Helios is a design for a spacecraft propulsion system such that small (0.1 kiloton) nuclear bombs would be detonated in a chamber roughly 130 feet in diameter. Water would be injected into the chamber, super-heated by the explosion and expelled for thrust. It was a precursor concept to the Orion project. Like Orion, it would have achieved constant acceleration through rapid "pulsed" operation.
This design would have yielded a specific impulse of about 1150 seconds (compared to a modern chemical rocket’s 450 seconds). However, a number of technical problems existed, most prominently how to keep the combustion chamber from exploding from the great pressures of the atomic detonations.
The Helios propulsion system was conceived originally by Freeman Dyson.