The ranges of specific impulse and thrust for different types of propulsion system.
Specific impulse is the ratio of a rocket engine's thrust to the weight of fuel burned in one second. Measured in seconds, specific impulse is an important gauge of the efficiency of a rocket propulsion system and, in particular, of a rocket propellant. It is similar to the idea of miles per gallon with cars. The higher its value, the better the performance of the rocket.