I've been fascinated by the idea of using a nuclear reactor (either pebble-bed or particle-bed)as a powerplant for an aerospace vehicle, but of course there is the problem of coupling the thermal energy to the propellant flow by direct contact without having bits of radioactive debris from the reactor coming out in the exhaust. MHD at least avoids this, by using electromagnetism to couple the reactor's power with the flowstream, instead of direct contact. To me, this is obviously the safest way to use nuclear power to get to orbit. My understanding is that NASA's recently discontinued Prometheus reactor was based on the SAFE-400 pebble bed design. http://en.wikipedia.org/wiki/Nuclear_electric_rocket I came across this mention of the Ajax, which was a Soviet design for a nuclear-powered MagnetoHydroDynamic hypersonic vehicle, apparently sled-launched (I don't know why they didn't have something that would take off under its own power) http://translate.google.com/transla...ets/ajax/ajax.htm&sl=ru&tl=en&history_state0= I was further thinking that ionization of the oncoming airstream would not only turn it into a good propellant, but would improve its flow characteristics as well, reducing frictional heating and turbulence. We also now know that control surfaces can be devised which use electrostatics to affect the slipstream around the aircraft even at high mach conditions. I've read that electrical control surfaces can even be used to reduce the stall speed of conventional aircraft. Another technology I was reading about was PIT (Pulsed Inductive Thruster) which seems to me to be the plasma-electric equivalent of the Pulse Detonation Engine. http://en.wikipedia.org/wiki/Pulsed_inductive_thruster Like PDEs, this seems to be an interestingly flexible form of propulsion, which seems very well suited to the instant-on-off cyclability of electric current. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930023164_1993023164.pdf It seems to me that with PIT, you could use it in a DC-X type of VTVL rocket, so that you could take off and land on Earth, Moon and Mars in the same way regardless. Unlike the hypersonic flight regime, you'd quickly clear the atmosphere and fly friction-free to orbit, and similarly you could also make powered landings. I'm then wondering what the opinions are on which is the superior form of nuclear-electric propulsion. For example, is it PIT or MHD - or is it something else? What are the pro's and cons of each? Comments?