Helios is a design for a spacecraft

propulsion system Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...

such that small (0.1 kiloton)

nuclear bombs A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...

would be detonated in a chamber roughly 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 Project Orion (nuclear propulsion), 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 originally conceived by Freeman Dyson.

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