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Nuclear propulsion research for aircraft was abandoned during the 50's.

Why wasn't it revived ever?

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    $\begingroup$ The thought of five thousand nuclear ships in the air is kinda unnerving. $\endgroup$ – Voldemort Nov 25 '17 at 9:43
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    $\begingroup$ @Voldemort: Specially at a time were early nuclear countries are trying to get out of the expensive nuclear plant mistake. $\endgroup$ – mins Nov 25 '17 at 9:59
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    $\begingroup$ The safety is just completely unworkable. $\endgroup$ – pjc50 Nov 25 '17 at 10:11
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    $\begingroup$ The same way steam propulsion for aircraft was proposed in the 1890's, but abandoned. Nuclear power is nothing more than a sophisticated steam engine, when you get the heat from nuclear reaction instead of the chemical reaction of burning coal. All the water boiler, turbines etc, have to be there and are heavy. $\endgroup$ – vsz Nov 25 '17 at 10:54
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    $\begingroup$ @vsz Not at all. As far as I'm aware, there was never a proposal for nuclear-powered aircraft to use steam turbines. One can loosely describe jet engines as pressurizing air by the heat from burning fuel, though the combustion products of the fuel also contribute to the pressurization. The plan for nuclear aircraft engines was to heat the air with the nuclear reactor, via heat exchangers. $\endgroup$ – David Richerby Nov 25 '17 at 12:40
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TL;dr - too heavy :-)

It's just not a good energy source for something like an aircraft.

Nuclear energy is superb for instances where you need continuous output over a long period of time, for example a satellite, which is going to be there for years without maintenance or refuelling. A very small amount of nuclear material in an RTG such as those on Voyager 1 and 2 can provide heat energy that can be used to power the satellite. This model works in this use case because the only real alternative (solar power) hasn't anywhere near the output required as the Voyager craft fly ever further away from the Sun.

Also, submarines are a good use case - if you remain under the sea for months or even years at a time, diesel or anything that uses oxygen is not suitable, so a nuclear power plant is ideal.

But while for a satellite the power unit can be very small as the actual output power required is small, to power an aeroplane or a submarine you require a large power plant, and that will also require a lot of shielding (as a side effect of nuclear reaction is radiation...)

On a submarine, that's okay - weight can be dealt with, but on an aeroplane weight is key. You just can't do it efficiently and safely. You need shielding that can cope with a crash - which is going to be massively heavy, so you won't have any passenger or cargo payload left.

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    $\begingroup$ Comments really are not for extended discussion. If you wish to discuss please use the chat room set up I have removed all comments not directly related to improving this answer (which is what comments are for). $\endgroup$ – Jamiec Dec 1 '17 at 11:58
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To a large extent, it wasn't revived because the only realistic use case became obsolete. The USA and USSR were both interested in nuclear-powered long-range bombers. The plan was to have a fleet of bombers loitering in the Arctic so that, if nuclear armageddon was required, they'd already be half-way to their target. They'd also be very hard to destroy as part of a first strike, since they'd be out of range of enemy fighters and widely dispersed. A nuclear-powered plane wouldn't need to refuel so the limitation on mission length would be basically crew resilience. With relief crew on board, you can easily imagine a mission lasting many days. All of this would give a credible nuclear deterrence.

However, the advent of high-altitude surface-to-air missiles in the late 1950s meant that any first or second strike with bombers would be vulnerable to the enemy's air defences. As a result, missiles became the preferred delivery method for both first- and second-strike capabilities. Survival against a first strike was now ensured by having widely dispersed silos on land, and missiles on submarines.

Reconnaissance is the only other application I can think of for a plane that can be aloft for multiple days at a time. But that's still vulnerable to anti-aircraft defences and the recon mission was also moving towards another platform (satellites) from the late 1950s.

