In the 1960’s U.S. Government laboratories, under Project Orion, investigated a pulsed nuclear fission propulsion system. Small nuclear pulse units would be sequentially discharged from the aft end of the vehicle. A blast shield and shock absorber system would protect the crew and convert the shock loads into a continuous propusive force.
In his book and companion series Cosmos, Carl Sagan discussed Project Orion, describing it as “providing a kind of putt-putt, a kind of nuclear motor boat in space.” He expounded Orion as the best use of nuclear weapons, “provided they do not depart from very near the Earth,” in order to reduce the risk of fallout.
The 1963 Partial Nuclear Test Ban Treaty, however, killed the Orion Project, as it “prohibits nuclear weapons tests ‘or any other nuclear explosion’ in the atmosphere, in outer space, and under water.”