In the 1950s and 60s the U.S. Air Force tested flying rockets and ramjets powered by nuclear reactors.
If it weren't for breakthroughs in chemical propulsion that occurred at about the same time, the nuclear missiles based throughout the U.S. and Europe could well be nuclear warheads sitting atop nuclear reactors.
The idea of a nuclear-powered rocket engine actually dates back to World War II. But research didn't begin until 1955 when teams at both Los Alamos and Livermore national laboratories were tasked with figuring out the basic engineering necessary for the project.
At its most basic level, rockets and ramjets work by superheating air and propelling it out the back of the engine. Conventional rockets and ramjets use chemical combustion to heat the air. The nuclear engines were designed to superheat the air using a heat exchanger hooked to a nuclear reactor.
The research into the rocket engines was dubbed "Project Rover," and the ramjet research was dubbed "Project Pluto."
Rover began tests in 1957, and a full-powered test in 1959 was a huge success. This led to an expansion of the program and the construction of new reactors, all of which underwent ground testing. The tests were largely successful, but extreme vibrations in some of the test engines caused a slowdown in progress through the 1960s.
Project Rover's initial successes allowed NASA engineers to briefly consider nuclear power for the first manned missions to the Moon, but the vibration problems were not worked out in time. Rover's engine then got the nod for a possible mission to Mars, but the mission was canceled. Without any immediate mission requirements, Rover was declared a technical success and shut down.
The nuclear rocket engine idea has been revived a few times since then, mostly when engineers start to seriously strategize manned missions to Mars. It was also briefly revived as a method of getting the Strategic Defense Initiative ballistic missile shield into orbit.
Project Pluto was even more successful from a technical standpoint. Each ramjet test reactor achieved every one of its major goals, and a number of the tests were declared flawless.
Like the Rover, all the tests were conducted on the ground. Also like Project Rover, Pluto was shut down in favor of chemical propulsion. America had found a way to strike the Soviet Union from across the world without having to fly nuclear reactors over their own land and troops.
Both concepts and their accompanying research are mothballed, waiting for a mission to potentially revive them.