In a March 1, 2018 speech before Russia's Federal Assembly, Russian President Vladimir Putin discussed new strategic weapons being developed to counter United States ballistic missile defenses. Two of these weapons are allegedly nuclear powered: a previously revealed intercontinental-range nuclear torpedo and a cruise missile. As Putin described them:

Russia’s advanced arms are based on the cutting-edge, unique achievements of our scientists, designers, and engineers. One of them is a small-scale, heavy-duty nuclear energy unit that can be installed in a missile like our latest X-101 air-launched missile or the American Tomahawk missile—a similar type but with a range dozens of times longer, dozens—basically an unlimited range. It is a low-flying stealth missile carrying a nuclear warhead, with almost an unlimited range, unpredictable trajectory and ability to bypass interception boundaries. It is invincible against all existing and prospective missile defense and counter-air defense systems.

Defense and nuclear disarmament experts did a double take. "I'm still kind of in shock," Edward Geist, a Rand Corporation researcher specializing in Russia, told NPR. "My guess is they're not bluffing, that they've flight-tested this thing. But that's incredible."

This is not the first time a government has worked on a nuclear-powered strategic weapon. Decades ago, the US developed engines first for a proposed nuclear-powered bomber and then for a hypersonic nuclear cruise missile. The US has also examined nuclear-powered rockets for space flight (that crazy Project Orion thing is a story for another time). These programs were all dropped, not because they didn't work but because they were deemed impractical.

Oh... and there was always that small problem of radiation spewing from the engine exhaust.

So when Putin announced that Russia has tested the cruise missile engine successfully, it got us thinking about those past experiments in nuclear propulsion. Is it actually possible to create a nuclear reactor small enough and powerful enough to propel a cruise missile? We broke out our calculators, checked some engine ratings, and asked some experts in the field of nuclear physics.

Not everyone is sure that Russia is really this far along in developing a nuclear-powered cruise missile. But there's plenty of evidence that they're trying hard. An unnamed Defense Department source told Fox News recently that Russia had already crashed several test missiles in the Arctic. Other sources have suggested that the engines are still in testing and have thus far only been run with electric power.

Nuclear-powered flight is certainly possible, but it's a terrible idea for all sorts of reasons. To understand how possible, yet horrible, it is, let's consider the history of this absolutely insane but perfectly workable idea.

Blame Enrico Fermi

The whole adventure into flying nuclear reactors began in 1942.

"The use of nuclear energy for aircraft and rocket propulsion was discussed almost from the time of achievement of the first fission chain reactor," physicist Robert Bussard and R.D. DeLaur wrote in Fundamentals of Nuclear Flight, "by Enrico Fermi and his associates in the wartime Manhattan District project." Once relocated to Los Alamos, Fermi and company kept speculating about uses for atomic energy other than in bombs. (One eventual use: powering the nuclear merchant ship Savannah.)

Before we had any idea about this thing called "fallout," nuclear power made a lot of sense for aircraft, because nothing else can generate the power density of a nuclear reaction. Most applications of nuclear energy simply swap a nuclear reaction out for whatever they previously used as a source of thermal energy. In nuclear power plants and shipboard nuclear propulsion, for example, fission took the place of coal and oil burned to turn water into the steam used to spin turbines. (Or, as we used to say in the Navy, "Hot rock make boat go.") The same principle, in theory, works for multiple types of aircraft propulsion, but getting past the weight-to-thrust ratio required for flight would require making reactors lighter and more compact.

By 1946, Fermi's nuclear airplane idea had evolved into a full-blown program funded by the military and called the Nuclear Energy for the Propulsion of Aircraft (NEPA) Project. This was a $10 million feasibility study contracted by the Army and Air Force to Fairchild Engine and Airplane Corporation (a bargain by current DOD standards, even in current dollars).

A group of MIT scientists brought in by the Atomic Energy Commission (AEC, the forerunner of the Department of Energy) to review the work in 1948 estimated that, while it was possible to build a nuclear aircraft engine, it would probably take "at least 15 years at a cost in excess of $1 billion." But these scientists also said that if the government felt the cost was warranted, it should double down and move quickly to get a full development program under way.

In 1951, the research from NEPA was rolled into the Aircraft Nuclear Propulsion (ANP) program, a joint AEC/US Air Force effort. ANP focused on what the MIT review had said was the easiest target to achieve: a nuclear-powered turbojet engine for a manned aircraft.

So, in a prelude to the many money sinks in defense procurement that would follow, Fermi's spitballing became a series of US Air Force and AEC projects that would span three decades. These initiatives would expend well over that $1 billion figure on multiple efforts—and produce zero nuclear-powered aircraft.