Editing Rocket (section)

===Specific impulse===
{{Main|specific impulse}}
{{Specific impulse|align=right}}
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|specific Impulse]], <math>I_{sp}</math>, 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:

{{block indent|<math>v_e = I_{sp} \cdot g_0</math><ref name="RPE7"/>{{rp|29}}}}

where:
{{block indent|<math>I_{sp}</math> has units of seconds}}
{{block indent|<math>g_0</math> is the acceleration at the surface of the Earth}}

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