the undamaged four and take in the three that can be used in the event of another micrometeorite strike holing 2 more?

An engineer's nightmare payload wise... but it does strike me as somewhat sensible.

Jim Baerg said...

For interplanetary drives with short travels times & modest handwavium I like the magnetic sail the M2P2 variant of it, & the high performance variant the magbeam.

These would have better performance than the solar sail & performance declines less than solar sails with distance from the sun or even remains constant with distance from the sun for the M2P2.

For high performance the magbeam requires a large installation at each end, but once that is installed reqular fast traffic is possible.

Thucydides said...

For the most part the answer is remote power beaming. The generator can be as massive and efficient as current engineering practice allows, but since the ship does not have to cart it around, the mass ratio becomes far better (and performance becomes much better as well).

This also works out for economics, a power station can service many ships, providing revenue on a much more consistent basis. The ships are cheaper as well, so they also generate revenue much more quickly for the owners. Separate corporations might exist to supply energy and supply shipping, so ship owners can look for competitive rates when shopping for boost.

This also supports one of my favorite tropes, Solar economic zones. Inside the orbit of Mars, it is most economical to generate energy using solar energy. From the Asteroids to Uranus is covered by the Jovian system, using energy harvested from the Jovian magnetosphere, while the beyond is powered by fusion generators running on 3He mined from the atmosphere of Uranus.

Since it is possible to beam energy beyond these zones, economic competition between the zones drives conflict (and provides plots for stories).

Sabersonic said...

Thucydides suggestion of broadcasted power from a centralized plant to remote spacecraft does solve the whole powerplant mass problem that would otherwise cut into payload and delta v, in theory. However, such a system does raises a few questions that should at least be debated upon like keeping the energy connected between the two points at such long distances. Not to mention a neccessity in back up, secondary power sources in case something happens to the central plant such as fuel cells and photovoltaic cells.

Though the biggest concern, depending upon how the energy is transferred, is how does one keep such a system with that much energy at its disposal from incenerating spacecraft through design, accident, or otherwise.

Either way, its likely that any plausible interplanetary travel would be restricted to either onboard power plant that cuts into deltaV and payload allowance of the spacecraft, broadcasted/beamed energy from a centralized source, or a hybrid of the two to power the plasma jet drives for some time to come until a reliable and economical fusion torch drive is created. That or someone accidentally invents a Warp Drive out of a washing machine and twine.

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nqdp said...

Sabersonic:

There are ways certainly ways to design a beamed power system that doesn't incinerate everything it touches. Specifically, you don't have to use a weapons-grade laser. After all, solar (photovoltaic) power works well here on Earth, and that's basically a beamed power source that can't do much more than give us sunburns after a couple hours' exposure. You could probably beam power a couple of times more intense than that without cooking the spacecraft. The problem would be that you would need a larger collection area, which means the ship would be more massive. I'm can't do the math to figure out how intense the beam should be or how big the collection area should be, unfortunately. Also (unfortunately in fiction) low-intensity beamed power can't really be weaponized, so for storytelling purposes it's a bit dull.

qraal said...

Sabersonic

Dammit don't give away the secret ingredients!

Alex Rosslyn said...

I'm going to have to ask, kilonewtons and meganewtons for a VASIMR? Can one really get it up that much (Last time I checked, under ten megawatts, by terribly reducing the exhaust speed, you could get something like 10^³ Newtons) or did you handwave the thrust?

Robert said...

Even with a propulsion system expelling large flow of plasma at higher velocity, the space exploration would be limited to our solar system. It is needed a system with higher density of energy that carries no extra propellants to allow fast interstellar travels within a human lifespan.

Sabersonic said...

to nqdp: Photovoltaic Cells are essentially beamed energy, however the idea was to focus said energy onto a single point rather then have it radiate in all directions like the sun. As Rick and yourself have noted, the solar panels would have to be big to power such a drive. Granted, beamed power does raises the question of what system could be used to beam the neccessary energy requirements at such a long range with acceptible loss of said energy due to the inverse-square law or some other law of physics that I am sure is involved.

One answer is to simply broadcast said energy not unlike what Nicola Tesla had envisioned early in the twentieth Century or even wi-fi internet connection. How, I haven't the foggiest. Well, not without resorting to space opera levels of magitech.

to qraal" What secret ingredients?

To Robert: From what I've seen of the video, it seems suspeciously similar to the Bussard Ramjet and potentially unable to deliver what it promised due to some unforseen variable such as electromagentic drag. I don't have anything against such interstellar drive systems, but barring magitech there doesn't seem to be an ideal IS-Drive that doesn't run into serious complications.

And then there's the unfortunate consideriation of interstellar matter and debris impacting said craft from relavistic velocities. No wonder people cling to space opera such as Star Trek and Star Wars so tightly.

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Tony said...

Elukka:

"This is a tad offtopic, but what I've always wondered about that picture is why they'd design the radiators so that they radiate right into each other."

The answer is: they don't. If you look closely at the illustration, they radiate past each other, similar to the radiator arrangement on deep space probe RTGs.

Tony said...

Re: beamed power

Nothing wrong with it in principle. The problem is practical application. The problem of keeping the beam centered has already been mentioned. Perhaps as big a problem is placing the power source so that it does not get occluded by planets and their satellites. For travel from Earth to Mars and back, one would have to place the power station away from the Earth, but in the Earth's orbit, probably ahead a few million km. That's going to add considerably to initial and ongoign costs.

Re: fission-fragment rocket

Where you going to test the thing? Nowhere on Earth, and that's where your industrial base is going to be in the plausible midfuture.

Milo said...

I would just like to reiterate my established view that any beamed power system capable of projecting enough energy to propel a spaceship at acceptable speeds, at a spaceship-sized target across interplanetary distances, would also be usable as a beam cannon of mass destruction.

That's fine if you're willing to have beam cannons of mass destruction in your setting, but keep in mind that weapons of mass destruction are not too easy to make.



Rick:

"a klugewerks of flat space modeling, sketching orbits, interpolation, and sheer guesswork indicates that reaching Mars in three months or Jupiter in a year calls for a mission delta v in the range of about 30-100 km/s,"

I have a program that calculates exact curved-space travel times, assuming instant acceleration (not really accurate for electric engines).

30 km/s does Earth-Mars in 2 months 14 days, and Earth-Jupiter in 13 months. 100 km/s does Earth-Mars in 29 days 15 hours, and Earth-Jupiter in 4 months 22 days.

Sabersonic said...

For some odd reason, the blogger deleted my previous comment despite the fact that it told me that it was sucessfully submitted earlier, so I'll have to do it again from memory.

To nqdp - Photovoltaic Cells are similar to the proposed beamed power, however the scale of such a electrical generation system for the plasma jet drive is directly perportional to the inverse-square law of the solar radiation. Beamed power generaiton technology attempts to circumvent this, from what I understand it, by focusing said energy onto spacecraft using whatever method delivers the most energy economically over distance. Granted, I'm not sure which laws of physics would enforce any potential range limitation from central power station to individual craft.

There is the idea of broadcasted energy not unlike what Nicolai Tesla had envisioned in the early twentieth century and wi-fi internet connection. How to impliment such an idea, barring space opera worthy magitech I got no idea.

To Robert - The proposed IS-Drive from what I've seen of the video seems supiciously similar to the Bussard Ramjet idea. I can't help but think that there is some design flaw in the idea that prevents it from being the ideal intestellar drive system, like electromagnetic drag or something similar.

And then there's the problem of interstellar matter, dust and gas impacting said starcraft at relevistic velocities. Not sure how the proposed drive is suppose to solve that particular problem.

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Scott said...

I'm going to have to ask, kilonewtons and meganewtons for a VASIMR? Can one really get it up that much (Last time I checked, under ten megawatts, by terribly reducing the exhaust speed, you could get something like 10^³ Newtons) or did you handwave the thrust?
Consider that wiki quotes 150MW delivered power for the S8G reactor used in an Ohio-class. It's heavy, but designed to run for 20 years or so at high power settings (ie, is just about orbitally-reliable). If we assume a similar-output reactor driving the VASMIR, that's at least 15KN thrust, assuming it scales linearly.

Personally, I'm interested in that Israeli physicist's electro-quantum mechanical drive. It (theoretically) uses no reaction mass, but instead generates thrust via a specific effect in the quantum vacuum using spin states.

Scott said...

curse the lack of an edit feature here.

to clarify: That theoretical drive generates thrust through the interaction of electromagnetic fields and spin states in a quantum vacuum.

I'm still looking for a link to the article.

Scott said...

Still no luck finding the article, but it looks like an application of the Aharonov-Bohm effect... I should have bookmarked that article when I saw it.

Tony said...

Re: Scott

Judging by the Wiki article on "Aharonov–Bohm effect", the proposed "drive" is at about the same stage of applicability as the Alcubierre warp drive. IOW, it's magitech with a plausible-sounding name.

Anonymous said...

Tony said:"Re: fission-fragment rocket

Where you going to test the thing? Nowhere on Earth, and that's where your industrial base is going to be in the plausible midfuture."

Initial proof-of-concecpt tests would be of a miniture version in a vacuum chamber through an MHD to slow the thrust and then into a collection/storage unit. The operational test would be conducted in orbit. The engine is only mildly radioactive before it is first used, so there shouldn't be any difficulty getting it into orbit. No one is proposing that you launch one of these things from Cape Canaveral! :0 It is for deep space propulsion only.

Ferrell

Tony said...

Ferrell:

"Initial proof-of-concecpt tests would be of a miniture version in a vacuum chamber through an MHD to slow the thrust and then into a collection/storage unit."

Not bloody likely, as my grandfather would have said. If the drive is as efficient as described, we're talking about exhaust velocities of a significant fraction of the speed of light, containing heavy, high-energy ions. The exhaust would in effect make a pretty good particle beam weapon, and a containment system would make a good Star Trek force field.

Elukka said...

Tony:
"The answer is: they don't. If you look closely at the illustration, they radiate past each other, similar to the radiator arrangement on deep space probe RTGs."
http://www.youtube.com/watch?v=Zj53rVWK5z0
You can see the radiators there edge-on at several points. It looks like half of the radiation would hit the tanks and the other half the neighboring radiator.

tkinias said...

Elukka:

You can see the radiators there edge-on at several points. It looks like half of the radiation would hit the tanks and the other half the neighboring radiator.

Yeah, that’s what it looks like to me, too. Draw a perpendicular from the plane of any one of the radiators and you hit the adjacent radiator. And, as Elukka notes, the tanks look like they mask almost half the radiator area.

But, as Tony mentions, that’s how (at least some) RTGs arrange their radiators: see the images at the Wikipedia RTG article. So what’s going on?

Maybe in RTG design compactness is important enough that the loss of efficiency from having radiators irradiate each other is worth tradeoff. And maybe the artist copied RTG radiator layout without thinking it through. Or maybe something else is going on that’s completely eluding Elukka and me. Tony?

That still doesn’t explain the masking by the fuel tanks, though...

Thucydides said...

High energy devices will be part of the plausible mid future (tm); they are part of the here and now and we live with them every day. There is a high pressure oil pipeline buried about 300m away from my house and another one several kilometers to the south. We are surrounded by high voltage power lines as well as the standard city utility lines, and there is a secondary highway about two kilometers east of my house with lots of truck traffic. My camera and old laptop have toxic Cadmium in the rechargable batteries, aircraft overfly my house on the approach to the local airport...perhaps I should move into a bunker? Ao long as the benefits outweigh the risks, most people will be willing to live with power beams or dusty fission fragment reactors in orbit.

WRT beamed power, you still need the most efficiency possible so as much of the energy you generate is actually salable. Some sort of handshake protocol will be needed to ensure the beam is on target and also to provide enough information to calculate any lead for the beam (an issue if the spaceship is light minutes away). Beam spillover will be looked upon as a sin by the accounting department, and minimized accordingly. Beam power will also be favoured because it is flexible, the energy source is separate from the power source, so if for some reason you really like steam engines, a Carnot cycle generator will fulfill your needs and still provide energy to the SS Minnow, who's Captain really does not care what the generator is, so long as he can receive a good quality power beam.

For the Solar economy, batteries of solar farms at the Earth Sun L1 point should provide a clear field of view for the power beams, and in this case they probably do have a secondary function as weapons of stupendous range; they can shoot down incoming asteroids, NEO's, comets or other space debris as well as making people think twice about sending laserstars or KKV's in the direction of the Earth Moon system.

The ships themselves might not need huge receivers, metamaterials have the ability to manipulate the optical paths of incoming beams and focus them to any arbitrary degree. OF course, there is no real reason you couldn't use an aluminum foil receiver of immense size either, even with bracing and control servos it will still mass far less than a nuclear reactor.

WRT the dusty fragment reactor, I believe the exhaust is moving at something like .03 c; since we routinely deal with heavy ions moving at a high fraction of c in particle beam accelerators, I don't think the technology to deal with small test reactors is out of reach. Still, the NIMBY brigade will probably ensure that nations without environmental protocols or respect for human rights will actually build the test articles and use them.

Anonymous said...

Tony said:
"Not bloody likely, as my grandfather would have said. If the drive is as efficient as described, we're talking about exhaust velocities of a significant fraction of the speed of light, containing heavy, high-energy ions. The exhaust would in effect make a pretty good particle beam weapon, and a containment system would make a good Star Trek force field."
I love it when people who don't know what they're talking about try to disagree with people who do. I've had 20 years experiance with high powered elecronic equipment and I can tell you that not only is the test possible, it is possible with today's technology. It may take a year or two to build the test chamber, but we can. Also, that force field? It's called a magnet.

Anyway, back to the OP; Jim's magsail sounds like the rocketpunk version of a clipper ship! Beamed power always seemed a tad bit iffy; It doesn't seem like it would be very reliable as far as targeting goes; also, what about junk moving through the beam?

Ferrell

Anonymous said...

Thucydides, I seem to have posted past you! I should type faster! :)
I agree with you; your post seems to have answered my questions. Thanks!
Ferrell

Tony said...

Ferrell:

"I love it when people who don't know what they're talking about try to disagree with people who do. I've had 20 years experiance with high powered elecronic equipment..."

Since it's not electronic equipment we're talking about, but electrical -- with a considerable amount of nuclear engineering -- your experience is relevant how? Be specific, no fudging or dodging please.

Also, we're not talking about a power reactor, which might conceivably be run at low enough energy output to manage, but rocket levels of power.

Tony said...

Elukka:

"You can see the radiators there edge-on at several points. It looks like half of the radiation would hit the tanks and the other half the neighboring radiator."

YMMV, but looks to me like the parts of the radiators that aren't pointed at tankage radiate past their neighbors. There's no fundamental reason why the whole radiator fin has to be radiating surface.

Anonymous said...

Tony: I'm sorry, but nothing I could say would would satisfy you: it is obvious that you have made up your mind on this subject and no amount of facts or logic could persuade you to do anything but disagree with anything anyone would say about this subject. Now, to answer your question, I've worked with high-powered electron beam devices (for communications), but the principle is the same; charged particles are slowed, accelerated, and their EM charicteristics manipulated; whether they are electrons, protons or ions, the princile is the same. I'm not going to send you my resume, so I guess that you'll just have to take my word for it. I'm not saying that the hypothetical test of the fission-fragment rocket wouldn't need some very heavy-duty equipment, but the principle is the same; granted that such an engine also emitts a certain amount of neutrons makes it imparitive that it is shielded, but nothing that cannot be done. I don't have to be a nuclear scientist to know that engineering principles and practices for the safe containment of a few fractions of a nanogram is achevible. Now, you can take this or leave it, but I don't care to argue with you anymore about everything I write: Please don't address me again and I will do the same.

Ferrell

Tony said...

Re: Ferrell

"[H]igh-powered electron beam devices (for communications)"? Could be cathode ray tubes, for as forthcoming as you've been.

I'll let others decide for themselves what you know and how relevant it is.

Rick said...

