Thursday, April 22, 2010

Wars of the Rings

Rings of Saturn
Traveling through the rings of Saturn, no one sees anything. The jeweled crown of the Solar System is a hailstorm swirling without end around Saturn, a place where ships might pass close enough for visual recognition but never see each other.

I was reminded of this by a commenter who, a couple of posts back, linked to a section of the ever growing Atomic Rockets site that I had entirely missed. The Solar System's leading tourist attraction, gorgeously visible through a backyard scope, is a weird and wonderful place. The laws of physics are the same, but the circumstances are so different that most of our conventional rules don't apply.

The fine structure of the major rings is magnificent, but most astonishing is how enormously compact they are along the 'vertical' north-south axis. The main bulk of the brightest and most massive of the rings, the B Ring, is estimated to be about only about 5-15 meters deep on average.

Within this narrow band the clutter is amazingly dense. The B Ring extends from 92,000 km from the center of Saturn outward to 117,600 km, giving it a cross sectional area of 1.69 * 10^10 square kilometers. It has an estimated mass of 2.8 * 10^19 kg, thus 1.66 million kg per square kilometer, or 1.66 tons per square meter of cross section. It is mostly water ice, so if the whole B Ring could be compressed into a flat solid disk it would be about 1.66 meters thick - about a sixth of its actual average thickness.

In fact the B Ring is so dense along its center plane that the iceballs may coalesce into 'solid' skeins, endlessly dissolving and reforming.

Hollywood? Are you paying attention?

If there is any place for real, classic style space fighters, it is the rings of Saturn. Relative speeds in the immediate ring plane will be slower than jets, more like highway speeds, since you're flying through the hailstorm, and even the clear lanes between rubble skeins probably have a good many smaller bits and the occasional big chunk drifting through them. The farther you get above or below the plane - on a scale of tens of meters - the clearer the going and the better the seeing, but the more exposed you are.

Wingmen, cruising on each side of the center plane, switching off now and then to throw off watchers? Raiders slipping along the center plane, working their way like experienced rivermen between the skeins?

Any craft with a human crew is much hotter than the rings, more than 200 K hotter, and would stand out in the mid IR. But if you are also coasting along through the rings, your view along the ring plane is practically nil; you are driving, or drifting, though the hailstorm. An observer far from the ring plane has a better view, but the rings still provide an lot of background clutter.

And an observer far from the ring plane is fully exposed to view, and therefore fully exposed to fire.

If there are denser clumps among the rings - and a 400 meter moonlet has been found in the B Ring - these could provide a place of concealment for larger ships or habs. And those billions of tons of ice drifting around might even allow that Holy Grail of space stealth, hiding your waste heat signature.

The B ring is at about -200 C, so melting a ton of B ring ice requires about 280,000 Kcal, or 1.1 GJ. Thus you can get rid of a gigawatt of waste heat by melting a not quite a ton of ring rubble each second. A moonlet 100 meters in diameter has up to half a million tons of ice (somewhat less if it is a loose rubble pile with voids). Stick a heat pipe into the center and pump away; it will take up to a week for the melt to reach the surface - and until it does, the heat is all trapped inside.

At the end of the week, you just gather another rubble pile (less than a square kilometer of ring), and start heating its interior. Your abandoned heat sinks will gradually cool off, but at a very slow rate, its surface barely warmer than its neighbors, its minimal signature lost amid all the random jostling in the rings. A rogue hab could drift through the ring system leaving only the most ghostly trace of its presence.

And the rings are vast, the B Ring alone 25,000 km wide and more than half a million in circumference, so there is plenty of room to lose yourself in.

Yes, there is something tacky about planning space battles in the rings of Saturn, sort of like visiting the Grand Canyon and thinking of what a great shoot 'em up Western you could film there. But hey, spectacle you want, spectacle you got.

Now all you need is an excuse to be there. The only sure reason for humans to go to the rings of Saturn is that they are so cool, but battles between rival tour operators would kill the business. Gathering He 3 for fusion reactors is a popular explanation for going to Saturn, but not a convincing one. If you have fusion technology and deep space access, you can use dirty reactions to breed clean fuel. All you need is ice and a good safe distance from human habitats, neither of which requires going 10 AU.

More convincing (IMHO) is that we will go to Saturn because it is cool, and if we go in sufficient numbers we will develop an economy to sustain ourselves there. And humans can always find ways to get into conflicts, if that is what the plot calls for.

The image, from Cassini via Astronomy Picture of the Day, is backlit by the Sun, itself eclipsed by Saturn.


Thucydides said...

Cool tourist resort, many moons full of valuable ice (with a lower overall escape velocity than the Jovian system), Nitrogen and Hydrocarbons on Titan and the potential to "vanish" among the rings for outlaws and dissidents; what's not to like?

Using the ice bodies in the rings as disposable heat sinks makes it a good base for warships as well, so lots of different groups have greater or lesser reasons to come to Saturn. Saturn might become very cosmopolitan with the eclectic mix of people and interests settling there. Jovian settlers may be motivated more by tapping the energy of the vast magnetosphere, while I will say Uranus becomes the 3He capital of the system, with a low enough escape velocity that a 150,000 km "skyhook" could be used to directly mine the atmosphere.

The physical description of the rings is very interesting, I remember a real grade "B" science fiction movie where the climactic scene involved taking a giant spaceship through the rings; the movie reflected the earlier view of the rings as a cloud of boulders in space. Since the ship had giant domes full of plants and animals, this didn't seem to be a very smart move but was pretty spectacular to watch. (can't say the same about the rest of the movie).

Anonymous said...

Well, Titan is just next door to the Rings and there are plenty of reasons to go, it's not inconceivable that a human presence in the Saturn system might wind up spawning a rogue element. (the Federal Titan Space Militia Armed Utility Ships chasing the Free Colony of Boogietown Raiders through the Rings...)


Luke said...

The easy, dirty form of fusion involves burning deuterium and tritium. You can't get tritium, because it only lives for a decade or two, so you need to make it by irradiating lithium with neutrons. So to get your clean fuel, you not only need ice (for its deuterium) but a supply of lithium. Whatever tritium you don't burn in your reactor eventually (within a decade or two) turns into helium-3.

Instead of using fusion, you can also make helium-3 by irradiating lithium with neutrons from a fission reactor (again creating tritium, which decays into helium-3 after a while). This requires lithium and uranium or thorium, rather than water.

You can fuse deuterium with deuterium, but it is actually easier to fuse deuterium with helium-3, and D/He-3 is much cleaner than D/D. For the ultimate in clean fusion you can fuse protons with boron-11, the latter of which is moderately abundant, the former of which is extremely so. Unfortunately, p/B-11 has difficulties which might prevent it from ever being useful.

Not that it really matters for Saturn. Uranus has a much lower escape velocity, and a higher concentration of helium, than Saturn.

Carla said...

Space fighters dodging through the rings of Saturn - what a cool idea! How big could the fighters be (the equivalent of single-pilot fighters, or something bigger)? Is there an equivalent of leading enemy ships onto a sandbank?

Byron said...

