New! Unique! Almost worth the free download! It's the one and only Rocketpunk Manifesto Travel Planner, available in all flavors of plain vanilla Excel. I did this workbook for my own amusement, inspired by this recent post, then decided to offer it to an indifferently awaiting world.
It is tailored for a high specific impulse drive, and trips across planetary distances conveniently measured in AU. If the ship pops through an FTL rabbit hole at the midpoint, that is between you and Albert Einstein. It has no effect on this worksheet, except that the given travel distance is misleading.
Each worksheet is laid out as three columns: mission characteristics, ship characteristics, and (inevitably) cost estimates. User entry values are across the top, in light blue.
The mission column models a steady acceleration, coasting phase, and steady deceleration, all in flat space. The ships I was modeling are fast enough, with transfer speeds in the dozens of km/s, that solar space beyond 1 AU is fairly flat for them. Real ships don't have constant acceleration, either - acceleration increases as propellant is burned off - but this gives a decent first approximation of travel time.
Providing a coasting phase is more efficient than a brachistochrone orbit, because you aren't putting on speed only to almost immediately take it off again. A coasting distance of zero corresponds to a brachistochrone. (If you change the distance, reduce the coasting phase first - Excel will freak out if you make the trip distance less than the coasting phase.)
Note that I round off 1 g to 10 m/s/s, versus ~9.8 m/s/s, just as I round off 1 AU to 150 million km. Acceleration is in milligees; drive specific impulse is in seconds (not exhaust velocity in km/s). The worksheet computes travel time, burn time (accelerating and decelerating), mission delta v, peak travel speed, and initial and final acceleration.
The second column is for basic ship characteristics. The first user entry is departure mass - the ship, fully loaded and fueled. The second is drive fraction - the percentage of the ship's arrival mass (what reaches the destination) that is given over to the drive engine, including shielding, radiators, etc.
The third user entry is keel fraction - the portion of all-up departure mass given over to the framework that holds drive engine, tankage, and payload together, along with bells & whistles such as the navigation and comms equipment. Since these are low acceleration deep space ships I used a modest 5 percent for this figure. (Each section of the ship would also have its own internal bracing.)
The final user entry here is tankage fraction, as a percentage of propellant load. l used a generous 10 percent, since for liquid hydrogen fuel the tankage must include a cryogenics plant.
There is no user entry for the payload. It is calculated (and shown on the right hand column, just below the user cost entries). The payload is simply whatever mass allowance remains once necessary mass has been assigned for the drive engine, keel, tankage, and of course propellant.
I also don't give a figure for the dry mass of the whole ship, because this depends on the payload. The payload figure given is gross payload - the whole payload section, including any hab compartment, cargo bays, the cargo itself, whatever. I do give a figure (also over on the right) for the mass of the bus, the ship structure minus the payload. So the dry mass is bus mass plus whatever part of the payload is 'fixed.'
Finally, the right side of the worksheet permits some cost estimates. The first is fuel energy cost (not propellant cost), measured in US cents per kilowatt hour. The figure I used, 2 cents per kW/h, is roughly the current cost of nuclear electric power, approximating the case of having to breed fusion fuel. This is the price to beat if you're mining He-3 from Out There somewhere.
The next user entry is hab mass per passenger berth, probably several tons. I used 5 tons per berth for trips of a few weeks, up to 25 tons per berth for an 18 month mission requiring long term life support.
The next entry is ship cost, per ton of dry mass, with a unit value of $1 million, which is about what a jumbo jet costs. If you want cheaper ships, just enter something less, like 0.3 for $300,000 per ton. The final figure, 'annually,' is what the ship costs you each year, for paying off the building loan plus operating costs, as a percentage of building cost. I use 20 percent.
All of this lets me calculate, i.e. guesstimate, the cost of a passenger ticket, helpfully broken down into berth charge ('rental' of the berth space, for duration of the trip), plus a carriage charge, the energy cost of actually making the trip.
A host of details are ignored, such as a reserve propellant margin, that you don't want to ignore in practice, along with other details such as necessary downtime between missions. And you know what resides in the details.
It is not exactly a 'ship design' tool - it doesn't even give the official mass and cost of a ship, but it is a way to suss out some ship characteristics required for long space trips with high specific impulse drives, whether Realistic [TM] midfuture drives or more advanced torchlike drives.