Beamed power has much the same relationship to self contained rockets as electric railroading does to steam locomotives.

Technical and security considerations aside, I would expect beamed power to develop when there is heavy, steady traffic along particular routes.

The beam installations need not lend themselves to practical weaponization; energy could be thousands of times more concentrated than sunlight and still merely scorch the paintwork of a target.

Rick said...

I forgot to add a welcome to a new commenter!

Anonymous said...

Hey can anyone help me with this? Whats the formula for the exhaust velocity of a VASIMR engine? the ship in the story I'm working on has 300 kN thrust from 4 engines at 300 km/sec exhaust velocity. I used the kinematic equation and got something like 16 gigawatts necessary power. Assuming 50% reactor effeciency and 80% drive effeciency plus about 4 gigawatts for other purposes on the ship, I got a requirement for 128 gW overall. That is a metric crapton of power, and the power plant likely weighs more than the ship. Just wondering if anyone had any additional details on exhaust velocity.

Thom

Jedidia said...

the ship in the story I'm working on has 300 kN thrust from 4 engines at 300 km/sec exhaust velocity. I used the kinematic equation and got something like 16 gigawatts necessary power.

Thrust power is always equal to thrust(N) times exhaust velocity(m/s), no matter what drive you use. That would in your case result in 90 Gigawats, not 16.

Rick said...

Welcome to a new commenter!

This is a good sample problem to illustrate my simple but clumsy method of calculating this stuff. 1 kg per second of exhaust at 300 km/s provides a thrust kick of - this is handy! - 300 kN of thrust.

Each kg of exhaust packs 300,000^2 / 2 = 45 GJ of kinetic energy, so expelling it in 1 second equates to 45 GW of thrust power. (!)

I'm not sure where the discrepancy with 16 GW comes from.

If the drive is 80 percent efficient it requires 56.25 GW of input power, plus 4 GW onboard power, so the reactor must provide a total 60.25 GW, and at 50 percent efficiency its total power output including waste is 120.5 GW.

Pretty close to 128 GW, though the intermediate steps don't all match. Either way it is beaucoup power, an eight of a terawatt total output.

Let's reverse engineer some possible characteristics of the ship. 300 kN can push 10,000 tons at 3 milligees, so let the ship have a departure mass of 14,000 tons and arrival mass of 7000 tons, a mass ratio of 2.

Mission delta v is 200 km/s, and full power burn duration time is 7 million seconds, about 80 days. The flat space brachistochrone distance is about 700 million km, about 4.3 AU.

This profile gets you to Jupiter in about 90 days, or with a coasting phase reaches Saturn in about 130 days. (For Uranus and beyond you'd want to adjust the drive to somewhat higher ISP and lower acceleration.)

So, we have a big ship capable of Pretty Fast travel to Jupiter and Saturn.

Let's continue the estimated mass breakdown:

3000 tons drive engine
1500 tons keel, tankage, etc.
2500 tons gross payload
____

7000 tons mass (less propellant)

The power plant puts out 60 GW from 3000 tons = 20 kW/kg power density, comparable to jet engines. This is an order of magnitude better than could be expected of any conventional nuke electric plant; you'd need fusion or some other semi speculative power plant.

The ship may be entirely different from what I reverse engineered, but the drive specifications seem appropriate for what might be called the further reaches of the plausible midfuture.

Rick said...

Three solutions to the sample problem, three different answers. Further class exercises are probably in order. :-)

(Isn't there a division by 2 in the formula?)

Robert said...

To Sabersonic:
The concepts are different:
1. The Bussard Ramjet is an expelling-mass fusion rocket using enormous electro-magnetic fields as a ram scoop to collect and compress hydrogen from the interstellar medium.
2. The Phased-shifted Thruster is based on spaced-apart phase-shifted electrodynamic forces acting/reacting directly on the interstellar medium; it is powered by an onboard aneutronic fusion reactor.

About matter and debris impacting the craft, perhaps a strong titanium alloy hull more a powerful magnetic field generated by superconducting magnets can minimize the problem.

Jedidia said...

(Isn't there a division by 2 in the formula?)

Indeed you are right. My fault.

Tony said...

Re: phase-shifted thruster

Neat in theory, I suppose, but the phase-shifts have to propagate as fast as light. Problem is, the control signals can propagate no faster than light, and they still have to be processed at each stop. You would have to guarantee exactly the same processing time at each propulsion node, or the dirve would phall out of phase and cease to work.

Also, claims are made that it would be more efficient than some kind of rocket. Weullll...to achieve a certain propulsive effect, one has to put in a certain amount of energy. You may not have megatons of reaction mass, but you will have megatons of power supplies and their fuel.

Markus said...

A long post incoming, about beamed power:

It is strange how everyone's talking of beamed power, and yet I still have to see a single proposal on what's going to project that beam.

In the long-winded laserstar vs. kinetic threads earlier, there was talk of visible light lasers with 10 meter mirrors. Using the laser spot size equation from Atomic Rockets (s = r * lambda / d, where r is range, lambda is wavelength and d is mirror diameter, there was also some factor of 0.8 or something like that but screw that since our estimates are order-of-magnitude), we can get some estimates on how long is the effective range of the beam devices.

Let's assume our ship has a solar collector of 100 m diameter. That might weigh 15 metric tons if it was one millimeter thick. With 10 m mirror and 500 nm visible light laser as the beam device, we get a 100 m spot at 2 million kilometres, i.e. about 1 % of an astronomical unit. After that, you're wasting energy into space and getting less power. This is not nearly enough range to get you to Mars.

If the laser emits at 100 MW, the collector achieves after efficiency consideration (20%) about 1300 W/kg. In this case, each time we double the distance from the beam device we have double the spot size, thus quadruple area: it follows a 1/r^2 law after 1% of an AU.

At short ranges, one has to defocus the beam not to burn the collector.

The relationship of beam device performance and ship's collector size can be, of course, tweaked. We ain't gonna be witnessing x-ray lasers in the midfuture and I wouldn't bet on 1 km mirrors either, so to increase the effective range of the beam system we either have to have a more powerful laser (which still, after 1% of an AU, starts wasting energy into space) or have a larger collector surface (which adds mass and thus slows us down).

To achieve a range of one tenth an AU (15 million km), which might be enough to at least give a good kick to a ship bound for Mars, one needs a collector 750 m in diameter, weighing about 900 tons. To achieve 1 kW/kg at the collector, your beam device has to emit at 4.5 GW (if collector is 20% efficient). The power you get at the ship is 900 MW. I think these are actually quite reasonable figures in "plausible midfuture".

And just as a closing remark, at one light second that beam intensity is 25 MW/m2, enough to burn through 1 cm of steel in 20 seconds or so.

- Markus

Milo said...

Thom:

"Hey can anyone help me with this? Whats the formula for the exhaust velocity of a VASIMR engine?"

The formula goes: power = 1/2 * exhaust velocity * thrust, so you can fill in any two of those values and extract the third.

In your case, 300 kN thrust * 300 km/s exhaust / 2 = 45 GW - unless you meant 300 kN per engine, in which case it's four times that value.

Another way to calculate it is: (power plant mass / ship mass) * power density = 1/2 * exhaust velocity * acceleration, where power density is constrained by your tech. Keep in mind that 2 MW/ton is about an upper limit for fission technology, barring a massive breakthrough.

Scott said...

@Tony:
Be specific, no fudging or dodging please.
Some of us *are not allowed* to be specific. Not one of you needs to know some of what I've done, period.

@Anonymous: 4 gigawatts extra power? Got a couple big lasers you're driving?

Most wet-navy ships run less than 10 megawatts for ship's service electrical generation. I don't think aircraft carriers exceed 100MW electrical generation.

Robert said...

The concepts are different:
1. The Bussard Ramjet is an expelling-mass fusion rocket using enormous electro-magnetic fields as a ram scoop to collect and compress hydrogen from the interstellar medium.
2. The Phased-shifted Thruster is based on spaced-apart phase-shifted electrodynamic forces acting/reacting directly on the interstellar medium; it is powered by an onboard aneutronic fusion reactor.

About matter and debris impacting the craft, perhaps a strong titanium alloy hull more a powerful magnetic field generated by superconducting magnets can minimize the problem.

Robert said...

To Tony:
The control system doesn’t need to be as fast as light for keeping the system working.
One solution is, in every time, sensors at each dipole can send feedback to a unit control for adjusting the phase-shifters to guarantee that each dipole is receiving the correct phase.

The rocket equation tells us, for increasing propellant-efficiency it is needed to increase the exhaust velocity, and the required power will increase with the square of exhaust velocity, what means that most of the energy goes away with the propellants leading to an outrageous waste of energy for keeping a spacecraft accelerating close to mere 1g-force.

Tony said...

Scott:

"Some of us *are not allowed* to be specific. Not one of you needs to know some of what I've done, period."

With apologies in advance to Rick and others for being so straightforward:

I've held a Secret clearance twice. I can't (and wouldn't, for very good reason) tell you everything I did WRT that clearance, but I can tell you I was a Marine, what my MOS was, what my training was, and what my experience (outside of the materials and activities covered by the clearance) is.

My grandfather held a Top Secret clearance for decades. I still know that he designed production tooling for the Redeye and Stinger guidance systems. I know what his general qualifications to do that were.

Almost anybody that works in a technological field for twenty years, as Ferrell claims, can tell anybody who asks what his training was and generally what kind of things he worked on -- certainly with much more specificity than Ferrell has mustered so far -- regardless of any security constraints he may have worked under. If he's one of the rare individuals that truly can't reveal anything about what he knows and how, he wouldn't say anything to begin with that would have people asking, "Now how do you know that?"

The bottom line is that we've all seen the outing of internet frauds. Ask yourself: how many signs of fraudulent behavior has he already shown? (FWIW, I count two really classical ones.)

Tony said...

Re: Robert

From what my google fu can find out: "the speed of traveling transverse force must be as fast as light waves". That reads to me that the phase-shifting has to propagate at the speed of light. Not that that couldn't be accomplished by sending timing signals via radio -- it's just that each drive node would have to read and implement the timing signal at near enough to exactly the same speed that the drive won't fall out of phase. Theoretically possible, but highly questionable for practical purposes.

Also, this is not a reactionless drive. In the most credible form, it uses the mass of the interstellar medium for thrust. (I'm not going to hold my breath on galactic/intergalactic magnetic fields, force at a distance against stars/planets, or interactions with spacetime.) That makes it an inside-out electrical rocket. Which means that the rocket equation is just as relevant to this type of drive as any other.

Robert said...

Re: Tony
With nowadays technologies (e.g. “phased array radars”), to keep the signal correctly phase-shifted is not so hard, even more with frequencies in a range of MHz instead of GHz.
It is not a reactionless drive. It will need to act and react with something, E/M fields, gases and cosmic dusts, and so on.

Tony, I believe theories, concepts, ideas, constructive thoughts, hypothetical claims, and expected results, should not be considered frauds, even more if they are broadly shared on the internet for open discussion.

Tony said...

Re: Robert

I'm not talking about how fast a circuit can be cycled. I'm talking about the apparent requirement that the phase-shift itself propagate at the speed of light. If I read that wrong, then please tell me how it should have been read. If I didn't, pleasetell me how it is practical.

I never labeled an idea fraudulent. (Though I have labeled more than a few impractical, no matter how well grounded in theory they may seem.) When I used the word "fraud", it was in reference to something else.

Tony said...

Re: Robert

Oh yeah...like I already said, I skeptical of propulsively useful interaction with the non-material medium. I can guardedly accept interactions with any gas and dust that happens to be floating around in the spacecraft's vicinity. But, like I also said, then you're talking about an inside-out electric rocket -- and one of only speculative efficiency at that. And it would be interesting to see how relative velocity between the vehcile and the medium would affect propulsive efficiency. My guess would be that it would go down.

Thucydides said...

A laser weapon will have a mirror of limited size in order for it to be retracted into a protective enclosure, and also to allow it to swivel rapidly in elevation and azimuth to engage targets without becoming defocused through rippling or excessive vibrations.

A beam power station will have far fewer restrictions of that sort (the target is cooperative and moving in a fairly predictable path, so the beam mirror can move slowly), and will be attached to a much larger structure than any spacecraft, which facilitates a larger mirror and mounting as well.

I also think that since the power collector is optimized for the standard beam power wavelength, the efficiency can be much higher than the 20% of standard solar cells, but that is a pretty flexible standard, so 20% might be considered a kind of baseline.

Rick said...

Umm, a couple of commenters need to cool off a bit.

Ferrell is one of the longest term regular commenters on this blog (and consistently an advocate of balanced good sense). Jumping from some dispute I'm admittedly not trying to follow to a serious accusation against a fellow commenter is ... problematic.

Continue the discussion of the substance, but let this thing drop.

So far as credentials go, I have none whatsoever that are relevant to any of the topics I discuss on this blog. So credentials as such carry no particular weight here. OTOH, a number of commenters have much greater technical knowledge than I do, which certainly improves the discussion.

Some commenters also appear to have classified clearances, and must walk delicately around particular subjects.

But arguing over credentials is entirely unhelpful to the outside observer.

Robert said...

Re: Tony
It’s hard to explain; I think first you need to understand how linear motor works. It is a linear motor using dipoles instead of electromagnets to get less inductance more AC current and consequently, more electric output power.
Now, only imagine, the phase shifters splitting RF waves into six phases (0°, 60°, 120°, 180°, 240°, 300°), they are correctly phase-shifted, now each phase is correctly connected to each spaced-apart dipole, each phase-shifted signal is out there flowing at speed of light and this speed don’t care, what care is the overall effect that is similar to a multiphasic linear motor generating a moving electric/magnetic field along the array of dipoles, producing electrodynamic forces virtually as fast as EM waves, adjusted by (f ≥ c/L).

Well, I believe it will produce thrust force more efficient than any conventional expelling-mass rocket, whether it will or not, in fact, I think nobody is able to assure without sending it to outer space.

"Heavier-than-air flying machines are impossible." –Lord Kelvin, president, Royal Society, 1895
"Space travel is utter bilge." –Dr. Richard Wooley, Astronomer Royal, space advisor to the British government, 1956

Tony said...

Robert:

Please don't trot out the same old cliches. My skepticism is based on a little bit more than haughty disregard for speculations outside of my field. I never said the thing wouldn't work in principle. I said I have trouble seeing the practicality, and extreme skepticism regarding some of the claims about the mechanism.

A linear motor gaining useful amounts of thrust against dust and gas at insterstellar concentrations? Really?

Maintaining propulsive efficiency as velocity increases relative to the medium? Really?

Reacting against spacetime or even galactic magnetic fields? Really?

Robert said...

Re: Tony
It’s hard to explain; I think first you need to understand how linear motor works. It is a linear motor using dipoles instead of electromagnets to get less inductance more AC current and consequently, more electric output power.
Now, only imagine, the phase shifters splitting RF waves into six phases (0°, 60°, 120°, 180°, 240°, 300°), they are correctly phase-shifted, now each phase is correctly connected to each spaced-apart dipole, each phase-shifted signal is out there flowing at speed of light and this speed don’t care, what care is the overall effect that is similar to a multiphasic linear motor generating a moving electric/magnetic field along the array of dipoles, producing electrodynamic forces virtually as fast as EM waves, adjusted by (f ≥ c/L).

Well, I believe it will produce thrust force more efficient than any conventional expelling-mass rocket, whether it will or not, in fact, I think nobody is able to assure without sending it to outer space.

"Heavier-than-air flying machines are impossible." –Lord Kelvin, president, Royal Society, 1895
"Space travel is utter bilge." –Dr. Richard Wooley, Astronomer Royal, space advisor to the British government, 1956

Thom said...

OK. Thanks guys. I don't know how that wierd discrepancy got in there.

Robert said...