I'm not so sure about He3 not being used. Tritium takes a long time to decay, and I bet that we'll go to Saturn initially to meet demand, and then keep going there because of infastrucure. Also, the fact that it's a natural He3 source is quite useful in a conflict setting. In a story I'm writing, a war breaks out between Earth and Luna, which controls He3. Saturn becomes an important source for He3 for Earth, leading to battles. The same could happen in any setting where one side has control of He3.

Anonymous said...

The setting for my novel has the colonization of Saturn's moons as a background detail. The colonists were a group of technophilic transhumanists who wanted to get away from Earth. Their transhumanist plans have fallen by the wayside, since all of their biotech resources have gone into maintaining lifesupport, but the colonies have otherwise done fairly well.

Spacefighters in the rings of Saturn is a nice bit of colour for that background. It's also a useful extra explanation as for why none of the larger powers have moved in on Saturn. Saturn's defence forces are designed to fight in an environment where no one else has any experience.


Citizen Joe said...

Magnetosphere of Saturn = free power. Ice rings of Saturn = free remass. Minimal reactor and no remass = very high payload fraction.

J Morris said...

Hi, I've been working on a comic book set on Titan, and so I've been doing some research. I spoke with one of the Cassini probe team members (one of the orbital physicists and also an expert on planetary rings) and he is of the opinion that the rings are actually to be avoided.

The issue is fine particles moving at fast speeds. According to him, one particular (a pun!) worry about the Cassini probe is that it could be hit by some ring matter traveling fast enough to junk the probe.

Any ship in the rings is going to need very heavy armor to survive being continuously beaten with ring matter. I'm not sure how practical it would be.

Citizen Joe said...

I'm pretty sure everything at a specific orbit is moving at the same velocity. So yes, they are moving fast to an outside observer, but no they aren't moving fast relative to themselves.

Now, for the Cassini probe, we're talking about coming in at a non-orbit vector and passing through. Yes, there are velocity differentials that could scrub the mission.

Rick said...

Lithium is pretty cheap for a fusion fuel, about $100/kg, and shouldn't be that hard to find among rocky bodies. So breeding He3 sounds a whole lot cheaper than scooping He3 off of gas giants.

The problem is that the same things that make gas giant scooping so cool also make it exceptionally difficult and therefore exceptionally expensive.

Accountants may be cool people in their private lives, but when they put on the green eyeshades, Generally Accepted Accounting Practices include being totally impervious to the power of cool. Which is the challenge of space travel in a nutshell.

If Saturn's rings are as narrow as reported, small ships are definitely favored - too big and the ship would stick out on each side of the center plane.

My thinking on impacts is that the rings must be a very conformist place, or they could not be so narrow; even collisions at walking speed would spread the stuff out to a kilometer or so deep.

(Anyone want to do that math? How much will a lateral kick of 1 m/s change your angle of inclination, and how far north and south the center plane does that carry you each orbit?)

On the other hand I have not discussed this with a Cassini team member! Traveling through the rings at high relative speed would be VERY dangerous, but there may be enough loose bits flying at odd angles to make any travel near the rings risky, even if your are orbit matching.

If you CAN travel safely through the rings, you can only do it at limited relative speed, which will make getting around a surprisingly slow process.

Admitting that we're pretty much looking at possibilities for Romance, not demi realistic futurism, it is easy to see the rings as a lawless zone where people go who don't want to be found, and where the authorities are reluctant to go looking for them.

Thucydides said...

There may not be space fighters in the rings of Saturn, but one or more States will be hiding their ICBM equivalents inside.

The actual difficulty isn't so much getting a matching vector and orbit so your doomsday torpedoes don't get filled with holes, but how do you unobtrusively insert them in the first place?

A burn to match velocities is easily seen from across the Solar system, while long looping orbits and gravitational billiards might do the job, but leave your weapons drifting around for long periods of time and negates some of the low observation advantages of hiding in the first place.

Dissidents need room so it is easier to burrow under the surface of an ice moon than try to hide out in the rings, but the future equivalent of a safety deposit box might be placed there with appropriate booby traps for people seeking the blackmail holos and Swiss bank account numbers. said...

Makes me wonder what the dust ring around a protostar would be like to maneuver through.

Or what's left of one, like Fomalhaut's:

If you look in the lower right corner you'll see Fomalhaut b.

Anita said...

Way off topic, but just reminder. Today is the 20th anniversary of the Hubble Telescope launch.

Salute and thanks for expanding our vision.

Luke said...

Lithium will be a bit more expensive, because natural lithium consists of two isotopes - Li6 and Li7. Both have their uses, but you will want to enrich your lithium in one isotope or the other for optimal utility (Li6 gives you additional energy when producing tritium, so that is good if you are running a power reactor. Meanwhile, Li7 gives you tritium and an extra neutron that can produce more tritium, but this takes energy and only works with high energy neutrons, so more Li7 can help with breeding tritium and thus ultimately He3, but at the cost of lower power production).

However, that is a minor detail compared to the main drawback of deuterium-tritium fusion - the enormous neutron flux produced by an operating reactor. This has many issues. For example, you will need some sort of structural element between your fusing plasma and lithium blanket, in order to keep molten lithium out of the fusion chamber. The extreme neutron irradiation will eventually ruin the structural properties of the separater, and will also activate the separater, making it radioactive. As a result, periodically (I've seen estimates of every few years) you will need to replace the separater and dispose of it as radioactive waste (it will not be as problematic of radioactive waste as spent fission fuel, but it will be an annoyance and a potential health and environmental risk).

The tritium produced will also be a concern. Tritium is a form of hydrogen, and hydrogen is ubiquitous in the environment, highly mobile, and rapidly taken up by life. Tritium accidents and leaks would lead to tritium getting into the environment, entering the watersheds, being taken up by plants (such as food crops) and incorporated into their sugars and carbohydrates, and then be eaten by animals (such as us) and incorporated into our bodies. At low levels, this would not be much of a concern (we deal with constant low level doses of natural radioactivity anyway) but enough tritium would eventually raise the planet-wide radiation background to the point that it could start being problematic.

Maintenance on deuterium-tritium fusion reactors would also be annoying. After shut down, you will need to wait a month or so before the neutron activation products decay away to a safe level for human technicians to enter and do their things.

So, if you can get deuterium-helium3 fusion to work, and if you can acquire the helium3, it would ameliorate a lot of problems. You wouldn't fully eliminate the neutron radiation (deuterium-deuterium fusion side reactions will produce some neutrons) but you will cut down on the neutrons by a very large amount.

Markus said...

@Rick, on the collisions and inclinations in the rings:

I decided to do the math. Correct me if I'm wrong. If I calculate correctly, 100000 km away from the Saturn equator (about 160000 km radius from Saturn center), somewhere within the rings, the orbital velocity is about 15.4 km/s. A lateral delta-v of 1 m/s would, according to trigonometry, cause a 0.0037 degree change in inclination (arctan 1/15400). The diameter of the ring system at this point is ~321000 km. Again with trigonometry, we can obtain that the 0.0037 degree inclination would cause a 10 km "altitude" at the extreme "north" and "south" of the modified orbit (the ring system radius times sin 0.0037 degrees), that is one fourth of the orbit from the place where the 1 m/s lateral bump happened.

