On the afternoon of 14 May 1961, in a tin control shed on a stretch of Nevada desert called Jackass Flats, a group of Lawrence Livermore engineers held their breath. Two miles away, bolted to a railroad flatcar and painted fire-engine red, sat a machine no one had ever dared switch on before: a nuclear reactor designed not to make electricity, but to breathe. When the valves opened and a ton of pressurised air per second slammed through its white-hot ceramic core, the world’s first nuclear ramjet roared to life.

It ran for only a few seconds, at a fraction of its rated power. It did not catch fire, as some Atomic Energy Commission officials had quietly feared. And it proved that the most extreme weapon concept of the Cold War — a missile that could fly essentially forever — was physically buildable.

The weapon was called SLAM, the Supersonic Low Altitude Missile. The reactor program was Project Pluto. The engineers, with the dark humour of people building the apocalypse, called the missile itself the flying crowbar.

A Missile With Unlimited Range

The physics is seductively simple. A ramjet has no moving parts worth mentioning: air rams in at the front, is heated, expands, and blasts out the back as thrust. Every ramjet before and since has heated that air by burning fuel — which means range is limited by tankage. Pluto’s designers replaced the burner with an unshielded 500-megawatt-class nuclear reactor. As long as the core stayed critical, the engine ran. Range stopped being a number and became a question of patience.

The operational concept made full use of it. Boosted to ramjet speed by conventional rockets, SLAM would loiter for weeks in circles over the ocean, riding out the opening hours of a nuclear war. On command, it would drop to treetop height and accelerate to Mach 3 — below every Soviet radar horizon — following terrain-comparison guidance of the kind later used by cruise missiles. Along its route it would toss out a dozen or more thermonuclear weapons over widely separated targets.

And the missile itself was a weapon three times over. Its shock wave at treetop height was calculated to injure people below; the noise was estimated at around 150 decibels on the ground; and the unshielded reactor sprayed neutron and gamma radiation, plus fission fragments in its exhaust, over everything it overflew. Built like “a bucket of rocks,” in one engineer’s words, it could in principle keep flying its pattern after its bombs were gone — a radioactive lawnmower at Mach 3.

Ceramics From a Beer Company

The engineering problem was savage. A flight reactor had to be compact enough to fly, survive aerodynamic loads estimated at five times those of the X-15, and run at temperatures — around 1,370 °C in the core airstream — that turn high-temperature alloys to taffy. Director Ted Merkle’s answer was ceramics: nearly half a million pencil-sized ceramic fuel elements, manufactured to extraordinary tolerances by the Coors Porcelain Company of Colorado — yes, that Coors; the porcelain works predated the brewery.

The margins were terrifying even on paper. The reactor’s base plates had an auto-ignition point only about 80 °C above peak operating temperature. Control motors had to work red-hot in an intense radiation field. When exotic coatings failed on electric-motor armatures, the fix that finally worked was exhaust-manifold paint ordered through an advert in Hot Rod magazine.

Testing the thing required a private industrial empire in the desert. Site 401 at Jackass Flats got a remote-controlled disassembly building with concrete walls up to eight feet thick — the government bought an aggregate mine just to supply it — and a fully automated railway to carry the lethally radioactive reactor between test stand and teardown bay. Simulating Mach 3 flight took a million pounds of compressed air stored in 25 miles of welded oil-well casing, blown over 14 million heated steel balls to bring it to ramjet temperatures.

Five Minutes at Full Power

Tory II-A’s brief 1961 run proved the principle. Its successor, Tory II-C, was the real thing: a lighter, flight-representative reactor. On 20 May 1964 it ran at full power for five minutes — roughly 500 megawatts, with published figures between 461 and 513 MW — producing the equivalent of about 35,000 pounds of thrust while dozens of AEC officials and Air Force generals watched from a safe distance. Less radiation escaped in the exhaust stream than predicted. The nuclear ramjet worked.

The celebration was pure Livermore: the team loaded a “borrowed” piano onto a flatbed truck and drove to the nearest bar in Mercury, Nevada, with Merkle at the keyboard. The next morning the celebrants lined up at the medical tent for vitamin B-12 shots, then the era’s preferred hangover cure.

This declassified 1963 progress film from the program shows the Tory hardware and the Jackass Flats facility as the engineers saw it:

The Question Nobody Could Answer

Six weeks after Tory II-C’s triumph, on 1 July 1964, the Air Force and AEC cancelled Project Pluto. The reasons stacked up fast. Atlas and Titan ICBMs now reached their targets in 30 minutes; the flying crowbar would arrive hours later, prompting critics to joke that SLAM really stood for slow, low and messy. At a projected $50 million per missile, it was ruinously expensive. And its flight path was a diplomatic impossibility: to reach the Soviet Union undetected it would deafen, flatten and irradiate America’s own allies on the way in — before dropping a single bomb.

The killing question, though, was simpler. Where do you flight-test a reactor with no shielding and no pilot?

One serious proposal was to fly it in figure-eights near Wake Island and then sink the radioactive carcass in 20,000 feet of Pacific. Another suggested a very long tether in Nevada. Neither survived contact with common sense. Pluto had become, in historian Gregg Herken’s phrase, a weapon without a mission — too provocative to deploy, too dangerous to test, too capable to be anything but destabilising.

Total program cost: about $260 million over seven and a half years. Merkle hosted a “last supper” at a country club near Livermore and handed out SLAM tie tacks and bottles of Pluto mineral water. The Tory II-C reactor sat at Jackass Flats until 1976 before it was finally taken apart.

The Crowbar’s Long Shadow

Project Pluto refuses to stay buried. Its ceramic-materials work fed into later turbine and space-reactor research. And in 2018, Russia unveiled the 9M730 Burevestnik — a nuclear-powered cruise missile of essentially the same theology, six decades on, with the same unanswered question about what its test failures scatter downrange.

The American engineers of 1964 had their answer ready all along. They built the most relentless engine ever run on Earth, watched it work flawlessly — and concluded that some machines are simply too dangerous to fly. That conclusion, more than the 500 megawatts, may be Project Pluto’s real achievement.

Sources: Gregg Herken, “The Flying Crowbar,” Air & Space Magazine (1990); US DOE/NNSS Project Pluto fact sheet; Air & Space Forces Magazine; Lawrence Livermore historical records; Wikipedia (Project Pluto, SLAM).