# Specific Impulse

Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. It represents the force with respect to the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass (such as in kilograms), then specific impulse has units of velocity. If it is given in terms of weight (such as in kiloponds), then specific impulse has units of time. The conversion constant between the two versions of specific impulse is g. The higher the specific impulse, the lower the propellant flow rate required for a given thrust, and in the case of a rocket the less propellant is needed for a given delta-v per the Tsiolkovsky rocket equation.

The actual exhaust velocity is the average speed that the exhaust jet actually leaves the vehicle. The effective exhaust velocity is the speed that the propellant burned per second would have to leave the vehicle to give the same thrust. The two are about the same for a rocket working in a vacuum, but are radically different for an air-breathing jet engine that obtains extra thrust by accelerating air. Specific impulse and effective exhaust velocity are proportional.

Specific impulse is a useful value to compare engines, much like miles per gallon or litres per 100 kilometres is used for cars. A propulsion method with a higher specific impulse is more propellant-efficient. Another number that measures the same thing, usually used for air breathing jet engines, is specific fuel consumption. Specific fuel consumption is inversely proportional to specific impulse and effective exhaust velocity.

Read more about Specific ImpulseGeneral Considerations, Units, Specific Impulse As A Speed (effective Exhaust Velocity), Examples

### Other articles related to "specific impulse, specific impulses":

Solid-fuel Rocket - Propellant Families - Composite Modified Double Base Propellants
... nitrocellulose, improving the overall specific impulse ... The aluminum also improves specific impulse as well as combustion stability ... in Trident II D-5, replace most of the AP with HMX, further increasing specific impulse ...
High Power Electric Propulsion - Specifications
... The thruster itself is in the 20-50 kW class, with a specific impulse of 6,000-9,000 seconds, and a propellant throughput capability exceeding 100 kg/kW ... HiPEP produced 670 mN of thrust at a power level of 39.3 kW using 7.0 mg/s of fuel giving a specific impulse of 9620 s ... Downrated to 24.4 kW, the HiPEP used 5.6 mg/s of fuel giving a specific impulse of 8270 s and 460 mN of thrust ...
LE-7A - LE-7A Specifications
... lbf) Rated thrust (vacuum) 1,074 kN (241,000 lbf) Specific impulse (sea level) Specific impulse (vacuum) 429 seconds Long nozzle Rated thrust (sea level) 870 kN (200,000 lbf) Rated ...
Specific Impulse - Examples - Model Rocketry
... Specific impulse is also used to measure performance in model rocket motors ... Following are some of Estes' claimed values for specific impulses for several of their rocket motors Estes Industries is a large, well-known American seller of model rocket components ... The specific impulse for these model rocket motors is much lower than for many other rocket motors because the manufacturer uses black powder propellant ...
Rocket Engine Nozzle - Specific Impulse
... rocket engine nozzle can be defined as and for perfectly expanded nozzles, this reduces to The specific impulse, is the ratio of the amount of thrust produced to the weight flow of the ... exhaust gas velocity at nozzle exit, m/s = specific impulse, s = Gravitational acceleration at sea level on Earth = 9.807 m/s² In certain cases, where ...

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