Specific impulse

Specific impulse (usually abbreviated Isp) 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. 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 (g0). 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 Isp 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 Isp describes efficiency in terms of amount of propellant, and does not include the engine, structure or power source. Higher Isp means less propellant needed to impart a given momentum. Some systems with very high Isp (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 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.