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 ~, 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. History As early as 1944, Stanisław Ulam and Frederic de Hoffmann contemplated the idea of controlling the power of nuclear explosions to launch space vehicles. After World War II, the U.S. military started the development of intercontinental ballistic missiles (ICBM) based on the German V-2 rocket designs. Some large rockets were designed to carry nuclear warheads with nuclear-powered propulsion engines. As early as 1946, secret reports were prepared for the U.S. Air Force, as part of the NEPA project, by North American Aviation and Douglas Aircraft Company's Project Rand. These groundbreaking reports identified a reactor engine in which a working fluid of low molecular weight is heated using a nuclear reactor as the most promising form of nuclear propulsion but identified many technical issues that needed to be resolved. In January 1947, not aware of this classified research, engineers of the Applied Physics Laboratory published their research on nuclear power propulsion and their report was eventually classified. In May 1947, American-educated Chinese scientist Qian Xuesen presented his research on "thermal jets" powered by a porous graphite-moderated nuclear reactor at the Nuclear Science and Engineering Seminars LIV organized by the Massachusetts Institute of Technology. In 1948 and 1949, physicist Leslie Shepherd and rocket scientist Val Cleaver produced a series of groundbreaking scientific papers that considered how nuclear technology might be applied to interplanetary travel. The papers examined both nuclear-thermal and nuclear-electric propulsion. Nuclear fuel types A nuclear thermal rocket