Nuclear-powered ramjet

Project Pluto was a nuclear-powered ramjet, intended for use in a cruise missile. Rather than combusting fuel as in regular jet engines, air was heated using a high-temperature, unshielded nuclear reactor. This raised the specific impulse of the engine by stupendous amounts, and the ramjet was predicted to be able to fly for months at supersonic speeds (Mach 3 at tree-top height). However, there was no obvious way to stop it once it had taken off, which is a great disadvantage. Unfortunately, because the reactor was unshielded, it was dangerous to be in or around the flight path of the vehicle (although the exhaust itself wasn't radioactive).