By Dr Peter Layton[1]

Birds and aircraft have a fundamental problem: their range and endurance are limited. To remain aloft requires the expenditure of energy. Eventually, birds must land and rest, and aircraft must refuel. The invention of nuclear power in the 1940s appeared to offer a way to cut this Gordian knot. A nuclear-powered aircraft could, it seemed, provide dramatically improved range and endurance compared to chemically fuelled powered aircraft.

Such ambitions were strengthened as the Cold War between the US and the USSR worsened. The Cold War released immense funding for military purposes while providing an operational rationale: a requirement for very long-range bombers able to strike military-industrial complexes deep in the Soviet heartland. The generous funding now available meant numerous new high technology possibilities could be considered, built, trialled and if successful enter mass production. An obvious candidate to research and investigate seemed nuclear-powered aircraft.

The original ideas about using nuclear power for aircraft propulsion had appeared around 1944. These led to a minor research program, the Nuclear Energy for the Propulsion of Aircraft study, beginning in mid-1946. Undertaken by Fairchild, this examined reactor technologies and engine transfer systems. These studies proved encouraging and so in 1951, with the Cold War deepening, the United States Air Force (USAF) proposed to begin actively developing manned aircraft nuclear propulsion. Contracts were let for three main elements: two X-6 prototype test aircraft, a nuclear propulsion system (reactor and turbojets) and an NB-36H reactor flight-test aircraft.

Convair received the X-6 contract. The aircraft was envisaged as being of comparable size to the company’s B-36 Peacemaker bomber with a length of 50m, a wingspan of 70m and an empty weight of some 100 tonnes. The X-6 was planned to have 12 turbojets; eight conventionally fuelled used for take-off and landing, and four nuclear-powered used during in-flight trials. This was an ambitious but expensive test program and was cancelled by the incoming Eisenhower administration in 1953 on budgetary grounds. However, the other two elements continued.[2]

General Electric was awarded the propulsion contract, progressively developing across 1955-1961 three direct-cycle nuclear power plants under the ground-based Heat Transfer Reactor Experiment (HTRE) test-rig program. The final HTRE-3 propulsion system featured a solid moderator using lightweight hybrided (sic) zirconium instead of water, a horizontal reactor to meet aircraft carriage requirements and produced sufficient heat to power two X-39-5 (modified J-47) turbojets simultaneously. HTRE-3 had several firsts including demonstrating an all-nuclear turbojet start, having a primary shield able to handle radiation levels expected in flight and in being designed for in-flight stresses, air pressures, temperatures, and G loadings.[3]

The third element was to flight test a reactor. In mid-1952, Convair was contracted to modify two B-36 aircraft: one for a ground test, the other for flight test and designated as the NB-36H. The major modifications involved firstly, the crew compartment and avionic cabin being replaced by an 11-tonne nose section lined with lead and rubber to protect against reactor radiation and secondly, the rear internal bomb bay being altered to allow fitment of the 16-tonne reactor. Less apparent were the cockpit glass transparencies being some 30cm thick and nine water-filled shield tanks in the fuselage to absorb any escaping radiation.[4]

In the meantime, the USAF was firming up its requirements. In March 1955, General Operational Requirement (GOR) No. 81 was issued seeking a nuclear-powered weapon system, WS-125A. Aspirations included a range of about 10,000nm, an operating altitude of 60,000-75,000ft and an endurance of perhaps more than a week airborne.[5]  WS-125A was to have a cruise speed of at least Mach 0.9, desirably offer supersonic dash in the target area and enter service with operational units in 1963.[6]  Realising such high ambitions was to prove problematic.

In July 1955, the NB-36H began flight test with the reactor going critical in flight for the first time in September. The reactor did not power the aircraft, instead of being tested to verify the feasibility of a safe, sustained nuclear reaction on a moving platform. For each NB-36 flight, the one-megawatt reactor was winched up into the bomb bay at a dedicated pit at Convair’s Fort Worth plant and then removed again after landing.[7] When in flight, the aircraft was accompanied by a radiation-monitoring B-50 (a slightly updated B-29) and a C-119 transport aircraft carrying paratroopers able to be dropped to secure any crash site and limit bystander exposure to radiation.[8] In total, the NB-36H made 47 flights, ceasing flying in March 1957.

The results of the nuclear propulsion tests and the NB-36H were mixed. HTRE-3 had proven nuclear-power turbojet feasible and that a flyable propulsion unit could be built albeit technical challenges remained. The major problem was that it was hard to build a nuclear reactor small enough to fit into aircraft, but which produced the operationally significant energy output required. It seemed that using contemporary technology would mean nuclear-powered aircraft were relatively slow. For a time, concepts of ‘nuclear cruise, chemical dash’ were investigated; supplemental aviation fuel would allow supersonic dash in the target area.[9]

Moreover, the NB-36H flight programme highlighted the hazards associated with operating such nuclear-powered aircraft. While well-shielded aircraft would not normally pose radiation dangers to air or ground crew, there were worries that accidents and crashes might release fission products from the reactors, and about the dosage from prolonged human exposure to leakage radioactivity.[10] In this, the test flights mainly served to draw attention to the real difficulties that would arise in working with nuclear fuel in operational service conditions.[11]

WS-125A was accordingly cancelled in early 1957. However, there remained occasional flickers of renewed interest in nuclear-powered aircraft into the early 1960s. The Continuously Airborne Missile Air Launcher (CAMAL) concept called for a nuclear-powered strike aircraft able to stay aloft on airborne alert for 2-5 days. This led into Dromedary, a turboprop design capable of an airborne alert for 70-100 hours and able to stand-off outside hostile territory and launch the 600-1000nm Skybolt ballistic missile.[12] These ideas meant research into aircraft nuclear propulsion continued although in only a fairly desultory fashion. This finally ended in 1961 when the new Kennedy administration reallocated funding.

