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. 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.
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). Helium-3 propulsion is a proposed method of spacecraft propulsion that uses the fusion of helium-3 atoms as a power source. Helium-3, an isotope of helium with two protons and one neutron, could be fused with deuterium in a reactor. The resulting energy release could be used to expel propellant out the back of the spacecraft. Helium-3 is proposed as a power source for spacecraft mainly because of its abundance on the moon. Currently, scientists estimate that there are 1 million tons of helium-3 present on the moon, mainly due to solar wind colliding with the moon's surface and depositing it, among other elements, into the soil. 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. Helium-3 is also produced via beta decay of tritium, which in turn can be produced from deuterium, lithium, or boron.
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