Hi Nick, Sirs, I’m sorry for this off-topic discussion.
Nothing is perfect and this concept too.
But it is less exotic than metamaterials, and, similarly to “phased array radars”, it can receive megawatts of electric power without big complications.
I believe this concept is the best option that we have to replace expelling-mass propulsion.
I’m stopping it here. Thanks.

Scott said...

Not so much 'walk delicately around' but having to cite other (open) sources for our info.

@Thom: nuclear-electric drives are obscenely powerful, to say the least, but don't forget that there's a difference between thermal energy and electrical energy.

That 150MW plant in a submarine is measuring delivered power, not total output. Total output is significantly more. No, I don't know how much higher, too many variables involved and too much information not known to the general public (or me). I'd make a rough guess at a total thermal output close to 500MW, possibly higher.

An aircraft carrier has up to 8 of those onboard (USS Enterprise), so that's 2GW thermal as a rough guess. Does your rocket sound that extreme anymore?

Thom said...

@ Scott:

When you consider that my ship has to deal with 128 gW vs 2, yes. It does.

Rick said...

It is extreme in terms of our current capabilities, but for an outer system liner of 2268?

Well, maybe not a liner. I join Scott in raising an eyebrow at the 4 GW onboard power requirement, which hints at some major armament.

I differ with Thucydides on the need for laser stars' armament to be mounted turret-wise. I see laser stars as rather like railway guns; range and hitting power trump everything else.

Thom said...

True. 4 gW is ridiculous. This was an exccedingly rough approximation, done mostly in my head in the back of a bus. However, it is not entirely out of the question. I had been thinking of the ship as sort of a "one size fits all" vessel. Need some lasers for orbital guard or asteroid mining? Take out the cargo bay, fill it with a row of capacitors, and you get an extremely powerful tool/weapon. In-Situ resourse utilization? Crack water off of an ice asteroid. The possibilities go on. However, I had been planning an H2 cracker on board. molecular hydrogen, stored cryogenically, would be broken up into atomic hydrogen just before entering the engines. I confess my ignorance of exactly how much power this would require (though I believe it would be less than the 4 gW) or how great the increase in effeciency would be. Seemed like a good idea at the time.

In summary though, you are right. 4 gW extra power is nothing short of ridiculous.

Anonymous said...

Thank-you Rick.
Back on topic...
One way of maximising Beamed-power systems is to have several different stations; as you move out of range of one, you are picked up by another...of course that meens that you need to build and position several of these stations around the Solar System. A variant that isn't as efficent as 'pure' beamed power propulsion is a hybrid system where the beamed power is used to power the craft, the off-board power heating or ionizing the propellant. While you have to carry propellant, you don't have a heavy powerplant. Give and take, power density is lower for off-board power vs beamed-power propulsion (like an enhanced light-sail), but it still seems like a massive construction project...who knows, maybe in a century or two we might feel the need.

Ferrell

Anonymous said...

Thom said:
"In summary though, you are right. 4 gW extra power is nothing short of ridiculous."
Well, yeah...unless you have a LOT of magnetic shielding. :)

Ferrell

Thucydides said...

Rick, I drew the inference the weapons laser mirror needs to have some sort of fast acting mount based on previous space war threads in this blog, particularly discussions of how much overmatch a KKV armed force needs to overwhelm a laserstar or constellation.

A laserstar on piquet duty will have to face an oncoming swarm of hundreds to thousands of oncoming KKV's. Depending on the way it is programmed, it needs to shoot down a large fraction in order to ensure its own survival, and have a Pk close to .9 or higher in order to ensure it protects the rest of the constellation (If you program the piquets to sacrifice themselves for the greater good, you might be able to reduce the Pk a bit, but not too much). The mirror is going to have to move quickly in order to defeat the threat, even if the ship is opening the engagement at stupendous range.

Lunar Helium and Electric, on the other hand, is dealing with a handful of cooperative targets moving on predictable paths, so their engineers can concentrate on issues like reducing diffraction and dealing with waste heat over prolonged periods, rather than combat surges of power (the laserstar's mirror might rely on a heat sink for combat and a secondary radiator to cool down between engagements while the power beam mirror needs a large primary radiator on constant duty. A system using a maser would have different tech details, but have many of the same issues).

A few other "beamed" power systems have been proposed, mostly involving the exchange of momentum between various objects and the ship. These can range from a "catcher's mitt" taking momentum from large object launched from a stationary mass driver to "hot beam" systems blasting particle beams at cooperative magnetic sails. An interesting "in between" concept has a beam station launching tiny solar sails (from postage stamp size to "nano sails" the size of dust motes) in swarms which impact the receiver and transfer momentum that way (Some Novel Space Propulsion Systems Copyright © 1997, Forrest Bishop) www.foresight.org/Conferences/MNT05/Papers/Bishop/

You can take this idea how you will, but it mostly shows there are no real show stoppers and lots of work arounds to the idea that the energy source is separate from the spacecraft.

Elukka said...

Perhaps this hints that we may require more types of laserstar (or larger laserstars with more varied weaponry) than the Big Gun variant.

Use the big guns against other ships, while using smaller, more mobile turreted lasers as point defense.

Thom said...

Ferrell--
I hadn't thought of that use. High level magnetic shielding for working close to a star? If orbital power becomes economical, that has serious potential. Thanks man.

Milo said...

Ferrell:

"One way of maximising Beamed-power systems is to have several different stations; as you move out of range of one, you are picked up by another..."

The problem is that space is empty. Except for planets and their immediate environment, there's nowhere to put your stuff. And there's a lot of space between the planets.

The transmission station on Earth has to be strong enough to carry you until you're halfway from Earth to <wherever you're going>.

Elukka said...

You don't need ground to build on in space, though. There's nothing stopping you from building a station in empty space.

Hey, couldn't you use simple automated satellites to refocus the beam?

Byron said...

I've been lurking so far, but I thought I'd weigh in. Why is fusion being ignored in this? I know it's 20 years off, and has been for the last 50, but if we can actually make it work, it has a lot of potential, particularly in being more throttleable than VASMIR. It's not known to be feasible, but there are models that say it should work rather efficiently.
The problem with beamed power is that using it puts you between a rock and a hard place. If you degrade the power to the point at which it isn't dangerous, you need a lot of receiver area, and thus mass. And even if the beam isn't dangerous at interplanetary distances, it could still be a weapon at intraplanetary distances. It might be possible to design this out, but I'm not going to bet on it.

Tony said...

Re: Byron

Nothing wrong with fusion, if you can make it work. As Rick pointed out, VASIMR is, in many ways, a de-rated fusion drive. But however you go about it, the drive problem in abstract is a question of energy density.

Beamed power, for example, seeks to put the inefficient part of the system off of the driven craft, so that effective energy density is calculated based on delivered kilowatts of thrust per kilogram of receiver + motor. Whether beamed power is a solution is a function of trading off all of the variables discussed, plus some others. Maybe it's viable, maybe it's not -- but there's no fundamental reason why it won't work, if the practical details can be managed.

WRT the weaponization of power beams, I wouldn't worry too much about that. Remember, power beam stations have to be put several million kilometers (at least) away from planets so that the planets will not occlude the beam for any great length of time.

The same goes for most reaction drives. Nuclear-electric schemes, from ion motors through fusion, should also worked, practical consideration allowing. The same goes for nuclear thermal.

More exotic types, like fission fragment and nuclear salt water rockets, have implementation safety issues, no matter what some people will tell you. Even if you believe that the exhaust of test systems could be safely contained*, it's hard to believe anyone would be allowed to use such machines this side of the Moon. They have essentially the same radiological contamination problem that caused Freeman Dyson gave up on Orion, even for orbit-to-orbit operations. Too much of the exhaust would be captured by the Earth's magnetosphere and returned to the Earth.

(*And that's not a given -- these machines toss radioactive fission by-products overboard at exhaust velocities of 3% of c or higher. These are not relatively light protons or electrons, but ions with an average atomic weight of 118. I wasn't exagerating when I compared these machines to particle beam weapons.)

AdShea said...

The question of beamed power v. High power drive (especially those drives that dump their remass in the form of high-mass particles) is that with beamed power it's possible to have centralized control of the Kzinti-Lesson weapons (just shut off the beam if a ship is pointing his jet wrong) as opposed to every tramp-freighter pilot having a death ray at his command.

Milo said...

Elukka:

"You don't need ground to build on in space, though. There's nothing stopping you from building a station in empty space."

Yeah, but the problem is that this station will spend much of its time being nowhere near your intended route, on account of it orbiting independantly from both your source and your destination.


"Hey, couldn't you use simple automated satellites to refocus the beam?"

Automated satellites require something to orbit. Which counts as "ground".

Byron said...

Yes, you can shut off the beam if the freighter's drive is pointing wrong. However, that beam is an even more lethal weapon. Spacecraft drives will not be usable as weapons at anything other than very close range. They are not designed to be collated as a weapon, and will likely be designed to minimize the consequences if someone tried that. And then there are practical issues. How in the world do you maneuver so you can point your drive at the enemy, and keep them there, at close enough range to be damaging?
The power beam, on the other hand, has to relay energy over interplanetary distances. It's going to be lethal at close range no matter what.

Tony:
I believe you actually mean specific power, not energy density.
And the beam might be a million kilometers away from Earth. But if it's designed to, say, deliver a 10 m beam at 10 million kilometers (I'm making these up as I go), then it will have a 1 m beam at Earth. With any sort of reasonable power, it will be a weapon.

KraKon said...

Someone proposed to be that I use my ICF exhaust as some kind of close-in plasma weapon against incoming kinetics....maybe because my drive power rating was in the order of 450MW :/

On beamed power:
Yes you can cut off the power to a tramp freighter, but centralized power also translates into major vulnerability in case of attack. I want to immobilize all (or most) of your ships? Concentrate fire on your large beaming satellite. Want a powerful level during negotiations? Hijack one of these satellites and you've got a Ravening Beam of Power without having to build one. Or you can build one, but say its for your navigation project....

KraKon said...

Following avidly...

Byron said...

If you think that's a lot, my Aurek uses a 335 GW D-He3 magnetic engine. And on takeoff, it vaporizes about 30 cubic meters of basalt. It might be useful against kinetics, but only from a single direction, and even then, a big enough one might get something through.
I see security as the biggest issue of beamed power. It's far too easy to turn the laser into a weapon, while a ship will have some very contorted maneuvering to do so.

Tony said...

Byron:

"I believe you actually mean specific power, not energy density."

Actually, we're talking about something that I'm not sure has an agreed upon technical term. We're not talking about the power-to-weight ratio of the engine itself -- which would be specific power -- nor are we talking about the energy density of the fuel itself. What we're talking about is the amount of propulsive force that the entire propulsion system can generate per kilogram of mass, fuel, electrical power source, and engine mass combined. Additionally, we're talking about what actually has to be driven, not components that may be necessary but can be left off of the ship, like power beam generating stations or mass projectors.

I think "energy density" is a good, if not particularly accurate, term. If you have a better one, please feel free to nominate it.

"And the beam might be a million kilometers away from Earth. But if it's designed to, say, deliver a 10 m beam at 10 million kilometers (I'm making these up as I go), then it will have a 1 m beam at Earth. With any sort of reasonable power, it will be a weapon."

As previously stated, I think it more likely that several million kilometers would be more accurate. It's a balance between accessibility, position relative to serviced trajectories, and keeping the angular diameter of possible occluding bodies as small as possible.

But we have to recognize that the power beams have to be effective over hundreds of millions of kilometers, not tens of millions. That would mean that even ten million kilometers might be a little close for comfort.

On the other hand, the 9/11 attacks didn't bring commercial jet travel to an end. I would think the threat of a beam attack that might not be nearly as destructive wouldn't be much of a deterrent. (Even at potentially destructive energy fluxes, the atmospere is a pretty good shield against beam weapon attacks.)

Thucydides said...

As noted, beamed power (or momentum if you are into large mass drivers delivering momentum to the ship) is a way of putting the mass ratio in your favour by taking the large, massive and bulky stuff out of the ship and leaving it in orbit or on a suitable asteroid or moon. Since range is an issue, ships might be designed to absorb large amounts of energy for short periods of relatively high g to boost, then coast the rest of the way. I would think that a viable fusion drive also has a place in the plausible midfuture (tm), mostly for military vessels and ships that do not follow heavily traveled orbits (mining vessels looking for McGuffinite?).

In fact, there would be room for an entire spectrum of drives, predicted mostly on a cost/benefit ratio. Passengers would need to get from point A to point B as fast as possible to reduce life support and liability costs [being sued for radiation damage, confinement stress or osteoporosis due to inadequate gravity would be hellish on the company books], and to generate as much revenue as possible by keeping the ship full of paying passengers as often as possible, supporting the development of demi-torchdrives, beamed power systems or whatever other means that can do the job for a reasonable price. Some cargo might be shuttled by unmanned NTR busses using water remass for logistical simplicity (core design would be a treat); with an ISP on the low 400 range at best, this would make for years long voyages. Solar, magnetic or electrostatic sails might also do for this sort of duty. Low value bulk cargo might end up in orbital “pipelines” filled by shooting ISO containers on minimum energy trajectories via mass driver, for deliveries decades down the road. Futures traders will love this scenario.

Orion drives will make an appearance as a military system, the only plausible midfuture drive capable of “torch” performance (a missile accelerating at 100 G is way beyond any other conceivable technology barring the discovery of magitech), there may also be some sort of niche for nuclear salt water rockets or dusty fission fragment drives in deep space, especially if VASMIR, fusion or beamed power is unsuitable for any reason

Byron said...

I think the term for that is specific impulse. Wait, that's already in use. Actually, it might be the proper term, if we lump the rest of the engine in with the fuel.
(I actually typed that without remembering that it was a real term.)
As to why a beam will be a threat:
Any beam will be placed so as to cover a location (planet, moon, trojan colony, etc). As it will be dealing with incoming and outgoing ships from said location, its range must be significantly larger than the separation between it and the location. Thus, if it has serviceable power flux at the maximum range, it will have far greater flux at the location. Maybe not enough to hit stuff on Earth's surface, but it could do a job on the stuff in orbit. The specific separation is largely irrelevant.
You could play games to make it work, but it would severely limit flexibility.

Tony said...

Byron:

"I think the term for that is specific impulse."

Sepcific impulse is essentially a measurement of total thrust output per unit of reaction mass. It doesn't reflect the mass efficiency of the propulsion system as a whole.

"As to why a beam will be a threat..."

All of which goes without saying. The point is that just because something poses a threat doesn't mean that it can't be used. It's a risk management issue.

And, before anybody goes there, yes the same thing can be said about radiological contamination from drives that expel radioactive isotopes. The difference is that a laser or microwave attack can be deterred or at least realiated against. Radioactive contamination, once it's in the environment, is there for hundreds or thousands of years, regardless of any person's or group's specific intent.

Anonymous said...

I think that most of the drives that have been discussed on this thread are plasusible, although almost all have some sort of drawback in some situations. For instance, I wouldn't use fission-fragmet or NSWR for planetary lift-off, nor do I think that ion drive is suitable for Earth-to-Lunar missions; using chemfuel to go to Jupiter probably isn't a good idea either; You just have to pick your drive to match your mission profile. Some exotic chemical fuels are unsuitable for use in Earth lift-off due to their toxic nature; nuclear drives (both fission and fusion), produce various levels of radioactive exhast, although some of it is so heavy and moving so fast that it is bent by Earth's magnetic field, but the great majority of it should escape both Earth orbit, but Solar orbit as well. High energy beamed power (depending on the frequency) can do much damage to a city or forest that is short of catastrophic levels, but still quite serious. The safty issues go on and on...just like with most modern travel technology; we just need to be careful and handle it with the respect that it deserves.

Anyway, space drives are tricky due to their suitability (or lack thereof) for a given mission. I think that mix-and-match drives/habs/payloads will become the standard during the last decades of this century. We can only hope...

Ferrell

Scott said...

128GW thermal doesn't sound that big, honestly. What's the thermal yield of a Saturn V first stage?

