Fusion rocket
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A fusion rocket is a rocket that is driven by fusion power. The process of nuclear fusion is well understood and recent developments indicate this technology may be able to provide terrestrial based power within 30 years (as of 2006). However, the proposed reactor vessels are large and heavy making them unsuitable to use on spacecraft in the foreseeable future. A smaller and lighter fusion reactor might be possible in the future when better methods have been devised to control magnetic confinement and prevent plasma instabilities.
For space flight, the main advantage of fusion would be the very high specific impulse, the main disadvantage the (probable) large mass of the reactor. In addition, a fusion rocket may produce less radiation than a fission rocket, reducing the mass needed for shielding. The surest way of building a fusion rocket with current technology is to use hydrogen bombs as proposed in Project Orion, but such a spacecraft would also be massive.
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[edit] Electricity generation vs. direct thrust
Many spacecraft propulsion methods such as ion thrusters require an input of electric power to run but are highly efficient. In some cases their maximum thrust is limited by the amount of power that can be generated (for example, a mass driver). An electric generator that ran on fusion power could be installed purely to drive such a ship. One disadvantage is that conventional electricity production requires a low-temperature energy sink, which is difficult (i.e. heavy) in a spacecraft. Direct conversion of the kinetic energy of the fusion products into electricity is in principle possible and would mitigate this problem.
An attractive possibility is to simply direct the exhaust of fusion product out the back of the rocket to provide thrust without the intermediate production of electricity. This would be easier with some confinement schemes (e.g. magnetic mirrors) than with others (e.g. tokamaks). It is also more attractive for "advanced fuels" (see aneutronic fusion). Only 20% of the power produced by the D-T reaction could be used this way; the other 80% is released in the form of neutrons which, because they cannot be directed by magnetic fields or solid walls, would be very difficult to use for thrust.
Even if a self-sustaining fusion reaction cannot be produced, it might be possible to use fusion to boost the efficiency of another propulsion system, such as a VASIMR engine.
[edit] Confinement concept
To sustain a fusion reaction, the plasma must be confined. The most widely studied configuration for terrestrial fusion is the tokamak, a form of magnetic confinement fusion. Currently tokamaks weigh a great deal, so the thrust to weight ratio would seem unacceptable.
The main alternative to magnetic confinement is inertial confinement fusion, such as that used by Project Daedalus. A small pellet of fusion fuel (with a diameter of a couple of millimeters) would be ignited by an electron beam or a laser. To produce direct thrust, a magnetic field would form the pusher plate. In principle, the Helium-3-Deuterium reaction or an aneutronic fusion reaction could be used to maximize the energy in charged particles and to minimize radiation, but it is highly questionable whether it is technically feasible to use these reactions. Both the detailed design studies in the 1970s, the Orion drive and Project Daedalus, used inertial confinement.
A popular confinement concept for fusion rockets is inertial electrostatic confinement, such as in the Farnsworth-Hirsch Fusor. In an article[1] in the Analog Science Fiction and Fact publication, Tom Ligon (who also wrote several science fiction stories) suggested that the fusor might make the basis for a highly effective fusion rocket. It was also featured in this role in the science fiction novel The Wreck of the River of Stars, by Michael Flynn. However, there are difficulties with the concept, related to energy losses through bremsstrahlung radiation or maintaining a highly non-Maxwellian ion energy distribution or both, that appear to be fundamentally insurmountable.
A still more speculative concept is antimatter catalyzed nuclear pulse propulsion, which would use tiny quantities of antimatter to catalyze a fission and fusion reaction, allowing much smaller fusion explosions to be created.
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