running a heat engine, boiling water or some other fluid, sending it though a turbine, and condensing it with the cold of space. A heat engine works with a temperature difference, turning part of the energy from the heat source into usable work, while discarding the rest.

Incidentally, radiation of waste heat has be neglected by almost everybody in proposing power supplies for deep space travel.

In space there is no gas to carry away waste heat, hence it must be radiated. The tradeoff is something like this: (Here, I assume that the temperature of space is absolute zero, which is not true, but a good enough approximation for these purposes.)

The thermal power radiated by a surface exposed to the cold of deep space equals

albedo * Boltzmann constant * (absolute temperature)4
= A * B * T4

On the other hand, the highest efficiency available from a heat engine equals

1 - (lower temperature)/(upper temperature)

= 1 - T / U, where T means the same thing here as above.

If T = 0, then conversion is 100% efficient, which can't happen because T = 0 is an impossible condition.

Realistically, T will be somewhere in the range 300 to 400 kelvins. U, the output temperature of the fission reactor, will be somewhere around 1200 kelvins, giving a maximum conversion efficiency of 75%. In the real world, one is lucky to get 40%, although 50% IS possible with organic vapor bottoming turbines.

To increase efficiency, it is necessary to vastly increase the surface area of the radiator, which is not feasible.

Consider a reactor producing 250 megawatts thermal and rejecting 150 megawatts while converting 100 megawatts into electricity.

The radiation into deep space of a blackbody radiator at 300 kelvins is roughly 500 watts per square meter.

At 400 kelvins, the radiation is (4/3)4 roughly 3.2 times as great.

Dumping 150 megawatts = 150,000,000 joules per second at 300 requires a radiator area of 150,000,000 / 500 = 300,000 square meters, which is rather large. Increasing the temperature to 400 kelvins reduces this to about 100,000 square meters. Assume that we are rejecting heat at 400 kelvins. 100,000 square meters is the area of a rectangle about 316 meters on a side. This is a huge structure, which would need to include coolant pipes, pumps, etc, etc.

Not that this is impossible, but it IS most definitely challenging.

I know of nothing else that would work AT ALL for deep space propulsion.

David F Mayer