Specific Impulse
A measure of rocket engine efficiency, expressed as the total impulse delivered per unit of propellant consumed.
Explanation
Specific impulse (Isp) is the rocket equivalent of fuel economy in cars — higher Isp means more thrust per kilogram of propellant. It is measured in seconds in the imperial system and in meters per second in SI. The value depends on exhaust velocity, which is determined by propellant chemistry and nozzle design. Chemical rocket engines typically achieve Isp between 250 and 450 seconds. Hydrazine monopropellant thrusters achieve around 220-230 seconds. Bipropellant engines like the RL-10 reach about 450 seconds in vacuum. Electric propulsion systems like ion thrusters achieve Isp values of 1,500 to 5,000 seconds, far exceeding chemical rockets, but with very low thrust. The tradeoff is captured by the rocket equation: higher Isp reduces propellant mass for a given delta-v, but low-thrust systems require longer burn times and cannot be used for launch from Earth's surface.
Why It Matters
Isp directly drives the propellant mass fraction of any spacecraft. A 10% improvement in Isp can translate into significantly more payload mass, longer mission life, or smaller launch vehicles. It is the single most important efficiency metric in propulsion.
Concept Map
How Specific Impulse connects to other glossary terms:
Frequently Asked Questions
Why is Isp measured in seconds?
In the imperial system, Isp equals exhaust velocity divided by standard gravity. The result is in seconds, which is physically meaningful as the burn time for one pound of thrust from one pound of propellant.
Does higher Isp always mean a better engine?
Not necessarily. High Isp often comes with lower thrust or higher mass. The right engine depends on the mission: high-thrust for launch, high-Isp for in-space maneuvers.
Sources
Last updated: July 1, 2026