So, indeed the rings have to be quite conformist place as Rick said.

Also, I would like to point out a thing about using the ice as a heat sink. In the post, Rick assumed the ice block would be melted. The problem is that the pressure in space is almost nonexistent, which means we cannot have liquid water in vacuum, which means we cannot melt the ice. We can heat it to the melting point and then it just sublimates.

Assuming the -200 C temperature mentioned in the post, final temperature of 0 C and a 2.09 kJ/kgK specific heat capacity, a metric ton of ice could take 420 MJ of heat before starting to boil/sublimate.

If the ice was further sublimated, which starts happening at about 0 C, it would then take 2.26 MJ/kg or 2.26 GJ/ton. But if it was boiled away, you would eventually turn the rings into water vapor...

- Markus

Citizen Joe said...

How well would Silicon Carbide work as a first line separator?

I used replaceable SiC shells as first line shielding for my Wildcat IIb D-D breeder reactors in my fictional setting. After a specified time, the shells would be replaced and ground up, then stored in barrels labeled "Fusion Core" with a radioactive symbol. Grounders would think that these things would explode if struck, but it would just be more of a radiation spill. Often the spacers would devise pranks around these misunderstanding. Likewise, this crushed SiC would be deployed as kinetic kill clouds against the aliens in the setting. Those aliens used visible light lasers well and thus had ships with highly reflective and angled surfaces. A SiC cloud would scratch the surface enough that the human lasers became more effective.

UmbralRaptor said...

That sounds like a fairly sensible material. Most Carbon (Carbon-12) can capture a neutron without becoming radioactive. And most Silicon can capture 2(!)

I'm not sure if carbon's tendency to slow neutrons is good or bad in this scenario. Also, would mechanical damage from neutrons bouncing off/pushing around nuclei be a problem for the core after a while?

Luke said...

Carbon plays nice with neutrons. It has a low neutron capture cross section. There is a neutron in, alpha out reaction that becomes significant at energies above 8 MeV (D-T fusion neutrons are at 14 MeV). This eventually turns your carbon into helium.

Just looking at Si28 (the other two isotopes are relatively low abundance) there are a few inelastic reactions that high energy neutrons can produce - neutron in, proton out; neutron in, deuteron out; neutron in, alpha out - that could degrade your silicon and become important at energies above 6 MeV or so. These reactions turn your silicon into other things.

Bulk silicon carbide is relatively resistant to being pushed about by neutrons. All in all, SiC seems like a least-bad choice for keeping the lithium out of your reactor.

Citizen Joe said...

There is of course various Boron Carbide possibilities. But I've mentioned that Boron is not very abundant whereas carbon and silicon are. So, maybe something like a disposable SiC shell, with lithium blanket to breed tritium, followed by a non replaceable Boron-11 Carbide shell and then some lead shielding to catch any incidental gammas from bremstraulung.

Prep for change out and emergency shutdown procedures probably involve a Boron-10 lavage.

Of course the magical part of this reactor is the part that separates out the bred tritium and He3.

This seems to be diverging from Saturn... Umm... so... Nitrogen on Titan... Ya, I had ammonia ice fabricated at Titan/Saturn and then shipped via Hohman trajectory to Mars for atmosphere enrichment. Meanwhile, way out on Triton of Neptune, that nitrogen was saved for interstellar hops. The nature of the FTL drive meant that your vessel's mass dictated how close you could get to the Sun's gravity well. The big Interstellar ships were limited to around Neptune orbit.

Rick said...

Anita, I knew something was up this morning from the Google logo. I just used the occasion to post on a lesser anniversary, tomorrow being the 3 year mark for Rocketpunk Manifesto.

Welcome to some new and returning commenters!

Based on Markus' numbers, typical drift speeds must only be about a millimeter per second. It makes me think of (the now vanishing!) Arctic sea ice, without the sea.

My idea for concealing waste heat ignored many devils in the details, but my general idea is to heat the interior of a big, big snowball. If you can cleverly seal the surface (a freezing water spray?) the molten interior won't just evaporate - which would produce a distinct IR signature, though still amid lots of background noise.

Evaporating the rings - yikes! But the vapor might form ice crystals and re-coalesce. Saturn distance blackbody temperature is chilly.

I certainly would not allow breeder reactors on Earth. And though worrying about environmental concerns in outer space (apart from human habs)) seems frivolous, maybe it isn't. If you are going to have a permanent, large scale human presence in space, be careful what you turn into a radioactive death zone.

So? What is involved in recovering He3 from a gas giant? Do you put refineries on balloons, and then launch the stuff up? That will be fun, getting to orbit from a gas giant. The other alternative I've heard is scooping, but if you're scooping up a decent amount of stuff you melt your scoop.

That sounds snide, which I don't really intend. But the requirements for this sound like Xtreme space travel - everything that makes ordinary space travel difficult, but more so.

That said, it is way cool technology if you can do it, and once again, ring ring Hollywood, cluephone on Line One.

(Not that Hollywood REALLY cares about whether there's a scientific pretext for cool special effects.)

Saturn it has to be, unless you intend some very broad comedy. Gas from Uranus. There, it had to be said.

Ahem. How much He3 does Neptune have, for Citizen Joe's starships?

The best reason to get it from Saturn, if you have the tech, might be that you're already there, and beyond Saturn the highway does get a bit long and lonesome.

The disk around Fomalhaut could be a mighty rugged travel environment. I doubt it is as orderly as the rings of Saturn. But it probably has a complex internal structure, perhaps with clear lanes.

Luke said...


You would probably have a gas refinery in a balloon. You would need both chemical and isotope separation on board.

You may joke, but Uranus simply looks like the best place to get helium 3 for the rest of the solar system that isn't around a gas giant. Saturn and Jupiter require extremely large delta-V's to escape from their gravity, or to brake into orbit. Saturn also is relatively depleted in helium (it has been condensing out of the upper atmosphere and dropping into the unreachable depths of its core for billions of years). Neptune is farther from everything, so if money is an issue, it will probably be initially ignored in favor of Uranus.

In actual numbers, Jupiter has 10 ppm of He3 in its atmosphere, Saturn has 3 ppm, Uranus has 15 ppm, and Neptune has 19 ppm.

Thucydides said...

For the Helium wildcatters out there:

Breeding it out of Lithium in high energy nuclear reactors is probably a mugs game, since the huge costs of the reactor and constant replacement of irradiated parts would overwhelm the benefits of getting 3He from that source. Can you imagine a clumsy employee dropping a dewar of 3He at that cost?

Boiling the stuff out of Lunar regolith will be the first step, simply because it is close and relatively easy to do. It is still costly, but the late 21rst century Lunar economy will probably be driven by 3He production, since Helium-3 is worth 7.9 million dollars/kg in 2000 dollars.

That sort of price will be the incentive to find ways to extract it from the Gas giants, and once a viable means is developed to harvest and deliver 3He in bulk, Luna will rapidly decline into a 22nd century "Rust Belt" world.

Once the wildcatters have seeded Uranus with balloons or start dropping ramscoops into the atmosphere, there will be consideration for really BIG projects to harvest 3He. I have seen an outline in a book on space elevators suggesting a 150,000 km skyhook could have one end dipping into the atmosphere and have enough momentum at the free end to "whip" payloads to Earth on minimum energy trajectories. Working out the details will be interesting indeed.