Naturally I took it for a test drive, with the results that you'll see. Drive engine characteristics (except for specific impulse) are calculated, not user input, but I fiddled each sim to represent a ship with a hefty 1000 ton main drive engine. For the first two sims this drive puts out 1 gigawatt of effective thrust power, the performance level of a nuke electric or solar electric drive that we could probably develop now if we put Pentagon money into it.
With this drive a ship with a mass of some 5500 tons should be able to reach Mars in three months, burning off about 2000 tons of propellant and delivering a 900 ton payload section. As a transport it could carry about 120 passengers, each paying about $1.2 million for the one way trip. Cargo rides cheaper, about $130,000 per ton.
The same drive plant, with a different specific impulse setting, will send a 1700 ton payload to Saturn in 18 months, one way - a passenger ticket costs $17.5 million, but a measly half million to ship a ton of cargo.
The next few sims step up drive performance to 10 gigawatts from a 1000 ton unit. With this drive a 3500 ton ship can just barely reach Mars in 21 days (at opposition), carrying a nominal 35 ton payload, enough for a handful of passengers. But a slightly slower trip, 30 days, allows a vastly more economical ship. For this mission our ship has a departure mass of 6400 tons and delivers a 1700 ton payload, carrying more than 300 passenger. A one way ticket to Mars now costs $440,000; a ton of cargo can be shipped for $80,000.
A longer and slower trip, three months, turns out to cost more for passengers, due to (assumed) greater life support requirements, but cargo cost falls to $40,000 per ton. Notice the enormous size of this ship: Departure mass of 35,000 tons, and a 16,000 ton payload section.
Then I tried the drive out for going from Earth orbit to Moon orbit, which turns out to be approximately a 38 hour one way trip. And for once the ticket prices are not exorbitant - $9000 for a cozy passenger berth, or $3000 per ton for cargo.
There is an important lesson here. What makes my other ticket prices so high isn't that spaceships are expensive, though they are, but that they are so unproductive, only delivering a few passenger and cargo loads per year, if that. For short trips with rapid turnaround the cost of space transport (under these assumptions) falls to levels comparable to global first class air travel.
This is another argument for settings that involve local space travel, such as Earth orbital space, or for that matter Saturn orbital space.
Moving right along, a few more sims. A 4000 ton ship fitted with a 1000 ton, 10 GW drive is just barely able to reach Saturn in 180 days, carrying only a nominal 5 ton payload. Extending travel time to 200 days yields a more practical ship, though a ticket still costs $25 million. If you're willing to take a year to get there, Saturn can be yours for less than $10 million, and the freight charge is a mere $300,000 per ton.
Finally I stepped the techlevel up another jump, to 100 gigawatts - verging on the low end of the torch range here. This is sufficient to reach Saturn in 90 days, with a peak speed above 270 km/s - nearly 0.1 percent of the speed of light, nothing to a photon but a lot for you and me. (I cut the energy cost by a factor of 10 too; the energy bill for that kind of speed was still most of the cost of the ticket.)
So there you have it. Go play with it, and see what results you come up with. Here's the link again: Rocketpunk Manifesto Travel Planner. And a recent post on getting to Saturn, among other places.
Image of the ISS from Astronomy Picture of the Day.
Update: The download is being a bit glitchy. I've tried renaming the file and re-uploading - let me know in comments if you're able to download it now.
Update II: Link fixed, thanks to Winch of Atomic Rockets. (See comments for brief, embarrassing explanation.)
Tuesday, June 29, 2010
Saturday, June 19, 2010
A week ago a poster over at SFConsim-l, Gregory Muir, raised an interesting question: 'What has changed in your assumptions about SF and when did it change? Which changes surprised you?' The resulting thread is one excuse for my having been remiss in posting here. (Thanks to Yahoo! Groups' wretched threading it is hard/impossible to follow in full, and parts have drifted into all too typical Internet political debates. You've been warned.)
Rocketpunk Manifesto is, in effect, my endlessly extended rumination on just that question. I gradually became dissatisfied with the SF setting I had created over decades (but never really did much with). It had one bit of para-prediction that looks good in retrospect - the Fall of the Terran Empire as crash of an interstellar real estate bubble. But on the whole it was a generic space opera setting, complete with FTL and Wild West planets. Bat Durston rides again!
On formal grounds All That Stuff is pure fantasy element, right up there with dragons and magic swords. And on one level, creating an essentially operatic universe and then belaboring the technical details of fusion torch drives is an exercise in missing the whole point.
