Specific Impulse

One of the most important parameters in rocket engine performance, specific impulse is given in units of seconds. It is the number of seconds that 1 lb of propel-lant will provide 1 lb of thrust.

Specific impulse = thrust force divided by propellant "weight" flow rate

If the atmosphere is providing the working mass, then specific impulse goes up significantly, because the propellant weight flow rate refers only to onboard propel-lants, not to exterior derived working mass.

The propellant "weight" flow rate is actually the mass flow rate times the standard acceleration of gravity at the surface of Earth.

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