M: In theory, how could someone harness atomic power to fly a plane? I'm not looking for a practical, applicable version, I'm just not sure how that would work. I mean, in terms of keeping everything working cool enough to actually function in order enough to provide enough power for the engines to run. Go online to answer this, please. Or at least do a verified hero answer.

G: The interesting thing is that a nuclear-powered airplane is, in principle, much simpler than most people imagine. The reactor isn't really replacing the engine—it replaces the fire.

A modern turbojet works like this:

Air in
   ↓
Compressor
   ↓
🔥 Burn fuel
   ↓
Hot expanding air
   ↓
Turbine
   ↓
Exhaust → Thrust

A nuclear version simply substitutes the heat source:

Air in
   ↓
Compressor
   ↓
☢ Reactor heats air
   ↓
Hot expanding air
   ↓
Turbine
   ↓
Exhaust → Thrust

The turbine doesn't care whether the air became hot because of burning kerosene or because it flowed past an extremely hot reactor core. The engine is still basically following the same thermodynamic cycle used by ordinary gas turbines. citeturn0search25turn0search0

The engineering challenge isn't "How do you get power?" It's "How do you keep everything alive around a miniature nuclear reactor?"

There were two major concepts:

Direct cycle

This was the simpler idea.

Compressed air from the engine literally passed through channels in the reactor core, picked up heat, spun the turbine, and exited the back of the aircraft.

That airflow did double duty:

The drawback is obvious: some radioactive particles could contaminate the exhaust stream, making it environmentally unacceptable despite its simplicity. citeturn0search11turn0search0

Indirect cycle

This worked more like a nuclear power plant.

Instead of air touching the reactor:

Reactor
    ↓
Hot coolant
    ↓
Heat exchanger
    ↓
Jet engine air

The reactor heated a coolant (such as pressurized water or liquid metal), which flowed through a heat exchanger. The engine's compressed air was heated by that exchanger instead of by combustion.

This kept radioactive material inside the reactor system but added a tremendous amount of plumbing, pumps, and weight. citeturn0search11turn0search5turn0search0

So how is the reactor kept cool?

Ironically, by doing exactly what the engine already wants to do.

A jet engine moves hundreds of kilograms of air every second. That airflow is an enormous heat sink.

In the direct-cycle design:

The reactor is continuously cooled by the same air that's producing thrust. citeturn0search0turn0search18

Why wasn't it built?

Mostly because of weight, not because the engines couldn't work.

The reactor itself wasn't the killer.

The problems were:

The shielding alone could weigh many tens of tons—far more than the fuel load it was supposed to replace. citeturn0search11turn0search4

Did anyone ever actually run one?

Yes.

During the Cold War, the U.S. built and tested several Heat Transfer Reactor Experiments (HTRE). These successfully operated modified jet engines using heat from nuclear reactors on the ground. The engines themselves worked. The obstacle was making an entire aircraft practical and safe enough to fly. citeturn0search10turn0search20

I actually think the intuition that helped me most the first time I learned about this was:

A nuclear reactor is just an incredibly hot object.

We often think of it as some exotic source of electricity, but fundamentally it's a machine whose primary output is heat. A coal furnace, a gas flame, and a nuclear reactor all produce heat by different physical processes. If your engine only needs a source of very hot gas, then in theory you can swap one heat source for another. The extraordinary engineering difficulty comes from everything that accompanies a reactor—radiation, shielding, control systems, and safety—not from the basic idea of heating the air.

M: Okay, I mean, that's kind of how I thought it would work. I guess my question is whether or not anyone recently has looked into it, particularly in terms of weight. I think the transcriber screwed up. Please go online and see if anyone has made any kind of effort in this direction. I don't see anyone doing it as a practical business thing, but it's the kind of stuff that millionaires would be interested in doing just as a hobby, it seems. Or at least that's how it appears to me.

G: People absolutely are still thinking about it—but the focus has shifted in a way that I think is actually more interesting.

The short answer is:

The biggest change is that reactor technology has become dramatically smaller.

Back in the Cold War, a reactor that could produce enough heat for a large aircraft was enormous. Today there's an entire industry building microreactors—reactors producing anywhere from hundreds of kilowatts to a few megawatts—that are designed to be shipped by truck, rail, or even aircraft. In February 2026, the U.S. military even airlifted an unfueled 5 MW microreactor to demonstrate that these systems can be rapidly deployed. That's obviously very different from using one for propulsion, but it shows just how much reactor packaging has improved. citeturn0search0turn0search5

The catch is that the reactor shrinking wasn't actually the biggest obstacle.

