Nuclear propulsion is the use of energy released by a nuclear reaction to provide thrust directly, as distinct from nuclear-electric propulsion.1 A nuclear propulsion system derives its thrust from the products of nuclear fission or fusion, and was first seriously studied by Stanislaw Ulam and Frederick de Hoffman in 1944 as a spin-off of their work on the Manhattan Project. During the quarter-century following World War II, the Atomic Energy Commission (superceded by the Department of Energy in 1974) worked with various federal agencies on a series of nuclear engine projects, culminating in NERVA.2
One way to achieve nuclear propulsion is to heat a working fluid by pumping it through a nuclear reactor, then let the fluid expand through a nozzle. Considering that nuclear fission fuel contains more than a million times as much energy per unit mass as chemical fuel does, this sounds promising. But the approach is limited by the temperature at which a reactor and key components of a rocket, such as a nozzle, can operate. The best working fluid to use is hydrogen, because it is the lightest substance and therefore, at any given temperature, consists of the fastest-moving particles. Chemical rockets cannot produce hydrogen as an exhaust because hydrogen is not the sole product of any practical chemical reaction. With unlimited nuclear power, however, it is not necessary to react or burn anything; instead, hydrogen gas could simply be heated inside a nuclear reactor and then ejected as a high-speed exhaust. This was the idea of the NERVA project.
Other concepts in nuclear propulsion have sought to circumvent the temperature limitation inherent in circulating the working fluid around a reactor by harnessing the power of runaway nuclear reactions. Among the most important historically of these has been the nuclear pulse rocket, designs of which were developed for Project Orion and Project Daedalus. See also antimatter propulsion.
NASA has recently revitalized interest in nuclear propulsion with its Project Prometheus. One of the possible systems that could be developed in the future by NASA as part of Project Prometheus is based on the idea of microwave-induced fusion propulsion.
1. Kommash, Terry, ed. Fusion Energy in Space Propulsion. Reston,
Va.: American Institute of Aeronautics (1995).
2. Abbot, Alison. "Rubbia proposes a speedier voyager to Mars and back," news item about an idea by Carlo Rubbia for a new type of NERVA-like nuclear rocket.