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# Specific Impulse

As can be seen from the thrust equation, the effective speed of the exhaust controls the amount of thrust produced from a particular quantity of fuel burnt per second.

An equivalent measure, the net impulse per weight unit of propellant expelled, is called specific Impulse, and this is one of the most important figures that describes a rocket's performance. It is defined such that it is related to the effective exhaust velocity by:

{\displaystyle v_{e}=I_{sp}\cdot g_{0}}v_{e}=I_{sp}\cdot g_{0}[2]:29

Thus, the greater the specific impulse, the greater the net thrust and performance of the engine. Impulse is determined by measurement while testing the engine. In practice the effective exhaust velocities of rockets varies but can be extremely high, ~4500 m/s, about 15 times the sea level speed of sound in air.