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A thermal rocket is a rocket engine that uses a propellant that is externally heated before being passed through a nozzle, as opposed to undergoing a chemical reaction as in a chemical rocket. Thermal rockets can give high performance, depending on the fuel used and design specifications. ==Theory== For a rocket engine, the efficiency of propellant use (the amount of impulse produced per mass of propellant) is measured by the specific impulse (), which is proportional to the effective exhaust velocity. For thermal rocket systems, the specific impulse increases as the square root of the temperature, and inversely as the square root of the molecular mass of the exhaust. In the simple case where a thermal source heats an ideal Monatomic gas reaction mass, the maximum theoretical specific impulse is directly proportional to the thermal velocity of the heated gas: : where is the standard gravity, is Boltzmann's constant, T the temperature (absolute), and m is the mass of the exhaust (per molecule). For reaction mass which is not monotomic, some of the thermal energy may be retained as internal energy of the exhaust, and this equation will be modified depending on the degree of dissociation in the exhaust, frozen-flow losses, and other internal losses, but the overall square-root proportionality will remain. A more detailed equation for the maximum performance of a thermal rocket can be found in Chung.〔Chung, Winchell, ("Choose Your Engine" ), ''Atomic Rockets'' (accessed 9 January 2015).〕 Thus, the efficiency of a thermal engine is maximized by using the highest feasible temperature (usually limited by materials properties), and by choosing a low molecular mass for the reaction mass. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「thermal rocket」の詳細全文を読む スポンサード リンク
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