The United States has a long history of developing space nuclear power and propulsion programs. The most common systems have been the various types of radioisotope power systems used for outer planet spacecraft such as Voyager and Cassini (see Table 1.2 for details). However, many other technologies have been studied and occasionally progressed to advanced testing or even launch.
Starting in the mid-1950s, the United States initiated a program to develop nuclear propulsion for spacecraft. The basic technology involved passing hydrogen through a very high temperature nuclear reactor, where it expanded and blasted out of the reactor at high velocity. Eventually NASA and the Atomic Energy Commission jointly ran two main programs, a reactor technology development program named Rover and a program to develop a flight-capable nuclear rocket engine known as NERVA.
Under the Rover program, nuclear reactors were built at the Los Alamos National Laboratory’s Pajarito Site and tested at very low power, and then shipped to the Nevada Test Site for higher-power tests. Laboratory work also included developing and testing the fuel elements that powered the reactors. Phase one of Project Rover was called Kiwi and entailed building and testing eight reactors between 1959 and 1964. Phase two, called Phoebus, involved advanced nuclear reactors.
NERVA began in 1961 and by the mid-1960s progressed to hardware development tests in the Nevada desert. The NERVA engine utilized a hot-bleed cycle in which a small amount of hydrogen gas is diverted from the thrust nozzle to drive the turbine that pumps fuel into the engine. NERVA reached an integrated system component demonstration readiness level. At various points NASA planned on using NERVA as a rocket upper stage, a space ferry for lunar missions, and a propulsion stage for human missions to Mars. However, after Apollo, none of these projects was approved, and NERVA therefore had no dedicated mission. Congress showed greater support for NERVA than did the White House, but the program was eventually canceled by 1972.
The nuclear pulse drive was conceived by H-bomb designers Stanislaw Ulam and Cornelius Everett at Los Alamos in 1955 and sponsored primarily as a study project by the Atomic Energy Commission and then the U.S. Air Force under the name Project Orion.