For anything where you don't need huge endurance, nuclear power doesn't make a lot of sense. The two main problems are the inevitability of crashes and weight. The B-36 that was modified as a testbed had a 12-ton shielded cockpit module and at least five tons of lead between that and the reactor, along with tanks of water that acted as both shielding and cooling. All of that is heavy (and needs to be close to the engines) so it needs to be in the middle of the plane. So, if you wanted to build a nuclear-powered airliner (e.g., for non-stop Europe–Australia/New Zealand flights), you'd need another shield behind the reactor and the reactor and shielding would seriously decrease the size of the passenger cabin and, hence, potential revenue.

On the plus side, you'd save carrying 150 tons of kerosene on your long-haul flight but it seems that nuclear is still heavier overall. For example, there were plans to build a 15,000ft runway at Carswell AFB to allow the proposed Convair X-6 to take off. For comparison, even at large commercial airports, the longest runways tend to be in the 10-13,000ft range.

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    $\begingroup$ we know that nuclear reactors can be miniaturized. world-nuclear.org/information-library/nuclear-fuel-cycle/… $\endgroup$ – user8792 Nov 25 '17 at 15:06
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    $\begingroup$ @anonymous Sure but I didn't mention the weight of the reactor. Shielding can't be miniaturized, because the only way of doing it is by putting a bunch of mass between you and the reactor. $\endgroup$ – David Richerby Nov 25 '17 at 15:56
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    $\begingroup$ +1 on the actual answer, but the shielding does not scale the way you assume when talking about why passenger planes would be even worse. You shield the reactor, the same as with military aircraft. Any extra shielding would be a shadow shielding between the reactor and the rest of the fuselage, proportional to cross-section of the fuselage. I doubt anyone ever seriously considered nuclear passenger planes, but spacecraft designs were made. And are still kind of relevant as using nuclear power would make solar system much more accessible. $\endgroup$ – Ville Niemi Nov 26 '17 at 4:56
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    $\begingroup$ Also that 150 tons of kerosene are burned in the way. The cooling water (hoppely) not $\endgroup$ – jean Nov 27 '17 at 10:14
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    $\begingroup$ A note, since so much is talked about shielding: The mass is needed to block the neutrons (mostly). Neutrons are needed for reactors to create a chain reaction, but a nuclear power plant that relies on the natural decay of radioisotopes can use fuel that minimizes penetrating radiation and get away with much less shielding. Obvious example are the radiothermal generators used by spacecraft where mass would be even bigger concern than with aircraft. So you probably could build an aircraft using nuclear energy without heavy shielding. Just not with a reactor. $\endgroup$ – Ville Niemi Nov 29 '17 at 13:12
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Because there was no practical purpose such an aircraft would've served. First and most important, the safety concerns over such an aircraft design would make its use in civil aviation more or less impossible, especially when more and more countries are shying away from nuclear power.

This leaves the military. There are quite a few issues with such an application. The most important of these would be the weight. The cost in weight for shielding the aircrew and the weapons (which by default would have to be nuclear), would be prohibitive. For example, the first (and only) 'nuclear' aircraft , the Convair NB-36H had a 11 ton shielded cockpit.

NB-36H cockpit

Special shielded cockpit being installed in the NB-36H; image from aviation-history.com

That's about a third of the aircraft payload. Add this to the reactor shielding, and the aircraft would not have any meaningful payload. Even in case of ships, the nuclear powered ones come with signinficant weight penalty, saved only by the low fuel cost (and space) associated with it.

There is a significant cost associated with maintaining nuclear weapons and safely maintaining them, which would be prohibitive for a fleet of nuclear powered aircraft. USAF sunk nearly a billion dollars in the nuclear aircraft without anything to show for it. Nuclear tipped missiles are more cost effective and more survivable that these lumbering beasts in air.

The only virtue of a nuclear aircraft is its virtually unlimited range and endurance , which is useful only in case of strategic bombers on continuous patrol with massive free-fall nuclear weapons, which has no meaning today (ironically due to nuclear submarines with missiles). Advances in aircraft and propulsion has resulted in a number of aircraft with >10,000 range, which is more than enough for all practical purposes and can be extended through air-to-air refueling.

