What is specific impulse?
Also called: Isp
Specific impulse measures how efficiently a rocket engine uses propellant — its miles per gallon. A Merlin engine manages 282 seconds at sea level and 311 in vacuum.
Higher is better: the same mass of propellant produces more change in speed. It is measured in seconds, which is an accident of the units rather than a duration of anything.
The vacuum version of the same engine does better because there is no atmosphere holding the exhaust back — Falcon 9’s Merlin Vacuum reaches 348 seconds, the highest of any flown US hydrocarbon engine.
Engine designers trade specific impulse against thrust. A high-efficiency engine that pushes gently is useless for leaving the ground.
See it happen
Push & GoWhy rockets flySource: Merlin rocket engine family