Specific Impulse

specific impulse [spə′sif·ik ′im‚pəls]

(aerospace engineering)

A performance parameter of a rocket propellant, expressed in seconds, equal to the thrust in pounds divided by the weight flow rate in pounds per second. Also known as specific thrust.

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Specific impulse

The impulse produced by a rocket divided by the mass m_p of propellant consumed. Specific impulse I_sp 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.

(1) 

Its equivalent, specific thrust F_sp, that is sometimes used alternatively, is the rocket thrust divided by the propellant mass flow rate F/m_p. See Thrust

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

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Specific Impulse 

(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.