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Project Pluto: The Nuclear-Powered Cruise Missile

Nuclear power – carbon-free, hugely efficient, and catastrophically dangerous when it goes wrong. Today, the world is a bit iffy on nuclear power, whether the risks outweigh the benefits and if we can deal with climate change without it, but back in the day nuclear power wasn’t as controversial. Granted, that was partly because we hadn’t fully internalized what too much gamma radiation does to a person, but that wasn’t the only reason. Just like how airplanes were initially seen as the first step towards everyone having their own personal helicopter, nuclear power was initially met with a great deal of optimism: it was seen as the energy of the future, and that one day it would power everything – cars, planes, your toaster, and more.

We’ve covered before some of the truly ridiculous manners in which scientists in the 50s and 60s attempted to find new applications for nuclear power, from building a pointless harbor in Western Alaska to giant flying aircraft carriers. This is another one of those ridiculous things, but perhaps slightly less ridiculous overall – and yes, it’s another military project. This is the story of Project Pluto, the time the United States tried to build nuclear missiles that ran on nuclear power.

Novel Ideas

Our story begins in the late 1950s, when the US Air Force and the US Atomic Energy Commission asked the Lawrence Radiation Laboratory in Berkely, California, to “study the feasibility of applying heat from nuclear reactors to ramjet engines”. Now, if you don’t know how nuclear reactors orramjet engines work, this might make things a little confusing, so here’s your two-minute science lesson for the day.

A nuclear reactor, very basically, shoots neutrons at unstable elements like uranium or plutonium. This causes the elements to absorb those neutrons, which causes their atoms to split. This split creates two lighter elements, known as fission products, and, crucially, more neutrons. This means that if you have enough material lined up in a specific way, you only need one neutron to split a lot of atoms, like a line of dominos hitting each other. This process generates heat – a lot of it. In a nuclear power plant, this heat is used to boil water into steam, which then turns a bunch of steam turbines, which then creates electricity. Genius.

Reactor hall in Forsmark nuclear power plant
Reactor hall in Forsmark nuclear power plant. By Vattenfall, is licensed under BY-NC-ND

As for jet engines, they essentially take cold air and very rapidly turn it into hot air, which expands and shoots out behind them, causing the engine to go forwards. Ramjet engines work by using the forward motion of the engine to pull in and compress air; basically, when a ramjet engine is going fast enough, the air will just fly into it and compress without needing any extra parts like a normal jet engine would. That’s the “ram” part of “ramjet”.

To summarize the details that are important to us, nuclear reactors create heat, and ramjet engines need hot air to go forward. So, the Air Force and Atomic Energy Commission looked at these two side by side and took the logical next step in concluding that these two pieces of engineering were just made for each other.

Jokes aside, nuclear propulsion in missiles, if it was feasible, offered some key benefits. Because of the efficiency of nuclear power, a nuclear-powered missile in effect has infinite range. Estimates at the time said that a nuclear-powered cruise missile could, well, cruise for up to 113,000 miles before its fuel ran out. For context, the circumference of the Earth is just under 25,000 miles. Sure, ballistic missiles can reach anywhere on Earth, too, but because cruise missiles fly low to the ground, they’re practically undetectable by comparison.

So, the rewards for a successful design of a nuclear powered nuclear missile are clear, which explains why the project went ahead. How did they do? Let’s find out.

Novel Problems

Initial testing of the idea was carried out at Livermore, California, before the work was moved to Nevada at a brand new 8 square mile test site, built for $1.2 million in a place called (and yes, this is real) Jackass Flats. Nevada, if you didn’t already know, is basically the woods behind America’s house where they light off all their dangerous imported fireworks. If America’s military has a weapon, it was probably tested in Nevada.

So, the work was moved to this new location, known as Site 401, where the team behind this project set about solving the insane problems that came with launching a nuclear-powered cruise missile. As with most things engineering, the concept was simple enough – air is drawn in through the front of the engine under ram pressure, a nuclear reactor heats the air, and then the hot air shoots out the back and provides thrust. Simple, right?