Even is all these technological issues are overcome, nuclear powered aircraft would be a overkill- there is no point in having an aircraft of unlimited range and endurance is the the crew can't eat- even in case of nuclear submarines, the food is the critical resource. Nuclear propulsion maybe used in an interstellar craft which requires fuel supply for very long with severe limitations in fuel mass and volume, but in case of an aircraft, it would be of no use.

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    $\begingroup$ Sure, the shielding's heavy, but it's much lighter than the fuel you don't have to carry any more. Also, the Tupolev Tu-95LAL was another nuclear-"powered" plane like the NB-36H. (But +1 for everything else.) $\endgroup$ – David Richerby Nov 25 '17 at 14:38
  • $\begingroup$ we know that nuclear reactors can be miniaturized. world-nuclear.org/information-library/nuclear-fuel-cycle/… $\endgroup$ – user8792 Nov 25 '17 at 15:06
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    $\begingroup$ @anonymous Sure, but shielding can't. Basically, the only way to shield is to use mass. $\endgroup$ – David Richerby Nov 25 '17 at 15:50
  • $\begingroup$ @DavidRicherby : But not all aircraft these days need people in them so maybe shieldning in those cases are unnecessary. $\endgroup$ – mathreadler Nov 26 '17 at 16:22
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    $\begingroup$ @mathreadler Far from people, there's not much to surveil and we already have surveillance drones that are less likely to provoke WWIII than is flying a gigantic nuclear-powered thing over somebody else's territory. Communications and weather services are provided by satellites. Post is delivered quite happily by regular aircraft and wouldn't benefit much from extreme long-distance non-stop flights. The lack of mission is a huge driver, here. $\endgroup$ – David Richerby Nov 26 '17 at 21:28
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What if the nuclear powered aircraft crashes? It would be near impossible to design a reactor that could withstand a 500+ mph impact, and you'd have a serious radiation mess to try to clean up.

The 'direct cycle' engine, where air is heated directly by the reactor, irradiates the air and leaves a radioactive trail behind it. Both the Soviet TU95LAL and the US SLAM nuclear cruise missile had plans for a direct cycle engine. With the SLAM, the radioactive trail was considered part of the weaponry. Of course, neither side fully explained how they could protect their people around the area where such a monstrosity would be launched.

The Convair NB36H had plans for an indirect cycle engine, which didn't leave a radioactive trail, but it never progressed beyond the stage of hauling an operational reactor aloft.

In the end, the perfection of aerial refueling in the 1950's obsoleted the need for the extra range that a nuclear powered aircraft promised, while the complexities and potential dangers were never fully resolved.

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    $\begingroup$ "Of course, neither side fully explained how they could protect their people around the area where such a monstrosity would be launched." I think you launch the SLAM, the thinking is "they are f*cked anyway, press the button". You would then retire to your bunker to medidate in front of a painting of the Kurfürst of Brandenburg or someone of equal stature. $\endgroup$ – David Tonhofer Nov 26 '17 at 6:53
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    $\begingroup$ @mathreadler Thermoelectric generators are also waaay below the power you need to drive a plane. $\endgroup$ – David Richerby Nov 27 '17 at 9:46
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    $\begingroup$ @mathreadler Look. Even a small two-seat plane like a Cessna 150 has a 75kW engine. RTGs generate a couple of kilowatts at most. They also rely on plutonium-238, of which the world's entire stock is enough to make only a few generators. RTGs are completely and utterly unsuitable for aviation use. Please do some basic research, and please stop using the comments on this page as a chat room. $\endgroup$ – David Richerby Nov 27 '17 at 10:04
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    $\begingroup$ @mathreadler If you would like to ask a question about why RTGs are completely unsuitable for powering planes, please ask a question instead of continuing to try to discuss the subject here. If you wish to have a discussion, please use the chat room. $\endgroup$ – David Richerby Nov 27 '17 at 10:31
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    $\begingroup$ @mathreadler google "flight recorders after recovery" or something similar, I wouldn't even put an RTG in something that gets that mangled after impact (not than an RTG would work with that much shielding around it anyway) $\endgroup$ – Kaithar Nov 28 '17 at 16:49
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Already great answers here but I`d also like to add...

