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in the rocket, and thus corresponds to an "effective" exhaust velocity.

The specific impulse in units of velocity is defined by

where is the average thrust.

The practical meaning of the measurement varies with different types of engines. Car engines consume onboard fuel, breathe environmental air to burn the fuel, and push against the ground beneath them. There is no reaction mass. In this case, is momentum per fuel burned.

Chemical rocket engines, by contrast, carry their fuel, oxidizer, and reaction mass

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