Specific impulse (often shortened to "I") is a way to describe how well a rocket performs. It is also used to describe how well a jet engine performs. It can be used to compare rockets of different sizes. It is a way to know how much force a given engine makes for each bit of fuel. To do this, one needs to know how much fuel is in the engine. A high specific impulse means that a rocket needs less fuel to perform as well. If the specific impulse is lower, it means that the rocket needs more fuel to perform as well. 