section_path: Physics/Specific impulse
section_name: Specific impulse
paragraph_text: 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, I s p {\displaystyle I_{sp}}, 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:
  
  where:
  
  Thus, the greater the specific impulse, the greater the net thrust and performance of the engine. I s p {\displaystyle I_{sp}} 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.