Spencer & Jaffe, in 1962/3, produced this JPL study which examined the feasibility of interstellar probes, finding there was no physical reason why fusion or fission propelled multi-stage vehicles could not approach the speed of light. With five stages and a mass-ratio of ~240,000, a D-He3 fusion propelled vehicle could reach ~0.8 c. However the initial mass would of necessity be very large and the engine power levels required would be unlike anything in previous experience. However the physics was clear, interstellar travel was feasible. But how fast would a probe accelerate to burn-out speed? Heating limitations meant the acceleration that could be achieved would be low – a nominal 5 light-year mission would require five stages and 55 years. Fusion plasma containment vessels and high-temperature radiators could only handle so much waste heat from the reactions – about 20% would be lost as chiefly x-rays via bremmstrahlung.
Dwain Spencer explored the characteristics of fusion-propelled probes in a more focussed piece:
Fusion Propulsion System Requirements for An Interstellar Probe
His work reinforced the original finding that fusion propelled probes would be hampered by heat-rejection issues. In one sense nothing has changed since, as magnetic confinement fusion reactors inherently absorb a large fraction of waste heat from the reactions they contain. Newer materials might reduce the intercepted x-rays, instead returning them to the fusion plasma, but waste heat needs to be efficiently handled for fusion rockets to achieve reasonable voyage times.
Freeman Dyson, based on his work on “Project Orion”, led him to sketch a high performance interstellar version of “Orion” with a wholly different relationship to reaction heat. Instead of absorbing and re-radiating the heat, blow it away with the propellant in a massive thrust, with a very short heating period. Interstellar “Orion” used pure deuterium fusion devices with very high burn-up fractions (“burn-up” is the fraction of fusion fuel that actually fuses) which allowed high-performance. Dyson argued, for reasons of energy efficiency, that the mission velocity be kept to a low multiple of the effective exhaust velocity – his fusion devices had an exhaust velocity of 15,000 km/s (0.05c) and his “Orion” had a mass-ratio of just 4, meaning a total delta-vee of ~20,000 km/s. This meant trips to Alpha Centauri lasting ~130 years. A slightly higher mass-ratio would drop that to just 100 years – “Project Icarus” time-frames.