There is a valid counterargument. Most fantasy has non-fantasy elements, and the general modern consensus is that these more realistic elements ought to be done 'right.' If people are going to fight with swords, some of them may be magical, but they should still be functional as swords. In a pinch, if all else fails, you should be able to skewer someone with it.
Likewise, if your starship has to travel a few AU in normal space before the lady singing in Welsh can have her desired effect, it is reasonable and appropriate to equip it with a credible torch.
All the same, the thread at SFConsim-l points to a growing dissatisfaction with the consensus tropes of SF. Why go boldly where Firefly already went? This dissatisfaction has been building for a while; 'Mundane SF' emerged to challenge the consensus back in 2002, and without quite intending to I jumped on the Mundane bandwagon by launching this blog.
But the full picture strikes me as more complicated and textured than simply Space Opera v Mundane SF, and it goes to the tensions inherent not just in SF but in the broader genre of Romance. In SF we (usually) imagine futures, though experience suggests that our best efforts to realistically portray the world of 2100 will, by 2100, be as laughably or charmingly retro, or both, as the future of 1900 seems to us. Or for that matter the future of 1950 with its circular astrogation slide rules.
Future shock is not confined to SF. Jane Austen, circa 1800, 'wrote what she knew,' about young women of the minor English gentry - a world that would likely seem commonplace to any young female (distant) relative of the Tooks or Brandybucks, but is thoroughly fantastical to us.
So I will re-pose here the question asked at the start of this post, along with a related one: Where is SF now, where is it going, and where should it be going?
Related Posts: From the earliest days of this blog, ruminations on Romance, and a couple of looks at the retro-future.
The image, as often, is swiped from Atomic Rockets.
Posted by Rick at 6/19/2010 07:30:00 PM
Saturday, June 12, 2010
My last post turned out to have some bogus calculations - adding/dropping zeroes, a rich source of careless error - which by one of those semi-happy accidents turned out to yield a correct final result - unless you looked at it too closely. H/T to commenter nqdp for wondering out loud why my figures didn't all quite make sense.
I've corrected the errors (or so I hope), while preserving the original text for posterity.
The stakes in this case were remarkably low; at worst I wasted a few hours of your collective time. In real world applications, of course, the cost of brain dead errors is higher, and the errors will hardly ever be neatly offsetting. The example that comes first to mind is the infamous feet/meters fail that did it for the Mars Climate Orbiter. There are of course many others.
Murphy is an Old Testament style prophet, not a gentle father confessor. He comes into your lab or shop to thunder a warning, not give absolution after an error has blown up in your face.
The image of the Gulf of Mexico is from NASA.
Posted by Rick at 6/12/2010 09:12:00 PM
Thursday, June 10, 2010
Reader JP emailed to ask about the practical sequence of spaceship design, where 'practical' means suited for created settings, stories or games, not spaceworthy for actual travel. In other words, do not try these tricks anywhere but at home.
And if you haven't done so already, this is a good time to consult relevant sections of Atomic Rockets, including the handy page of equations (from which I also swiped the image above).
I get the feeling that there's a specific order that I have to go in in order to determine the following:The short answer is there is no one 'right' way to attack this interlinked web of performance traits. Acceleration (thrust), specific impulse, and propellant flow are all very closely tied together, along with propellant fraction, which in turn constrains payload. Drive power density is also in this mix, constraining acceleration on the one hand and payload fraction on the other.
-Propulsion system specific impulse
-Available payload capacity
-Propulsion system mass flow
Define one parameter and all the others can vary around it. Define two parameters and the rest become much more constrained. Each defined parameter reduces the degrees of freedom for the remaining ones, until you lock it down. Great in principle, not so helpful in practice. I have my own approach and rules of thumb, shaped by my workflow habits and biases as well as mission requirements. But I gotta say something, so here goes:
The first parameter of all is mission delta v - how fast you want the ship to go - because that will drive everything else. After that, start is with parameters that are fixed by your techlevel, because that sets the ground rules that possible designs have to play by. And the parameters that are most fixed by techlevel are specific impulse and drive power density (power/mass ratio).
