The NERVA nuclear rocket used a sorta-conventional nuclear reactor as its basis. As the reactor was brought up to power, the solid fuel elements would heat up; hydrogen would be sent through coolant channels. The hydrogen would of course heat up, cooling the reactor. When operated properly, the temperature in the reactor would reach a steady state; a temperature limited by the structural capabilities of the solid reactor materials.
The hotter the reactor, the hotter the hydrogen, and the better the rocket performance. However, the hotter it gets, the softer the reactor structural elements get; at some point it fails structurally, and perhaps even melts. So the temperature must be limited. And the temperature limits limit performance. NERVA was quite cool compared to the combustion temperature of a conventional hydrogen/oxygen rocket engine. However, the extremely low molecular weight of the pure hydrogen exhaust compared to the H2O exhaust of the chemical rocket makes up for that, providing about twice the specific impulse.
In order to greatly improve specific impulse, the temperature of the hydrogen must be increased; and to do that the temperature of the reactor must be increased. It quickly becomes impossible to have a solid-core reactor, and one must accept that the uranium will not be a solid. So rocket designers of the early 1960’s came up with the liquid-core nuclear rocket, with molten uranium; and then the gas-core rocket where the uranium is so hot it has actually vaporized. But the problem was containing the uranium. Typically centrifugal force (by spinning the rocket around its central axis) was employed; the uranium liquid or gas would, hopefully, be stuck to the outer wall of the rocket reaction chamber, while lighter hydrogen would migrate to the core, and then out the nozzle. The thickness of the uranium liquid or gas would be less than the distance from the chamber wall inwards to the nozzle, so the hope was that ther uranium wouldn’t be able to flow out the nozzle. A simple idea made extremely difficult in actual practice… massive, extremely hot nuclear reactors being spun at high rate around their central axis, with little to no vibration while being injected with liquid hydrogen? Not exactly the description of a straightforward engineering problem. With all the effort involved, uranium was still expected to leak out the nozzle, making the propulsion system both filthy (a minor concern in deep space) and wasteful of uranium (a serious concern).
Another solution was devised in the mid 1960’s that in principle would combine the best of both worlds… uranium brought up to plasma temperatures, and uranium contained physically so that it could not escape. This concept was known as the Nuclear Light Bulb.
TO BE CONTINUED…