Which brings us back to Saturn. Since it has lots of the elements for 22nd century primary industries (mining ices from the moons, harvesting Nitrogen and hydrocarbons from Titan) and lots of secondary and tertiary economies that can exist independent of the primary sector (tourism, military basing and contracting), a viable civilization can exist there, and one that is more cosmopolitan than places like Luna, Jupiter and Uranus which would be dominated by a single resource extraction industry (harvesting energy from the Jovian magnetosphere is trivially easy in theory; just deploy electrodynamic tethers. Once again, the devil is in the details).

How would the relations between the various "nations" play out? Would the sheer industrial and economic muscle of Jupiter and Uranus dominate Saturn (making it a 22nd century "Vienna"), or will the Saturnians be canny enough to form their own Serenìsima Repùblica Vèneta and dominate the others through trade and diplomacy?

Elukka said...

Rick - I've been inspired! Or rather I'll have to steal your idea of a hidden ring-habitat because that's pretty much exactly what my story needs.

Here's a shameless plug (and also because everything is better with pictures) of some ancient doodles I did on the setting:

I use helium-3 as the reason for the colony's existence. I figure an universe with ten thousand inhabited worlds and Magic Hyperdrives has enough demand for it that some corporations will fund colonies.

I haven't really decided how they get the gas up to orbit, though. It's only very incidental to the story as it doesn't even involve the he-3 industry as such, but it's fun to think about.
I think that, ideally, the ascent vehicles could be refueled up on orbit, thus not needing very expensive floating propellant depots down in the atmosphere.

Rick said...

Welcome to another new commenter! Saturnian biplane, huh? I think you get away with it, too; it has the look of a plausible configuration.

Balloon refineries are what I would most expect. In spite of the obvious zeppelin wank potential, surely the whole operation down there would be automated or remotes, like on Earth seabeds.

But if you want people down there for story reasons, you are on your own. Only convincing pretexts get the Rocketpunk Manifesto seal of approval.

Let me think about the price of He3, on order of $8 million/kg. Fusion bombs yield very roughly 1 kt/kg - a different reaction, but I'm only looking at orders of magnitude here.

So, on order of 4 TJ for $8 million, or $2 per megajoule, $7/kWh. A figure that, now that I have it, is so crude as to be nearly worthless. A little fusion fuel goes a long ways, though I'd guess that most of the cost of fusion power, when/if it is available, will be the capital cost of the installation.

There are entire discussions to be had about the possible 'topopolitics' of each giant planet moon (and ring) system, let alone the entire Solar System.

Duly noted as a way to feed the beast. :-)

Citizen Joe said...

Doesn't electricity cost like 15 cents per kilowatt hour? That would seem to make fusion cost on the order of 50 times as much.

Elukka said...

I'm thinking it's probably more practical to use more easily available fusion fuels for terrestrial applications anyway, since mass isn't a particular concern.
Although, if you already have a significant he-3 industry to support spacecraft, I suppose it may become economical for power plants too.

Rick - I don't really want people there. My main issue is getting the he-3 up to orbit. Using chemical rockets (which are cheap and plentiful) will require a staged vehicle, and refueling and integrating the stack seems like a major pain with no surface and no crew.

Of course, since it's The Future, they've got fusion torches and NSWRs and antimatter rockets, but those are all expensive and will have to be ferried in by starship.

Currently, I'm thinking of using reusable, high isp SSTO vehicles. Fusion thermal rockets? Could integrate hydrogen propellant harvesting to the balloons.

Curiously, a NASA document about mining the gas giants (I'll go dig it up if someone's interested) seemed to bypass the issue of ascending to orbit almost entirely.

Luke said...


The fusion of one deuteron with one helium-3 nucleus nets you 2.93E-12 J of energy. One gram of helium-3 is about 2E23 atoms, so the complete fusion of one gram of the stuff will give you 5.86E11 J or 1.63E8 kWh. One kg nets you 1.63E11 kWh. Since you can harvest the charged particles directly for power, you can expect efficiencies of turning this fusion energy into electricity on the order of 80% to 90%, lets make it 85% for 1.3E11 kWh per kg. If we use your figure of $8,000,000/kg, this gives us 1,625 kWh per $1.00, or $0.06 per MWh (not kWh). That's just the fuel costs, I'm not factoring in the infrastructure and maintenance here.

Citizen Joe said...

D-He3 nets you better power, but it involves two reactants. If the D-D reacted, you'd get T and He3 as byproducts, which would then react and throw neutrons.

'My setting" uses He3-He3 fusion on Earth to avoid any contamination. D-He3 fusion is used on the 5 mile long warship. That ship requires the length for a linear accelerator to cause the reaction. The magnetic confinement for He3 and D-He3 was too much for a compact torus shape and would wreck a ship. Thus the long Linear Accelerator. The magnetic fields for Earth based He3 reactors don't have the same problems as space born reactors.

The significantly more compact magnetic confinement for D-T thrusters made it the choice for mercantile vessels.

Luke said...

Citizen Joe

Basically right, but it is a bit more complex than that. When you get D-D fusion, 50% of the time you have D + D -> He3 + n and 50% of the time you have D + D -> T + p. The T immediately undergoes D + T -> He4 + n. So you do get some neutrons.

Fortunately, D-He3 is easier to fuse than D-D. Only about 5% of the power is in the form of neutrons (as opposed to 80% of the power for D-T).

As far as I can tell, He3-He3 has no advantage over p-B11. It takes higher temperatures for ignition, higher temperatures for optimum performance, has a lower power per volume at optimum performance by a large margin, is generally harder to make work at all, and emits more bremsstrahlung power than it produces by fusion reactions (meaning the fuel mix cools faster than fusion can heat it and you need to dump more power in for heating than you are getting via fusion). The latter issue is a big problem. If you can solve it, you can run a p-B11 reactor as well, and produce more power for less money.

Thucydides said...

p-B11 may be more efficient (and Focus Fusion and EMC2's "Polywell" IEC reactors are predicted to use this reaction), but mining Gas giant planets is just plain cool.

If I were to take a stab at this, I would use a highly articulated "Waverider" for scooping. The ship would fold up into a very slender dart shape for entry and scooping, extend a little to use aerodynamic forces for orientation to orbit, then kick in the fusion drive (augmenting the plasma exhaust with some of the scooped and compressed gas for thrust) to fly out.

Once in free space, the waverider extends fully to present the maximum surface area for radiating heat while the on board machinery separates the hydrogen from the helium and dumps the hydrogen overboard as part of the cooling cycle (this can be in conjunction with orbital manouevres to the base).

More information here:

Thucydides said...

There are entire discussions to be had about the possible 'topopolitics' of each giant planet moon (and ring) system, let alone the entire Solar System.

While each colony, asteroid or moon might be considered a "city-state", I would argue that the protective function of the State would encompass a fairly large volume due to the threat of high energy collisions from comets, space debris, off course spacecraft and deliberate malice.

