Jet Propulsion/Specific impulse

Specific impulse is defined as the thrust (N) divided by the fuel weight flow rate (N/s). The resulting measure is usually quoted in seconds and defines the weight fraction that is necessary to give a particular delta V for a rocket or range for an aircraft with a given lift to drag ratio.

For a jet engine the specific impulse can be determined from the specific fuel consumption. For example an engine that has an SFC of 15mg/N-s will have a specific impulse of 6800s. Current turbofan engines have cruise SFC below 15.

For a chemical rocket the specific impulse ranges to about 500s.