What is specific impulse?

Engineers write it Isp, and say it out loud as “I-S-P”. 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 — it is how long one kilogram of propellant could produce one kilogram of thrust.

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, and it gets there mostly through a much larger nozzle — an expansion ratio of 165 to 1 against the stubby bells at sea level.

The propellant matters more than the engine. Kerosene engines land somewhere around 270 to 360 seconds; hydrogen engines reach 370 to 465. That is a large gap, and it is the reason hydrogen keeps being used despite being a nuisance to store.

Engine designers trade specific impulse against thrust. A high-efficiency engine that pushes gently is useless for leaving the ground, because it cannot beat the rocket’s own weight. This is why big rockets often use kerosene low down, where raw push matters, and something more efficient higher up, where there is time.

It is also the number that governs how far a rocket can go. Specific impulse feeds straight into the rocket equation, which is why a small gain in efficiency is worth so much more than it sounds.