At a minimum, there would have to be a Saturnian system wide space traffic control system, and an enforcement arm. In today's context, we would be thinking of a Coast Guard or Navy, but other scenarios (which have been discussed in previous blog posts here, no less!) can be raised. I like the idea of insurance companies hiring "Kepler cowboys" to patrol the space lanes and round up stray space debris and malfunctioning equipment, but pick your own model.

Jupiter and Uranus will have similar organizations, and the Trojan asteroids are good candidates as well. Earth will probably remain Balkenized (with multiple STC sectors.

Smaller polities like Mercury or Luna will need smaller STC zones, but on the other hand the traditional definition of territorial waters was defined by the range of shore batteries (hence the "three mile limit"), so energy rich polities like Mercury, Jupiter or Uranus could feed huge Xaser batteries and fire at targets a light minute or more out.

Citizen Joe said...

So sovereignty extends to the tip of your sword.

I set up a system where Earth dealt with stuff inside the orbit of Mars (and acted as a central communications hub). Mars base dealt with Mars, the Martian Lagrange points and the asteroid belt. Jupiter base dealt with Jupiter and the Trojans. Saturn base just dealt with the local area around Saturn (which is a lot of stuff to track). Uranus was again a local sovereignty. Way out at Neptune was the primary battleship for Sol and it had reign over everything outside of Uranus orbit. The FTL drive could skirt around the Sol gravity well rapidly at that range, but inside it was a long haul using rockets.

Most of those authorities were heavily influenced by the various mining operations.

Nick P. said...

Still thinking of He3 extraction, this is the method I have in mind:

The extraction vehicle is a fission nuclear-thermal SSTO rocket that uses hydrogen as a mono-propellant. After it enters the atmosphere it'll pops a short series of drogue parachutes to slow itself down to the point where it can safely deploy a balloon envelope, this is inflated and then kept warm to generate lift by waste-heat from the reactors in the engines.

The NTR engines will also power the payload: A refined product tank and extraction plant. While (Or more likely, after, considering the relative proportions.) extracting He3 from the atmosphere the refining plant will also purify hydrogen and liquefy to refuel the ascent fuel tanks.

Once the payload tank and fuel tanks are full (I wonder how long this will take, weeks? Months? Years? Things to figure out for sure.), the vehicle severs itself from the balloon, fires the rockets and returns to orbit.

On orbit the extraction vehicle meets up with a waiting tanker/repair ship which inspects the extractor for damage, replaces the expended drogue parachutes and the balloon envelope.

Once product transfer, inspection and chute' replacement is complete it goes back down for another cycle.

I'd imagine that the parachutes and balloon envelope will be locally manufactured considering that they're relatively simple and would be needlessly expensive to ship in from elsewhere.

Citizen Joe said...

Don't forget that you're doing your extraction in a hurricane on steroids. Wind speeds on Saturn push 1800 km/h.

By contrast, Uranus winds top out at 900 km/h. I like the idea of high pressure deep dives into the Uranus atmosphere down to 100 atmospheres. At that depth, ammonia ices and other ices are present for scooping. I figure that the crew all wear pressurized hard suits inside a pressurized operational compartment. By breathing Heliox instead of Nitrox, the crew can maintain the depth much longer. By having multiple pressure shells, the ship can be variably pressurized while still maintaining a survivable pressure in the crew compartment. Once the tanks are filled at (ambient) high pressure, the ship uses the atmosphere itself as remass for ascent and ejection into a capture orbit. Then a dedicated tender/tug swoops in to take the dive ship to one of the bases at Oberon or Titania.

Byron said...

If anyone is interested in the economics of He3 fusion, read Harrison Schmidt's Return to the Moon. It's about lunar He3, and highly technical. And why would Luna become a rust belt. If they are in any way independent, I'd expect them to work the hardest on gas giant He3 to avoid losing their monopoly.
On fusion reactors, so much depends upon the parameters of the reactor. I've done some work on that, but it was for a ship's drive. The problem is that there's a limited band where the reaction is optomized, and even then, you're still getting about two percent in neutrons and another couple in x-rays. At torchship powers, that's still a lot. On the ship I built this for (details to follow) I had to do a lot of work to get the shielding to a reasonable level.

Thucydides said...

Byron, the reason for thinking Luna would become a "rust belt planet" is based on the observation that 3He is in limited supply and needs to be boiled out of regolith, and that Luna is also deficient in the volatile elements needed for life. Things are just plain expensive on the Moon, even if you are making $8 million/kg on your 3He sales.

A lunar mining machine excavating a square kilometre of regolith to a depth of 3 metres will provide enough material to provide 33 kg of 3He. Given the corrosive and abrasive nature of regolith, this will be pretty hard on the machinery, so constant replacement and repair is going to be needed.

A single ramscoop or balloon in the atmosphere of Saturn or Uranus could probably process and extract 3He by the tonne lot, drastically undercutting the price of 3He system wide and putting the Lunar miners out of business. Since ices and volatile elements are already in place around the Gas giants (either in the rings or the moons, if not the atmosphere), the cost of living will also be much lower.

With their 3He priced out of the market and people and investment flowing to the outer solar system, I can easily see Luna becoming a "rust belt planet"

Byron said...

I understand the mechanics of Lunar He3 extraction, but I still question the fact that cheaper He3 will make them a rust belt. If they have any kind of foresight, they'll invest in gas giant mining. Also, Saturn is a long way away, meaning a couple years shipment time, while Luna is close. Also, the fact that they're a source of a lot of important materials close to earth and with a low gravit well will likely make them a play for a long time.

Citizen Joe said...

The moon provides the initial fuel for both proof of concept on the fusion reactors and then the rockets bound for the gas giants. Once the Helium starts flowing back in, the moon would get turned into a strategic reserve just in case the mining colonies go on strike.

Thucydides said...

3He is a fungible resource like oil, so as far as the end consumer is concerned, it does not matter where it comes from.

Market conditions and price swings will affect the smallest, highest cost and marginal suppliers far more than the stable bulk suppliers, and small sources won't really act as a "reserve" (particularly when the large suppliers have 3He containers heading in on minimum energy orbits with futures contracts stretching decades).

Citizen Joe said...

Well, if a stray rock wrecks one of those tankers, Earth would be out of power for months. Although probably more expensive, lunar regolith He3 production could fill the gap until the next tanker arrives.

Jim Baerg said...

"Well, if a stray rock wrecks one of those tankers, Earth would be out of power for months"

Actually I doubt it.
Since nuclear reactions release roughly a million times as much energy per unit mass as chemical reactions, it is trivial to keep a few years fuel supply on site, as insurance against such an interuption.

Note that current fission reactors keep the (partly) used fuel of the past few decades on site, even though only about 1% of the available U has been fissioned & the mass is about 100 X the amount of fuel that would be needed otherwise.

I will add that I'm skeptical that fusion will actually be better than fission. Something like this
could safely & cleanly provide several kW to each of a few billion people for the next billion years.

Citizen Joe said...

Humans, like viruses, expand to fill their environment. If we can provide two tankers worth of He3 for fusion reactors each year, then we will ramp up our power requirements to use up two tankers a year. At that point, if we lose one tanker, we will experience a 50% shortfall in energy.

