Specific Impulse

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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.

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.

Specific impulse

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.

Its equivalent, specific thrust Fsp, that is sometimes used alternatively, is the rocket thrust divided by the propellant mass flow rate F/mp. 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

References in periodicals archive ?
Note that the specific impulse remains below that of the aerospike at all altitudes except 12,000 feet.
This energy manifests itself in the velocity of the charged particles that escape from the end of the GDM device, which serves as a magnetic nozzle, thereby producing very high specific impulse.
This engine develops 1/50th of a pound of thrust, much less than chemical rockets, but with its high specific impulse, it can journey in space for years.
High specific impulse means a lower propellant-mass flow rate to produce a given thrust and less propellant consumption to produce a given total impulse.
The 2,480-lb engine features a relatively high area ratio (106:1) that provides a specific impulse of 350 seconds.
The study recommends pursuing improvements in tank weight, nuclear engine thrust-to-weight ratio, and specific impulse-a performance parameter of rocket propellant that measures the thrust-producing energy content of the propellant; the higher the specific impulse, the less propellant mass is needed to perform the mission.
The objectives of this activity are to improve the specific impulse thrust density and reliability of the existing micro-colloid thruster technology by the application of mass production manufacturing techniques developed for terrestrial electrospray applications e.

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