|
Specific impulse (usually abbreviated ''I''sp) is a measure of the efficiency of rocket and jet engines. By definition, it is the impulse delivered per unit of propellant consumed, and is dimensionally equivalent to the thrust generated per unit propellant flow rate.〔(【引用サイトリンク】title=What is specific impulse? )〕 If mass (kilogram or slug) is used as the unit of propellant, then specific impulse has units of velocity. If weight (newton or pound) is used instead, then specific impulse has units of time (seconds). The conversion constant between these two versions is the standard gravitational acceleration constant (''g''0). The higher the specific impulse, the lower the propellant flow rate required for a given thrust, and in the case of a rocket, the less propellant needed for a given delta-v, per the Tsiolkovsky rocket equation. Specific impulse is a useful value to compare engines, much like ''miles per gallon'' or ''liters per 100 kilometers'' is used for cars.〔 A propulsion method and system with a higher specific impulse is more propellant-efficient.〔 While the unit of seconds can seem confusing to laypeople, it is fairly simple to understand as "hover-time": how long a rocket can "hover" before running out of fuel, given the weight of that propellant/fuel. Of course, the weight of the rocket has to be taken out of consideration and so does the reduction in fuel weight as it's expended; the basic idea is "how ''long ''can any given amount of ''x'' hold itself up". Obviously that must mean "...against Earth's gravity", which means nothing in non-Earth conditions; hence ''I''sp being given in ''velocity'' when propellant is measured in mass rather than weight, and the question becomes "how ''fast'' can any given amount of ''x'' accelerate itself?" Note that ''I''sp describes efficiency in terms of ''amount of propellant'', and does not include the engine, structure or power source. Higher ''I''sp means less ''propellant'' needed to impart a given momentum. Some systems with ''very ''high ''I''sp (cf. ion thrusters) may have relatively very heavy/massive power generators, and produce thrust over a long period; thus, while they are "efficient" in terms of propellant mass carried, they may actually be quite poor at delivering high thrust as compared to "less efficient" engine/propellant designs. Another number that measures the same thing, usually used for air breathing jet engines, is specific fuel consumption. Specific fuel consumption is inversely proportional to specific impulse and the effective exhaust velocity. The actual exhaust velocity is the average speed of the exhaust jet, which includes fuel combustion products, nitrogen, and argon, as it leaves an air breathing engine. The effective exhaust velocity is the exhaust velocity that the combusted fuel and atmospheric oxygen only would need to produce the same thrust. The two are identical for an ideal rocket working in vacuum, but are radically different for an air-breathing jet engine that obtains extra thrust by accelerating the non-combustible components of the air. Specific impulse and effective exhaust velocity are proportional. ==General considerations== The amount of propellant is normally measured either in units of mass or weight. If mass is used, specific impulse is an impulse per unit mass, which dimensional analysis shows to have units of speed, and so specific impulses are often measured in meters per second and are often termed effective exhaust velocity. However, if propellant weight is used, an impulse divided by a force (weight) turns out to be a unit of time, and so specific impulses are measured in seconds. These two formulations are both widely used and differ from each other by a factor of ''g0'', the dimensioned constant of gravitational acceleration at the surface of the Earth. Note that the rate of change of momentum of a rocket (including its propellant) per unit time is equal to the thrust. The higher the specific impulse, the less propellant is needed to produce a given thrust during a given time. In this regard a propellant is more efficient the greater its specific impulse. This should not be confused with energy efficiency, which can decrease as specific impulse increases, since propulsion systems that give high specific impulse require high energy to do so.〔http://www.geoffreylandis.com/laser_ion_pres.htp〕 It is important that thrust and specific impulse not be confused. The specific impulse is a measure of the ''impulse produced per unit of propellant'' expended, while thrust is a measure of the momentary or peak force supplied by a particular engine. In many cases, propulsion systems with very high specific impulses—some ion thrusters reach 10,000 seconds—produce low thrusts.〔(【引用サイトリンク】title=Mission Overview )〕 When calculating specific impulse, only propellant that is carried with the vehicle before use is counted. For a chemical rocket the propellant mass therefore would include both fuel and oxidizer; for air-breathing engines only the mass of the fuel is counted, not the mass of air passing through the engine. 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「specific impulse」の詳細全文を読む スポンサード リンク
|