In the darkest years of the Cold War, American engineers designed a weapon so appalling that many of the people building it were relieved when it was cancelled. It was a cruise missile with an unshielded nuclear reactor for an engine, and it was meant to fly for weeks.

The engine program was called Project Pluto. The missile was SLAM, the Supersonic Low Altitude Missile. Its own director nicknamed it the flying crowbar.

It never flew a single mission. This is the story of a machine that was, quite deliberately, too terrible to build.

A ramjet with a reactor for a heart

A ramjet is the simplest jet engine there is: it has no spinning blades, just a tube shaped so that air rammed in at high speed is compressed, heated, and blasted out the back. Normally you heat that air by burning fuel. Project Pluto's engineers had a darker idea. They would heat it with a nuclear reactor.

Strip out the fuel tanks and a nuclear-powered aircraft can fly almost forever. That was the whole point. SLAM could loiter for weeks, then dash toward its targets at three times the speed of sound, so low that no 1960s radar or interceptor could touch it.

The flying crowbar

The plan was as ruthless as it was simple. SLAM would race across enemy territory at rooftop height, dropping hydrogen bombs one after another along a winding route, hitting city after city on a single mission.

Its blunt, purposeful shape earned it a grim nickname from the project's own director, and the name stuck because it was accurate: this was not an elegant weapon, it was a club.

A weapon that killed just by passing

Here is the part that horrified even its creators. The reactor at the heart of SLAM had no shielding to save weight, so the missile left a trail of radioactive exhaust everywhere it flew. Its passage at low level produced a shockwave and a noise so violent that it could injure people on the ground.

In other words, SLAM could kill you without dropping a thing. Just having it scream overhead, spewing fallout and a deafening boom, was a weapon in itself. That made it almost impossible to test, because where do you fly a machine that poisons everything beneath it?

Tory roars in the desert

The reactor itself was real, and it worked. Engineers built test units called Tory-IIA and Tory-IIC and ran them at a remote site in the Nevada desert, bolting them to railcars because letting one fly was unthinkable.

In 1964, Tory-IIC roared to full power for a few minutes, proving that a nuclear ramjet could actually breathe fire. It was a genuine engineering triumph, attached to a genuinely monstrous idea.

Too terrible to deploy

By mid-1964 the ground had shifted. Intercontinental ballistic missiles had arrived, delivering warheads faster and more cheaply than any low-flying nuclear cruise missile ever could. And SLAM raised a queasy question its planners could not answer: a weapon this provocative, this uncontrollable in testing, might be more dangerous to its owners than to the enemy.

In July 1964 Project Pluto was cancelled. The flying crowbar never left the ground, and more than a few of the people who built it were quietly glad. Some weapons, it turns out, are so effective that even a superpower flinches.

Sources: Wikipedia, Fourmilab Project Pluto archive, Air & Space