Not simple. For starters, the project had specified that the design would be for Supersonic Low-Altitude Missiles, or SLAM missiles, and I honestly can’t think of a better name for a missile than SLAM. Comical names aside, the fact that these missiles would need to maintain a constant speed in low-altitude conditions meant that the reactor could reach temperatures of 2500 degrees Fahrenheit. That’s a big problem, because that kind of temperature is just going to melt the missile, and it goes without saying that missiles that melt before they hit their targets are not very good missiles.

The solution to this problem came from forgoing metal parts altogether for the internal structure. Instead, specially developed ceramics from the Colorado-based Coors Porcelain Company were used, and they were presumably resistant to the extreme heat of the reactor. We’d love to give you more details on that, but there’s not much publicly available information on this project, either because it’s still classified or because no one’s given it any thought for sixty years. Oh well.

The second problem that presented itself was that the reactor, unlike those in power plants, was completely open to the atmosphere. This meant that as the reactor burned through its fuel, it would expel radioactive particles into the air as the missile flew. This problem… wasn’t solved, because again, we didn’t really pay attention to what gamma radiation did to people back then. In addition, those involved figured that in the event of a nuclear war breaking out between the US and the USSR, the fact that their missiles might cause a bit of cancer as they made their way to blow up an entire city… well, let’s just say it wasn’t that high on the priority list.

The last and simplest problem was that ramjet engines, by design, can’t be used to launch anything. No motion, no thrust. That was easier to solve – they could just attach some regular rockets to the sides to get it in the air so the ramjet could work. This was a tried and tested solution; the British ‘Bloodhound’ missile, in use up until the 1990s, operated with a similar design.

With all of that in mind, here is the final version that they came up with. The missile, a SLAM, would be launched from the ground with a cluster of rocket boosters attached to the side, which would get it into the air and up to ramjet speed. Once it reached cruising altitude, the nuclear reactor would kick in, and the missile would then cruise in circles over the ocean due to its effectively unlimited range. Then, when it received a target, it would alter its course to reach it. But here’s a major change that was made – the missile would carry multiple nuclear warheads that it would drop onto targets as it flew. In effect, this missile was now an unmanned bomber. After it had dropped all of its warheads, the missile would finally cruise to its last target, where it would impact and detonate its own nuclear warhead.

What an intriguing design, if you set aside the terror that comes with a nuclear apocalypse. And, funnily enough, the initial concept actually worked. On May 14, 1961, the nuclear ramjet engine was successfully tested for a few seconds, and three years later, an upgraded version ran for a full five minutes. Thus, the concept was proven – nuclear fission can successfully power a cruise missile. But despite this success, it simply was not meant to be.

Pluto Isn’t a Planet

After these tests, the Pentagon decided to cancel the project. They had a number of reasons, the most obvious being the “raining nuclear radiation on everything underneath it” thing. But aside from that, the weapon was, ironically, considered to be too good. Specifically, the Pentagon believed that the weapon was “too provocative”, and that adopting it into widespread use would compel the Soviet Union to do the same, and there was no theoretical counter at the time to such a weapon. In addition to this, ICBM technology turned out to be easier than expected. Not easy, just easier; it’s literal rocket science, after all. Because of this, there was suddenly not much of a need for a nuclear-powered cruise missile.

And so, on July 1, 1964, exactly seven years and six months after the project was begun, Project Pluto was cancelled. But that wasn’t the end of the idea, for in 2018, Vladimir Putin announced that Russia was developing their own nuclear-powered cruise missile, called the Burevestnik, or “Petrel”. In Russian, the word literally translates to “Storm-bringer”, a reference to an old sailor’s legend that petrels are harbingers of storms. And then, one year later, a test missile exploded and killed several scientists working on it. Probably not the storm they were thinking of when they named it.

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