On August 29, 1949, the Soviet Union conducted its first-ever atomic weapons test, thus ending America’s monopoly on the most destructive weapon system ever conceived by man. An arms race that had already begun immediately kicked into high gear, with both nations working frantically to develop new weapons and capabilities that were powerful enough to keep the opposition in check.

From our modern vantage point, the Cold War between America and the Soviet Union seems like an exercise in overblown budgets and paranoia. But it’s important to remember the context of the day. Many senior leaders in both Washington and Moscow had seen not one but two World Wars unfold during their lifetimes. After the uneasy alliance between the Soviet Union and the rest of the Allied Nations failed to last beyond the final shots of World War II, many believed a third global conflict would be coming in short order. And terrifyingly, most — including those with their fingers on the proverbial nuclear buttons — believed it would begin with a nuclear exchange.

Although the destructive force of the atom bombs dropped on Hiroshima and Nagasaki had been so monstrous that they changed the geopolitical landscape of the world forever, both the U.S. and Soviet Union immediately set about developing newer, even more powerful thermonuclear weapons. Other programs, ranging from ballistic missiles to unguided bombs, sought new and dynamic delivery methods for these powerful nukes.

Project Pluto and the SLAM Missile

One such effort under the supervision of the U.S. Air Force was a weapon dubbed the Supersonic Low Altitude Missile or SLAM (not to be mistaken for the later AGM-84E Standoff Land Attack Missile). The SLAM missile program was to utilize a ramjet nuclear propulsion system being developed under the name Project Pluto. Today, Russia is developing the 9M730 Burevestnik, or Skyfall missile, to leverage the same nuclear propulsion concept.

As Russian President Vladimir Putin recently pointed out, nuclear propulsion offers practically endless range, and estimates at the time suggested the American SLAM Missile would likely fly for 113,000 miles or more before its fuel was expended. Based on those figures, the missile could fly around the entire globe at the equator at least four and a half times without breaking a sweat.

The unshielded nuclear reactor powering the missile would practically rain radiation onto the ground as it flew offering the first of at least three separate means of destruction the SLAM missile provided. In order to more effectively leverage the unending range of the nuclear ramjet, the SLAM missile was designed to literally drop hydrogen bombs on targets as it flew. Finally, with its bevy of bombs expended, the SLAM missile would fly itself into one final target, detonating its own thermonuclear warhead as it did. That final strike could feasibly be days or even weeks after the missile was first launched.

Over time, the SLAM missile came to be known as Pluto to many who worked on it, due to the missile’s development through the project with the same name.

Nuclear Propulsion

The nuclear ramjet developed for SLAM under Project Pluto was designed to draw in air from the front of the vehicle as it flew at high speed creating a significant amount of pressure. The nuclear reactor would then superheat the air and expel it out the back to create propulsion. This ramjet methodology is still in use in some platforms today and plays a vital role in some forms of hypersonic missile programs.

The onboard nuclear reactor produced more than 500-megawatts of power and operated at a scorching 2,500 degrees — hot enough to compromise the structural integrity of metal alloys designed specifically to withstand high amounts of heat. Ultimately, the decision was made to forgo metal internal parts in favor of specially developed ceramics sourced from the Coors Porcelain Company, based in Colorado.

The downside to ramjet propulsion is that it can only function when traveling at high speeds. In order to reach those speeds, the SLAM would be carried aloft and accelerated by rocket boosters until the missile was moving fast enough for the nuclear ramjet to engage. Once the nuclear ramjet system was in operation, the missile could remain aloft practically indefinitely. This would allow it to engage multiple targets and even avoid interception.

The nuclear-powered ramjet was so loud that the missile’s designers theorized that the shock wave of the missile flying overhead on its own would likely kill anyone in its path — and if not, the gamma and neutron radiation from the unshielded reactor sputtering fission fragments out the back probably would. While this effectively made the missile’s engine a weapon in its own right, it also made flying the SLAM over friendly territory impossible.

A Missile Carrying 16 Hydrogen Bombs

While the doctrine of Mutually Assured Destruction made the launch of just one nuclear weapon the start of a cascade that could feasibly end life on Earth as we know it, Project Pluto’s SLAM Missile was practically apocalyptic in its own right. The nuclear powerplant that would grant the missile effectively unlimited range would also potentially kill anyone it passed over, but the real destructive power of the SLAM missile came from its payload.

Unlike most cruise missiles, which are designed with a propulsion system meant to carry a warhead to its target, Project Pluto’s SLAM carried not only a nuclear warhead but 16 additional hydrogen bombs that it could drop along its path to the final target. Some even suggested flying the missile in a zig-zagging course across the Soviet Union, irradiating massive swaths of territory and delivering its 16 hydrogen bombs to different targets around the country.

Doing so would not only offer the ability to engage multiple targets but would almost certainly also leave the Soviet populace in a state of terror. A low-flying missile spewing radiation as it passed over towns, shattering windows, and deafening bystanders as it delivered nuclear hellfire to targets spanning the massive Soviet Union would likely have far-reaching effects on morale.

How Do You Test an Apocalyptic Weapon?

Project Pluto’s nuclear propulsion system made testing the platform a difficult enterprise. Once the nuclear reactor onboard was engaged, it would continue to function until it hit its target or expended all of its fuel. Any territory the weapon passed over during flight would be exposed to dangerous levels of radiation, thus limiting the ways and the places in which the weapon’s engine could even be tested.

On May 14, 1961, engineers powered up the Project Pluto propulsion system on a train car for just a few seconds. A week later a second test saw the system run for a full five minutes. The engine produced 513 megawatts of power, which equated to around 35,000 pounds of thrust — 6,000 pounds more than an F-16’s Pratt & Whitney F100-PW-229 afterburning turbofan engine with its afterburner engaged.

However, those engine tests were the only large scale tests Project Pluto would ultimately see, in part, because a fully assembled SLAM missile would irradiate so much territory that it was difficult to imagine any safe way of actually testing it.

A weapon That’s Too Destructive to Use

Ultimately, Project Pluto and its SLAM missile were canceled before ever leaving the ground. The cancellation came for a litany of reasons, including the development of intercontinental ballistic missiles and the introduction of global strike heavy payload bombers like the B-52 Stratofortress. There were, however, some other considerations that led to the program’s downfall.

Because the SLAM would irradiate, destroy, or deafen anyone and anything it flew over, the missile could not be launched from U.S. soil or be allowed to fly over any territory other than its target nation. That meant the missile could really only be used from just over the Soviet border, whereas ICBMs could be launched from the American midwest and reach their targets in the Soviet Union without trouble.

There was also a pressing concern that developing such a terrible weapon would likely motivate the Soviet Union to respond in kind. Each time the United States unveiled a new weapon or strategic capability, the Soviet Union saw to it that they could match and deter that capability. As a result, it stood to reason that America’s nuclear-spewing apocalypse missile would prompt the Soviets to build their own if a U.S. one entered into service.

Project Pluto and its SLAM missile program were canceled on July 1, 1964

This article was written by Alex Hollings and originally published on Sandboxx.