https://en.wikipedia.org/wiki/Project_Pluto
The proposed use for nuclear-powered ramjets would be to power a cruise missile, called SLAM, for Supersonic Low Altitude Missile. In order to reach ramjet speed, it would be launched from the ground by a cluster of conventional rocket boosters. Once it reached cruising altitude and was far away from populated areas, the nuclear reactor would be made critical. Since nuclear power gave it almost unlimited range, the missile could cruise in circles over the ocean until ordered "down to the deck" for its supersonic dash to targets in the Soviet Union. The SLAM as proposed would carry a payload of many nuclear weapons to be dropped on multiple targets, making the cruise missile into an unmanned bomber. After delivering all its warheads, the missile could then spend weeks flying over populated areas at low altitudes, causing tremendous ground damage with its shock wave and radiation from its unshielded reactor. When it finally lost enough power to fly, and crash-landed, the engine would have a good chance of spewing deadly radiation for months to come.
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",[2] and it was believed that it would compel the Soviets 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.
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