The radioactive dust had barely settled on Hiroshima before military scientists on both sides of the Iron Curtain turned their attention to the seemingly unlimited power offered by nuclear fission. Bizarrely to the cautious modern mind, years before safe and practical commercial nuclear power stations came online, compact nuclear generators were considered as an energy source for aircraft.
Much as nuclear submarines can stay submerged for months crossing oceans, the Cold War drove a need to have surveillance aircraft and bombers aloft scouting for signs of enemy activity across longer duration missions than ever before.
The fission past
In 1946, the US Air Force launched the Nuclear Energy for the Propulsion of Aircraft (NEPA) project to conduct preliminary studies on the feasibility of nuclear powered aircraft. This served as a precursor to the 1951 Aircraft Nuclear Propulsion (ANP) programme which used a small, high-output liquid thorium fuelled reactor to achieve the required duration - theoretically up to three weeks aloft - in a compact format.
The reactor was trialled with two different engine designs by established manufacturers; General Electric developed a direct air cycle engine and Pratt & Whitney produced an indirect air cycle variant. Based around its established J47 turbojet engine, General Electric's GE X-39 proved the more practical of the two as it was simple, reliable and lighter and was used in the first ever operation of a nuclear aircraft engine.
6 and 9 August 1945 - Atomic bombings of Hiroshima and Nagasaki.
28 May 1946 - The US Air Force starts the Nuclear Energy for the Propulsion of Aircraft (NEPA) project.
May 1951 - The Aircraft Nuclear Propulsion (ANP) programme replaces NEPA to investigate two different types of nuclear-powered jet engines: General Electric's direct air cycle and Pratt & Whitney's indirect air cycle.
12 August 1955 - the USSR orders two military design bureaus involved in the production of bombers to begin nuclear aircraft research.
31 January 1956 - The first operation of a nuclear aircraft engine occurs using a modified General Electric J47 turbojet engine.
2 December 1957 - The Shippingport Atomic Power Station, the world's first full-scale atomic electric power plant devoted exclusively to peacetime uses, reaches criticality.
26 March 1961 - The ANP programme is terminated after withdrawal of its budget.
1961 - 1969 - The USSR Tupolev bureau fits a small reactor in the bomb bay of a Tupolev Tu-95M bomber to create the Tu-95LAL, which carried out 40 research flights, but only a handful with the reactor in operation.
By the time the ANP programme was terminated in 1961, the costs topped $24bn, of which engine development cost $2bn - unprecedented sums for the time. While useable nuclear fission powered aircraft never resulted from the programme, spin-off technology greatly advanced nuclear science.
While it wasn't as historically significant as the space race, a nuclear aircraft rivalry inevitably sprung up between the US and the Soviet Union, and in 1961 the USSR threw the weight of its Tupolev and Myasishchev design bureaus, known for their work on bombers, behind its efforts. It culminated in housing a VVRL-100 reactor within the bomb bay of a Tu-95M bomber creating the Tu-95LAL (Letayushchaya Atomnaya Laboratoriya or flying nuclear laboratory) testbed. It flew over 40 missions, but only a few with the reactor switched on, as its main aim was to test radiation shielding.
And therein lays the biggest barrier to nuclear powered aircraft. Although military test pilots are used to taking huge risks to be the first in their field - break the sound barrier, walk on the moon, land on a moving aircraft carrier - sitting next to a nuclear reactor may have been one of the most extreme demands made on their courage. Aside from the potential dire outcome of being shot down or crashing, the most immediate danger to the pilot was radiation sickness. Engineers never adequately addressed the problem, and when intercontinental ballistic missiles were introduced in the 1960s, programmes on both sides of the Iron Curtain were abandoned.
The fusion future
In recent decades, the prospect of nuclear fusion as a source of energy has proven an attractive alternative to fission as it is more efficient, creates no nuclear waste to dispose of, and the fuel can be readily recycled. However, every announcement on the technology, whether from top-league universities or bedroom scientists, has always put a useable version a tantalising "20 years away", seemingly a moving target.
But when the research wing of an established military contractor - in this case Lockheed Martin's Skunk Works - announces that it has been working on a fusion reactor for some years and aims to have a work working prototype capable of ignition within five years , the world sits up and notices.
Fusion occurs when two atomic nuclei collide to create a heavier single nucleus, releasing as much as four times as much energy as a fission reaction. Lockheed's "high beta concept" compact fusion reactor (CFR) uses a version of magnetic confinement fusion, much like the International Thermonuclear Experimental Reactor (ITER) project underway in France. However, while the 'doughnut'-shaped ITER tokamak will be 16m in diameter, Lockheed claims its solution uses a "high fraction of the magnetic field pressure". In practical terms, that means a CFR will be able to fit on the back of a truck - or in a military transport aircraft.
Like its fission predecessors, the CFR acts as a source of heat, with temperatures reaching hundreds of millions of degrees, which it releases in a controlled fashion to turbine generators fitted with heat exchangers in place of the combustion chamber.
Aircraft propulsion is just one of the applications of Skunk Works' CFR, but the existence of the project was first brought to light by an aviation publication, hence the initial focus of interest. It could also be used on ships, for renewable commercial electricity generation, to reduce the cost of water desalination and even for space travel. Perhaps the Philae lander wouldn't have got into trouble if it had had a fission reactor on board rather than relying on solar power so far from the sun.
Lockheed is now looking for industrial and academic partners to take the project forward, making working versions of each part of the system in turn as steps towards a functioning prototype.
But will it work?
Cynics - and when it comes to claims of imminent practical fusion energy, there are many - point out that no fusion projects announced to date have yet resulted in a significant working prototype. Lockheed's Skunk Works certainly has many hurdles to overcome, not least that the associated equipment, such as cryoplant and steam generators, take up far more space than applications such as powering aircraft would allow. But Lockheed already has a number of patents pending on its developments to date, and the fact it has openly asked for help for the next stages of the project could signal a genuinely new, compact solution to future energy demands.