Specific Impulse
The specific impulse is a figure of merit of the performance of a rocket propulsion system. It represents the thrust per unit propellant weight flow rate or, in other words, it is the measure of how efficiently a rocket engine creates thrust.
For a constant thrust and constant propellant flow, the equation reduces to
where:
- the numerator is the total impulse, It, essentially proportional to the total energy released by the propellant;
- the denominator is the effective propellant weight, w, given by the product of total effective propellant mass expelled, mP, and the standard acceleration of gravity, g0.
In the International System of Units (SI), the specific impulse is in seconds [s]. However Is does not indicate a measure of elapsed time but the thrust force per unit “weight flow rate”.