Skipjack wrote:Well high thrust and high Isp are not mutually exclusive. It depends on the total amount of energy. The more the expelled mass is accelerted, the higher the Isp for the same amount of thrust (or more thrust for the same Isp, depending on how you handle it). For fission rockets that means they need to get hotter.
And run the lowest molecular weight of reaction mass possible. Hydrogen is a pest but it gets the exhaust velocity up better than anything else, unless you can break it down to atomic hydrogen.
There is a problem with hot-running fission reactors. Normal fission reactors run on thermal neutrons, and they lose reactivity as they get hotter. So making a reactor with sufficient reactivity to run hotter than normal means it is a touchy mutha when cold. Most modern fission reactors are pretty safe and it is difficult to make them explode, but fission rocket engines are more dangerous.
Rover story, told to the best of my recollection from what RWB said. Some blamed fool didn't understand the thermocouple color codes (red is usually negative). The first four thermocouples were hooked up correctly, but then all the rest, hundreds of them, were wired backwards. They found this out after they ran a short power blip on it. This was a problem. The reactor was now hotter than a two dollar pistol and they could not fix all those backward TCs. But they were under the gun because there was a rain storm coming and these test reactors had the exhaust nozzle pointed up, a big funnel to collect rain water in a prompt-critical reactor that normally ran without water as a moderator. They really did not want it full of water.
Some poor schnook got to run out and reverse the first 4 thermocouples, which could be done quickly enough not to cook him, and they could then get useable reading off them all.
We have not mentioned the VASIMIR option, at least for deep space. If you use the fission reactor to drive an electrical powerplant, you can then make any Isp you want. However, that's a lot of complex hardware, not to mention needing huge heat radiators that probably out-mass the reactor, so this means a worse power to mass ratio than the simpler versions. This is much better with a p-B11 Polywell providing the juice.