Specific impulse

Specific impulse is the ratio of the thrust delivered by a rocket engine, expressed in newtons, to the flow rate multiplied by the weight of ejected propellant.

It is expressed in seconds and represents the time during which 1 kg of propellant is capable of producing a force of 9.81 newtons. For example, for the pair H₂-0₂ it is equal to between 430 and 480 seconds.