ISP = (Thrust * Time) / WeightPropellant
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
ISP = Ev / 9.81 m/sec2 (Metric)
ISP = Ev / 32.2 ft/sec2 (Standard)
ISP = Specific Impulse
Ev = Exhaust Velocity (m/sec or feet/sec)
EXAMPLE: We have an engine that has an exhaust velocity of 8,093 meters per second. What is it's ISP?
8,093 / 9.81 = 824.97 ISP