An electric propulsion system, also known as an ion propulsion system or electric thruster, is a type of propulsion technology used in spacecraft to generate thrust by expelling ions or charged particles at high velocities. Unlike traditional chemical propulsion systems, which rely on the combustion of fuel and oxidizer, electric propulsion systems use electricity to accelerate ions and produce thrust. This results in higher exhaust velocities and much greater efficiency compared to chemical rockets.
Here's how an electric propulsion system typically works:
Ionization: The first step involves ionizing a propellant gas, usually xenon, by using a process such as electron bombardment or radiofrequency discharges. This transforms the neutral gas atoms into positively charged ions and free electrons.
Acceleration: The ions are then accelerated using electric fields. This is achieved through a combination of electrodes, grids, and magnetic fields. The positive ions are attracted to negatively charged grids, which accelerate them out of the thruster.
Exhaust: The accelerated ions exit the thruster at extremely high speeds, creating a high-velocity exhaust jet. Due to the conservation of momentum, the spacecraft experiences an equal and opposite force, generating thrust.
Reaching High Velocities: The key advantage of electric propulsion systems is that they can achieve much higher exhaust velocities compared to chemical propulsion. This leads to greater fuel efficiency and allows spacecraft to reach higher speeds over time.
Continuous Operation: Electric propulsion systems can operate continuously for extended periods of time, gradually building up velocity over time. This is especially useful for deep space missions, where constant low thrust is more efficient than short bursts of high thrust.
Efficiency and Specific Impulse: The efficiency of an electric propulsion system is often described by its specific impulse (Isp), which is a measure of how effectively the system converts propellant mass into thrust. Electric propulsion systems typically have much higher Isp values compared to chemical rockets, resulting in greater efficiency.
There are several types of electric propulsion systems, including ion thrusters, Hall-effect thrusters, and electrostatic thrusters, each with its own design and operational characteristics. These systems are commonly used for missions requiring precise trajectory adjustments, stationkeeping, orbit raising, and interplanetary travel, where their high efficiency and long-duration operation are advantageous.
While electric propulsion systems offer significant benefits in terms of fuel efficiency and continuous operation, they usually provide lower thrust levels than chemical rockets. As a result, they are better suited for missions with longer travel times and less need for rapid changes in velocity.