Project Pluto was the late-1950s-early-1960s project to create a nuclear ramjet cruise missile:
In the movie Dr. Strangelove, the pilot of a low-flying B-52 assures his crew that ‘they might harpoon us, but they dang sure ain’t going to spot us on no radar screen.’ Pluto would operate on the same strategic principle. In order to reach ramjet speed, it would be launched from the ground by a cluster of conventional rocket boosters. Not until it was at cruising altitude and far away from populated areas would the nuclear reactor be turned on. 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. Relying upon the same terrain comparison guidance system (TERCOM) used by modern cruise missiles, Pluto would come in below enemy defenses to hit its targets with pinpoint accuracy. Unlike modern cruise missiles, however, one SLAM would be able to strike up to a dozen widely separated targets.Because of its combination of high speed and low altitude, Pluto promised to get through to targets that manned bombers and even ballistic missiles might not be able to reach. What weaponeers call ‘robustness’ was another important advantage. ‘Pluto was about as durable as a bucket of rocks,’ says one who worked on the project. It was because of the missile’s low complexity and high durability that physicist Ted Merkle, the project’s director, called it ‘the flying crowbar.’
(Hat tip to Boing Boing.)