specific impulse (often shortened to "i"sp) 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.