For chemfuel, both of these are sharply defined. Specific impulse (for H2-O2) is about 450 seconds, or 4.5 km/s exhaust velocity, and power density is on order of 1 MW/kg. Chemfuel engines put out such prodigious thrust relative to their weight that you can pretty much ignore drive power density (and therefore engine mass) unless you intend multi-g acceleration.
Example 1: Suppose an orbital 'gunship' with 5 km/s mission delta v, and an H2-O2 chemfuel drive. Total mass is an arbitrary benchmark (you can have big ships or small ones), but let us say 100 tons full load mass.
Right off the bat we know that full load mass ratio must be 3.04, a value determined by mission delta v, specific impulse, and the basic rocket equation. This means that out of our 100 tons departure mass, 67 tons will be propellant, the remaining 33 tons everything else - engines, fuel tankage, ship structure and equipment (including crew), plus payload.
Old Sir Isaac tells us that 1 kg of propellant burned in these engines in a second produces 450 kg of thrust (~4500 Newtons, for the picky), with a thrust kinetic energy of 10.125 MJ. Suppose we want a maximum acceleration of 2 g, requiring 200,000 kg or 2 MN of thrust. We need to burn 444 kg/second of propellant to get, which will produce 4.5 GW of effective thrust power. By my simple rule of thumb the engines will have a mass of 4.5 tons.
This performance requirement happens to be very close to that of a Space Shuttle Main Engine, SSME, now verging on retirement. It has a mass of 3.2 tons, but turned out much less rugged than hoped, so I'd stick with 4.5 tons for a combat capable engine.
That leaves 28.5 tons for the rest of your ship. Say 7 tons for fuel tankage and fittings and 5 tons each for overall structure, equipment, and crew pod including the crew, leaving 6.5 tons for additional payload such as a weapon pod.
A couple of things to note. As you burn off propellant, performance increases. Once this ship has burned off half its fuel, acceleration is 3 g, and the ship has about 3 km/s of delta v remaining. Assuming your mission involves going somewhere, blowing someone up, and returning, your combat acceleration will be higher than full load acceleration, something to remember in design.
The other thing to note is that you cannot overload a ship in space. It will not sink, crash at the end of the runway, or even bottom out its suspension. It will merely be sluggish. Thus full load, maximum load, or whatever you call it, are all really terms of art, and for some ships will be almost meaningless - for example a drive bus, that can be mated to a small payload section for fast travel or a big payload for slow hauling.
Example 2: Now suppose a deep space ship, such as the one I outlined last post for travel to Titan. In that case I first specified a 'travel speed' of 100 km/s, corresponding to a mission delta v of 200 km/s.
I assume some sort of 'plausible midfuture' nuclear-electric plasma drive, without going into details, even fission v fusion. Drives of this type need not have a fixed exhaust velocity (specific impulse). The maximum is very high, hundreds or even thousands of km/s, but these drives lend themselves to VASIMR style variable specific impulse. The key techlevel parameter is power density, how much oompf you can fit into a given mass.
But for this discussion I leave techlevel itself a bit open. Instead I pick an exhaust velocity of 200 km/s (~20,000 seconds Isp), equal to mission delta v, and specify an average acceleration of 1 milligee. Departure mass is, arbitrarily, 1000 tons.
Since mission delta v and exhaust velocity are the same, we know that the mass ratio is e, 2.72. Thus our ship departs with 632 tons of propellant, and the ship itself (plus payload) has a mass of 368 tons. Acceleration will increase as fuel is burned off, so let us say that full power delivers 600 kg of thrust, or 6 kilonewtons. Departure acceleration is thus 0.6 milligee, while acceleration at fuel exhaustion is 1.63 milligee.
Burning 1 kg/second in the engine described produces 20 tons of thrust, and requires 20 GW of power. So to meet our requirement we will burn 0.03 kg/second, and our drive engine puts out 600 megawatts of thrust power. (Last post I said 10 GW, because I was working really quick & dirty.)
Suppose that our drive tech can develop 10 kw/kg - much less than chemfuel rockets, but comparable to jet engines, and about 100 times better than a first generation nuke electric drive might put out. Thus our drive engine must have a mass of 60 tons, including shielding and radiators, leaving 308 tons for the rest of the ship. Hydrogen propellant is bulky, and you'll need a cryo system for long missions, so let the fuel tankage be 20 percent of fuel mass or 74 tons, leaving 234 tons. Say 34 tons for structure, 50 tons for equipment and fittings, 100 tons for the hab, including crew/passengers, leaving 50 tons for additional payload.