The Sulaco (of Aliens fame) is pushing 3.6 terawatts electrical, so make that 3.6/.3=12 terawatts of waste heat, to push some 78,000 tons around at a max of 1.8 gee. No, I don't know why the Sulaco doesn't turn itself into a glowing ball of blue-white plasma as soon as they start the reactor.

DP9's Jovian Chronicles ships seem to have forgotten drive efficiency multipliers, but they are still dragging around multiple GWs. The classic 'Valiant-class' has 4 reactors delivering 800MW each, plus secondary generators. If you add in a thermal-to-electric efficiency of .3, that's 800MW/.3, roughly 2.7GW thermal per reactor, for a total of nearly 11GW thermal. The Valiant-class mass about 50,000 tons, and can accelerate at .8 gee.

Keep in mind that those are basically NTR-gas drives, but using a fusion reaction as the heat source instead of fission. I'm not sure what effect that would have on the exhaust velocity, since nobody has gotten a fusion reaction to last long enough to get numbers (yet).

I haven't worked backwards through the Atomic Rocket formulas to see what kind of thermal signature the DP9 ships should have, yet.

Tony said...

Ferrell:

"The safty issues go on and on...just like with most modern travel technology; we just need to be careful and handle it with the respect that it deserves."

Sorry, but that's incorrect. It ignores the qualitative difference between things like energy beams and jet airplanes, on the one hand, and sources of radiological contamination on the other. The first can do damage, but the damage they do can be contained and contolled -- or deterred altogether. Radiological contamination, especially that released in orbit, will distribute itself uncontrollably through the biosphere and reamin a hazard for centuries or millenia. Trying to draw an equivalence between localized applications or releases of energy and radiological contamination is misleading.

Scott said...

It's nice to be done with my finals this semester.

Just crunched some numbers. The drive power on the Valiant-class is roughly 350GW per engine. Assuming that the 'plasma combustion chamber' drives are effectively NTR-Gas designs writ BIG (they *are* outside the NTR-gas MAX level on the Atomic Rockets engine chart), then 800MW reactors feeding the engines just don't sound right.

Can you really get more rocket KE out of an engine than you get thermal energy?

Continuing to run the numbers from the fleetbooks.
Using MHD generation, 800MW electrical at 60% efficiency makes for about 1.4GW thermal. In theory, the exhaust could dump all that, and then some. I don't even know where to start looking for confirmation of that idea.

From my navy experience, I would assume that the reactors would spend most of their time running at a relatively low total output, maybe 25% max capacity, so you'd need a total of 4800m^2 radiator area for radiators @1600K, plus some big wings to dump life-section heat. The ring section around the habs is roughly 20m long and 285m in diameter, which gives you a veritable crapton of radiator area, but is complicated by getting the heat into the rotating section. However, that is actually enough to dump 4MW of waste heat at 300k.

Byron said...

Tony:
I know. Maybe we should use drive section specific impulse, which includes everything involved.
Can you please provide some numbers on the radioactive contamination issue? You're acting like it completely rules out any radioactive-exhaust spacecraft, at least near Earth. How much radiation would your average person get from one trip by one of these ships? Is there a way to minimize the danger? The burden of proof here is on you.

As for beams being not dangerous, you're basically putting a really big laserstar in orbit. If it falls into the wrong hands, very bad things can happen.
The concept also seems like a case of putting all your eggs in one basket. If someone didn't like the beam, they just send a cold kinetic at it. It's going to be ballistic, or moving in a fairly small area, so the projectile probably won't be detected until two seconds before impact, when the spreader charge goes off.

Actually, D-He3 drives are pretty clean, as the only radiation is neutrons from secondary D-T reactions and the tritium itself.

Scott:
Chemfuel rockets can't really be speced in thermal energy. We usually use thrust power. A Saturn V had about 400 GW thrust.
Your numbers rely heavily on everyone using the same process, which can't be relied upon. For example, according to wikipedia the Sulaco uses a lithium hydride fusion reactor, which means it's D-T. You're going to have 80% of output power in neutrons, but the other 20% can be almost straight electric. I'm not sure about the other ships. They might be using He3 reactors, which can be far more efficient. Or they might be using direct thrust, which means they can skip thermal issues.
A direct exhaust D-He3 fusion torch can have a specific impulse as high as 10^6 seconds, and I'd estimate that a D-He3 fusion reactor could produce electricity at around 90% efficiency, as all the products are charged particles.

Tony said...

Byron:

"I know. Maybe we should use drive section specific impulse, which includes everything involved."

I think we need to decouple the idea of engine specific efficiency from that propulsion package efficiency. I also think system mass is a factor, but I don't think specific power is the right term, because the mass of the fuel/reaction-mass/electrical-power-supply are also factors. The closest thing I can think of is what is done in chemical rocketry with the payload-to-orbit number, but that presumes roughly the same efficiency (somewhere between 270 and 450 seconds Isp) in roughly comparable vehicle classes. I'm thinking of a figure that could be used in tradeoff studies between wildly varying technologies.

But then again, maybe that's the point -- you can't think in terms of one efficiency number, but have to consider a range of factors, picking those that are relevant to a given application.

"Can you please provide some numbers on the radioactive contamination issue?"

Nope, but then again I'm making my case on qualitative, not quantitative grounds. Radiological contamination over time is simply not the same thing, qualitatively, as immediate destruction due to an application mechanical, chemical, or electromagnetic energy.

What I'm trying to do here -- and this is entirely and absolutely impersonal, despite recent unpleasantness -- is counteract a mode of thinking. This mode of thinking maintains that if we want to do a certain thing, and a certain type of machine seems expedient, then only the will to do the thing need matter. Any safety measures taken will be entirely within the context of minimizing damage.

Well, that's not how the real world works. Certain types of damage, like radiological contamination, are to be minimized altogether, regardless of the cost in convenience to do this that or the other thing. IMO, disregarding this very real imperative is irresponsible, even in theoretical discussion.

Why do I harp on radiological contamination from rockets so much? Because Freeman Dyson was turned off to Orion by it, even for orbit-to-orbit schemes. Many of the more exotic nuclear rocket schemes envision releasing, as a matter of normal operations, significant amounts of radioactive materials. Some not as much as Orion, to be sure, but then again many of these are expected by their supporters to be used regularaly for decades or centuries, perhaps even for millenia. The total contaminaiton is simply beyond calculation.

But whether it is small or large, the idea that radiological contamination is no different from other types of risks is simply mistaken. It isn't something that can be deterred or mitigated. Once it happens, that, as they say, is that. It can't be rebuilt, or reparated, or retaliated against. It's a part of the environment until it decays.

"As for beams being not dangerous, you're basically putting a really big laserstar in orbit..."

Factually true, but missing context. Use of power beams as weapons can be deterred and/or retaliated against. So can the military targetting of power beam stations. In their normal operations they present no environmental risks.

Scott said...

Unfortunately, I'm an international business major, not a physics geek.

It still just doesn't look right to have an 800MW electrical reactor somehow powering a 350GW drive, even if that reactor is making 1.6GW of heat.

Byron said...

Tony:
So there is no situation where any level of radioactive contamination is allowable at all? I find that hard to believe. You do raise a valid point, but radioactive fallout (not the proper technical use, but close enough) is not something to be avoided at all costs in my book.
I'm not so much worried about planetary neighbor using his beam against me, as I am about someone hijacking my beam and turning it on me. And what about terrorism? The beam would be an ideal target, either to destroy or hijack.

Scott:
Assuming the engine is electric, it can't. I was slightly uncertain what your earlier comment referred to.

Scott said...

The 'Plasma Combustion Chambers' used in Jovian Chronicles are closer to a gas-core NTR with a MHD 'afterburner', I think, but I may be mistaken.

=====
As a former radiation-worker, there are a few times that required maintenance will require some contamination of tools or whatever. This usually means that you are going to throw the tools away in a lead-and-concrete dumpster at the end of the work. However, large-scale environmental contamination is always unacceptable.

Unless you're talking as an aftereffect of instant sunshine (which is still unacceptable, but for different reasons).

Scott said...

I was using NTR-Gas numbers for exhaust velocity to figure out the drive power.

The description of remass and operation is consistent with a fusion-NTR.

Thom said...

Scott:
All of the ships you mentiones were around 5 times larger than my example (Rick was dead on: 14,000 tons total. The only difference was an extra 1100 tons to the engines out of cargo). Also, none of them have pathetic 3 milligee acceleration, but all but the one from aliens produce less waste heat. This is a ship for just beyond the end of the plausible midfuture, when the F150 of SPACE!! becomes viabl and necessary. I just chose effeciency numbers that can be reasonably expected to exist by around 2300.

Elukka said...

If the possibility of any radioactive contamination is unacceptable, then nuclear reactors (whether in space or on the ground) are unacceptable. RTGs too.

Milo:

"Yeah, but the problem is that this station will spend much of its time being nowhere near your intended route, on account of it orbiting independantly from both your source and your destination."

Won't it orbit just the same regardless of whether it's orbiting a planet (that's on a solar orbit) or on a solar orbit itself? It's still a big problem, but it's the same problem whether there's a planet or not.

"Automated satellites require something to orbit. Which counts as "ground"."

The Sun. :P
Okay, let's call them automated stations instead of satellites.

Thucydides said...

Weaponizing beam stations is a valid concern, but a cost-benefit analysis with all the various concerns factored in would make the case for or against very clear.

I would think that under some circumstances, you would want a weapon grade beam, the cluster of stations at the Earth-Sun L1 point would be able to blast asteroids, NEO's or comets that are approaching the Earth-Moon system, and the Jovian equivalent would be appreciated if another comet Shoemaker-Levy event were to happen.

Attacking or hijacking the beam station would be another factor in the cost benefit calculus; competition would probably ensure multiple stations are built so the essential space service markets will continue to get coverage, or old stations can be brought out of mothballs. Since the station itself would be a multi-billion dollar investment, the company will have as much security as practical on or around the station, and national governments might also have observers and forces deployed at or near the station to "protect" it.

The beam might not be useful as a terror weapon against Earth, since it will be tuned for whatever specific wavelength is standardized among shipping companies (the shortest practical wavelength in order to have the longest reach). As well, since the mount is optimized to track cooperative targets at long range (and thus moving slowly in the beam's field of view), you would not be able to respond quickly to military targets. A terrorist or military takeover might be more focused on sabotaging the equipment so they cannot produce energy and ships will be stranded without their power beams. The economic impact will outweigh any physical damage the beam could do.

Milo said...

I'm a little biased against measurements called "specific anything", since they seem to usually be an excuse to multiply by some arbitrary hidden factor that has no business being there. (Like specific impulse having a hidden 1 Earth gravity factor, and measuring something that has no business being described in units of time.)



Elukka:

"Won't it orbit just the same regardless of whether it's orbiting a planet (that's on a solar orbit) or on a solar orbit itself?"

If it's orbiting a planet, then it's staying near that planet. Since planets are where most interesting stuff happens, that means orbiting a planet is a good way to make something conveniently reachable.

However the space between the planets is costantly shifting due to them orbiting at different rates. Charting a route using this station would require you to wait for an orbital alignment when the three objects (source planet, station, destination planet) line up appropiately for a timely route.

KraKon said...

The 800MW electrical generator could be just for the magnetic nozzle or magnetic containment fields...

Elukka said...

Yeah but if you're going from planet A to Planet C, it doesn't really matter if there's a planet B or just a deep space beam station in between.

---

One thing that's puzzling me is the immense power densities nuclear thermal rocket reactors have. I know it's thermal power, not electrical, but it's not just a few times more than the 1 kw/kg figure bandied about for power reactors, it's hundreds of times more.

It's surprisingly hard to find numbers on NERVA or other engines, (or maybe I'm just not digging deep enough) but these are engines massing some tens of tonnes with a thermal output measured in hundreds or thousands of megawatts. There's also a launch vehicle, around Saturn V scale (a bit lighter, I believe) in an "Advanced Propulsion Study" document with, I'm going from memory here since the document isn't available, about 125 gigawatts of thermal power. One source mentions a Russian NTR with a power density of 300 kW/kg.

How is there such a huge difference in the power densities of power reactors and rocket reactors?

Thucydides said...

How is there such a huge difference in the power densities of power reactors and rocket reactors?

I'm sure other people will have more detailed explanations, but the key difference is a rocket motor is designed to have high power densities. The rocket is needed to go at full power for a very short period of time, while a power reactor is needed to work on a continuing basis 24/7. I believe CANDU reactors can run for up to 500 days without needing to be shut down and refueled, and nuclear powered navy vessels also run their reactors for prolonged periods between shutdowns (at a minimum, a nuclear attack or ballistic missile sub is on patrol for months at a time).

An easy comparison is to look at a dragster and your stock road car. They both use IC engines, but the dragster needs to run all out long enough to get down the 1/4 mile track. Every part is stressed to the design limits and the entire engine needs to be stripped down and rebuilt after every run. The engine in your car is very similar in design, but must last at least 100,000km or whatever the warrantee period is.

Now using a NERVA type core to generate electrical power would give you very high power densities, right up until the reactor disintigrated...

Byron said...

I'm not sure how this MHD afterburner would work. MHD is usually a means of getting power from something like that. And if it's a "plasma combustion" drive, then it wouldn't need input power. However, I seriously doubt that people like DP9 lack Ve numbers. Do they have such things as dry mass, loaded mass, and delta-V specified?

Elukka:
Things go around the sun at different rates. The only way to stay stationary relative to the planet on a solar scale is to be inside the hill sphere or at one of the lagrange points.
Otherwise, you won't be in the proper place very often.

Thucydides:
Yes. That's another problem I have with beams. Economic warfare becomes really easy, as you just have to destroy the beams. And then blame terrorists.
As for power density, you can skip all the radiators, pumps, turbines, etc. It just has to heat the primary coolant and expel it.

Milo:
Actually, most specifics (specific energy, specific power) are just x/unit mass. I was thinking Newton-seconds per kilogram when I first typed specific impulse (which is what it should be). And I agree with you on it. I always use Ve for specing rockets.

Tony said...

Byron:

"So there is no situation where any level of radioactive contamination is allowable at all?"

That would obviously be an absurd position to take. There will be some level of contamination from terrestrial power reactors and supporting industries, no matter how well run. Same-same for nuclear medicine. I wouldn't put a regional nuclear conflict out of the realm of possibility either.

What we're trying to avoid is accumulating too much contamination. We should be interested in balancing the introduction of new contaminants with the decay of old ones. Nuclear power, medicine, and other applications will continue to have benefits we would probably rather not do without.

Nuclear spacecraft, on the other hand, are a convenience. In that context, nuclear electric bears risks that are probably acceptable, since they don't release radiological contaminants as a normal function of their operations, and their nuclear components can probably be disposed of by launching them out of the solar system at the end of their service lives.

Maybe solid core nuclear thermal rockets are acceptable too. They do release some low level radioactives, but perhaps not too many.

More exotic rockets, that release relatively large amounts of radioactive isotopes as a part of their normal operations? The engineering and physics are complex -- certainly above my pay grade -- but when educated people who have worked on such systems think twice about the consequences, that's enough for me to just ignore them as possible solutions for a plausible midfuture.

Citizen Joe said...

RE: weaponizing beamed power. Due to range, cost issues, physical properties and design considerations, beamed power stations will likely be operating with vacuum frequencies, like UV and beyond. Earth's atmosphere absorbs those making it unfeasible (or at least horrible inefficient) to be used against terrestrial targets.

Lasers, in the sense of space weapons, aren't really cylindrical shaped, they are conic with a focal point where you get the best damage. The reason for this is that you have to bounce your energy off a mirror in the first place to get it to the target. Without focusing, the bounce energy density would be the same as at the destination. If it could melt a ship at destination, it will melt your mirror at the source.

