Antimatter rocket

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An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several types of design that attempt to accomplish this goal. The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket.

Antimatter rockets can be divided into three types: those that directly use the products of antimatter annihilation for propulsion, those that heat a working fluid which is then used for propulsion, and those that heat a working fluid to generate electricity for some form of electric spacecraft propulsion system.

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Direct use of reaction products

Antiproton annihilation reactions produce charged and uncharged mesons, in addition to gamma rays. The charged mesons can be channelled by a magnetic nozzle, producing thrust. This type of antimatter rocket is a beamed core configuration. It is not perfectly efficient; energy is lost as the rest mass of the charged and uncharged mesons, lost as the kinetic energy of the uncharged mesons (which can't be deflected for thrust), and lost as gamma rays.

Positron annihilation has also been proposed for rocketry. Annihilation of positrons produces only gamma rays. Early proposals for this type of rocket, such as those developed by Eugen Sänger, assumed the use of some material that could reflect gamma rays, used as a light sail to derive thrust from the annihilation reaction. No viable means of specularly reflecting gamma rays has been proposed (no solid material has this property, and plasma is not sufficiently reflective to gamma rays under practically attainable conditions). However, the momentum of gamma rays can indeed be partially transferred to matter by Compton scattering.

Antimatter heating of an exhaust fluid

Several methods for heating an exhaust fluid using the gamma rays produced by positron annihilation have been proposed.[1] These methods resemble those proposed for nuclear thermal rockets. One proposed method is to use positron annihilation gamma rays to heat a solid engine core. Hydrogen gas is ducted through this core, heated, and expelled from a rocket nozzle. A second proposed engine type uses positron annihilation within a solid lead pellet or within compressed xenon gas to produce a cloud of hot gas, which heats a surrounding layer of gaseous hydrogen. Direct heating of the hydrogen by gamma rays was considered impractical, due to the difficulty of compressing enough of it within an engine of reasonable size to absorb the gamma rays. A third proposed engine type uses annihilation gamma rays to heat an ablative sail, with the ablated material providing thrust. As with nuclear thermal rockets, the specific impulse achievable by these methods is limited by materials considerations, typically being in the range of 1000–2000 seconds.[citation needed]

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