High Quality Content by WIKIPEDIA articles! In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. In this kind of thermal rocket, the nuclear reactor`s energy replaces the chemical energy of the propellant`s reactive chemicals in a chemical rocket. Due to the higher energy density of the nuclear fuel compared to chemical fuels, about 107 times, the resulting propellant efficiency (effective exhaust velocity) of the engine is at least twice as good as chemical engines. The overall gross lift-off mass of a nuclear rocket is about half that of a chemical rocket, and hence when used as an upper stage it roughly doubles or triples the payload carried to orbit.