The reactor itself can now be surprisingly compact.

The problems that haven't shrunk nearly as much are:

Those are still the elephants in the room.

There have been a few modern academic studies revisiting atmospheric nuclear aircraft, usually asking, "Would today's materials and small modular reactors change the answer?" The conclusion is generally that advances in reactor design, high-temperature materials, and compact heat exchangers improve the engineering picture—but shielding and safety remain the dominant issues. citeturn0search2

Your comment about millionaires is actually not far off in spirit, but the people spending large amounts of money aren't generally trying to make a nuclear airplane. Instead, they're funding things like:

Those technologies solve many of the same engineering problems—lightweight reactors, high-temperature materials, compact heat exchangers—but without having to convince regulators that a reactor should be flying over cities every day. citeturn0search8turn0search0

One thing that also surprised me when I dug into this is that weight is no longer the first thing nuclear engineers bring up. Modern reactors have made progress there. The first question today is usually:

"If this aircraft crashes into a mountain, can we guarantee the reactor and fuel remain safely contained?"

That's a much harder certification problem than simply making the airplane fly.

One place where I wouldn't be surprised to see nuclear propulsion eventually appear is not passenger aviation but very large, long-endurance unmanned aircraft—for example, strategic surveillance platforms intended to stay aloft for months. Even there, though, I don't know of any public program that is actually building such a vehicle. The regulatory, political, and safety hurdles remain enormous, even if the underlying reactor technology has become far more capable than it was during the Cold War. citeturn0search2turn0search12

M: Maybe not during this administration, but I would think that sometime recently, someone would be looking to this as an emergency aircraft at least, you know? Like for, I don't know, Air Force 4 or 5, something that would be able to carry a ridiculous amount of, well, not a ridiculous amount of weight, but fly for a ridiculous amount of time without refueling in case of a problem. One big enough that a air crash leading into contamination would have been the least of the problems at that point.

G: I think your intuition matches one of the original motivations of the Cold War program.

The U.S. Air Force wasn't trying to build a nuclear airplane because it was "green." They wanted a bomber or command aircraft that could stay airborne for days or even weeks, reducing dependence on vulnerable airfields. That mission is remarkably similar to what you're describing. citeturn0search1turn0search15

The interesting part is that modern militaries have solved that problem in different ways, so they haven't had much incentive to revisit nuclear flight.

For example, the U.S.'s airborne command-post aircraft—the so-called "Doomsday" planes—are still conventionally powered. They rely on aerial refueling. A tanker rendezvous every several hours is far easier, cheaper, and safer than flying a reactor over populated areas. The replacement entity["vehicle","E-4C Survivable Airborne Operations Center","United States Air Force"] follows the same philosophy rather than reviving nuclear propulsion. citeturn0search16

That said, there is a trend that looks like the first half of a nuclear airplane program.

The U.S. military has been investing heavily in transportable microreactors—not for aircraft propulsion, but to provide reliable power to remote bases, command centers, radar sites, and communications networks. Programs like Project Pele and its successor, Janus, are pushing reactor technology toward lighter, more compact, and more rugged designs. citeturn0news21turn0news22turn0search6

So the technology tree is progressing like this:

…but "put it in an airplane" is still missing, because the cost-benefit analysis hasn't changed enough.

One place where I think your idea becomes more plausible is if you change the mission slightly.

Imagine an unmanned strategic aircraft:

Now you've removed the crew shielding problem and greatly reduced the political consequences of losing the aircraft. That's the sort of concept that aerospace engineers occasionally speculate about, although there are no publicly acknowledged U.S. programs building one that I'm aware of. citeturn0search19turn0search1

One thing I find particularly telling is that the Air Force has continued to spend money on making its command aircraft survivable, but not nuclear-powered. That suggests the military has concluded that the combination of conventional jet engines, global tanker fleets, and multiple backup airfields is still a better trade than accepting the complexity and risks of an airborne reactor. citeturn0search16turn0search1

So I don't think the concept has been forgotten. Rather, the research has been split apart: the reactor technology is advancing under military microreactor programs, while the aviation side has continued down the path of more efficient engines and better aerial refueling. If those two trajectories ever converged—say, if reactors became dramatically lighter or there were a compelling need for aircraft to remain aloft for months rather than days—the idea could become attractive again. As of today, though, no public evidence suggests that a government or private organization is actively developing a nuclear-powered airplane for atmospheric flight.