If we look closer at the rocket equation

we see that is proportional to . This means that if the motor is throwing out gas molecules faster, the force per unit mass of propellant is higher. Thus, is a measure of how efficient the rocket motor is. The exhaust velocity parameter is very important when designing a rocket motor, and we can actually calculate it since we know that it is the same as dividing the total impulse by the propellant mass . Since it is specific to propellant mass we often call it specific impulse, and it is thus given as

Since is the same as the exhaust velocity, the unit is, in fact, . We have here described as being the same as the exhaust velocity. In some cases the specific impulse is defined differently, as

where . This definition is often used in the US, and the unit is seconds.

Common values of the specific impulse on expensive rocket motors is 2500–4500 m/s. The most efficient motor to date (2016) is the RL10 motor, an upper stage on the American Delta IV rocket. It has a specific impulse in vacuum of 4 567 m/s.

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*This article is part of a pre-course program used by Andøya Space Education in Fly a Rocket! and similar programs.*