21-09-2003 16:31									#61
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18 months is a lot shorter time than the astronauts would have to spend in space.

Either they go at Earth's closest approach to Mars (6 months in space then have to wait a couple of years to return) or they come back before that which would take 2 years to fly back.

	21-09-2003 17:08									#62
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Ironically enough, one of the biggest problems seems to be cooling the ship down. We had a talk at CERN about this very topic last week. The best proposal seems to be using an ion drive, which gives approx the same force as the weight of a peice of paper on Earth. The constant acceleration allows them to pick up huge speeds over the long flight (they turn around half way of course). It is powered by fission reactors (>1 in case of failure, and for stability/symmetry reasons).

In the design the speaker showed us, the fission reactors were on long 'arms' keeping them as far away from the ship as possible, and there was a huge amount of sheilding on the inner pod where the crew live, because of the radiation given off by the reactors rather than that of space.

But the big problem is keeping the reactors cool. How do they do it in nucl. subs? I don't understand why they can't use the same method. Unfortunately I didn't get the chance to ask this question after the talk.

	21-09-2003 22:37									#63
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There is no shame in copying Russian rocket designs. In its heyday (through about 1988), the Russian space program was a lot more active than the American one. Also, the Russians copied the German designs, so there's some precedent. The US went a step further in emulation, by capturing the German rocket scientists outright.

As for the Chinese program, if they want to waste their money on this stuff, then they are welcome to it. As I said earlier in this thread, there are some benefits to everyone for Chinese inolvement, but with the US and Russia, the benefits have not proven to match the costs for a robust program. That's why nobody has a robust program any more.

	21-09-2003 23:41									#64
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quote: Originally posted by Rogan Josh But the big problem is keeping the reactors cool. How do they do it in nucl. subs? I don't understand why they can't use the same method. Unfortunately I didn't get the chance to ask this question after the talk.

I would think that water cooling would be efficient on a sub. Conduction and convection are a lot faster than radiation (you have to wait till you get up to a thousand degrees absolute before any appreciable blackbody power loss). So on a sub you drink in some seawater, run it through pipes wrapped around the reactor, then dump it outside. Not an option in space.

You would need to set up some sort of manifold system with coolant (cfcs?) running through it to get as much radiative area as possible.

	21-09-2003 23:44									#65
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quote: Originally posted by Rogan Josh But the big problem is keeping the reactors cool. How do they do it in nucl. subs? I don't understand why they can't use the same method. Unfortunately I didn't get the chance to ask this question after the talk.

I don't see what the problem is here. These rockets are in space, how much colder can you get? All it would take are a few tiny vents in the reactors.

No, not vents, that isn't what I mean. If the reactors are made in a way so that there is an open column going through the center (if they are built in an 'O' shape) than the constant flow of cold space passing around and through them should keep them cool. (This is assuming that they are fixed to wings or stalks and are held away from the body of the spacecraft.

	21-09-2003 23:45									#66
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On some experiments I did as an undergrad RA that's what we did. Problem was that the manifold had to be perfectly leak tight. Environment needed to be less than .01% humidity and a few parts per million O2...

	21-09-2003 23:51									#67
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quote: Originally posted by iamlod I don't see what the problem is here. These rockets are in space, how much colder can you get? All it would take are a few tiny vents in the reactors. No, not vents, that isn't what I mean. If the reactors are made in a way so that there is an open column going through the center (if they are built in an 'O' shape) than the constant flow of cold space passing around and through them should keep them cool. (This is assuming that they are fixed to wings or stalks and are held away from the body of the spacecraft.

Cold space? Space isn't a fluid. Amount of H2 etc. comes to a few atoms per cubic meter. Only real source of cooling is radiative, which follows the stefan-boltzmann law P=AeST^4 where P is the radiated power (energy per unit time), A is the radiative surface area, e is the emissivity of the surface material (perfect blackbody=1) S is the stefan-boltzmann constant (related to kB, the boltzmann constant) and T is the temperature of the surface in Kelvin.

EDIT: added word "area"

	21-09-2003 23:54									#68
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S is a very small number, so T has to be a big number to cool appreciably. And machines don't like to work at 2000K. They have a bad tendency to melt.

	21-09-2003 23:56									#69
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Some of you probably are wondering why they don't run an air conditioner or refrigeration system onto the core. The answer why not is the same reason I don't leave my refrigerator door open inorder to cool my apartment down...

	22-09-2003 02:38									#70
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You lost me, but sure.

(This is why I'm not designing these spaceships...)

Last edited by iamlod on 23-09-2003 at 06:20

	22-09-2003 02:53									#71
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quote: Originally posted by Rogan Josh Ironically enough, one of the biggest problems seems to be cooling the ship down. We had a talk at CERN about this very topic last week. The best proposal seems to be using an ion drive, which gives approx the same force as the weight of a peice of paper on Earth. The constant acceleration allows them to pick up huge speeds over the long flight (they turn around half way of course). It is powered by fission reactors (>1 in case of failure, and for stability/symmetry reasons). In the design the speaker showed us, the fission reactors were on long 'arms' keeping them as far away from the ship as possible, and there was a huge amount of sheilding on the inner pod where the crew live, because of the radiation given off by the reactors rather than that of space. But the big problem is keeping the reactors cool. How do they do it in nucl. subs? I don't understand why they can't use the same method. Unfortunately I didn't get the chance to ask this question after the talk.

