A nuclear rocket
From 1955 to 1973, the United States sought to develop a nuclear rocket for use in long-duration missions. The basic idea was to “use fission to heat liquid-hydrogen and expel it as thrust at rates that would supersede those of chemical rockets,” according to NASA. The original idea was to create atomic aircraft for the U.S. Air Force, but the program shifted to a space-based application, requiring NASA to take over. The program reached its penultimate phase, an effort to build a Nuclear Engine for Rocket Vehicle Application (NERVA). A second-generation NERVA engine was built and tested on the ground, but the program was terminated before any in-flight tests could be conducted.
