In the 1960s NASA began testing designs to superheat hydrogen gas inside a nuclear reactor in order to produce thrust from a rocket nozzle. Termed the NERVA project, this idea seemed to produce practical solutions for space travel as long as the temperatures were kept low enough inside the reactor. To approach relativistic velocity, however, such a reactor would have to be heated far beyond the melting point, so NERVA was not considered a practical solution for interstellar propulsion. In 1955 an alternative to using a nuclear reactor was devised, one which could utilize the upper theoretical limits of nuclear energy. In the 1960s this idea became a highly classified project under NASA and the Airforce because of its nature as a spacecraft propelled by atomic bombs. It was called Orion.
Fission-fragment propulsion is a propulsion design whereby fission fragments from a fissioning reactor core are utilized to provide thrust, thus theoretically capable of achieving speeds of .03c (3% of the speed of light)*