Specific impulse

Specific impulse measures rocket or jet propulsion efficiency as thrust produced per unit weight flow of propellant. It is commonly expressed in seconds.

Thrust: Thrust is the force exerted by a propulsion system on a vehicle. Specific impulse compares this force with the propellant’s weight flow.
Chemical rocket: A rocket that obtains energy from chemical reactions between propellants. Chemical engines trade relatively high thrust for specific impulse limited by reaction temperature and exhaust chemistry.
Thrust-to-weight ratio: Thrust-to-weight ratio is thrust divided by the weight of a propulsion system or vehicle. It compares force with engine or vehicle weight, not propellant weight flow.
Weight: Weight is the gravitational force acting on an object with mass. The conventional definition divides thrust by propellant weight flow rather than mass flow.
Mass flow rate: Mass flow rate is the amount of mass passing a point per unit time. It supplies the propellant-flow term used to calculate specific impulse.
Electric propulsion: Space propulsion that accelerates propellant using electrical energy. Electric thrusters can achieve high specific impulse while producing low thrust.
Propellant mass fraction: Propellant mass fraction is the share of a vehicle’s total mass made up by propellant. It describes vehicle mass allocation rather than propulsion efficiency during operation.
Standard gravity: Standard gravity is the defined reference acceleration of 9.80665 metres per second squared. It converts specific impulse in seconds into effective exhaust velocity.
Effective exhaust velocity: Effective exhaust velocity is the equivalent exhaust speed that relates thrust to propellant mass flow. In rocket propulsion, it is specific impulse multiplied by standard gravity.
Ion thruster: An electric thruster that accelerates ions to produce spacecraft thrust. Its high exhaust velocity gives it high specific impulse, but its thrust is small.

Linked from 17 pages