The propulsive efficiency () of an ion thruster rocket engine, like other rocket engines, is often expressed in terms of the specific impulse (), which is a measure of the efficiency of the engine. Ion thrusters, also known as electric propulsion or ion propulsion systems, use electric power to ionize and accelerate propellant (usually xenon gas) to produce thrust.
Ion thrusters typically have very high specific impulse values, often in the range of 1,000 to 5,000 seconds or even higher. The high specific impulse allows ion thrusters to operate more efficiently in terms of propellant utilization compared to traditional chemical rockets.
The propulsive efficiency is a crucial parameter in evaluating the performance of a rocket engine, indicating how well it converts propellant energy into useful thrust. Practical rocket engines often have propulsive efficiencies less than 100% due to factors like incomplete combustion, heat losses, and other inefficiencies in the propulsion system.
The propulsive efficiency () is defined by the following formula:
where,
- is the propulsive efficiency,
- is the thrust produced by the rocket engine,
- is the effective exhaust velocity of the rocket,
- ṁ is the mass flow rate of the propellants,
- is the specific impulse of the rocket engine,
- g0 is the acceleration due to gravity (approximately 9.81 m/s²).