I have yet to see a fission reactor that directly produces electricity. They are all just abundant heat sources that turn a turbine to make power. That isn't terribly efficient. That is where fusion plants shine. Second generation fusion power throws protons instead of neutrons. That lets their efficiency skyrocket.

Fission plants shine in that they don't require powerful magnetic fields to operate, thus less input energy. Along those lines, betavoltaics and RTGs work well.

Anonymous said...

I'm gonna skip back and say something about the original topic of this thread. Titan is a wonderful place to have colonies: scientific, economic, and military reasons; exploration, those seeking excape from imagened or real governmental oppression, people seeking to get rich in the "new world", and all other sorts would find a reason to go there (provided it was cheap enough to be within their means). Having a large enough human population on Titan would mean that either local governments would form police/militaries or that Earth would send some to Titan...that would tend to cause some friction among the locals...


Elukka said...

A problem with Titan is that it isn't exactly hospitable. The atmosphere is poisonous and COLD, which means you not only need to live in a closed habitat as in everywhere else, but you also need to have a potent heat source because the thick atmosphere is constantly robbing you of heat.

This doesn't make it impossible to colonize, of course, but it'd probably be easier to build space colonies unless there's something on Titan that you want.

Citizen Joe said...

The trick to almost any habitat is to turn its features into a commodity. The supposed problem with being cold on Titan makes it an excellent location for nuclear furnaces with ridiculous waste heat.

Anonymous said...

Build a dome of water ice, build a slightly smaller dome of some conventional material inside that one, evacuate the narrow space beteween them, and you have an insulated habatat...if there is life-as-we-do-not-know-it (lawdnki), then there is a reason to put research colonies there; if there is enough organic material, then agricultural colonies could be built; if a source of energy, or rare (but important) industreal material is discovered, then mining/refining colonies would be founded for the space colonies elsewhere in Saturn space. Titan is a world with a wide variety of resources, an atmosphere, weather, liguid/rain cycle; Titan can support a human civilization indefinatly, even at *merely* current technological levels. If Titan wasn't a moon, it would be classified as a planet; not a dwarf planet, or planetisaml, or minor planet, but a full fledged planet.

Besides, humans have a tendency to inhabit places for reasons that aren't strickly logical or based on hard economical facts.


Rick said...

Well, actually the original topic was the rings, not Titan. :-)

One other problem with colonizing Titan: The lower atmosphere is about 5 percent methane, so a leak either direction might produce an explosive mix. That's looking for more trouble.

Anonymous said...

Picky, aren't we? "Well, actually the original topic was the rings, not Titan. :-)

One other problem with colonizing Titan: The lower atmosphere is about 5 percent methane, so a leak either direction might produce an explosive mix. That's looking for more trouble."

Hince the 'double-domed- design with the vacuum space inbetween the lots of leak/methane detectors both on the inter-dome space and in/on/around the inner dome...I never said it would be easy or without risk, but it could be done; living on the bottom of the ocean, Luna, a space hab, or inside a hollowed-out asteroid also involves risks, each differing (more or less) from each other. But your point does remind me that internal combustion engines would work on Titan; just have the caruator pull in the methane-laden air and have liquid oxygen in the 'fuel' tank... The main reason I mentioned Titan was that it would be easier to establish a Saturn-wide infrastructure on it's surface than to build a lot of space structures first, then colonized Titan. Anywhere that has water ice for rocks,rocket fuel for air, and a lesser excape velocity then Earth, would get my vote for the primary site for colonization when we do get around to going to Saturn. Anywhere is dangerous, whether it be Colorado, Alabama, or Titan; you just need to address
that area's unique threats and engineering needs...:)


Rick said...

You wouldn't want vacuum between the layers, but some neutral gas. Nitrogen would do fine, and just requires scrubbing the methane out of atmospheric gas.

A problem for long term human habitation on Titan, or any of the smaller bodies, is that they probably don't have enough surface gravity for human health. Spinning a space hab is pretty easy; spinning a surface base on a huge turntable is possible but a big hassle to do.

Cloud cities in the atmospheres of Venus, Saturn, Uranus, and Neptune do not have this problem. 'Surface' gravity (meaning the visible cloud layer) is near 1 g, though both Saturn and Neptune are a shade heavy footed.

But getting to orbit is tough, especially from any of the gas giants.

Anonymous said...

Ok, Nitrogen, or Argon...Well, trade-offs exist in everything...I will, however, hold my opinion as to low G worlds and human health until we have a generation born on Luna or Mars...of course, you don't have to worry about radiation shielding on Titan, what with that thick atmosphere...Still, an inhabited Titan would dominate any Ring colonies through sheer proximity.


Citizen Joe said...

Although I had originally hypothesized about Uranus cloud cities servicing the miners with most of the operations taking place on the moons, I'm looking at some of the issues with it. Specifically, at one atmosphere, the air outside is like 75 kelvin... On the plus side, there are calm belts analogous to our tropics of Cancer and Capricorn.

Another drawback is that the air outside is already the lightest gasses so you'd need essentially heated helium balloons for lift. I only hope that whatever processing is going on there produces enough waste heat to counter the loss to cold atmosphere.

Citizen Joe said...

Hmm... it seems that Saturn, Uranus and Neptune are all around 70K at 1 bar pressure altitude. Additionally, Saturn and Neptune have ridiculous wind speeds.

So, cloud cities would probably just be on Venus and maybe Uranus (if you have a good heat source). Uranus cloud cities would probably be in higher pressure depth with heated helium as the lift gas. The lift balloon may actually double as the radiator.

Luke said...

If the air outside is 80% H-2 and 20% He at 100 kPa and 70 K, then simply by heating outside air to 300 K and releasing it inside your lifting volume, you get a buoyancy offsetting 0.3 kg/m^3. Your lifting ability drops as you descend because the temperature difference decreases, but rises as you descend because the air gets thicker. Assuming dry adiabatic compression/expansion of the atmosphere, I find that you get a maximum of about 1.4 kg/m^3 at pressures of between 2 MPa and 5.8 MPa (173 K and 240 K, respectively) with the actual maximum of 1.55 kg/m^3 located at depths where the pressure is 3.7 MPa (209 K).

Using helium instead of outside air as the lift gas will decrease your lift. If you use exhausted gas from a He-3 separation process which is relatively enriched in hydrogen (because first you separate out all the He before taking out the He-3), you will increase your lift.

The people living on bubble cities will not be able to live on normal air at 2 MPa - the safe limit for breathing normal air is about 500 kPa before nitrogen narcosis sets in. Trimix can get you to at least 1 MPa, and healthy people have survived dives as deep as 3 MPa while breathing trimix. Of course, you will sound funny, and will also cool off faster, if you breathe trimix. You will also need decompression time if ascending from high pressures to someplace with more normal pressures.

Alternately, you could have the habitable part of your floater cities inside a pressure vessel. The added weight of the pressure vessel may make this undesirable.

If the reason for your gas giant floater city is to extract He-3 for fusion, then presumably you have practical fusion and can use a small percentage of the He-3 and D you extract to heat your gas bag.

Citizen Joe said...