In the 50's it was the atomic age. We thought splitting the atom was the best thing since sliced bread. But that was at a time when we, as humans, had no idea of the long term effects of radiation and radiation poisoning. It really was not until a decade had passed since the dropping of the bombs in WWII that the long term effects became apparent and were simply horrible. In the mean time we had done all sorts of above ground testing with "volunteers" in the trenches exposed to blast radiation and fallout.

Until we woke up to that, all sorts of fanciful ideas came up that today we would say are ludicrous. Among those was the notion of putting a reactor inside an aircraft and flying it over populated areas.

Today we know better, and the probability of anyone doing such a thing, except perhaps for some remote drone to fly way out over the ocean perhaps, is zero to none. At least in developed nations.

There were other weird ideas at the time too. Microwave heating was discovered in that era too. Someone thought we could heat our homes with microwaves instead of furnaces. The house would be cold, but our bodies would be toasty warm.

The point is when new technology comes alone, folks usually try to use it in novel and different ways. It isn't till later that a sanity check kicks in.

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    $\begingroup$ +1 for the best answer. It is of course a totally ludicrous idea. $\endgroup$ – Koyovis Nov 27 '17 at 4:27
  • $\begingroup$ I don't think the chronology is right here, at all. The atomic bombing of Japan was the first time that we'd seen acute radiation syndrome (i.e., "radiation poisoning") but that kills people very quickly. Plenty of people had been exposed to lower levels of radiation during the first half of the 20th Century (e.g., the "radium girls" of WWI) so people were fairly familiar with the effects. In any case, the US Aircraft Nuclear Propulsion programme wasn't ended until 1961, much more than a decade after the atomic bombings, so you can't just blame early '50s naivete. $\endgroup$ – David Richerby Nov 27 '17 at 9:36
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    $\begingroup$ @DavidRicherby yes acute poisoning was known, but the long term cancer effects were far less understood at the time since they took a decade to really develop. Add in that the development itself was also lengthy, starting only a year after the end of WWII. I'm not saying that's the only reason, or even the prime reason, just adding the naivity was a factor in the development. $\endgroup$ – Trevor_G Nov 27 '17 at 11:07
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    $\begingroup$ @DavidRicherby ya well, you know the military... they don't do anything fast, esp with black projects. ;) $\endgroup$ – Trevor_G Nov 27 '17 at 12:02
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    $\begingroup$ I can remember when people had a snit-fit about a nuclear space probe with less than 100lbs of fuel on-board being launched (via traditional rockets) over the Atlantic. Watching someone try to sell them on nuclear planes flying over cities would be ... entertaining. $\endgroup$ – T.E.D. Nov 28 '17 at 18:40
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Along with all the other reasons, there's another - nuclear reactors contain a lot of energy but are not so good at power.

An aircraft takes off using 100% throttle. After reaching cruise it throttles back to somewhere around 55 to 70%. During a war mission, they would throttle back up to 100%, basically instantly.

Nuclear reactors do not like to throttle. The one near my home can throttle about 15% over a 24-hour period. It is possible to make ones that are better than that, like on nuclear submarines, but you start getting more and more complex.

So that's where they were at in the 50s when the Atlas was beginning to mature. Why bother with all that complexity when you can have the same bomb just sitting in a silo ready to go when you push a button?

