# How could this small nuclear reactor power two jet engines?

In the 1950s, the US was working on an Aircraft Nuclear Propulsion and it is stated that they successfuly operated two modified General Electric J47 jet engines using a 2.5 MW (thermal) nuclear reactor.

Now, looking at the specifications of the J47 jet engines, it is clearly stated that its specific fuel consumption is 103.36 kg/kN/hr, and since the engines they used had 23 kN of thrust, I calculate that the fuel consumption of this engine is 0.66 kg/s, and since the specific energy of the fuel is about 42 MJ/kg, then the engine requires a power of 27.72 MW to operate.

So my question here is how could this relatively small nuclear reactor power two of those jet engines?

• How much of the chemical energy do you think goes out the back of the plane without producing any thrust? (I don't know the answer beyond "not zero", but it seems to be the very first question to ask.) Aug 28, 2016 at 21:28
• Come to think of it, the Carnot efficiency gives you a lower limit on the answer to that question. Aug 28, 2016 at 21:37
• @dmckee I think the question would better be: how much of the chemical energy heats the gases since we are comparing it to a power of 2.5 MW (not thrust). Aug 28, 2016 at 21:41
• I haven't looked for your mistake, but your numbers don't make sense. The max thrust of the biggest modern turbofan engines is around 100,000 lbf, and their maximum power output is around 50MW. Your 6,000 lbf thrust J47 can't possibly need 28MW, unless GE don't know how to design jet engines. Aug 29, 2016 at 2:08
• @alephzero your approach is only partially correct. Please let me explain why. Firstly, the most powerful turbofan engine is the GE90-115B at 110,000 lbf and output power of 83 MW, check it here: en.wikipedia.org/wiki/Engine_power#Examples Secondly, I calculated the 28 MW number according to the fuel consumption of the J47 engine, which can not be compared with the consumption of a modern airliner engine because these large and modern engines are much more efficient, check it here: en.wikipedia.org/wiki/… Aug 29, 2016 at 18:56

How could this small nuclear reactor power two jet engines?

The answer is, it couldn't and was never even utilised in that way.

This nuclear powered bomber concept, first thought of in the 1950's, had two major problems to contend with, neither of which were solved. The first was how to harness the nuclear power, either directly as a propulsion unit or indirectly using a heat exchanger and the second was how to protect the crew, either by directly shielding the reactor, which made the aircraft unstable to fly, due to concentration of weight, or by distributed shielding, which was not as efficient but spread the weight around the aircraft in a more balanced fashion.

The B36 shielding test aircraft, accompanied by an aircraft with a cleanup crew in case the B36 crashed.

On September 5, 1951, Convair was formally granted a contract to construct, or rather convert, one of their   B-36 aircraft to enable it to carry out the testing of the reactor and the shielding associated with it.

To illustrate how stupidly ridiculous this idea was, every test flight of the B 36 had to have a troop carrying aircraft flying behind it, in case the B 36 crashed. The soldiers could then attempt to clean up the nuclear waste.

Two methods of nuclear propulsion were proposed. The first was the indirect cycle, which was designed to heat up incoming cold air by the use of hot air from the reactor, and the second was the direct cycle engine, which simply used atmospheric air as a working fluid and ignored the environmental effects of exhausting nuclear waste straight out of the engine tailpipe.

One early design of indirect nuclear propulsion power plant.

A illustration of a direct nuclear propulsion power plant .

Image Sources and excerpts from Nuclear Powered Aircraft

This was the only known airborne reactor experiment by the U.S. with an operational nuclear reactor on board. The NTA flew a total of 47 times testing the reactor over West Texas and Southern New Mexico. The reactor, named the Aircraft Shield Test Reactor (ASTR), was operational but did not power the plane, rather the primary purpose of the flight program was shield testing. Based on the results of the NTA, the X-6 and the entire nuclear aircraft program was abandoned in 1961.

In reality, because this was part of the cold war, I would tend to disbelieve claims such as any power outputs quoted.

The Soviet program of nuclear aircraft development resulted in the experimental Tupolev Tu-119, or the Tu-95LAL (Flying Nuclear Laboratory) which derived from the Tupolev Tu-95 bomber. It had 4 conventional turboprop engines and an onboard nuclear reactor.

It actually turns out that the Soviets were prepared to risk pilots who volunteered to endure the inevitable radiation hazards. The Soviets used little or no shielding to save weight during 40 test flights, and used direct cycle nuclear reactors. Most of the aircrew died through radiation induced illness.

The obvious potential of the ICBM made the expensive program superfluous, allied with the fact that these slow bombers were easy targets for surface to air missiles and around the mid-1960s, both countries cancelled their projects.

You can watch an hour long video of the story at Nuclear Powered Bombers

• Thanks for the answer. There is one more thing I want to add though. It seems that the 2.5 MWt reactor was just a scaled down reactor used just for testing. The photo of the direct method you gave above used the HTRE-3 reactor, which according to this: ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19640019868.pdf could operate at 34 MWt. Also the HTRE-1 which was used to power a single J47 engine is stated to have power of 15,100 BTU/sec which is about 16 MW. But as you said, these reactors were never used on an aircraft, but things make sense now. Aug 29, 2016 at 0:00
• Hi Abanob, that Wikipedia article is wrong. Watch the video, it's worth a look and its a good story. Then see which convinces you. The video has interviews with the scientists and technicians involved, as well as another mad idea to send a cruise missile into Russia powered by a nuclear engine. The problem they had was aircraft weight. I thought the picture I used was the same as on Wikipedia, but because, as you say, it was not used directly I was not as careful. But it does seem the Soviets got further than the Americans, but at the cost of the crew. It's totally insane stuff, imo....
– user108787
Aug 29, 2016 at 0:14