Specific impulse

Specific impulse (usually abbreviated as Isp) a physical quantity defined as the ratio of change in momentum (impulse) to the mass used, usually fuel. It typically uses units of metres per second (a SI unit) or feet per second (in imperial units). It is equivalent to thrust (a force, in newtons or pounds) per mass flow rate (in kg/s or lbm/s).

It serves as a measure of how efficiently an engine, such as a rocket or jet engine, generates thrust from propellant. Isp is the effective exhaust velocity used in the Tsiolkovsky rocket equation which calculates how much a vehicle's velocity can be changed with a given quantity of fuel.

Normalized specific impulse is the ratio of specific impulse to Earth's standard acceleration of gravity, g (in m/s2 or ft/s2). It is measured in seconds, and conveniently is the same number in both SI and imperial units. It can be understood as the time that one kilogram of fuel can produce one kilogram of thrust, which is equal to the time that one pound of fuel can produce one pound of thrust.

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