Specific Impulse

Specific impulse (usually abbreviated I_sp) is a way to describe the efficiency of rocket and jet engines. It represents the force with respect to the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass (such as in kilograms), then specific impulse has units of velocity. If it is given in terms of weight (such as in kiloponds), then specific impulse has units of time. The conversion constant between the two versions of specific impulse is g. 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 is needed for a given delta-v per the Tsiolkovsky rocket equation.

The actual exhaust velocity is the average speed that the exhaust jet actually leaves the vehicle. The effective exhaust velocity is the speed that the propellant burned per second would have to leave the vehicle to give the same thrust. The two are about the same for a rocket working in a vacuum, but are radically different for an air-breathing jet engine that obtains extra thrust by accelerating air. Specific impulse and effective exhaust velocity are proportional.

Specific impulse is a useful value to compare engines, much like miles per gallon or litres per 100 kilometres is used for cars. A propulsion method with a higher specific impulse is more propellant-efficient. 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 effective exhaust velocity.