specific impulse

Specific impulse is a measure of rocket engine efficiency, defined as the thrust produced per unit rate of propellant consumption. It is typically expressed in pounds of thrust per pound of propellant used per second.1

Rockets generate thrust by expelling mass at high velocity, a principle rooted in Newton's third law of motion. The thrust is directly proportional to the rate of mass discharge and the exhaust velocity. While a high mass discharge rate can increase thrust, it depletes propellant quickly. Therefore, achieving a high exhaust velocity is generally preferred for efficiency.2

The effective exhaust velocity, often expressed in seconds and referred to as specific impulse in engineering, is a key figure of merit. It indicates thrust per unit mass of propellant consumed. Higher exhaust velocities lead to greater efficiency, with values varying based on propellant type and energy source.2