Fusion rocket

A fusion rocket is a rocket that uses a fusion reaction to power it. There is currently no known method of producing a self-sustaining artificial fusion reaction, although the theoretical power can be calculated, and the nuclear reaction has been verified in laboratory experiments and nuclear weapons. The advantage over a fission rocket is that less radiation is produced (depending on the fusion reaction), which requires less shielding, and that there is greater energy density in the fuel. Of the speculative spacecraft propulsion systems that have been proposed, fusion is likely to be feasible in the medium term as steady progress is being made towards self-sustaining fusion reactions.

For space based application there are several configurations for a fusion drive.

One would mirror a terrestrial fusion reactor like a tokamak. The fusion plasma would be vented to provide thrust. Currently tokamaks weigh about the same as an oil tanker, so the thrust to weight ratio would seem unacceptable.

The most practical approach might eventually be a Farnsworth-Hirsch Fusor. A fusor uses inertial electrostatic confinement. Since an electron volt equals 11,604 degrees, electrostatic confinement can and has achieved fusion in large vacuum tubes. The reactors still have not broken even, but the problems may be soluble.

Fusors have three important advantages:

They can react fuels that no other design could. For example, they might be able to fuse protons and Boron 11. This reaction produces neither gamma nor neutrons.
They are mostly vacuum, and therefore very light weight, suitable for vehicles.
They might be able to generate electric power directly, The ionized reaction products would be permitted to fly from the reaction site through a high voltage field of several million volts, and then hit a grid. This would create a small current at several million volts. This power could either be used directly, or pulsed, and used to operate a transformer to get more normal voltages.

Another proposed configuration would resemble an orion drive using inertial fusion. A small pellet of fusion fuel (with a diameter of a couple of millimeters) would be ignited by an electron beam, a laser or even a tiny amount of antimatter. A magnetic field would form the pusher plate. The preferred fuels would have an aneutronic fusion reaction to minimize radiation. Most researchers have focused on a combination of Helium-3 and Deuterium. Helium-3 is very rare on Earth and might have to be obtained from the Moon or a gas giant.

Fusion-based bombs could (and almost certainly would) be used in an Orion design, improving efficiency substantially while a pellet based inertial fusion approach has been extensively studied by the British Interplanetary Society during their "Daedalus" interstellar probe design study in the 1970s.