# Specific Impulse

## Description

Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. It represents the force with respect to the amount of propellant used per unit time. If the “amount” of propellant is given in terms of mass (such as in kilograms), then specific impulse has units of velocity. If it is given in terms of weight (such as in kiloponds or newtons), then specific impulse has units of time (seconds). The conversion constant between the two versions of specific impulse is g. The higher the specific impulse, the lower the propellant flow rate required for a given thrust, and in the case of a rocket the less propellant is needed for a given delta-v per the Tsiolkovsky rocket equation.

In rocketry, where the only reaction mass is the propellant, an equivalent way of calculating the specific impulse in seconds is also frequently used.

Related formulas## Variables

I_{sp} | specific_impulse (s) |

v_{e} | average exhaust speed along the axis of the engine (m/s) |

g | standard acceleration due to gravity(for an object in a vacuum near the surface of the Earth : 9.80665 m/s^2) (m/s^{2}) |