Nuclear thermal rocket

Recent articles about NASA working with DARPA on a nuclear rocket had me wondering what is going on. The the nuclear heat source generated electricity for ion acceleration, it would make sense but a thermal rocket had me wondering because there are limits to high temperature materials per Wiki:

As thermal rockets, nuclear thermal rockets work almost exactly like chemical rockets: a heat source releases thermal energy into a gaseous propellant inside the body of the engine, and a nozzle at one end acts as a very simple heat engine: it allows the propellant to expand away from the vehicle, carrying momentum with it and converting thermal energy to coherent kinetic energy. The specific impulse (Isp) of the engine is set by the speed of the exhaust stream. That, in turn, varies as the square root of the kinetic energy loaded on each unit mass of propellant. The kinetic energy per molecule of propellant is determined by the temperature of the heat source (whether it be a nuclear reactor or a chemical reaction). At any particular temperature, lightweight propellant molecules carry just as much kinetic energy as heavier propellant molecules and therefore have more kinetic energy per unit mass. This makes low-molecular-mass propellants more effective than high-molecular-mass propellants.

Because chemical rockets and nuclear rockets are made from refractory solid materials, they are both limited to operate below ~3,000 °C (5,430 °F), by the strength characteristics of high-temperature metals. Chemical rockets use the most readily available propellant, which is waste products from the chemical reactions producing their heat energy. Most liquid-fueled chemical rockets use either hydrogen or hydrocarbon combustion, and the propellant is therefore mainly water (molecular mass 18) and/or carbon dioxide (molecular mass 44). Nuclear thermal rockets using gaseous hydrogen propellant (molecular mass 2) therefore have a theoretical maximum Isp that is 3x-4.5x greater than those of chemical rockets.​

Ok, this sort of makes sense but powering an ion engine makes more to me.

Bob Wilson

 

I suspect a nuclear rocket either thermal or ion needs to be in low earth orbit before going critical. But there are some interesting molten salt, thorium cycles that appear very interesting.

Bob Wilson