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specific impulse[spə′sif·ik ′im‚pəls]
(also specific thrust). The specific impulse of a rocket engine is the ratio of the thrust developed by the engine and the weight of propellant consumed per second. It is a measure of the efficiency of a rocket engine in newton-seconds per kilogram or kilogram-force-seconds per kilogram. As of 1976 the specific impulse of the best liquid-propellant rocket engines was 4.5 kilonewton-seconds per kilogram (kN-sec/kg); for solid-propellant engines it was 2.5–3 kN-sec/kg.
The impulse produced by a rocket divided by the mass mp of propellant consumed. Specific impulse Isp is a widely used measure of performance for chemical, nuclear, and electric rockets. It is usually given in seconds for both U.S. Customary and International System (SI) units.
The impulse produced by a rocket is the thrust force F times its duration t in seconds. The specific impulse is given by the equation below.
Calculation of specific impulse for the various forms of electric rockets involves electrothermal, resistance or arc heating of the propellant or its ionization and acceleration to high jet velocity by electrostatic or electromagnetic body forces. Ions in the exhaust jets of these devices must be neutralized so the spacecraft will not suffer from space charging or other effects from the plumes of the devices' operation. See Electrothermal propulsion, Ion propulsion, Plasma propulsion, Rocket propulsion