With beamed power, you may be feeding someone at 1 AU distance or 20 (well probably not). This means a lot of adjusting the mirror to focus on to a small fast moving object... and VERY precisely at that fast moving object. Instead, send off a broad columnar beam which has the same power density at any distance (within reason). Then the ships are responsible for catching that beam with directional focused mirrors.

Now, if you want to use solar beamed power, you need a concentrator mirror and then a targeting mirror. Whether that station is at mercury orbit or Jupiter, you can maintain the same power output by changing the size of your concentrator mirror. Without doing the math, I'd say that the Jupiter mirror would have a thousand times the collector area as the mercury one.

Now if I were to design this system, the ship's collector array would be sized for Earth orbit solar energy density. Transmission stations would then be placed such that the signal lag to report course of ship was safe. i.e. If you can spot and plot course around hazards in 5 minutes, then transmission stations should be set at 5 light minute intervals. If you can only detect problems a few seconds out, then you need a lot more stations. Of course, should you maneuver too quickly, it just means you'll lose (main) power for a few minutes until the station can regain a lock on you.

Now, if you've got lots of ships and not so many stations, it may simply be a matter of purchasing power time from the station when you need to do course correction burns. And even without the direct rays, you can still direct your collector at the sun for a lower yield. And when you compare drive power to life support, it probably won't be a problem.

Scott said...

Thom said: "Also, none of them have pathetic 3 milligee acceleration, but all but the one from aliens produce less waste heat."

I'd call that a sign that nuke-electrics may not be a good idea, then. Either that, or someone's completely missed some math someplace!

Byron said: "I'm not sure how this MHD afterburner would work. MHD is usually a means of getting power from something like that."

Any electric motor is a generator operated in reverse. What you'd do is apply power to the MHD generator coils, which would further accelerate the exhaust.

"... if it's a "plasma combustion" drive, then it wouldn't need input power. However, I seriously doubt that people like DP9 lack Ve numbers. Do they have such things as dry mass, loaded mass, and delta-V specified?"

They abstract the delta-V capabilities into 'burn points', but the dry mass and loaded mass are there. Hey, it's a game, not a simulation.

Byron said...

So how much is each burn point? I know that it's a game, but there has to be a number assigned somewhere.
And in this case, if you're anywhere close to correct on power, the MHD afterburner isn't worth anything.

Anonymous said...

Just to expand on what Thucydides say about NTRs; while power reactors are designed to convert a portion of their heat energy to electricity, NTRs are designed to use their heat directly to produce thrust.

Ferrell

Thom said...

Scott:
I'm more willing to bet that someone messed up on the math on the other examples. The VASIMR system that we are discussing has about 60% waste heat/40% converted into thrust. The most effecient internal combustion engine today--a low speed diesel used on large ships-- is about 50% effecient. The average diesel is 40%, and the average gas is 30% or less. Nuke-electric combines decent effeciency, relatively high thrust when needed, and extremely high exhaust velocity when required. Is there better stuff out there in the plausible midfuture? Probably. He3-Dt (I'm no fan of tritium) drives will have higher exhaust velocities, no neutrons, and likely a hella lot more thrust. However, if advances in fission reactor technology make it 10X more energy dense in 2-3 centuries, I see no reason to discard nuke-electric. They'll be less complicated, less dangerous (That hunk of metal you see in the portside monior orbiting Earth is the remains of the Mriya space station, destroyed because one idiot used his fusion drive in dock) and far less expensive.

A ord on fission power density: As of now, there has been virtually no need to build a multiple gigawatt reactor with a low mass. When we enter space, necessity (i.e; price) will generate demand for lighter weight reactors. New materials will likely allow higher operating temperatures and greater effeciency, and it is likely possibl that uing what is cutting-edge or near future technology today, we would be able to advance that technology signifigantly.

In summary, the lack of a need is why we see such low power densities in fission reactors today.

Tony said...

Fusion will only replace nuclear electric if fusion can be made to work. That's not a done deal. In fact, long before there may be fusion-only propulsion systems, there may very well be borderline fusion systems that use fission reactors for supplemental power.

Also, where do such low efficiency numbersfor VASIMR come from? Hall effect thrusters (HET) are rated somehwere between 60% and 70% efficiency. One would think a more advanced technology could be made at least as efficient.

And if not, one wonders what the real advantage is. Okay, you can run VASIMR in a higher gear, but only at the cost of added mass and complexity in power generation and heat management. Seems to me one might just as effectively run uprated HETs or some other kind of ion motors off of a somewhat smaller nuclear power source and get the same ultimate delta-v. Okay, you can't run those systems at higher thrust for Earth departure. Add a small expendable kicker stage for a component of departure to make up for that. Basedo n the logic of staging, it might be a better idea to begin with.

Byron said...

In my research on VASMIR for Rocketverse I saw efficiency numbers in the 70-75% range. So I'm not sure where the low numbers came from.

And yes, fusion has to be made to work. But is it any less likely than large-scale space presence? (And I know that's not realistic either, Tony. Don't start.)
I was just pointing out that they have a lot of potential, and had been ignored up to that point.

Tony said...

Byron:

"And yes, fusion has to be made to work."

Why?

Byron said...

You misunderstand. I'm saying that if it is to be used, it has to be made to work. As in, it's not ready yet. Not that it absolutely must be made to work.

Tony said...

Byron:

"You misunderstand. I'm saying that if it is to be used, it has to be made to work. As in, it's not ready yet. Not that it absolutely must be made to work."

Ahhh...

I don't think a large scale space presence is all that unlikely. I just don't think it's going to develop ver quickly. I've said before that I could see an order of magnitude increase in peak off-Earth population per century. Clearly, a space population of up to 10,000 could be accomodated by by a relatively moderate improvement in chemical launch cost efficiency ($2000-2500 per kilo, in 2010 dollars) and nuclear electric interplanetary propulsion. (Figuring only 10% -- or less -- of the population needs to be moved over interplanetary distances in any one year.) So improvements, over what we are already in sight of, don't need to be made for another 300 or so years.

And I'm not so sure that a fusion rocket would be all that higher performance. Compared to nuclear electric, it would only have to increase performance from milligee to centigee level accelerations to be a big improvement. And given the likely mass of a fusion reactor, that may be all you get.

Thucydides said...

If beam stations become the standard, then space warfare tropes become very different.

Victory is achieved by hijacking or disabling the station to ensure the beam is interrupted and service is disrupted to customers. (destroying the station outright might not be acceptable, as the new owners will be deprived of the possibility of ransom or economic rents from becoming the new owners). The defenders won't be constellations of laser and kineticstars, but the company police and insurance company guards.

Citizen Joe said...

But there would be more than one station... probably hundreds. If you went after one, none of the others would task for you so you'd get stranded.

tsz52 said...

I don't want to commit economic heresy or anything, but don't all of the meta-projects discussed here necessitate a resurgence in the relative (to transnational entities) strength of the nation-state, as they always have?

If they're extremely expensive, then only states (or coalitions of states) will be able to afford to build them.

They will only be built if necessary, and therefore become - by definition - strategic assets: they will be protected and rendered as safe as possible in *roughly* the same way that CVNs are today: they seem to be fairly terrorist-resistant....

Thucydides said...

If fixed infrastructure like beam stations (or momentum transfer tethers) become the standard means of space travel, some might be built by governments, but economics suggests there will be plenty of private capacity either piggybacked or new private capacity built alongside.

By analogy, governments built roads for military purposes since Roman times, but the roads supported expanded trade and commerce. National railways were often followed by private ones (the Canadian Pacific Railway paved the way for the Canadian Northern Railway and the Grand Trunk Railway//National Transcontinental Railway system in Canada).

Slightly OT, but related in terms of space settlement and industrialization is this piece from NextBigFuture. Note it is premised around the Jules Verne nuclear cannon, which is implausible for many reasons, but some of what is suggested makes sense (especially the social and political climate for bootstrapping wealth, and the size and scale for successful economic takeoff):

http://nextbigfuture.com/2010/12/setting-up-industrial-village-on-moon.html#more

Rick said...

Welcome to a couple of new commenters, and hello to all - I've been offline since Tuesday.

On beamed power, the biggest problem I see is one that Markus (I believe) pointed out way upthread, that they pretty much have to get ships up to speed over a distance much shorter than interplanetary distances, requiring a correspondingly higher acceleration.

On the other hand, I don't think they would necessarily be so easy to weaponize - angels, like devils, lurk in the details, and could make beam generators unsuited for use as weapons even though they put out beaucoup power.

Regarding nuke electric drives, I would guess that most of the mass of present day nuclear power plants is not the reactor or even its shielding, but everything else - basically a steam turbine power plant with the reactor serving as firebox.

And as noted a bit upthread, there's been little need to shave every gram of weight from nuclear plants, even shipboard. Subs need the weight, since their submerged density must be 1.0, while for a large surface ship a thousand tons deep in the hold is rather benign, tending to add stability. Nuke electric space drives will call for much more weight saving than any present era power application.

Jedidia said...

Regarding nuke electric drives, I would guess that most of the mass of present day nuclear power plants is not the reactor or even its shielding, but everything else - basically a steam turbine power plant with the reactor serving as firebox.

Which brings up another question that has so far been untouched: Once you get the beam to the target, what's the best means to convert it back to electricity? How much efficiency can we expect from that, and how heavy would the machinery doing this task be?

I think we can discount Photovoltaic conversion, since then you'd still need one hell of a receiver area. I also don't know how strong you can focus a beam so that photovoltaics can still handle it.

Also, we'd still need radiators. Even with a very efficient conversion we'd still have to dump apreaciable amounts of waste heat (let's not forgett the drive efficiency either). In the end, it might turn out that we can save unexpectedly small amounts of mass...

Markus said...

@Jedidia:
The only other way of harnessing the beam energy that comes to mind is concentrating the incoming radiation on a pipe where the coolant flows, and then driving a steam turbine with that (i.e. solar thermal power). But I fail to see how that would reduce the surface area we need for the collector. After all, the intensity of the beam at some range depends only on the beam emitter properties, so the only way a ship can get more energy is to increase surface area or efficiency.

I'm pretty positive the solar thermal scheme takes up more mass than photovoltaics because of the steam turbine. PV is not much more than the quite thin panel. And photovoltaics also has the advantage of functioning as its own radiator. The light which is not turned into electricity just heats up the photovoltaic panel, which then radiates the heat away.

Taking, for example 5x solar intensity (~6500 W/m2) and 20% efficiency, we are left with 5700 W/m2 as heat, which corresponds to 560 K = 290 degrees C (per Stefan-Boltzmann Law for a black body I = k * T^4, I is irradiance W/m2, k is the constant 5.67e-8 W/m2/K4, T is temperature in K). Not something that would damage your panel.

I think it might even be possible to dual-use the PV panel as radiator for other systems, too. But it might be better to use separate radiators for them.

- Markus

Elukka said...

How about using the beam to heat the propellant directly? Sorta like a nuclear thermal rocket except the beam replaces the reactor as the heat source. Don't know if that'll give you great isp but it probably won't require a terrible amount of heavy and expensive machinery.

Citizen Joe said...

I just saw a video where they used a 1 meter parabolic mirror to focus the sun's light to a point. At that point, it was hot enough to melt stone. You could use that as a thermal rocket and you wouldn't need much as far as radiators because the heat is being dumped into the reaction mass. If fuel processing isn't an issue, you could use Lithium Deuteride/tritium as the primary, packed into a hohlraum, with some other reactant as remass. A magnetic field manipulates the charge into position and the concentrated beam detonates the fusion microcharge to produce thrust. Between detonations, the sunlight is instead directed into the ship's on board power/batteries/capacitors which are discharged to form the magnetic fields, etc.

tsz52 said...

@ Thucydides:-

"If fixed infrastructure like beam stations (or momentum transfer tethers) become the standard means of space travel, some might be built by governments, but economics suggests there will be plenty of private capacity either piggybacked or new private capacity built alongside.

"By analogy, governments built roads for military purposes since Roman times, but the roads supported expanded trade and commerce. National railways were often followed by private ones (the Canadian Pacific Railway paved the way for the Canadian Northern Railway and the Grand Trunk Railway//National Transcontinental Railway system in Canada)."

[I really don't have any economic axe to grind here, but am merely interested in discussing such things, and how they might inform the future.]

Whilst it is always best to be charitable with analogies, I think that there are three main differences here (all compounding each other), which might upset, and invalidate, the model:-

1 The business enterprises that rode upon the back of the grand meta-transportation-projects (road, rail, canal) were far more willing and able to 'bet the farm' on long-term, risky ventures than Anglo-US model publicly owned companies are;

2 Those 'roads' were capable of being used to support profitable 'tributaries' when even only 1% (say) built, and the 'tributaries' helped to defray their own costs, even as they were being built, with the little towns and businesses built alongside to service them.

The 'space road' has a more all-or-nothing nature: a long 'road' with few 'way-stations' (to be commercially exploited) along its length, with each long stretch having to be fully built before it can be commercially exploited.

Thus the entities that have built it will be in a strong position to dictate how it is used, and demand that any potential profits come directly into their own coffers, given the enormous expense (with possible resentment that 'business' didn't pay its share when it had the chance, and would have been helpful).

Add the strategic importance and potential danger of this infrastructure, and 'business' may be rendered a very small player on this playing-field (at least in the early days);

3 The 'space road' is relatively far more expensive (whatever the definition or index of 'cost') than the ground road.

tsz52 said...

Holy cats! I only pressed the 'Publish' button once....

Tony said...

Citizen Joe:

"I just saw a video where they used a 1 meter parabolic mirror to focus the sun's light to a point. At that point, it was hot enough to melt stone. You could use that as a thermal rocket..."

Solar thermal rockets can fundamentally be no more efficient than a nuclear thermal rocket using the same reaction mass. The limiting factor is the heat the reactor can withstand.

BTW, reflected sunlight is not going to create enough pressure to initiate a fusion reaction.

Jedidia said...

Taking, for example 5x solar intensity (~6500 W/m2) and 20% efficiency, we are left with 5700 W/m2 as heat, which corresponds to 560 K = 290 degrees C (per Stefan-Boltzmann Law for a black body I = k * T^4, I is irradiance W/m2, k is the constant 5.67e-8 W/m2/K4, T is temperature in K). Not something that would damage your panel.

I'm very unfamiliar with photovoltaics, doesn't their efficiency decrease the hotter they get?

AdShea said...

Something that everyone seems to be forgetting about He3-D fusion is that the He3-D fusion rate is only about 3x higher than the D-D fusion rate so you do get some neutron output from the D+D -> T+p, T+D (much much higher reaction rate than either D-D or He3-D) -> He4+n so you'd have both high energy protons and D-T fusion neutrons coming out. Not a game ender of course, but not the perfectly clean fusion that seems to be the hope around here.

Thucydides said...

Like Rick, I didn't specify how the beamed power was going to be used, since there are many different iterations. We can probably discount hot beams propelling magnetic sails for now, since there is plenty of near magitech involved, so the main contenders are thermal and electric.

Thermal energy can be harnessed directly by focusing the energy on the remass, this gives a fairly light and rugged rocket but limited to chemfuel or NERVA like ISP, with performance issues for long voyages. Thermal energy can be harnessed to generate electrical energy, but using a Carnot cycle steam generator is rather silly. Crank up the focus and use a MHD generator with @ 66% efficiency. Photovoltaics can be used if they are optimized for a particular wavelength and the power beam tuned to that frequency, performance will be increased to about 60% (based on an experiment I read about in AW&ST using a solar cell fed monochromatic laser light via fiber optic).

Microwave beams might be more practical for various reasons, in which case a rectenna can convert the beam with @ 80% efficiency as discovered in tests of the concept in the 1980's.

We still need radiators, power inverters and lots of other parts, but the set-up is still superior if the mass of the beam receiver and machinery is less than the mass of a nuclear reactor or other powerplant with the same output (even fusion reactors)

Rick said...

tsz52 - Blogger is up to its old tricks again, welcoming commenters by duplicating their posts. (I got rid of the dupes.)