I was positing the Uranus floating cities to provide a natural gravity environment. But that same gravity could be crucial for distillation process. I had already intended for a Heliox mixture in microgee environments, but if there's gravity (either spin or natural) then the helium would stratify out of the oxygen causing a dangerous situation.

Perhaps people could spend their day in the higher pressure mix, but then climb into decompression pods to sleep at night.

Room temperature doesn't occur until like 80 atmospheres, so that isn't really the goal. One atmosphere is right above the methane clouds and two atmospheres is just below them. I guess I'm looking for that sweet spot of habitability and lift capability.

Luke said...

Heliox will not stratify out into helium and oxygen over reasonable distances in livable habitats. If you have mixing (life support circulation, slight differences in temperature, etc) the gas composition will be uniform. If you do not have mixing, the oxygen and helium will settle down into independent exponential falloffs, with a scale height of 8 km for oxygen and 64 km for helium at 300 K temperature and 9.8 m/s^2 "gravitational" acceleration. If your ceilings are 100 meters high and you start off with 90% He, 10% O_2 at ground level, then at the ceiling level you will have 9.9% O_2 and 90.1% He at the ceiling.

You don't want to put the floating city at room temperatures - you couldn't float! However, as I noted above, as you descend your lifting capacity for a given volume increases until somewhere around 20 atmospheres. There will also be less heat loss due to the decreased temperature difference across the gas bag.

The difference in lifting for a given volume at 1 atmosphere and 20 atmospheres is a factor of about 4 or 5. If you just have a large enough gas bag, you can overcome this.

Regularly changing pressure, such as spending the day at high pressure and sleeping at low pressure, will be more problematic from a medical standpoint than just staying at the same pressure for long periods of time.

Citizen Joe said...

Well, if I'm translating this right... and it is entirely likely that I'm not since it is in many different units... it looks like people could take 16 atmospheres (about 16 bar) and get used to it by using Heliox or Trimix. That would bump ambient temperature up to 200K and stick the city around the ammonia clouds. That means they could pull nitrogen out of the ammonia for the mixture. I'm guessing that isn't at all comfortable, whether you get used to it or not.

However, assuming a floating city the size of an air craft carrier (100 Ktons), how large and hot of an air envelope would be needed?

Luke said...

For units, 100 kPa = 1 bar = 1 atmosphere. 1 MPa = 10 bar = 10 atmospheres.

Trimix or heliox is commonly used down to 10 atmospheres, and people have used it down to 30 atmospheres.

One additional possibility is adding sulphur hexafluoride to the mix. SF_6 is much heavier than air at a given pressure, and can help offset some of the annoyances caused by helium, such as squeaky talking and a high heat capacity (causing people to cool off too quickly). It will, however, magnify the problem that as the air gets denser (as it will under high pressures) it becomes more work just to move that mass in and out of your lungs, so you will get tired just by breathing.

At 100 kPa and 70 K, you get a lift of 0.3 kg/m^3. Thus, for a 100,000 ton city (100,000,000 kg) you will need 333,333,333 cubic meters of lifting gas. A spherical envelope 430 meters in radius (860 meters across) would do the trick.

At 2 MPa, where you get 1.4 kg/m^3 of lift, you will need about 70,000,000 cubic meters of lifting gas, or a spherical envelope of about 260 meters radius.

Your envelope can be slightly smaller if you are using separation exhaust gas depleted in helium.

Jim Baerg said...

"One additional possibility is adding sulphur hexafluoride to the mix. SF_6 is much heavier than air at a given pressure, and can help offset some of the annoyances caused by helium"

Probably not. My understanding is that *all* inert gasses produce narcosis like nitrogen & the greater the molecular weight the lower the pressure at which narcosis sets in. So at the 10+ Bar pressures you are talking about the mix better be mostly low molecular weight gasses like helium.

Citizen Joe said...

I'm thinking now that the Bespin cloud city from Star Wars was a pretty good design. The pendulum below would act as a keel to keep it upright. The main lifting body might be a torus with industry at the core and residential outboard. People could then derive power with stirling engines synced between the warm lifting gas and the frigid exterior. Obviously they would need a reactor for heating everything, but what I noticed about my D-D breeder reactors is that they through off a ridiculous amount of heat for the amount of He3 they created.

Yep, I'm really liking these cloud cities now.

What are some of the physical effects of such high pressures? There was the hard breathing with the heavier inert gasses. But if you're just walking around in a heliox/trimix at 20 bar pressure, what is that like?

Luke said...

At 20 atmospheres of pressure, with pure heliox at 0.2 atm partial pressure oxygen, the density is nearly three times higher than air at sea level on earth. I'm guessing that this will not be too noticeable for just breathing. However, all aerodynamic forces will be increased by about 3 (drag from breezes or motion through the air, lift of wings). Because the air is more massive, you will need to beef up the air circulation parts of life support.

The increase in speed of sound in heliox is well known, and consequent change in pitch of resonant cavities of enclosed air (such as flutes, didgeridoos, and human windpipes, but not violin strings, audio speakers, or drums, which are based on vibrations of non-air materials).

Heliox has a significantly greater thermal conductivity than air. People and equipment will more rapidly lose heat to cooler air or gain heat from warmer air than on earth. Other transport properties will also have greater conductivities - helium diffuses faster, there will be a greater viscosity (diffusion of momentum), and so on. Oddly, diffusive transport properties like these (thermal conductivity, diffusion of atoms, viscosity) are not affected by the atmospheric pressure or density, until either the air is so dense that it is no longer approximately an ideal gas (which would be quite lethal) or so diffuse that the mean free path between atomic collisions is of the same order of scale as the length dimensions of the object from which the diffusion is occurring (which would also be quite lethal).

Contrariwise, a given volume of heliox will have more than 13 times the heat capacity of an equivalent volume of sea level air. This gives the air a greater thermal inertia, so it will tend to maintain its temperature over longer periods of time.

That's all that comes to mind right now, although no doubt there are other effects I'm not thinking of.

As far as the cloud city design, why bother using heat engines between the lifting gas and the exterior? You've got a friggin' fusion power plant to heat the gas, just tap some of that for electricity. Further, running any sort of heat engine between the lifting gas and the outside transfers much more heat than work, so you will be losing heat faster than otherwise. The outside air and lifting gas will have an even higher thermal conductivity than heliox, so insulation is already a significant concern, no need to exacerbate the effect by intentionally dumping some of the heat outside. If you just tap the fusion plant for electricity, all of your waste heat goes into the lifting gas anyway, so you are not losing anything that way - all your electric needs don't take away from the heating effect (assuming the lifting envelope surrounds the city).

And why run a D-D reactor for generating He3? You've got He3 all around you, you just need to enrich it. Perhaps use a gaseous diffusion cascade, or refrigerate the outside gas until the hydrogen condenses out, and then further until the He4 goes superfluid and you can let it creep away, leaving only the He3.

Citizen Joe said...

My choice for the D-D breeder reactor stems from its availability (in that setting). There's a whole fleet of refinery ships that have multiple D-D breeders, so parts and supplies would be common place. The difference is that the city breeders are going more for the tritium than the He3. My hypothesis for the D-D reactors was that they were a failure as far as producing surplus energy (well not heat, they were very good at that). But the confinement field required almost all of the electricity that it could produce... Thus they were used primarily for breeding other fuels. In the case of the cloud city, heat production is fine and the service vehicles run with dirty D-T thrusters. So they would be mining both deuterium and helium-3, but also the nitrogen, sulfur, carbon and water for local use.

