Gas core reactor rockets use nuclear gas reacting to super heat and therefore pressurize hydrogen. They operate at about 25000 C. Why not use a high intensity plasma arc which routinely operate at about 13,000 C but if designed to can go much higher by at least several fold. I got bored today and found myself curious about the progress in ion and pulsed laser ion propulsion systems and some how found myself here. I think it may be feasible solid core rockets were desirable because of the heat produced well that and the heat producer was it's own energy source. People are funny though tell them you want to launch a bunch of nuclear fission rockets and they think you've gone mad. --I'm pretty sure thats quotable. Plasma arcs can produce a lot of heat but the downside is they require a fair amount of electricity to do so. http://en.wikipedia.org/wiki/Gas_core_reactor_rocket for anyone who isn't up on a gas core rocket and wondering what I'm talking about. Oh and I know wikipedia = not real research.