Direct thermal drive requires not just heating the propellant but doing so in a way that confines it to produce thrust instead of merely expanding and dissipating. The implied beam range is very short by interplanetary standards, but acceleration could be closer to 1 g than a milligee.

I'd expect beamed electric drives to be in the range of perhaps 10 milligees. The spacecraft would take a little over a day to reach 10 km/s, with a travel distance of about 500,000 km, comparable to Earth-Moon distance.

tsz52 said...

Cheers Rick.

Maybe the beam generator will be built as part of the prestige project of driving an interstellar sail probe (perhaps there'll be a race between Europe and the US against the emerging eastern powers...).

@ AdShea:-

"Something that everyone seems to be forgetting about He3-D fusion is that the He3-D fusion rate is only about 3x higher than the D-D fusion rate so you do get some neutron output from the D+D -> T+p, T+D (much much higher reaction rate than either D-D or He3-D) -> He4+n so you'd have both high energy protons and D-T fusion neutrons coming out. Not a game ender of course, but not the perfectly clean fusion that seems to be the hope around here."

Yeah, when I hear 'clean' or 'aneutronic' fusion I make sure I keep a hand on my wallet....

I see 'aneutronic' in pretty much the same way as 'non-lethal'; wishful thinking that falls far short when actually used (and the term later amended) - even a tiny percentage of neutrons adds up to an immense problem when multiplied by high output over time.

And D-He3 isn't even technically aneutronic, though it's still probably the best compromise between minimal neutrons, performance and minimal ignition power (maybe with a small D-T igniter, at least in the early days).

Especially if it's externally pulsed, rather than attempting to contain and sustain within your very hull (another assumption that generally accompanies fusion)... I'd much prefer any nuclear action to take place *outside* the ship....

Markus said...

"I'm very unfamiliar with photovoltaics, doesn't their efficiency decrease the hotter they get?"

Found a paper on that subject (hope I'm interpreting it right):
http://www.iea-pvps.org/products/download/pap2_033.pdf

At 55 K temperature rise one plant suffered 11 % loss. Another had 46-52 K rise and suffered 5-7% losses (all of these are annual figures). So, it varies between different photovoltaics, but there's definitely a decrease as you said. If we extrapolate linearly (breaks down at high temperatures), our 290 degrees C panel would suffer ~30% lower efficiency (if compared to room temperature).

Thucydides presented higher efficiencies than the 20% I've been using, and that of course makes everything more viable. But I'd like to contend the microwave beam solution. True, we have phased arrays for microwaves, which can be made larger than laser mirrors, but by how much? The wavelength is over four orders of magnitude longer, thus for similar focus we'd need four orders of magnitude larger phased array, tens or hundreds of kilometers in diameter. I don't know if this is possible with reasonable technology.

Though it might be better to waste some energy into space if we had much higher efficiency at the collector. But since you proposed 60% for wavelength-optimized photovoltaics, I'm still firmly standing behind a laser solution for the emitter.

- Markus

KraKon said...

What you could do is make a two step setup, ie several emitter stations.

You could have the central station benefiting from a large energy production facility, as well as a large mirror array, while the secondary station, being much closer to target (moon orbit? Lagrage points?) could supplant the ship's necessity for very large collectors, by providing a much better collimated beam, in the required wavelength.

This way the central station emitter could beam out a short wavelength beam (xray if you want), while the secondary station could emit a much better absorbed beam (maser?), which lightens the load upon the ship....

The secondary station could also afford to produce energy itself, to replace absorption losses, and aiming could be improved with shorter ranges, so mirror edge margins could be lowered (you do include safety margins on your mirror ships, right?).

AdShea said...

On the maser idea, you could actually do a 100km wide phased array to get the beam power and spot size you want, we're already doing this for the receiving end in the Very Long Baseline Array projects. It's also a whole lot easier to build a few thousand small transmitters than to build a huge mirror. You could even synchronize them to a coded pilot beam coming from the target which would ensure perfect targeting.

Scott said...

So how much is each burn point? I know that it's a game, but there has to be a number assigned somewhere.
Under cruise thrust conditions, 1 burn point is 1 m/s delta-V. They simplify out the differences between hydrogen and heavier element remass, though.

How about using the beam to heat the propellant directly?
There's stats for that on the Atomic Rockets engine page, under 'Laser Thermal' http://www.projectrho.com/rocket/enginelist.php#The_Drive_Table

I'd much prefer any nuclear action to take place *outside* the ship....
Why? Proper design will deal with that. I've spent a couple years of my life living within 300 feet of a working 150ishMW nuclear reactor. Not within 300 feet of the plant fence, within 300 feet of the reactor itself! My lifetime radiation dose from all types of radiation is *zero*, and I wore a dosimeter optimized for neutron detection. I think I could even say that my lifetime dose is less than most airline pilots (noting that Airline pilots don't need to worry about neutrons very often).

Tony said...

Scott:

"My lifetime radiation dose from all types of radiation is *zero*, and I wore a dosimeter optimized for neutron detection."

Aboard US nuclear surface ships (carriers today, but also cruisers in the past) dosimetry isn't even required unless you enter certain parts of the propulsion plant. One wouldn't even know he was on a nuclear ship except that there isn't any fresh water rationing. (With the surplus electrical power available, and no need to refuel, making a hundred gallons a day of fresh water per crewmember is no big deal.)

Having said that, I can understand the hesitancy to put nuclear reactors inside spaceships that can't afford the penalty mass of shielding common in naval nuclear installations. Even relatively cool RTGs are placed at the end of booms on many spacecraft so that their fast neutron leakage won't interfere with research and navigational instruments. I can't see a fusion reactor being any less of a problem. I'd want any kind of nuclear reactor installed on my spaceship to be on a boom and behind a shadow shield.

Scott said...

It seems that someone did mess up on some math: me. [where's that embarrassed smiley?]

It really helps if you look at the right line in the Atomic Rockets engine table for the PCC rockets they use in Jovian Chronicles. A closer guess would be He3/D fusion drives... which puts each engine up to 54TW of power!

Thom, the reason that you're dealing with such high power levels (ie, waste heat) is that you're using an electric drive. If you go to a thermal drive, like all the ships I used in comparison to your "F150 of space". You don't have the shipboard heat problem if you are dumping most of your reactor heat into the rocket exhaust.

Tony said...

Scott:

"You don't have the shipboard heat problem if you are dumping most of your reactor heat into the rocket exhaust."

Problem is, you still have to manage the heat of all of those machines that make it possible to funnel the plasma out the ass end. Cooling humungo electromagnets is not going to be thermodynamically cheap and easy.

Luke said...

Since people are mentioning beamed power, and discussed the efficiency of laser-photovoltaic beamed power in particular, I will note that there is no theoretical upper limit to the efficiency of a photocell turning laser light into electricity (other than 1, or 100%, of course). You need a direct band gap semiconductor (so silicon is out) and you need to tune the energy per photon to just slightly higher than the band gap. If you do this, in principle you can harvest all of the energy of any photon that is absorbed. In practice, of course, there will always be other effects that compete with you so efficiency will be lower - but it could still be high. A photo-conversion efficiency of 80% or 90% would be quite believable.

Thucydides said...

Of course the theoretical efficiency electric power generation from an aneutronic fusion reactor is on the order of 90%, so high energy devices might not always require massive generators and large radiators etc.

There will still be waste heat from the reactor (or beam converter), as well as from power conversion devices and other electrical/mechanical/photonic devices needed to run the drive, so radiator mass still has to be accounted for as well.

Still, when we need to deal with GW and TW of energy to move about in space, reducing waste heat will be the most efficient mans of improving the power/weight ratio's of plausible mid-future(tm) spacecraft.

KraKon said...

Keeping the nuclear explosions outside -a concept I adhere to has nothing to do with radiation shielding. What we want to limit is a massive reactor that absorbs terrawatts of waste heat directly. Even if the plasma is not in touch with anything, thanks to magnetic confinement, radiated heat makes sure that thete's a sizeable fraction of waste heat that you cannot do anything about intercepting. Keeping the explosions outside the ship makes sure that instead of all that radiation hitting your chamber directly, most of it escapes into space.
The other advantage is massively decreasing reaction chamber mass by reducing it to thin tungsten-edged arms forming the magnetic field.

tsz52 said...

@ Scott:-

Me: "I'd much prefer any nuclear action to take place *outside* the ship...."

You: "Why? Proper design will deal with that. I've spent a couple years of my life living within 300 feet of a working 150ishMW nuclear reactor. Not within 300 feet of the plant fence, within 300 feet of the reactor itself! My lifetime radiation dose from all types of radiation is *zero*, and I wore a dosimeter optimized for neutron detection. I think I could even say that my lifetime dose is less than most airline pilots (noting that Airline pilots don't need to worry about neutrons very often)."

Tony and KraKon have pretty much answered this, but to clarify: I meant in terms of propulsion (rather than power generation) and especially fusion.

By 'action' I meant something like 'raging nuclear inferno' or 'nuclear implosions/explosions'.

It's just that 'fusion' tends to get hit with the 'wishful-thinking/implausibility' stick because of what folks assume when they read the word: there's a world of plausibility separating externally imploded pellets (with lasers/plasma-guns), with as little physical structure as possible anywhere near that action; and a self-sustaining nuclear inferno, tidily and reliably contained with magfields, and that within a handy little container exposed to the heat (and neutrons and X-rays and gammas that don't get talked about), and all of that within your ship.... ;)

Unfortunately, folks tend to assume the latter where 'fusion''s concerned, and it gets a bad rep.

Dunno, of all of the standard-issue bits of SF-tech, the magbottle-MCF is the one that always makes me go "Really?..".

Tony said...

tsz52:

"Dunno, of all of the standard-issue bits of SF-tech, the magbottle-MCF is the one that always makes me go 'Really?..'."

No kidding...right along with the storable liquid hydrogen (of any isotopic flavor) to fuel it. But I guess if you can invoke one kind of magick, you can invoke another kind as well.

Perhaps the future really is going to be fission powered. Going back a few steps, the solar system opening McGuffinite will be simple fissionables to supplement the limited supply in the Earth's crust.

Byron said...

While I will admit that He3-D fusion isn't fully aneutronic, it is still far more efficient than D-T fusion. And the heat radiation is also a problem, but regenerative cooling might help that some.
For the fusion torch I designed, my first attempt was thwarted when I discovered that it was an excellent source of X-rays. Considerably better as that than as a drive, in fact. The second attempt was better, but there were still a lot of neutrons, and the shielding almost gave me fits. Despite it all, I got it working. This just illustrates the problems behind this sort of thing, but I don't think completely ruling out magnetic-confinement fusion is a good idea.

Tony said...

Re: Byron

Regenerative cooling presumes a cold fluid/gas to absorb the heat of the hot fluid/gas. The problem is you still have to refrigerate the cooling fluid/gas. In chemical rockets, the fuel used for cooling is refrigerated at a ground plant that itself produces a lot of heat, which is pumped into the atmosphere. Onboard a fusion spaceship...not so much. Keeping the hydrogen fusion fuel -- if that's what you're going to use for your regenerative cooling fluid -- cool enough to stay stored is going to generate plenty of heat. Also, since the whole point is to reduce fuel requirements by using energetic nuclear processes, you're probably not going to have enough fuel flow through the cooler, on the way to the reactor, to actually provide much cooling.

Heat management in a vacuum is what my generation would call a cast iron bitch. You can't escape the necessity, yet at the same time there are no simple solutions.

Byron said...

I'm referring to a direct-thrust fusion torch. You run the extra remass (which will be in use) through the magnet and shield, then dump it into the chamber. There's likely to be enough to carry away most of the heat.

Tony said...

Byron:

"I'm referring to a direct-thrust fusion torch. You run the extra remass (which will be in use) through the magnet and shield, then dump it into the chamber. There's likely to be enough to carry away most of the heat."

I wouldn't count on that being effective. Remember, we're trying to limit fuel/remass mass ratio in any propulsion system we would likely consider advanced. That means you just don't have that much expendable working fluid for the cooler. No matter how much handwaving you do, there will be an independent cooling cicuit and radiators.

Citizen Joe said...

He-3/He-3 fusion is fully aneutronic. It requires more containment energy and yields less overall energy, but it throws straight up protons which can be converted easily to electricity. I suspect that the huge advances we need to achieve D-He3 fusion confinement would be such that bumping up to He-3 confinement isn't that difficult.

Scott said...

I will agree that most people think that fusion is some kind of panacea. I'd love it to be, but I'm not holding my breath.

I've pointed out a couple times that your rad-shielding is going to get hot, especially your neutron shielding. Use that to drive generators before you dump the heat through a radiator.

Thucydides said...

Dumping remass into a fusion reactor for heat control might work, but will kill ISP. You will get a lot of thrust, though, so there may be a call for this sort of system.

Assuming a 3He fusion reactor is possible and portable, the true magic is the hot beam of protons exiting the chamber. If they are being "harvested" for electrical energy you get a stupendous amount of high quality electrical current, but if the beam is being directed out of a magnetic nozzle then you are getting an exhaust with an ISP in the range of 1,000,000. With that sort of drive you will be moving from point a to point b around the solar system pretty quickly....

tsz52 said...

[OK, whenever I start using numbers, you do well to check for yourselves... :( ]

He3-He3 has about 3/4 the Ve of D-He3 or D-T. It requires over 8 times the energy input to get it to fuse, compared to D-He3; which itself requires around 6 times D-T input (for around 50 times the input for He3-He3 compared to D-T).

Even the most enthusiastic papers about MCF I've read quietly doubt that He3-He3 will ever reach anything near to break-even point (D-He3 might not, for that matter... I mean assuming that any fusion ever actually does).

It will need to be very heavily subsidised by other power and heat management systems (and you get a lower Ve into the bargain).

I haven't explored it fully yet but there might be some way to fuse He3-He3 using a D-T igniter that, overall, gives a performance comparable to D-He3 using less power and with fewer neutrons (but with more complexity).

I've never come up with any contained, sustained, internal MCF that is superior to a fission-powered (at worst) external pulsed ICF/MTF... once you factor in Bremsstrahlung, shielding and cooling; let alone obvious-catastrophe-avoidance (if a component fails, it doesn't vape your ship) and plausibility.

[The ships in my setting fuse He3-He3 pellets only when within a certain distance from other ships/habs/ etc, but it sucks the ship's power dry and makes it run hot: the D-He3 pellet switchover is always welcome.]

@Tony: And my engineers assure me that they have found a way around the liquid hydrogen problem <*skulks off, whistling innocently...*>.

Nah, seriously, there are some not too exotic possible solutions to hydrogen storage, but I haven't nailed my colours to any of their masts yet, so won't say anymore.

Tony said...

Scott:

"I've pointed out a couple times that your rad-shielding is going to get hot, especially your neutron shielding. Use that to drive generators before you dump the heat through a radiator."

Which transfers a good portion of the heat to the generators, which have to be cooled in their turn. And any machine or instrument that uses the electricicty has to be cooled itself. You just wind up cycling the heat around the ship before finally having to radiate it anyway.

All work eventually turns to heat. You can't escape entropy. Even death will not release you.

Luke said...

Regarding He3-He3 fusion ... all estimates I've seen indicate that the bremsstrahlung radiation will be significantly greater than the fusion output, meaning you will always be losing more energy than you get from fusion and you cannot ignite your fusion plasma.

There may be ways around this, various athermal fusors, for example, that keep the electrons cold while the ions are hot, or possibly fusion in very high magnetic fields which might (or might not) be able to suppress bremsstrahlung. However, even in this case it looks like proton-boron fusion outperforms He3-He3 fusion in all respects.

Luke

Thucydides said...

Athermal fusion? Does this mean the Migma will be disinterred?