The US Navy had also occasionally expressed interest in nuclear-powered turboprop flying boats. In April 1955, Operational Requirement CA-01503 sought a nuclear-powered seaplane capable of high subsonic speeds primarily for the attack of ports and warships using conventional and nuclear weapons with the secondary roles of mining and reconnaissance. The USN desired to have a prototype available for its evaluation no later than 1961. By mid-1956 the Navy had decided a solely-USN power plant was unjustifiable and that the Navy’s aircraft would use the USAF’s WS-125A power plant. The cancellation of the WS-125A thus terminated the USN’s plans as well.[13]  At one stage, it seemed the UK might sell three mothballed Princess-class flying boats to the USN for nuclear-power trials, but funding oscillated and eventually was not forthcoming.[14]

Further afield, the USSR was also busy. In the late 1950s Tupolev designed but did not build two nuclear-powered bombers: the subsonic Tu-119 and supersonic Tu-120. The Soviet leadership thought the projected payloads and speed were inadequate for the costs involved. Tupolev was though authorised to continue research on nuclear aircraft.[15] Accordingly, a Tu-95 turboprop bomber was modified at a nuclear complex near Semipalatinsk in Kazakhstan to allow flying a nuclear reactor, becoming the Tu-95LAL (Letayushchaya atomnaya laboratorya – flying atomic laboratory).[16] Mirroring the NB-36H trails, some 34 Tu-95LAL flights were undertaken in 1961 with the reactor on board but without providing propulsion. The tests similarly revealed that a nuclear-powered aircraft was impractical with the technology of the time. The gain in performance from not carrying chemical fuel was consumed by the heavy reactor and shields and so Soviet interest in nuclear-powered aircraft declined.[17]

In the end, a better technological solution won out. For both the US and the USSR, the ICBM fitted with lightweight thermonuclear warheads offered a much better answer to the problem of a long-range, highly survivable nuclear strike. The considerable effort and funds expended in investigating nuclear-powered manned aircraft yielded much technical information and engineering expertise but ultimately little else. This was not for lack of interest in the defence aerospace industry. At the time, Kelly Johnson of Lockheed’s Skunk Works fame wrote:

After a half century of striving to make aircraft carry reasonable loads farther and farther, the advent of a [nuclear] power plant that will solve the range problem is of the utmost importance […] this unique characteristic is one to be greeted enthusiastically.[18]

Dr Peter Layton is a Visiting Fellow at the Griffith Asia Institute, Griffith University. His PhD is in grand strategy, and he has taught on this at the US National Defense University. He is the author of the book Grand Strategy.

Header Image: An NB-36H producing contrails in flight. (Source: Wikimedia)

[1] This post partly draws on the author’s Chapter in Michael Spencer (ed.), Nuclear Engine Air Power (Canberra: Air Power Development Centre, 2019). This book discusses contemporary nuclear-powered propulsion systems for aircraft and missiles.

[2].  Jay Miller, The X-Planes: X-1 to X-31 (Arlington: Aerofax, 1988), pp. 69-73.

[3].  F.C. Linn, Heat Transfer Reactor Experiment No.3: Comprehensive Technical report, General Electric Direct-Air Cycle Aircraft Nuclear Propulsion Program (Cincinnati: General Electric Company, 1962), pp. 15-18.

[4].  Raul Colon, Flying on Nuclear: The American Effort to Built a Nuclear Powered Bomber.

[5].  Theo Farrell, ‘Waste in weapons acquisition: How the Americans do it all wrong,’ Contemporary Security Policy, 16:2 (1995), p. 194; ‘Thoughts on WS-110A,’ Flight, 10 January 1958, p. 44.

[6].  Comptroller General of the United States, Review of the Manned Aircraft Nuclear Propulsion Program of the Atomic Energy Commission and the Department of Defense, B-146749, 28 February 1963, p. 133

[7].  Colon, Flying on Nuclear.

[8].  Miller, The X-Planes., p. 210.

[9].  Ibid., p. 73.

[10].  Brian D. Bikowicz, The Decay of the Atomic Powered Aircraft Program, 12 Nov 1992.

[11].  Bruce Astridge, ‘Propulsion,’ in Phillip Jarrett (ed.), Faster, further, higher: leading-edge aviation technology since 194 (London: Putnam, 2002), p. 134.

[12].  Peter J. Roman, ‘Strategic bombers over the missile horizon, 1957–1963,’ Journal of Strategic Studies, 18:1 (1995), pp. 208-13.

[13].  Comptroller General, Review of the Manned Aircraft Nuclear Propulsion Program, pp. 134-40.

[14].  Raymond L. Garthoff, ‘The Swallow and Caspian Sea Monster vs. the Princess and the Camel: The Cold War Contest for a Nuclear-Powered Aircraft,’ Studies in Intelligence, 60:2 (2016), p. 3.

[15].  Arthur J. Alexander, ‘Decision-Making in Soviet Weapons Procurement,’ Adelphi Papers, 18:147-148, (1978), p.32.

[16].  Garthoff, ‘The Swallow and Caspian Sea Monster vs. the Princess and the Camel,’ p. 2.

[17].  Piotr Butowski, ‘Steps Towards Blackjack,’ Air Enthusiast, 73 (1998), p. 40.

[18].  F.A. Cleveland and Clarence L. Johnson, ‘Design of Air Frames for Nuclear Power’, quoted in Bikowicz, Decay.