The cooling of the reactor is what drives the engine. Pressurised (so it doesn't turn into steam) water is run through the reaction chamber, the heat is tranferred to the water. The water then transfers its heat to water pumped in from the outside, which is unpressurized, so it turns to steam and drives a turbine.

	22-09-2003 03:21									#72
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I suppose you could try and make the reactor as flat as possible, although I doubt that's very flat. Another (inelegant) solution would be to have multiple engine assemblies that work in shifts, with one providing propulsion whilst the other one cools down.

	22-09-2003 03:39									#73
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Or you could make the engine more efficient and divert more energy backwards (or turn it into electricity) instead of it just being waste heat

	22-09-2003 03:55									#74
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I don't think that's really possible, barring some sort of miraculous breakthrough in materials science.

	22-09-2003 04:00									#75
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quote:

	22-09-2003 09:57									#76
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But waste heat is just heat you can use because you don't have a sufficient gradient. This is mainly a problem on earth because it is generally somewhat warm. In space, though, you can easily set up a huge, huge gradient.

	22-09-2003 12:02									#77
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MikeH,

Let's ignore the Venus fly-by option; So our astronauts spend two 6-month stretches in zero-G and two years at one-third of a g. The two zero-g sections aren't a problem by themselves. We don't know exactly what effects will be caused by a third of a g, but it's reasonable to assume that wastage will be slower than zero g, and that a stable state, if it exits, will be at a lower level than in full g. Most of the possible outcomes should be within human tolerance; only if mars gravity has a minimal effect is there the possibility of a less-than-full recovery.
But the only way to find out is to put a couple of astronauts in one-third-g for two years. In which case, Mars seems like an excellent spot to test this...

Rogan,

Best solution according to what criteria? Employing lots of particle physicists, by any chance?
While IMO ion rockets and space fission reactors are certainly worthy of development, there's nothing like off-the-shelf technology for getting a job done.

For cooling, the only option is large radiator fins. Ask the russians how they managed it; IIRC they have two radarsats with fission reactors aboard.

skwalker,

In a normal nuclear reactor they do use vapourized coolant to power turbines, but not in most designs for space-based reactors. The mass of the turbines, cooling system etc. is considerable, and if you intend to launch the reactor into orbit then it's better to enlarge the size of the core and use a thermoelectric generator, even though you only get ~5% efficiency.

But this leads to a heck of a lot of heat which just has to be dumped by any means possible.

	22-09-2003 13:11									#78
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You know we could just have he astronaights excerisze on resistance band machines and the problem would be solved.

	22-09-2003 13:35									#79
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quote: Originally posted by Curiosity MikeH, Let's ignore the Venus fly-by option; So our astronauts spend two 6-month stretches in zero-G and two years at one-third of a g. The two zero-g sections aren't a problem by themselves. We don't know exactly what effects will be caused by a third of a g, but it's reasonable to assume that wastage will be slower than zero g, and that a stable state, if it exits, will be at a lower level than in full g. Most of the possible outcomes should be within human tolerance; only if mars gravity has a minimal effect is there the possibility of a less-than-full recovery. But the only way to find out is to put a couple of astronauts in one-third-g for two years. In which case, Mars seems like an excellent spot to test this...

Fine. There's no way I'd risk it though.

	22-09-2003 16:53									#80
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Oerdin,

They already exercise on almost every device imaginable, including bungee-cord treadmills and the like, for up to 6 hours a day. The problem is very strange; they apply far vastly more overall force to their muscles and bones than someone who is bedridden, but loose muscle and bone at a much higher rate. There seems to be something special about how the body reacts to constant, low level compression. (Which is why I suspect that Mars gravity will not be too much worse than full-g, but that's just conjecture.)

MikeH,

OK - one less competitor for the seat.

	22-09-2003 17:17									#81
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quote: Originally posted by Curiosity skwalker, In a normal nuclear reactor they do use vapourized coolant to power turbines, but not in most designs for space-based reactors. The mass of the turbines, cooling system etc. is considerable, and if you intend to launch the reactor into orbit then it's better to enlarge the size of the core and use a thermoelectric generator, even though you only get ~5% efficiency. But this leads to a heck of a lot of heat which just has to be dumped by any means possible.

I was talking about in submarines

	22-09-2003 17:19									#82
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KH: Presumably the subs can't cool like that if hey are running on stealth because you would see the heat signature. They must somehow 'store' the heat. Anyone know how?

Curiosity: This was a talk from NASA, so there were no particle physicists involved.

	22-09-2003 20:24									#83
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AFAIK, though, sub detection is mainly via sonar, not heat sensing.

	23-09-2003 01:57									#84
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TCO would probably know, whenever he's sober and back home from the hurricane.

	23-09-2003 05:10									#85
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quote: Originally posted by Urban Ranger AFAIK, though, sub detection is mainly via sonar, not heat sensing.

You would be surprise if you knew how we find Subs.

	23-09-2003 05:12									#86
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quote: Originally posted by Rogan Josh KH: Presumably the subs can't cool like that if hey are running on stealth because you would see the heat signature. They must somehow 'store' the heat. Anyone know how? Curiosity: This was a talk from NASA, so there were no particle physicists involved.

Perhaps something with a high whatsitcalled (water has 1.0; it is volume/mass). Use it as a heat sink. Eventually you'll have to expell it or transfer the heat outside, of course.