I find it interesting that many "tabletop" setups like Migma, IEC, MTF, and Dense Focus Plasma have seemingly reached high milestones of plasma density, neutron production or high fusion triple products at a small fraction of the time and cost of more conventional approaches. While none have actually achieved fusion (and there may be subtle reasons why they cannot), this seems to mostly be a case of limited resources available to continue experimentation. I suspect the combined budgets of all these projects would not equal the budget of one conventional magnetic confinement project, much less laser Inertial Confinement.

Should one of these approaches be successful, not only will there be inexpensive energy for Earth, but most of these devices are much more compact than more conventional machines, making them ideal for spacecraft (low mass and high energy density). Hope springs eternal...

KraKon said...

It's not that we specifically want fusion, it's that nothing else gives me the 800km/s exhaust velocity we need for relatively zippy interplanetary travel. I'll leave the details to you guys, but it seems other propulsion methods are left behind on this terrain...

Tony said...

The problem with fusion...the problem with fusion is that it is regarded as a panacea by almost everybody, SF authors and "realistic" futurists alike. And, being that we really don't know if we can do it at all, ever, it really is a form of magitech. That takes it beyond serious discussions of the plausible midfuture.

Rick said...

The second attempt was better, but there were still a lot of neutrons, and the shielding almost gave me fits. Despite it all, I got it working.

I assume you mean 'for story purposes,' rather than using blog comments for a Nobel Prize level announcement. :-)

If a direct fusion drive expels only the fusion products, the exhaust velocity is very high - upwards of 10,000 km/s, I believe, if the drive is decently efficient at capturing the fusion energy as kinetic motion.

This is a suitable drive for interstellar probes, but for interplanetary travel it is like putting rocket engines on a bus. I'd expect an interplanetary fusion drive to mix the fusion products with a much larger mass of inert propellant for a drive exhaust velocity of a few hundred km/s.

Even so, the propellant flow will be much too low for effective regenerative cooling.

tsz52 said...

So the crew of the pion drive interstellar torchship are trying to distract themselves from the fact that their ship might just vape them at any moment... and their regets at having left the safer ICF/MTF interplanetary haulers behind....

"So yeah, I've had another idea about how MCF might be made to work_"

"Tsk - you and your magitech...." :D

Byron said...

Rick:
You're correct. I got the math working. The engineering won't be done for another 20 or so years. And the maximum Ve for a fusion torch is 98,000 km/s. However, nobody needs that sort of exhaust velocity, so mass mixing will occur. And you guys are probably right about radiators.

tsz52 said...

@ Rick: The more serious proposals I've read (ICF/MTF) seem to average around 8:1 expellant (hydrogen) to fuel.

The primary focus is in reducing transit time, to minimise the deleterious effects of being out in space: irradiated and low g.

So even (crewed) cargo haulers will benefit from a decent performance fusion drive, in reducing propellant and shielding mass (and even centrifugal g mass and complexity), and consumable stores etc.

Tony said...

Re: tsz52

There's no reason to crew a cargo vehicle. initially, cargoes are going to be one-way packages, from the Earth to wherever. As trade grows -- presuming trade grows -- cargoes will probably be boosted into a transfer orbit by some kind of tug, then caught at the other end and delivered the same way.

Luke said...

Thucydides,

Athermal fusion? Does this mean the Migma will be disinterred?

The most practical idea I've yet heard for an athermal fusor is Bussard's polywell design. It uses regions of cold trapped electrons to direct fast moving ions past (and into) each other. Since electrons are responsible for most of the bremsstrahlung, this gives very little radiant photon emission. In theory, anyway. The migma idea also looks interesting. Again, in theory, you might get reduced bremsstrahlung.

There is some wishful thinking that the high magnetic fields of a dense plasma focus can suppress bremsstrahlung enough to make proton-boron fusion practical. While the DPF people are making progress, they are still quite a way from even D-T break-even, let alone p-B11.

While none have actually achieved fusion (and there may be subtle reasons why they cannot) ...

I will note that a lot of methods have achieved fusion. While the fusion gives off less energy than was put into making it, there is still fusion occurring and it is useful for things like laboratory high intensity neutron sources. In my day job I do theory for experimenters who work with deuterium-tritium accelerator-based fusors for neutron sources.

KraKon,

Baryon decay catalyzed by topological defects is another vaguely plausible method for getting high energies and high exhaust velocities. While it does require a handy supply of topological defects (magnetic monopoles, cosmic strings, domain walls .. and the last two are impractically heavy) it might end up being less hand-wavy than sustained fusion (especially aneutronic fusion).

Luke said...

Tony,

The problem with fusion...the problem with fusion is that it is regarded as a panacea by almost everybody, SF authors and "realistic" futurists alike. And, being that we really don't know if we can do it at all, ever, it really is a form of magitech. That takes it beyond serious discussions of the plausible midfuture.

Serious discussions? Realistic? THIS IS ROCKETPUNK!
(er, ahem, enough '300' for me ...)

All,

An amusing comment from the introduction to Dr. Rider's thesis in electrical engineering at MIT on "Fundamental Limitations on Plasma Fusion Systems Not in Thermodynamic Equilibrium" ...

For the record, the author would like to apologize for apparently killing some of the most attractive types of fusion reactors which have been proposed. He advises future graduate students working on their theses to avoid accidentally demolishing the area of research in which they plan to work after graduation.

Tony said...

Luke:

"Serious discussions? Realistic? THIS IS ROCKETPUNK!
(er, ahem, enough '300' for me ...)"


Well, I do seem to be something of a wet blanket, but the midfuture I remember from my youth only started having fusion at the back end (starting with Niven and Pournelle, ISTR). Otherwise it was stricly fission. Intriguingly, Heinlein, Clarke, Piper, et al. knew fission and chemical rockets would work, even if they weren't quite perfected applications at the time of their writing. They didn't see the need to go any further, most of the time. When they did, as with Heinlein's torch or Piper's Abbot Lift_and_Drive, they knew they were delving into magitech and didn't try to justify them as a near future technology based on known principles. I think we can learn something from that.

Jedidia said...

Intriguingly, Heinlein, Clarke, Piper, et al. knew fission and chemical rockets would work, even if they weren't quite perfected applications at the time of their writing. They didn't see the need to go any further

At least Heinlein grossly overestimated them, though, which might be why he didn't see the need to go any further. Also, Clarke was using a fusion drive already in 2001, if I'm not mistaken (and later went on to cold fusion and (gosh!) reactionless drives, while Heinlein suddenly pulled the Mass-Energy conversion drive out of his hat. They weren't that innocent back then, really.

Jedidia said...

Edit for above post: Yes, I know that 3001 isn't mid-future, so that's a bit besides the point. 2064 still fits in comfortably, though. Not sure when Heinleins Mayflower made her trip to Ganymede, but it wasn't too far in the future either.

Thucydides said...

Baryon decay catalyzed by topological defects is another vaguely plausible method for getting high energies and high exhaust velocities. While it does require a handy supply of topological defects (magnetic monopoles, cosmic strings, domain walls .. and the last two are impractically heavy) it might end up being less hand-wavy than sustained fusion (especially aneutronic fusion).

You owe me a new keyboard! The only thing more magithech than that would be control over the Higgs field and elimination of inertia.

Still, you never know when the "black swan" event is going to happen....

Tony said...

Re: Jedidia

I think you're missing my point. It's not that the old SF writers didn't use magitech, it's that they kept it to a minimum and didn't try to BS their way through it with pseudo-scientific mumbo jumbo.

Luke said...

Thucydides,

Monopoles are predicted by most candidates for physics beyond the standard model. It would not be surprising if they were discovered. If found, they would be stable. With a source of monopoles, catalysis of baryon decay should be easy, and would not consume your monopoles. Thus, it only requires one hand wave (a source of monopoles) rather than a whole host of hand waves (overcoming plasma instabilities, dealing with radiation losses, maintaining suitably symmetrical implosions, et multiple cetera).

Admittedly, it is a big hand wave rather than a bunch of smaller hand waves - but controlled, sustained fusion looks like it is technologically a very hard problem, making its score on the plausibility meter drop with each passing decade. For fiction, I'll take either of them.

Geoffrey S H said...

@Tony:

"There's no reason to crew a cargo vehicle. initially, cargoes are going to be one-way packages, from the Earth to wherever. As trade grows -- presuming trade grows -- cargoes will probably be boosted into a transfer orbit by some kind of tug, then caught at the other end and delivered the same way."



I imagine that there might be some sort of cargo that might have humans monitering it... and slinging a cargo module on an orbit might not be the best frm of cargo transfer, or the chosen one of companies- nothing can be done about the module if something happens to it (yes, I know it will be said that nothing will, but I'm not inclined to be that optimistic).

Besides, that form of transfer would be so horrendously slow (unless the tug goes quite far out from the condainter's point of origin, thus expending alot of propellant) that trade would be impractical.

Besides, no crewed trade would completely kill any rocketpunk setting.

tsz52 said...

@ Tony: Re: Crewed Cargo Vehicles:-

Some vehicles will/won't be crewed according to the specific cargo (I include passengers as cargo, by the way), level of maturity of the cargo route, evolving tech, and narratives of the day regarding economics, morality and human value/purpose.

I think that the bit in the middle is more likely to be crewed than not: there's an infrastructure of some sophistication and complexity that needs maintenance but it isn't yet large enough to warrant permanent presence of mechanics on site, so your crews do a bit while they're there.

I don't assume any anime-slick tech level where anything exposed to space doesn't need regular fixing/replacing, or non-organic, intelligent, autonomous repairbots etc.

A dude with a lever and a rope always comes in handy....

Jedidia said...

I think you're missing my point. It's not that the old SF writers didn't use magitech, it's that they kept it to a minimum and didn't try to BS their way through it with pseudo-scientific mumbo jumbo.

Ah, yes. Having had the hell annoyed out of me by the first two seasons of Star Trek Voyager I can only wholeheartedly agree with that.

tsz52 said...

Tony: "I think you're missing my point. It's not that the old SF writers didn't use magitech, it's that they kept it to a minimum and didn't try to BS their way through it with pseudo-scientific mumbo jumbo."

Honesty above all else, so I agree with your point, but they were writing for a less hardfaced, more Romantic audience than today's.

If there had never been a Golden Age, and all them beautiful jokers like Clarke and Heinlein turned up now and started peddling that stuff, SF-heads would roll their eyes, go to town on the science errors/magitech, and do everything to criticise the finger, rather than even attempt to look at the sky that it is pointing to.

Against that level of mean-spiritedness (coupled with profound desire to be passively entertained, mind you), you kind of need some authoritative-sounding tech babble to win the audience over.

It's a matter of the luck of having been born into the right time, rather than moral high ground.

tsz52 said...

PS

Tony: "I think you're missing my point. It's not that the old SF writers didn't use magitech, it's that they kept it to a minimum and didn't try to BS their way through it with pseudo-scientific mumbo jumbo."

Jedidia: "Ah, yes. Having had the hell annoyed out of me by the first two seasons of Star Trek Voyager I can only wholeheartedly agree with that."

I didn't want to cause trouble by mentioning The Franchise... but I did wonder if you were referring particularly to this, Tony.

If so, then yeah. :)

Thucydides said...

Well if we really want to go for the big handwave (although at this point it is starting to look more like a layup or slam dunk in a basketball game), why not try stable "strange" matter nuggets.

A strangelet absorbing a neutron will undergo an exothermic reaction releasing ~ 20 MeV photon emission; a beam of light brighter than the sun!

Of course all we need to do is prove the Stable Strange matter hypothesis is true, locate or make a strangelet and keep it under close control while we feed a beam of neutrons to it...

Luke said...

Thucydides,

Sure, you could do that. A couple of complications immediately come to mind: (1) Normal matter would tend to be unstable to conversion to strange matter, so you could have accidents where entire planets turned into strange stuff, and (2) photons, especially 20 MeV photons, are not particularly useful for propulsion or power production (except insofar as they can run a heat engine - and you will get photoneutrons that cause all the usual problems with neutrons). If the strangelet is positively charged you avoid problem (1), but then you are back to the fusion problem - getting two positively charged bits to interact. Getting energy from reacting neutrons is not unique to strangelets, neutrons do the same thing with normal matter - just about every isotope has a neutron capture reaction that releases a gamma ray. The problem is getting the neutrons to run these reactions. There's also the minor detail that negative strangelets are predicted not to be stable by existing theories (less so than positive strangelets, anyway) and studies of pulsars indicate that positive strangelets can't occur either (otherwise the pulsar would be made of strange matter rather than neutronium).

Not that the catalytic baryon decay doesn't have interesting quirks ... monopoles are repelled from the diamagnetic cores of atoms, so a monopole encountering an atom with electrons orbiting it would tend to stay away from the nucleus - you would need fully ionized matter to get the catalysis or monopoles hot enough to overcome the diamagnetic potential barrier. Fortunately, you don't need the plasma to be anywhere near so hot as what is needed to sustain fusion - about 10 eV or so should do the trick. The baryon decay of an isolated a proton or neutron would release a lepton and a pion, both of which would be very energetic. The charged reaction products - positrons, electrons, and charged pions - can be contained and focused using magnetic fields, allowing all the fun stuff we can do with plasmas like high efficiency conversion to electricity and rocket thrust. The neutrinos just escape, of course, and the neutral pi mesons immediately decay into a pair of high energy photons that can drive photo-nuclear reactions among the other usual undesirable things. If the baryon decay occurs inside a nucleus, the pion will bounce around between the nucleons in a hadron-meson cascade which ends up fragmenting the nucleus. This will produce neutrons. Although the right choice of isotope (perhaps 3He) will reduce the number of neutrons produces, it seems that your choice is between ~65 MeV pion-decay photons or nucleus-fragment neutrons and a reduced but non-zero number of pion decay photons. In addition you will get the annihilation photons from the positrons. And, of course, you will want to re-capture your monopoles so they don't squirt off in the rocket exhaust.

Luke said...

Tony,

I remember from my youth only started having fusion at the back end

Whoa! TMI dude. HAHAHAHAHAHAHA ... sorry, couldn't resist.

Tony said...

tsz52:

"I didn't want to cause trouble by mentioning The Franchise... but I did wonder if you were referring particularly to this, Tony.

If so, then yeah. :)"


I'm heavily conflicted by The Franchise. I grew up on and certainly still like TOS, which was certainly informed by Golden Age sensibilities. Even the more outlandish plots/situations were redeemed by a relative lack of technobabble justifications. And everybody loves a good game of fisbin or a tribble infestation. But TNG just descended into PC hell and nothing ever redeemed it. And while DS9 tried to get into a little bit more realistic politics and economics, I could just never get into it. (YMMV, but Voyager and Enterprise aren't even Trek, AFAIAC.)

Luke:

"Whoa! TMI dude. HAHAHAHAHAHAHA ... sorry, couldn't resist."

Then I will eschew resistance myself...the reciprocating front end was a real crowd pleaser, back in The Day. ;) :P

Anonymous said...

OK, so since you brought up the subject of exotic drives (monopoles, etc.), I guess I'll bring one into play; if (and I stress IF), we can manufacture a microscopic black hole (I beleive it's refered to a a quantum singularity, but don't quote me) then if we can confine it and use it to accelerate a stream of matter, not into it but around and out an exhast nozzle, then might we not have a powerful rocket? More over, would it not be useful as a source of power? I don't have any idea how much Hawking radiation such a tiny object could produce, and how much of it could be harvested for power production? Or, could another way be used to produce power? Like spinning mass into the sinularity and generating energitic jets that could be utilized for power and/or propulsion? Just a thought.

Ferrell

Jedidia said...

It sounds a bit like you're missjudging the nature of a black hole. Especially a microscopic one.

a) I don't see how exactly you're going to accelerate anything meaningfull with a microscopic black hole. The particles would have to pass it at micro- to nanometer distances to feel much of the effect, resulting in a veeeery thin particle stream. Plus, it'll loose the energy again when passing it, unless you eject the black hole with it.

b) a microscopic black hole doesn't give you much Hawking radiation to speak of.

c) what little it gives will result in it decaying in a matter of seconds, maybe even less. You have to feed it to keep it alive, and a microscopic black hole isn't the all-devouring monster medias think it is. It is very hard to feed such a little bugger to keep it stable.

