Robert Bussard (1928-2007) was one of the original Nuclear Rocket Scientists, a nuclear physicist responsible for some major ideas in Space Propulsion, such as the Interstellar Ramjet in 1959. He worked on taming fusion energy from the late 1970’s to 2007. Fusion powered propulsion for high-performance interplanetary spacecraft systems thus featured in many of his later papers.
This Crowlspace episode focusses on one early design from 1988, that uses a “traditional” thermal fusion rocket system that uses fusion energy to add heat to hydrogen propellant. In the 1980’s Bussard focussed on a “cheap” tokamak reactor design, the Riggatron. Initially designed for the Deuterium-Tritium reaction, the Riggatron could be repurposed for the more difficult to start Deuterium-Deuterium reaction. Since tritium has to be made by nuclear reactors, the easiest to access fusion fuel in space is deuterium, so the DD fuel option is preferred.
Bussard’s Fusion Rocket system added large amounts of hydrogen to the fusion products (mostly helium) diverted from the reactor. In the case-study Bussard examined for this particular design, the spacecraft would carry 6,300 tons of hydrogen, about 1,000 tons of payload, and 2,700 tons of reactor, rocket and radiator for disposing of waste heat.
The spacecraft’s initial acceleration would be 1.72 milligee, rising to about 5 milligee as the hydrogen tank emptied. Such a system could achieve a delta-vee of 78.03 km/s, reaching Mars at its closest approach to Earth in just 32.6 days – 20.4 days accelerating and 12.2 days braking. Continuous acceleration!
The figures provided by Bussard allow us to compute the average Jet-Power of the drive:
PJ = 0.5 * (8,000 s * 9.80665 m/s2)2 * (6.3E+6 kg / 2.82E+6 s) = 6.875 GW
…a large fraction of the raw fusion energy from the reactor will be lost to neutrons and x-rays, so the fusion reactor will be working hard.
ROBERT BUSSARD. “Fusion as electric propulsion,” AIAA 1988-3167. 24th Joint Propulsion Conference. July 1988. https://doi.org/10.2514/6.1988-3167