The stirling engines turn heat into kinetic energy and as you stated, the heliox has a large thermal inertia. That means that fans, escalators, moving sidewalks, machinery, etc. could all have a consistent power supply from the thermal mass in the city rather than heavy batteries to store electricity inefficiently.

I am actually very concerned with heat loss through the envelope, but I was presuming that the D-D reactors were producing far more waste heat than the stirlings would use up. So much in fact that once the stirlings did their job, there would still need to be waste heat ejection.

Now, my D-D breeders require fairly regular replacements of their core shielding (Depleted silicon carbide shells, probably shipped in from Venus). So there would definitely be times when the city would operate on less than full capacity.

The high pressures seem to insist on slower movements since running would feel like you're running into a stiff wind. Hmm... assuming a fall from a high point, what would terminal velocity be? Could people glide down from upper levels? What would be the effects of firearms in that kind of pressure? The reduced pressure differential would probably mean lower muzzle velocity. The air resistance may also slow it way down. End result might be an irate victim squealing like a mouse at the superheated slug burning his jacket.

Luke said...

Since the air is three times as dense (more or less, I'm going to say 3 for ease of computation), and aerodynamic forces scale linearly with air density and quadratically with airspeed, the air speed of aerodynamic-limited mechanics will be reduced by a factor of the square root of 3, or about 1.7. Since terminal velocity for a skydiver in a spread-out posture is about 55 m/s on Earth, it would be about 32 m/s on the bubble city.

People can already glide on earth, using parasails or wing suits. Wing suits usually require an auxiliary parachute for the landing. If a wing suit has a 2:1 lift to drag ratio (typical of modern wingsuits) the forward airspeed will be 16 m/s. An impact at these speeds will likely lead to injury. My guess is that you will need to reduce forward airspeed down to 10 m/s or less for a safe landing (and 10 m/s is still on the edge of safety), requiring a 3:1 or better lift to drag ratio. Just a note, the lift-to-drag ratio is also the tangent of the angle of descent - if you have a 3 to 1 lift to drag, you will go forward 3 meters for every meter you drop (and this is independent of weight - if you weigh more you will drop faster in a steady state glide, but you will not glide at a steeper angle).

The range of bullets launched from firearms will decrease by a factor of three due to the increased density, due to aerodynamic effects (this does not account for dropping due to gravity). A .223 Remington bullet can go about 250 meters through sea level air before its kinetic energy is halved. In this bubble city, the distance to halve the kinetic energy will be about 83 m. Muzzle velocity will be unchanged.

Citizen Joe said...

Hmm... maybe a courier system with wing suits. Then they glide in at the uplift ports to slow the descent. This could be that 'fast way' through the lift torus from the refinery core to the outer habitat ring. It takes a special kind of crazy to dive that deep into a gas giant. People gliding around like that only reinforce the idea. Living in 20 atmospheres of trimix or heliox ain't like Kansas, so it is important to point out the differences.

Jim Baerg said...

Joe: As Luke pointed out you don't get your power by running heat engines off the temperature difference between the hot air balloon & the outside, but rather the hot air balloon is the low temperature heat dump for heat engines with the high temperatures coming from some sort of nuclear reaction.

Something of the sort that could be done with current technology is use a Pu238 RTG & stirling engine to power a probe that floats in the atmosphere of a gas giant & the waste heat coming out the cool end of the stirling engine keeps the 'air' in a hot air balloon warm so it keeps the probe floating.

This will be done long before the balloon cities because no one will go the balloon cities until we have a way to get from inside the atmosphere of a gas giant into orbit around the planet. The delta-V for that is way beyond current tech.

Citizen Joe said...

Well there's the balloon to orbit method. Given the 20 atmosphere pressure differential, a gradual ascent might not be such a bad idea. I'm gonna go with heated hydrogen lifting body. In fact, I'm going to stick a heated hydrogen dock at the coldest spot, the tropopause of Uranus. There the pressure is about 1/10th of an atmosphere. These would station in the calm bands around 25 degrees north and south latitude. Simple hydrogen tankage would be refined/liquefied here for remass. Primary lift is via the gas bag while axillary rotorlift can be kicked on during the docking procedure. Orbital craft could use aerobraking followed by lift craft capture and towed into port. Lifting body tankers, primarily prop driven would deposit their loads. Then the orbital craft would be boosted into space with a reusable rocket. The deep ports would be analogous to submarines while the high ports would be aircraft carriers.

Jim Baerg said...

Joe: by balloon to orbit are you talking about this proposal?

I don't see how it can work. Unless I'm missing something the air drag will totally overwhelm the thrust from the ion drives they propose to use to accelerate the airship.

They do however have a bunch of neat tech for high altitude ballons that can be used for eg: surveilance & communication relays.

Rick said...

I see the same problem. I don't know the mathematical relationships involved, but it sounds really off the wall.

Otherwise I'm quietly following this discussion!

Anonymous said...

Jim: "They do however have a bunch of neat tech for high altitude ballons that can be used for eg: surveilance & communication relays."

I know this will sound like a "duh" moment, but why don't they use chemfuel rockets to boost these orbital capable airships into orbit? Yes, ion engines could get them up into the upper atmosphere, but what's to stop you from transitioning to standard rockets at the point where you need to be inserted into orbit? If this is a dumb idea, please let me know...


Citizen Joe said...

My idea involves some sort of reusable rocket/tug whose job is to get orbital ships from "high port" to "orbital height" (beyond the drag line). At that point, the IP drives on the cargo vessel take over and the tug drops away into the Uranian atmosphere. The tugs would need multiple drive systems: A rotorlift in atmosphere and then probably a D-T thruster rocket. Remass would only be used during the ballistic phase where the tug is clear of anything that might be destroyed by the nuclear backwash. This is a little like the Shuttle stack with its boosters (the tug) tank and Orbital. Given some heaters, even the tank might be reusable by turning it into a balloon until recovered.

Hmm... looking at the performance of the various rockets at Atomic Rockets, I'm thinking that a fission rocket might have better thrust to weight needed for a tug. Maybe something like the Pebble Bed NTR. Unfortunately, that means a high reliance on imported fission fuel (probably from Venus).

Citizen Joe said...

I should point out to Firefly fans that the ship in that series uses two systems, a tilt rotor/jet and a larger nuclear primary thruster. The effects of the latter are shown during a full burn in atmosphere while evading Reavers.

Anonymous said...

Citizen Joe said:
"I should point out to Firefly fans that the ship in that series uses two systems, a tilt rotor/jet and a larger nuclear primary thruster. The effects of the latter are shown during a full burn in atmosphere while evading Reavers."

Something like an oil-tanker-sized FAE (fuel air explosive). Not something you'd want to happen anywhere near you...especially if it left radioactive residue.
It seems that a fission drive would be a good option for orbit-to-orbit spacecraft within the Saturn system. Maybe not the best option for interplanetary travel, but for zipping around the Rings, or between moons, more than adiquate.