 thumb|300px|sketch of nuclear thermal rocket  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. the thermal heater / inert propellant paradigm as opposed to the reactive propellants of chemical rockets turns out to produce a superior effective exhaust velocity, and therefore a superior propulsive efficiency, with specific impulses on the order of twice that of 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. a nuclear engine was considered for some time as a replacement for the j-2 used on the s-ii and s-ivb stages on the saturn v and saturn i rockets. originally "drop-in" replacements were considered for higher performance, but a larger replacement for the s-ivb stage was later studied for missions to mars and other high-load profiles, known as the s-n. nuclear thermal space "tugs" were planned as part of the space transportation system to take payloads from a propellant depot in low earth orbit to higher orbits, the moon, and other planets. robert bussard proposed the single-stage-to-orbit "aspen" vehicle using a nuclear thermal rocket for propulsion and liquid hydrogen propellant for partial shielding against neutron back scattering in the lower atmosphere.dewar, james and bussard, robert, "the nuclear rocket: making our planet green, peaceful and prosperous", apogee books, burlington, ontario, canada, 2009 the soviet union studied nuclear engines for their own moon rockets, notably upper stages of the n-1, although they never entered an extensive testing program like the one the u.s. conducted throughout the 1960s at the nevada test site. despite many successful firings, american nuclear rockets did not fly before the space race ended. to date, no nuclear thermal rocket has flown, although the nerva nrx/est and nrx/xe were built and tested with flight design components. the highly successful u.s. project rover which ran from 195
Nuclear_spacecraft_propulsion