Aviation Week has been reporting on and, in one case unwittingly, furthering the cause of nuclear-powered aircraft for more than 60 years. Spurred on by the promise of the ‘Atomic Age’ and the potential strategic benefits of limitless range and endurance, the U.S. Air Force launched the Nuclear Energy for the Propulsion of Aircraft (NEPA) project in 1946. This was succeeded by the Aircraft Nuclear Propulsion (ANP) in 1951 which was supposed to pave the way for a strategic missile carrier that would remain on continuous airborne alert for a week or more.
The idea was to develop a hybrid propulsion system combining turbojet engines with a reactor. ANP looked at two designs: General Electric’s “direct cycle,” in which the engine airflow cooled the reactor; and Pratt & Whitney’s “indirect cycle,” in which a liquid-metal coolant carried heat from the reactor to the engine. Ground tests of a modified J47 turbojet (dubbed X-39/HTRE-1) were conducted in 1956, although flight tests on the proposed Convair X-6 (a modified B-36) were cancelled when the project was axed in 1958.
One B-36 was, however, modified under the ANP project to carry a reactor and flew successfully 47 times between September 1955 and March 1957. Dubbed the NB-36H, the purpose of the test was to evaluate techniques for shielding the crew and systems from radiation. The Aircraft Shield Test Reactor (ASTR) was operational but provided no power to the aircraft or engines. Weighing 35,000 lb., the reactor was carried in the No. 4 bomb bay and was principally cooled by water which dissipated energy to the atmosphere through a water-to-air heat exchanger.
The crew was protected in a purpose-made nose section which incorporated a 12-ton lead and rubber shielded compartment. Various reports indicate the cockpit windows were between 6-inches to one foot thick, while the crew used a closed circuit TV to monitor the reactor. In addition, water jackets were installed between the crew and the reactor along with a four-ton lead disc shield amidships.
The NB-36H clocked up 215 hours of flight time, 89 hours of which were with the reactor active. The aircraft was escorted on each flight by a C-97 which carried a platoon of Marines who were ready to parachute down to surround and protect the aircraft if it crashed.
Aviation Week itself caused a stir in the nuclear propulsion field when it reported in its Dec 1, 1958, edition on flight tests of what appeared to be a Soviet nuclear-powered aircraft. The report, on what later emerged to be an elaborate hoax, led to concerns that the U.S.S.R was pulling ahead of the U.S. and helped to extend funding in nuclear aircraft research for a few more years. The feature also contained a very detailed timeline of associated nuclear aircraft plans in the U.S. and foreshadowed actual flights of a modified Tupolev Tu-95 equipped with a reactor in the early 1960s. The Soviet research in nuclear-powered aircraft, like that in the U.S., effectively ended within a few years as ICBM’s rendered the concept obsolete.
Soviets Flight Testing Nuclear Bomber, December 1, 1958 Aviation Week & Space Technology
The final death knell for the U.S. program was sounded in 1961 by President John F. Kennedy who ended the ANP by stating "15 years and about $1 billion have been devoted to the attempted development of a nuclear-powered aircraft; but the possibility of achieving a militarily useful aircraft in the foreseeable future is still very remote."
While several lower-level research efforts into nuclear-powered missiles and UAVs have been made right up to the 2000s, none have made it beyond the early experimental or concept stage. The bottom line for all these projects has been that, despite vast improvements in reactor technology, fission-based nuclear propulsion cycles remain heavy, complex and inefficient. The question now is, with the possible dawn of a viable, modular fusion capability, is the new atomic age for aviation finally on the horizon?
► Read the report in the December 1, 1958 issue of Aviation Week & Space Technology:
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