NEPA and ANP
In May 1946, the United States Army Air Forces started the Nuclear Energy for the Propulsion of Aircraft (NEPA) project, which conducted studies until the Aircraft Nuclear Propulsion (ANP) program replaced NEPA in 1951. The ANP program included provisions for studying two different types of nuclear-powered jet engines: General Electric's Direct Air Cycle and Pratt & Whitney's Indirect Air Cycle. ANP planned for Convair to modify two B-36s under the MX-1589 project. One of the B-36s, the NB-36H, was to be used for studying shielding requirements for an airborne reactor, while the other was to be the X-6; however, the program was cancelled before the X-6 was completed.
The first operation of a nuclear aircraft engine occurred on January 31, 1956 using a modified General Electric J47 turbojet engine. The Aircraft Nuclear Propulsion program was terminated by Kennedy after the President's annual budget message to Congress in 1961.
The Oak Ridge National Laboratory researched and developed nuclear aircraft engines. Two shielded reactors powered two General Electric J87 turbojet engines to nearly full thrust. Two experimental engines complete with reactor system, HTRE 3 and HTRE 1, are at the EBR-1 facility south of the Idaho National Laboratory 43°30′42.22″N 113°0′18″W / 43°30′42.22″N 113°0′18″W / 43.5117278; -113.00500.
The U.S. designed these engines for use in a new, specially-designed nuclear bomber, the WS-125. Although Eisenhower eventually terminated it by cutting NEPA and telling Congress that the program was not urgent, he backed a small program for developing high temperature materials and high performance reactors; that program was terminated early in the Kennedy administration.
In 1957, the Air Force and the U.S. Atomic Energy Commission contracted with the Lawrence Radiation Laboratory to study the feasibility of applying heat from nuclear reactors to ramjet engines. This research became known as Project Pluto. This program was to provide engines for an unmanned cruise missile, called SLAM, for Supersonic Low Altitude Missile. The program succeeded in producing two test engines, which were operated on the ground. On May 14, 1961, the world's first nuclear ramjet engine, "Tory-IIA," mounted on a railroad car, roared to life for just a few seconds. On July 1, 1964, seven years and six months after it was born, "Project Pluto" was cancelled.