@S1-A.ARPA,@MIT-MC.ARPA:DIETZ@RUTGERS.ARPA (06/24/85)
From: DIETZ@RUTGERS.ARPA There was some work back in the 60's on nuclear rockets. The idea is to use a high temperature reactor to heat hydrogen, then expel the hydrogen out a rocket nozzle. The low molecular weight of the xhaust gives the rocket really high Isp, potentially as high as 1200 (vs. ~400 for the SSME's). This idea has problems. It's hard to get a reactor powerful enough to lift a rocket from the ground. Temperatures and power densities in the reactor have to be very high. The rocket would be very nice for propelling a mission to Mars, but that wasn't (and isn't) in the budget. Some of the problems with a purely nuclear rocket may be avoided by using a nuclear/chemical hybrid. The idea here is to use a nuclear reactor to preheat some of the chemical fuel before its injection into the combustion chamber. Since the preheated fuel need not be at exhaust temperatures the temperature of the reactor can be lowered, easing some of the engineering problems. Also, the reactor need not supply all the energy to the exhaust: even a (say) 20% contribution would raise Isp significantly. Since the amount of fuel needed is an exponential function of Isp, there's a chance for real savings here. What are the big problems here? (1) reactor mass. This can be minimized by using a compact core of highly purified fissile materials. Only a few kg of U235 are needed, if properly moderated. (2) Reactor shielding. Neutron shielding should not be hard; we can afford to use exotic elements, like gadolinium, with very high thermal neutron capture cross sections. Gamma shielding is more of a problem. The reactor should be at the back of the rocket; personnel at the front, with fuel in between. It may pay to not use the reactor at full power until one is in less dense air to avoid gamma rays reflected back from the atmosphere. (3) Reactor waste. Unlike a nuclear power station, we can afford to replace and reprocess the core very oftwen (perhaps after every flight). This minimizes the quantity of nuclear debris released should the rocket explode. Waste from launch decaying during the flight will produce waste heat that must be radiated away, but since waste has not been builtup over long periods the power level should not be too high (and we can use this heat for on-orbit power). -------