Computing ISP

ISP = (Thrust * Time) / WeightPropellant

Where:

ISP = Specific Impulse of the Propellant/Engine System

Thrust = Thrust of the engine

Time = Length of time the engine operates in seconds

WeightPropellant = Amount of propellant consumed by the engine during it's operation

Note: All units must be internally consistent; e.g. if you use pounds of thrust, you must also measure propellant consumption in pounds.

EXAMPLE: We have a solid rocket motor which burns for 15 seconds, producing 24,000 lbs of thrust; consuming 1,400 lbs of propellant in the process.

(24,000 * 15) / 1,400 = 257.14 ISP