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Rocket propellant
Rocket propellant is a material used by a rocket as, or to produce in a chemical reaction, the reaction mass (propulsive mass) that is ejected, typically with very high speed, from a rocket engine to produce thrust, and thus provide spacecraft propulsion. Each rocket type requires different kind of propellant: chemical rockets require propellants capable of undergoing exothermic chemical reactions, which provide the energy to accelerate the resulting gases through the nozzle. Thermal rockets instead use inert propellants of low molecular weight that are chemically compatible with the heating mechanism at high temperatures, while cold gas thrusters use pressurized, easily stored inert gases. Electric propulsion requires propellants that are easily ionized or made into plasma, and in the extreme case of nuclear pulse propulsion the propellant consists of debris from nuclear explosions.
==Overview==
Rockets create thrust by expelling mass backwards in a high speed jet (see ''Newton's Third Law''). Chemical rockets, the subject of this article, create thrust by reacting propellants within a combustion chamber into a very hot gas at high pressure, which is then expanded and accelerated by passage through a nozzle at the rear of the rocket. The amount of the resulting forward force, known as thrust, that is produced is the mass flow rate of the propellants multiplied by their exhaust velocity (relative to the rocket), as specified by Newton's third law of motion. Thrust is therefore the equal and opposite reaction that moves the rocket, and not by interaction of the exhaust stream with air around the rocket. Equivalently, one can think of a rocket being accelerated upwards by the pressure of the combusting gases against the combustion chamber and nozzle. This operational principle stands in contrast to the commonly-held assumption that a rocket "pushes" against the air behind or below it. Rockets in fact perform better in outer space (where there is nothing behind or beneath them to push against), because there is a reduction in air pressure on the outside of the engine, and because it is possible to fit a longer nozzle without suffering from flow separation, in addition to the lack of air drag.
The maximum velocity that a rocket can attain in the absence of any external forces is primarily a function of its mass ratio and its ''exhaust velocity''. The relationship is described by the ''rocket equation'': , where is the final velocity, is the exhaust velocity relative to the rocket, is the initial total mass, and is the mass after the propellant is burned. The mass ratio expresses what proportion of the rocket is propellant (fuel/oxidizer combination) prior to engine ignition. Typically, a single-stage rocket might have a mass fraction of 90% propellant, 10% structure, and hence a mass ratio of 9:1 . The impulse delivered by the motor to the rocket vehicle per weight of propellant consumed is the rocket propellant's ''specific impulse''. A propellant with a higher specific impulse is said to be more efficient as more thrust is produced per unit of propellant.
Lower rocket stages usually use high-density (low volume per unit mass) propellants due to their lighter tankage to propellant weight ratios and because higher performance propellants require higher expansion ratios for maximum performance than can be attained when operated in atmosphere. Thus, the Saturn V first stage used kerosene-liquid oxygen rather than the liquid hydrogen-liquid oxygen used on its upper stages. Similarly, the Space Shuttle used high-thrust, high-density solid rocket boosters for its lift-off with the liquid hydrogen-liquid oxygen Space Shuttle Main Engines used partly for lift-off but primarily for orbital insertion.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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