In the 1950s, people dreamed of using nuclear energy to power all manner of transport — from cars to airplanes to airships. In the U.S. the father of the nuclear reactor, Enrico Fermi, envisioned a nuclear-powered aircraft, while in the USSR, the chief designer of the Soviet atomic bomb, Aleksandr Kurchatov, thought nuclear-powered "heavy aircraft" could be built.
A nuclear-powered bomber seemed a no-brainer since it could theoretically stay aloft indefinitely, providing an effective deterrent to a nuclear attack. Both the U.S. and the Soviet Union researched nuclear-powered aircraft, but neither country developed an active-duty version due to problems inherent in the design. These included shielding air and ground crews from radiation, and the possible effect of a crash.
To date, no civilian nuclear-powered aircraft has ever been created.
Nuclear-powered jet engines
In May 1946, the U.S. Air Force initiated the Nuclear Energy for the Propulsion of Aircraft (NEPA) program. In 1951, NEPA was supplanted by the Aircraft Nuclear Propulsion (ANP) program, which was run by the Atomic Energy Commission.
ANP studied two different types of nuclear-powered jet engines: General Electric's Direct Air Cycle, and Pratt & Whitney's Indirect Air Cycle.
In the Direct Air Cycle program, which was based in Evendale, Ohio, air was drawn directly through the nuclear core, where it was heated, then sent through a turbine and out the exhaust as thrust. The drawback to this design was that the air became irradiated and left a trail of radioactive particles in the plane's wake.
The Indirect Air Cycle method, which was based near Middletown, Connecticut, used a heat exchanger, where heat from the nuclear reactor heated either pressurized water or liquid metal. This, in turn, heated air which moved through a turbine and out the exhaust, providing thrust.
While the Indirect method was the safer of the two, its program never was able to produce hardware that was flight-ready before the program was canceled.
The Aircraft Reactor Experiment
While the propulsion came down to the Direct method, a reactor that could fly on board an aircraft was still needed. The U.S. Aircraft Reactor Experiment (ARE) was created to develop a high-power-density and high-output-temperature nuclear reactor for use on an aircraft.
The design that researchers settled on became the first molten salt reactor (MSR). It used molten fluoride salt (NaF-ZrF4-UF4) as fuel, and was moderated by beryllium oxide (BeO). The reactor achieved a peak temperature of 860° C. and a peak of 2.5 MWth.
The MX-1589 Project
Now that a propulsion system and a reactor had been created, a plane was needed with which to test. On September 5, 1951, the Air Force awarded a contract to Convair to fly a nuclear reactor on board its Convair B-36 Peacemaker aircraft.
Convair was the result of a merger between Consolidated Aircraft and Vultee Aircraft. In 1953, Convair was purchased by General Dynamics and became one of its divisions.
The B-36 Peacemaker was a strategic bomber that was flown by the U.S. Air Force from 1948 to 1959. Between 1948 and 1955, the B-36 was the primary nuclear weapons delivery vehicle of the Strategic Air Command (SAC), but was replaced by the jet-powered Boeing B-52 Stratofortress. All except five Peacemakers were scrapped.
At 230 ft (70.1 m), the B-36 had the longest wingspan of any combat aircraft ever built, and it was the first bomber capable of delivering any of U.S.'s nuclear bombs from within its four bomb bays without needing any modification.
The Peacemaker had a range of 10,000 mi (16,000 km), a maximum payload of 87,200 lb (39,600 kg), and it was capable of intercontinental flight without refueling.
The NB-36H Nuclear Test Aircraft (NTA) was created to study the shielding requirements for an airborne nuclear reactor. It had a modified cockpit and a raised nose, and from July 1955 to March 1957, it flew a total of 47 times over West Texas and Southern New Mexico.
While the reactor, dubbed the Aircraft Shield Test Reactor (ASTR), was operational, it did not power the aircraft. News of the flights leaked out to Russia, who misinterpreted it as a successful test of a nuclear-powered engine, and this spurred the Soviets into redoubling their efforts to produce a competing airplane.
The Russians came up with a test aircraft, "Aircraft 119", or LAL (Letayushchaya atomnaya laboratoriya), which translates to "the Flying Atomic Laboratory." Nicknamed The Swallow, the craft was a modified four-engine turboprop Tu-95 (NATO code-name Bear) which was the largest Soviet bomber at that time.
In the summer of 1961, just like in the American tests, The Swallow took flight with the reactor on board, but not providing any propulsion.
In September 1959, the first intercontinental ballistic missiles entered into service and eliminated the need for a nuclear-powered aircraft. On March 26, 1961, the new president, John F. Kennedy, canceled the nuclear-powered aircraft program, citing its high cost and the fact that no flight-worthy reactor had been produced.
Nuclear-powered jet engines
In 1956, the ANP program ran modified General Electric J47 jet engines on nuclear power from a reactor test assembly called Heat Transfer Reactor Experiment 1 (HTRE-1).
HTRE-1 used vertical control rods, while a successor, HTRE-2, featured a removable core. HTRE-3 had horizontal control rods which were better suited to an airframe.
Today, you can view the decommissioned HTRE-2 and HTRE-3 reactors and test assemblies at the Experimental Breeder Reactor I facility at the Idaho National Laboratory.
As an odd aside to the nuclear-powered aircraft story, the U.S. military considered solving the shielding problem by employing elderly crews to fly the nuclear-powered airplanes. Their thinking was that the crew would die of natural causes before the effects of radiation could kill them.