An interesting weapon system that never made it out of testing, though not because it wasn’t workable.
This article from February 1960 Popular Science magazine:
An A-powered missile, on the way for the Air Force, promises the U. S. its most formidable weapon
ALREADY the U. S. is grooming the weapon that may prove the successor to the ICBM. Its name is Pluto. An air-breathing ramjet missile of unlimited range, it will fly on atomic power. It will carry an H-bomb to any target on earth, with pinpoint accuracy, and will defy interception. It can double as a pilotless reconnaissance vehicle.
Preliminary studies have shown that the awesome weapon can be built, and now it’s official that Pluto will be developed as a weapons system for the Air Force. The newly revealed AF program is called Project SLAM, short for “Supersonic Low-Altitude Missile system.”
Much about Pluto is cloaked in secrecy – but facts so far released, on which our artist bases his conception of what it may look like, reveal its potency:
Its secret speed will be many times as fast as sound, requiring a cooling system for its skin. Instead of plunging out of space upon a target like a ballistic missile, it will skim the earth to its objective. Coming in “under the fence” set up by radar-warning nets, it will elude detection until too late for interception.
Chemical-rocket boosters will launch Pluto. Then its A-engine will take over. An array of guidance systems will come successively into play – beginning with inertial guidance, and ending with the advanced map-matching system illustrated above. Probably radar maps will be used. They will serve day and night, and, if map-making flights over the territory are impossible, relief models photographed by radar technique give “synthetic” radar maps almost as good.
Preparing Jnr the try-out. The Air Force has been conducting its Pluto study jointly with the Atomic Energy Commission, whose key part is to develop Pluto’s A-engine – a reactor that must operate at thousands of degrees. For trials, a safely remote proving ground has been established in Nevada, adjoining our A-bomb test site.
Following successful small-scale experiments, an AEC team headed by Dr. Theodore C. Merkle of Livermore Laboratory is now preparing to demonstrate a sizable A-engine called Tory II. Although a non-flying version, it’s said to be a scale model of the one for Pluto, and is a powerful brute itself. An output of 10,000 kilowatts of heat is only an intermediate step in bringing Tory II up to its secret “full power.”
An AEC drawing now gives a glimpse of this remarkable reactor. An air intake corresponds to a flying ramjet’s air scoop. Heated on the way through passages in the reactor, the air issues from a nozzle, as from a ramjet’s tailpipe. Instead of control rods, Tory II’s novel main controls are eight rotating cylinders of boron steel and graphite, which turn through 180 degrees to unleash or check its power rapidly.
Pluto has already proven “feasible,” and progress on it is going forward “rapidly,” sums up AEC Chairman John A. McCone. Adds Dr. Merkle, “I think it will put the ICBMs out of business.” -Alden P. Armagnac.
As always, there’s a Wikipedia article about it as well:
On May 14, 1961, the world’s first nuclear ramjet engine, “Tory-IIA,” mounted on a railroad car, roared to life for a few seconds. Three years later, “Tory-IIC” was run for five minutes at full power. Despite these and other successful tests the Pentagon, sponsor of the “Pluto project,” had second thoughts. The weapon was considered “too provocative”, and it was believed that it would compel the USSR to construct a similar device against which there was no known defense. Intercontinental ballistic missile technology had proven to be more easily developed than previously thought, reducing the need for such highly capable cruise missiles. On July 1, 1964, seven years and six months after it was started, “Project Pluto” was canceled.