We can also cross check a few numbers. With fuel consumption of 0.3 kg/second it will take 2.11 million seconds to burn through all the propellant and reach 200 km/s, and our actual average acceleration is 0.95 milligee. Close enough for quick & dirty!
I previously wrote about spaceship design in general, and life support. (Connoisseurs of SEO spam may note that this last post gets more spam comments than any other - perhaps the combination of design and life.)
Update: Commenter nqdp did some head scratching about my deep space ship's drive engine, and when I checked I found I'd committed multiple brain dead errors, adding or dropping zeroes, that ended up with a correct result - so long as you didn't check too closely. I have fixed the errors in the main text, but for the sake of completeness here is the original section, with errors lined out:
Since mission delta v and exhaust velocity are the same, we know that the mass ratio is e, 2.72. Thus our ship departs with 632 tons of propellant, and the ship itself (plus payload) has a mass of 368 tons. Acceleration will increase as fuel is burned off, so let us say that
Burning 1 kg/second in the engine described produces 20 tons of thrust, and requires 20 GW of power. So to meet our requirement we will burn
Suppose that our drive tech can develop 10 kw/kg - much less than chemfuel rockets, but comparable to jet engines, and about 100 times better than a first generation nuke electric drive might put out. Thus our drive engine must have a mass of 60 tons, including shielding and radiators, [and miraculous save, dropping a zero to end up with the correct result] leaving 308 tons for the rest of the ship ....
Well, I did say only to try these tricks at home!
Posted by Rick at 6/10/2010 05:59:00 PM
Saturday, June 5, 2010
Titan is one of only two worlds known to have open surface seas, and a cycle of evaporation, rainfall, and streams.
No industrial FUBAR is needed to fill Titan's seas with hydrocarbons, but its gleaming rivers and seas turn out to be short of acetyline, and its lower atmosphere deficient in hydrogen. Some curious chemistry appears to be going on there. H/T to commenter Thucydides for this link, which in turn takes you to this piece in New Scientist.
Hear these words of wisdom:
"Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed," says Mark Allen of NASA's Jet Propulsion Laboratory in Pasadena, California. "We have a lot of work to do to rule out possible non-biological explanations."
That said, if at some point we find distinctive evidence pointing to life on Titan, things get fairly interesting. I am not going to jump on the Titan Direct bandwagon. For one thing, just on practical grounds the time scale will be long.
It will be a few years at least before a follow up Saturn/Titan robotic mission is launched, and outer system missions at our techlevel are great teachers of patience, so perhaps 15 or 20 years before the next probe lands on Titan. More will follow, because a human mission to Saturn space is beaucoup demanding. At 100 km/s the flat space trip, about 9.5 AU or 1.3 billion km, takes 13 million seconds, 150 days if you could go the whole way at top speed, which you can't.
Your mileage and orbital mechanics may vary, but 100 km/s is at the high end of Realistic [TM] travel speed. I will hocus pocus a few intermediate steps - the sufficiently geeky can play the home game to check my results - and find that a 1 milligee drive pushing a 1000 ton ship puts out around 10 gigawatts, and will take 20 million seconds, give or take, for the flat space brachistochrone. So now we are up to some 250 days for the one way trip.
We don't need a torch drive for this, but you need a sort of demi-torch, call it a flashlight drive, designed like a torch drive but with less extravagant handwaving. It might be one of the fancier fission drives, or a fusion drive. Note that our ship must be designed to operate through a more than 100 to 1 range of solar intensity, from 1 AU to beyond 10 AU; one more complication for heat management.
I am not sure that we make a training mission to Saturn space. By the time we are ready to send humans to Titan, they should be equipped to do real work there, and by then we will have an idea what the work will be.
Titan may be a mission for the next century, and if many people follow and build a society in Saturn space, that could be the work of two centuries more - which still brings us merely to the 24th century, still the midfuture, a mere 100,000 day trip down the river of historical time.
Meanwhile we are free to speculate about what may be there now, and in the future.
The top image of a lake on Titan is a detail from Astronomy Picture of the Day.
Bonus Space News: Also via New Scientist, report of the successful launch yesterday of the SpaceX Falcon 9, which reached orbit with a mockup Dragon capsule. Good work, SpaceX! This was a big launch for them, and everything came together.
Posted by Rick at 6/05/2010 05:28:00 PM