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.
Through a boost of slightly more than 3 percent in mean specific impulse on the first stage with an aerospike, without accounting for any weight savings by using the DEAN engine on the upper stage(s), current AFIT modeling indicates the possibility of realizing a 6 percent gain in maximum payload to GEO.
Crimson founder and CEO Glenn Gow notes, "We have been working closely with Specific Impulse for three years on major web projects for clients such as Seagate, HP, and Verizon.
is currently developing the LEROS 1D apogee engine, designed to deliver specific impulse in excess of 330 seconds.
Based on the latest test data, our vacuum specific impulse will surpass 350 seconds.
Fuel efficiency, or specific impulse, is 30 percent higher than the NSTAR ion thruster and is a ten-fold increase over today's chemical rocket engines.
The LEROS 1b combines high thrust with high specific impulse (318 lbf-sec/lbm) to meet the control authority needs and provide thrust margin for heavy payloads and deep space probes.
This engine provides not only added assurance for orbit insertion but the higher specific impulse and longer life will increase time on orbit and satellite life," said Robert Huebner, Vice President - AMPAC-ISP Operations.
The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is a type of propulsion system that produces a plasma exhaust at temperatures similar to those in the interior of the sun.
The accelerated schedule included detailed manufacturing and quality planning processes, and an updated injector design to improve engine specific impulse performance, known as Isp.
Both cast and extruded types offer similar specific impulses.
With the radio-frequency ion thruster assembly, the company s engineers have created the technology to enable efficient means of operating satellites and probes in space using high specific impulses, while at the same time keeping fuel consumption very low.

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