# Rocket Engine Nozzle - Specific Impulse

Specific Impulse

Thrust is the force which moves a rocket through the air, and through space. Thrust is generated by the propulsion system of the rocket through the application of Newton's third law of motion: "For every action there is an equal and opposite reaction". A gas or working fluid is accelerated out the rear of the rocket engine nozzle and the rocket is accelerated in the opposite direction. The thrust of a 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 propellants. It is a measure of the fuel efficiency of a rocket engine. It can be obtained from:

 where: = gross rocket engine thrust, N = mass flow rate of exhaust gas, kg/s = exhaust gas velocity at nozzle exit, m/s = exhaust gas pressure at nozzle exit, Pa = external ambient pressure, Pa (also known as free stream pressure) = cross-sectional area of nozzle exhaust exit, m² = equivalent (or effective) 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 equals, then:

In cases where this may not be the case since for a rocket nozzle is proportional to, then it is possible to define a constant quantity which is the vacuum for any given engine thus:

and hence:

which is simply the vacuum thrust minus the force of the ambient atmospheric pressure acting over the exit plane.

Essentially then, for rocket nozzles, the ambient pressure acting over the engine largely cancels but effectively acts over the exit plane of the rocket engine in a rearward direction, while the exhaust jet generates forward thrust.

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

Solid-fuel Rocket - Propellant Families - Composite Modified Double Base Propellants
... oxygen deficit introduced by using nitrocellulose, improving the overall specific impulse ... The aluminum also improves specific impulse as well as combustion stability ... as NEPE-75 used 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. 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 ...
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 and emphasizes ...
LE-7A - LE-7A Specifications
... Rated thrust (sea level) 843 kN (190,000 lbf) Rated thrust (vacuum) 1,074 kN (241,000 lbf) Specific impulse (sea level) Specific impulse (vacuum) 429 ...

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