Interactive Rocketry Tutorial

SPECIFIC IMPULSE

The Specific Impulse (Isp) of a rocket is the ratio of the Force, or thrust, F, to the product of the flow rate of the mass which is used up in the engine and the acceleration due to gravity (g). Specific Impulse is measured in seconds.

Specific Impulse is similar to “miles per gallon” for a car. It is a measure of how efficient a rocket engine is. Conventional rocket engines do not have as high a specific impulse as ion and nuclear engines (about 200 - 450 seconds), and are thus less efficient. Ion engines have very high specific impulses (1000 - 2000 seconds) but low thrust. Nuclear engines have medium specific impulses (500 - 1000 seconds), and high thrusts.

The equation used to measure the specific impulse of a rocket is:

Specific Impulse, Isp = F . Mflow rate x g

Question.

If a rocket has a thrust, F, of 2000 Newtons, and a Mass flow rate of 1 kilogram per second, what is its specific impulse (Isp) ?

[Assume the acceleration due to gravity, g = 10 metres per second squared]

This page is maintained by Richard Osborne