Specific impulse (Isp) is the standard measure of rocket engine efficiency, defined as the thrust produced per unit weight flow rate of propellant, expressed in seconds. A higher Isp means the engine extracts more momentum from each kilogram of propellant. Chemical engines typically achieve 250-470 seconds, while electric propulsion systems can reach 1,500-10,000 seconds at much lower thrust levels.
Among chemical propellants, hydrogen-oxygen engines like the RS-25 achieve the highest Isp (~450s in vacuum), while methalox engines like SpaceX's Raptor reach approximately 380s. The tradeoff between Isp and thrust is fundamental: high-Isp electric thrusters are efficient but slow, making them suitable for station-keeping and interplanetary cruise but not for launch or rapid maneuvers.