Specific Impulse is like the gas mileage of the rocket engine. The unit of specific impulse is *seconds* $Specific Impulse (ISP) = \frac{thrust(lbf)}{flowrate(lbm/s)} $ It depends on the type of propellant. Vacuum ISPs of various engines are: - H2 (~450s) - LNG (~340s) - RP1 (~330s) Sea level ISPs are 20-30% lower Comments: - Lighter stuff wants to go faster. So H2 is higher specific impulse - Denser propellants are used in boost stage and upper stages uses $H_2$