KraKon said...

Fire up the monopoles Scotty! We've got to catch those strange-matter emitting hotrods! It's bad for the environment

Too much opera in two lines for me.
But, end-line, it's fusion or nothing? Better off sticking to ICF then.

Luke said...

Ferrell,

If the derivation in Wikipedia is to be believed, a black hole with a mass of 400 tons will completely evaporate due to hawking radiation in less than a second (releasing 600 million times the energy yield of the Hiroshima bomb in the process). This black hole will have a radius of less than 1/10,000,000 the radius of a proton. Unless you can shove 400 tons of matter per second through a sphere with a diameter ten million times less than that of a proton (and handle the consequent 600 million Hiroshima bombs per second), you cannot use low mass black holes to propel your spacecraft. This pretty much limits you to holes that are much more massive than 400 tons. For example, if your hole is the diameter of a proton (allowing it to feed merely excruciatingly slowly instead of mind-numbingly excruciatingly slowly), it would have a mass of 700 million tons. This is kinda hard to haul around. This 700 million ton hole would radiate at 700 MW, which gives you a rather poor specific energy of 1 mW/kg. Note that I meant mW/kg (milliwatt per kilogram), not MW/kg. These megawatts of radiation also make it harder to shove matter into it, as they tend to push the infalling stuff away.

Now if you had a magic thingamagizmo that could turn ordinary matter into a black hole, you could do nifty things. Because reasonable amounts of matter collapsed into a hole will evaporate essentially instantly, you have direct conversion of matter into radiation at an impressive scale. There is no method yet found in the wildest dreams of theorists yet that could accomplish this, however.

Other fun magi-tech toys could include things like non-orientable wormholes. If you use wormholes for FTL transportation in your setting, you might be able to carry around a non-orientable wormhole in your spacecraft. A non-orientable wormhole is just a wormhole that when you go through it turns you left-to-right. A left handed glove would end up as a right handed glove after going through. Dextro-glucose, which we can digest, would turn into levo-glucose, which we can't. So far, it's kinda nifty, but not really mind blowing. However, now I invoke CPT symmetry, which is thought to be inviolate under all possible physics in the universe. The T is time reversal symmetry, changing going forward in time to going backward in time. P is parity, switching left to right (as we did). C is charge conjugation, a physicist's way of saying it turns matter into antimatter and vice versa. So if you flip left-to-right, and you are still going forward in time, you have to turn into antimatter. So if you put on that now-right-handed glove, it annihilates your hand; and the levo-glucose is not only indigestible, it gives you the worst case of heartburn imaginable. But it becomes very convenient for powering your spacecraft. Just have some guy down in the boiler room shoveling matter into the wormhole, and it comes out the highest possible grade fuel for spacecraft propulsion. And this is a natural consequence of your One Big Handwave you made to allow FTL.

Odd particle-physics constructs called Q-balls can have similar consequences, reflecting matter into antimatter.

Thucydides said...

Fire up the monopoles Scotty! We've got to catch those strange-matter emitting hotrods! It's bad for the environment

Welcome to the implausible midfuture! ;)

Merry Christmas everyone

Anonymous said...

Ok, so micro black holes are out for rocket propulsion, but might have other uses. By the way, neat trick for turning matter into antimatter (I can just see Scotty shovling coal into a wormhole to power the engines). Hmmm...700 million tons for a useable micro black hole...even if you could use it to generate an Alcumbrie (sp?) type spacewarp, once you got to where you were headed, you wouldn't be able to scoot around the system; you'd be stuck in whatever orbit you settled into upon arrival. So your starship would jump from one system to another, but once you get there, your daughter ships would need to complete whatever your mission to that system was. You're gonna need a lot of asteroids to feed into your matter-crunching machine...

Ferrell

Milo said...

Luke:

As far as we currently know, topological defects do not exist. Maybe we'll find them someday, but that strikes me as more magitech than miniaturizing a reaction we can see up the sky each day - especially when, to find something that is so obviously rare, we will probably need to explore the wide reaches of outer space to begin with.


"Unless you can shove 400 tons of matter per second through a sphere with a diameter ten million times less than that of a proton (and handle the consequent 600 million Hiroshima bombs per second)"

Hey! I think I found a new refinement on the Orion design!

Luke said...

Milo,

However, pretty much every theory we have for figuring out how the strong nuclear force relates to the electroweak force predicts topological defects. We will probably never find any - our theories also predict that inflation spreads them out over such a huge volume that we would be lucky to have one monopole in our observable universe. If we get to use them, it will probably be because we find out how to make them. This might be as easy as just building a bigger accelerator (until we get a verified theory of strong-electroweak interactions, we wouldn't know how big of an accelerator you need, so there's still hope and a sort of vague plausibility), although the lack of monopoles in cosmic rays argues against this.

And I would claim that this is less magitech than miniaturizing what we see up in the sky each day - up in the sky you have proton-proton fusion. This requires a step that occurs due to the weak nuclear interaction. This makes the process really, really slow. And that is a good thing, from our perspective - we wouldn't want the sun to exhaust all of its fuel in a few seconds. However, it does mean that the sun has a specific power of only 0.2 mW/kg, and that's with an entire sun's worth of mass confining the plasma and an entire sun's thickness of plasma to re-capture the bremsstrahlung x-rays. With a set-up you can get in the laboratory, you will face plasma instabilities and be faced with a plasma that is optically thin to its bremsstrahlung, so your bremss power that is lost will be many many orders of magnitude larger than the fusion power that is gained. As far as I am concerned, any mention of proton-proton fusion for power or thrust production on a human scale is as magitech as it comes. If you want to use proton-proton fusion, set up a photovoltaic cell or a heat engine using solar concentrators.

Aneutronic fusion isn't nearly that bad, but it is pretty bad. D-3He is vaguely plausible, but you will get somewhere between 0.2% and 5% of your energy out as neutrons (depending on the specifics of temperature and whether your fusor captures the tritium that is produced in D-D side reactions). Everything else will require some unforeseen advance of which we currently have no inkling of how to bring about - the current understanding of exotic aneutronic fuels looks pretty bleak
http://dspace.mit.edu/handle/1721.1/11412.
So from my understanding of the situation, fully aneutronic fusion producing net power is just as magitech as catalyzed baryon decay - both require an unforeseeable advance in technology and both are theoretically possible.

Now D-T fusion I would say is more plausible than catalyzed baryon decay. Getting net power out of it is certainly very difficult, but if the plasma instabilities can be solved it should work. Of course, D-T comes with a whole host of technical difficulties even once you assume it works - namely the incredible flux of 14.1 MeV neutrons, which are just generally all around bad news. If you demand an external pulse thruster for D-T fusion, you are edging closer toward magitech - it is hard enough even when you can enclose it from all sides. Even then, you have the issue of the amazing neutron flux activating and embrittling and amorphizing the surrounding equipment and losing 80% of your fusion power to unusable neutrons.

Now I like high thrust external fusion thrusters as much as the next guy. They are fun to think about and certainly very impressive. However, from a plausibility point of view the less troublesome varieties seem about as likely as a breakthrough that allows non-conservation of baryon number and thus direct conversion of matter to radiation.

Rick said...

Regarding Golden Age drives, I don't think Heinlein ever specifically mentioned fusion. His tech progression varied slightly from story to story, but generally it seemed to go from nuke thermal (with chemfuel remaining in use for less demanding missions) to Ortega's mass conversion torch to the Horst-Milne-Conrad impellor, an unabashedly magitech drive.

Ortega's torch is interesting in terms of Luke's comment just above.

There may have been a hint of fusion in Citizen of the Galaxy, but no more than a hint. Heinlein also never used nuke electric drive, that I can recall - he probably regarded any drive without bone crushing acceleration as wimpie, no matter how well suited to interplanetary missions.

Clarke's golden age stories form less of a consistent future history, but I usually got the impression of a nuke electric drive.

Ja jeg har læst vilkårene for brug og ønsker at gå i gang med "google grupper" said...

FYI: new conventional rocket fuel discovered: http://www.physorg.com/news/2010-12-discovery-molecule-efficient-rocket-fuel.html

Rick said...

Welcome to another new commenter!

Most chemfuels are such basic stuff that I never imagined that new ones could be discovered. But apparently they can. The stuff in the link, trinitramide, is only the 9th compound found of oxygen and nitrogen without other elements. Most of the others have been known since the 18th century.

The actual performance isn't given in the article, and I suspect that the expected performance improvement is compared to existing solid fuels, not H2-O2. Still pretty remarkable ...

(Of course they still have to make the stuff in bulk!)

Anonymous said...

Even if it "only" doubles the payload, it would be a big step toward reducing costs for launches. Coupled with new construction materials, future rockets could be much more like those featured in Rocketpunk.

Ferrell

Tony said...

Re: Ferrell

Except that it wouldn't double the payload, because the increase in efficiency is WRT other solid fuels. What it would do is put solids in the same efficiency class as Kerosene/LOX mixtures. So you might get a few percentage points more payload with designs that include solid compnents.

Anonymous said...

Tony, obviously you missunderstood: I was talking about solid fuels and the increased efficency of this new compound over others of the same class. According to the article, every 10% increase in efficency of these fuels results in a doubling of payload capacity. qibble with the authors of the article, if you disagree.

Ferrell

Tony said...

Ferrell:

"Tony, obviously you missunderstood: I was talking about solid fuels and the increased efficency of this new compound over others of the same class. According to the article, every 10% increase in efficency of these fuels results in a doubling of payload capacity. qibble with the authors of the article, if you disagree."

I'm addressing what appears to be a misunderstanding. Whether it is your's or the authors' is irrelevant. The efficiency numbers are an apples to apples comparison. But, unless you're talking about rocket propelled weapons that are totally reliant on solid fuels, it's not an apples to apples exercise.

If you're talking about launch vehicles, only strap-on boosters are solid fuel rockets. Making those more efficient is of course desirable, but making just those components more efficient isn't the same thing as making the entire package more efficient. You're not going to double or tripple the payload by just making the strap-on boosters lighter and/or longer lasting. You'll gain some payload mass, but not that much.

Anonymous said...

Tony: Whatever; I'm sure all the other readers would appreseate us not arguing again, so let's change the subject:

Others have written about Mag-Sails or Plasma Sails that use the solar wind to get around the solar system...my question is how does this drive work (if at all) in Earth's mangetoshpere?

Ferrell

Stevo Darkly said...

Ferrell -- I remember reading Zubrin's original article about the magsail concept in Analog magazine. It will work within a planetary magnetosphere. (Assuming it works at all -- the high-temperature, high-strength superconducting loop would be considered magi-tech by all here, I'm sure.)

-- Stevo Darkly
I remember reading in that article that a magsail vehicle could even, in theory, lift a vehicle from Earth's surface into orbit using the Earth's magnetic field, taking off/landing from the Earth's magnetic poles -- again, given the right magi-tech materials and proper field strength. In practice, launching a vehicle with a miles-wide superconducting loop from Earth's surface, or landing it, would pose a lot of practical difficulties.

There's an article on Wikipedia, and an interesting-looking overview is on the website of Steve Jackson Games (sjgames dot com). Google "magsail" (one word) and check out the first few hits.

(I haven't posted here in quite a while, but have followed all discussions of the past couple of months with interest. Wish I had time to contribute more.)

Stevo Darkly said...

Oops, I misplaced my signature there. And it's totally superfluous here anyway. Sorry.

Thucydides said...

I suspect for practical purposes, magsails screened by the magnetosphere would work very slowly, or not at all. Even invoking a coupling to the Earth's magnetic field seems to be a large handwave, how do you decouple from the magnetic field and gain access to the solar wind? (given the slow accelerations of sails in general, I suspect that cutting away from the magnetic field would not be simple in practice).

Magsails benefit from a beamed propulsion system, such as M2P2 or magbeam, and since there is a direct coupling of the beam energy to the spacecraft (rather than converting beam power to electrical energy and then using electric power to accelerate the remass) the performance should be the best of all indirect systems, and comparable to many self propelled ships as well. since the Magsail can also be used without the power beam, it gives the ship a great deal of flexibility.

Since the lightweight, room temperature superconductor is still currently magitech, we can put this on our wish list, and of course it should show up in written works, games etc.

tsz52 said...

"I suspect for practical purposes, magsails screened by the magnetosphere would work very slowly, or not at all. Even invoking a coupling to the Earth's magnetic field seems to be a large handwave, how do you decouple from the magnetic field and gain access to the solar wind? (given the slow accelerations of sails in general, I suspect that cutting away from the magnetic field would not be simple in practice)."

Yup, you'd need some other sort of propulsion system as well for such transitions and for steering (can't tack etc the way a lightsail can).

The synchrotron radiation might be an issue in some places....

"Magsails benefit from a beamed propulsion system, such as M2P2 or magbeam, and since there is a direct coupling of the beam energy to the spacecraft (rather than converting beam power to electrical energy and then using electric power to accelerate the remass) the performance should be the best of all indirect systems, and comparable to many self propelled ships as well. since the Magsail can also be used without the power beam, it gives the ship a great deal of flexibility."

I guess the M2P2 could heat its plasma as the other, necessary, drive system (itself or beamed).

M2P2 also has the advantage, over most other sails, that it can fairly easily maintain a constant thrust by adding more plasma to the loop (and increasing the loop's size), rather than being a helpless slave to inverse-square.

tsz52 said...

Gah - gettin' old... can't remember nuthun!

I've just had a quick re-read of a few papers, and some types of magsails may be able to travel perpendicularly to the radial solar wind (or beam); some can't.

And I think that M2P2 auto-corrects for constant thrust, as the solar wind intensity decreases (plasma bubble automatically increases in size)... does that sound right? Some of the wording's quite ambiguous.

Didn't want to mislead anybody.

Anonymous said...

tsz52, Stevo Darkly, Thucydides, thank-you all for the info! I now have a better idea of how to portray magsails.

Ferrell

tsz52 said...

@ Ferrell:

I don't know what your project is or how 'hard' it needs to be, but I'd hate for you to be misled.

I did some more reading last night, and remembered why I'd put M2P2 on my 'seriously research sometime later...' list - it sent my 'way too good/cool/slick/elegant to be true' detector's needle off the dial.... :(

It seems that you really do need a superconductor for claimed performance, but (with current state of the art) that reduces thrust:mass by orders of magnitude from the claimed.

And another paper suggests that the original claim made an optimistic assumption about the momentum transfer mechanism (from solar wind to plasma-field loop to craft - this being the bit that got my gut dubious, when I first came across it) which is incorrect... by *ten* orders of magnitude....

So, unless you're an expert who can independently figure all this out for yourself (I'm certainly not), you might do well to leave M2P2 until a few major tech breakthroughs have turned up, and stick with the much 'harder' magsails of various stripes (sufficiently different that it's worth a little research, so you don't trip yourself up).

[Lol: I saw a brief and splendid animation of an M2P2 craft in operation last night (I really hope that such things do come to pass - beautiful!) and... couldn't resist supplying the 'sound effects': Zzzzzt as it unfurls, Pshyoow as it activates its plasma-mag loops (best thing ever!) and vwuum as it started to sail off....

Facepalm! Then I did it *again*! :D ]

Anonymous said...

OK, so when the newbie pilot starts gushing about the M2P2-drive ship in the next berth, the older engineer should tell him to curb his enthusiasm? Character development and tech exposition all in one conversation! Thanks!

Ferrell

Geoffrey S H said...

Joan D Vinage's "Outcasts of Heavens's Belt" had a delta-vee force with expendable chems... never found any numbers on them except in the books. I don't even think it said how many stages each craft had...

While wondrously ludicous as a concept, I am nevertheless curious.

Geoffrey S H said...

Sorry, that was meant to read Joan D Vinge

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