China's manned space flight program.

[MikeH]

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.

[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.

[DanS]

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.

[KrazyHorse]

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.

[iamlod]

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.

[KrazyHorse]

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...

[KrazyHorse]

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"

[KrazyHorse]

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.

[KrazyHorse]

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...

[iamlod]

You lost me, but sure.

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

[Kuciwalker]

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.

[Sandman]

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.

[Kuciwalker]

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

[Sandman]

I don't think that's really possible, barring some sort of miraculous breakthrough in materials science.

[Kuciwalker]