Why the Aircraft Nuclear Propulsion Program was Abandoned
The concept of propelling aircraft with nuclear power is not a new one. In 1946 the United States Air Force launched the Nuclear Energy for the Propulsion of Aircraft (NEPA) program to further develop this concept. The program ran until 1961, when it was transferred to the Aircraft Nuclear Propulsion (ANP) program which was also run by the United States Air Force. Unfortunately the program did not last long enough to see the first test flight of this type of aircraft.
The Aircraft Nuclear Propulsion program researched and developed two main systems of nuclear powered jet engines. The Direct Air Cycle program was the first of these. Developed by General Electric, this facet of the program was based in Evendale, Ohio. The Direct Air Cycle program was popular because it was simple, reliable and suitable and the engines were able to start quickly. It worked by letting compressed air run through the reactor of a conventional jet engine where it could be heated before being exhausted through the turbine. The X-39 engine was produced by this program and it proved to be highly successful with several upgrades made to the system at later stages. The final HTRE-3 would have most likely been used to power the X-6 nuclear propulsion aircraft if the project hadn’t been scrapped.
The other form of nuclear propulsion that was investigated was the Indirect Air Cycle Program developed by Pratt & Whitney near Middletown in Connecticut. While it was not the more popular of the two concepts, it did produce a lot less radioactive pollution. The system worked by way of one or two loops of liquid metal that served to carry heat from the reactor to the engine. The research and development of this project was extensive, but it was never even brought to a level where it could have been used to power a test aircraft.
The ultimate goal of the program was to use nuclear propulsion to power the Convair X-6. In preparation for this flight, the ANP decided to modify two B-36s in order to effectively study what sort of shielding requirements would be needed for an airborne reactor. The Nuclear Test Aircraft (NTA) became the only known US aircraft to carry an operational nuclear reactor on board. The NTA flew 47 times, during which extensive shield testing was conducted. It was largely due to the results of these test flights that the development of the X-6 nuclear-powered aircraft was stopped and the entire nuclear aircraft program was abandoned in 1961.