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  • $\begingroup$ I'm not convinced by this argument. If throttling were a such an obviously huge problem, nuclear-powered aircraft research would never have even begun. $\endgroup$ – David Richerby Nov 29 '17 at 21:38
  • $\begingroup$ Your assumptions about how aircraft use throttle is wrong, very often they don't take off at 100% throttle, and they usually throttle back shortly after take-off, not using take-off power until they reach cruise altitude. They also don't, during a "war mission" throttle up to 100%, it makes range very very short. $\endgroup$ – Ron Beyer Nov 29 '17 at 22:38
  • $\begingroup$ @RonBeyer You're right that not all aircraft use 100% power at takeoff (though many do, especially light piston aircraft,) but still, nearly all aircraft need to be able to go from ground idle to 90+% thrust in a matter of a few to several seconds at most, which was my understanding of the point this answer was trying to make. $\endgroup$ – reirab Nov 29 '17 at 23:03
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    $\begingroup$ @RonBeyer - I'm a commercial pilot Ron, you should tell me all about how aircraft work. $\endgroup$ – Maury Markowitz Nov 30 '17 at 12:08
  • $\begingroup$ Good point. Big rockets have the energy output of a medium sized country's electricity production. Try to lift all these nuclear power plants! $\endgroup$ – Peter A. Schneider Jun 11 '18 at 19:23
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The existing answers have already covered the weight, lack of need, and crash safety concerns well, but there is also an additional reason, especially for civilian airliners: hijacking. Securing a nuclear reactor at a power station is feasible because it sits in one place and we can build fences and walls around it with armed security. While submarines and aircraft carriers are not stationary, anyone that tries to attack one to get its fissile material is going to have a very bad day on the wrong end of a naval gun (or torpedo, missile, etc.)

Unfortunately, history has shown us that hijacking an airliner is much easier than attacking a carrier battle group or a nuclear power station. If we started putting significant amounts of fissile material onboard airliners, rouge countries or terrorist groups seeking such material would have more incentive to hijack airliners in order to acquire the fuel. Considering the remote corners of the world that airliners must visit, both for cargo and for passengers, it wouldn't be very difficult to hijack several of them in short order or possibly even steal the fuel from them while they sat on some remote ramp. Relative to the other ways significant amounts of fissile material can be acquired, this would be pretty straightforward.

Additionally, if someone tried another September 11-style attack, the airplane would now be a giant dirty bomb. Not good.

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  • $\begingroup$ Yes on manned aircraft would be far too unsafe. But more and more percent of aircraft these days are unmanned and automated. $\endgroup$ – mathreadler Nov 26 '17 at 21:54
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I suppose Nuclear powered airplanes never went from experiment because of the fear one of it falls on your backyard. A Steam-powered airplane went fully airborne, the Besler Steam; the possibility of a Nuclear Reactor driving propellers as in B-36, but through Steam Turbines, seems realistic. A similar concept was used for interplanetary probes, relying on heat from radioactive isotopes to produce energy. USSR was attributed considering a Nuclear Energy powered bomber, the USA attempted purchasing the Saunders-Roe Princess for converting it to Nuclear Power, but the Flying Boat was severely corroded from lack of money to properly preserve it. USR Nuclear powered bomber proposal -Aviation Week, Dec 1, 1958

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  • $\begingroup$ "USSR was attributed considering a Nuclear Energy powered bomber" - is is worth pointing out this is a myth. The aircraft in the picture is actually the M-50 Bounder, which was cancelled after the single prototype due to the ICBM becoming practical. However, US sources "leaked" it to AvWeek to create the myth of this being nuclear powered, in order to shore up support for their own nuclear program. This was known as "policy by press release", and what Ike was talking about in the "military industrial complex". $\endgroup$ – Maury Markowitz Jun 11 '18 at 21:55
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We can discuss this on a very general level, without diving into technical details.

  1. It is extraordinarily difficult to safely operate a nuclear reactor and its fuel even on land; the accidents are numerous.
  2. It's more difficult on sea and has led to substantial release of radioactivity into the environment.
  3. The level of difficulty involved in operating a nuclear reactor safely within the challenging frame conditions of manned aviation is prohibitive.

The increasingly demanding frame conditions in the succession "land — sea — air" apply not only to the reactor but also to the vehicle proper. On land and on sea loss of propulsion or some structural damage is usually not fatal (after all, a land based nuclear power plant is stationary by design); in the air it often is.

It is no coincidence that for example spent fuel containers are transported by train and truck, not flown around, even though that would avoid a lot of trouble with protesters.

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