Electric Propulsion

The future of rockets

Electric propulsion is a technology aimed at achieving thrust with high exhaust velocities, which results in a reduction in the amount of propellant required for a given space mission or application compared to other conventional propulsion methods. Reduced propellant mass can significantly decrease the launch mass of a spacecraft or satellite, leading to lower costs from the use of smaller launch vehicles to deliver desired mass into a given orbit or to a deep-space target.

In general, electric propulsion encompasses any propulsion technology in which electricity is used to increase the propellant exhaust velocity. There are many figures of merit for electric thrusters, but the mission and application planners are primarily interested in thrust, specific impulse, and total efficiency in relating the performance of the thruster to the delivered mass and change in the spacecraft velocity during thrust periods. While thrust is self-explanatory, specific impulse is defined as the propellant exhaust velocity divided by the gravitational acceleration constant g, which results in the unusual units of seconds. The total efficiency is the jet power produced by the thrust beam divided by the electrical power into the system. Naturally, spacecraft designers are then concerned with providing the electrical power that the thruster requires to produce a time thrust, as well as with dissipating the thermal power that the thruster generates as waste heat.

Electric thrusters propel the spacecraft using the same basic principle as chemical rockets — accelerating mass and ejecting it from the vehicle. The ejected mass from electric thrusters, however, is primarily in the form of energetic charged particles. This changes the performance of the propulsion system compared to other types of thrusters and modifies the conventional way of calculating some of the thruster parameters, such as specific impulse and efficiency. Electric thrusters provide higher exhaust velocities than gas jets or chemical rockets, which either improves the available change in vehicle velocity (called Δv or delta-v) or increases the spacecraft’s payload mass limit. Chemical rockets generally will have exhaust velocities of 3 to 4 km/s, while the exhaust velocity of electric thrusters can approach 100 km/s for heavy propellants such as xenon atoms, and 1000 km/s for light propellants such as helium.

Electric thrusters are generally described in terms of the acceleration methods used to produce the thrust. These methods can be separated into three categories: electrostatic, electromagnetic, and electrothermal. Common electric propulsion thruster types are described as follows

Resistojet

Resistojet is electrothermal devices in which the propellant is heated by passing through a resistively heated chamber or over a resistively heated element before entering a downstream nozzle. The increase in exhaust velocity is due to the thermal heating of the propellant, which limits the specific impulse to low levels (<5005)

Arcjet

An arcjet is also on an electrothermal thruster that heats the propellant by passing it through a high current arc in fine with the nozzle feed system. While there is an electric discharge involved in the propellant path, plasma effects are insignificant in the exhaust velocity because the propellant is weakly ionized. The specific impulse is limited by the thermal heating to less than about 700 s for easily stored propellants.

Ion Thruster

Ion thrusters employ a variety of plasma generation techniques to ionize a large fraction of the propellant. These thrusters then utilize biased grids to electrostatically extractions from the plasma and accelerate them to a high velocity at voltages up to and exceeding 10 kV. Ion thrusters feature the highest efficiency (from 60% to >80%) and very high specific impulse (from 2000 to over 10,000 s) compared to other thruster types.

Hall Thruster

This type of electrostatic thruster utilizes a cross-held discharge described by the Hall effect to generate the plasma. An electric field established perpendicular to an applied magnetic field electrostatically accelerates ions to high exhaust velocities, while the transverse magnetic field inhibits electron motion that would tend to short out the electric field. Hall thruster efficiency and specific impulse are somewhat less than that achievable in ion thrusters, but the thrust at a given power is higher said the device is much simpler and requires fewer power supplies to operate.

Electrospray, Field Emission Electric Propulsion Thruster

These are two types of electrostatic electric propulsion devices that generate very low thrust (<1 mN). Electrospray thrusters extractions or charged droplets from conductive liquids fed through small needles and accelerate them electrostatically with biased, aligned apertures to high energy. Field emission electric propulsion (PEEP) thrusters wick or transport liquid metals (typically indium or cesium) along with needles, extracting ions from the sharp tip by field emission processes. Due to their very low thrust, these devices will be used for precision control of spacecraft position or attitude in space.

Pulsed Plasma Thruster

A pulsed plasma thruster (PPT) is an electromagnetic thruster that utilizes a pulsed discharge to ionize a fraction of a solid propellant ablated into a plasma arc, and electromagnetic effects in the pulse to accelerate the ions to high exit velocity. The pulse repetition rate is used to determine the level.

Magnetoplasmadynamic Thruster

Magnetoplasmadynamic (MPD) thrusters are electromagnetic devices that use a very high current arc to ionize a significant fraction of the propellant, and then electromagnetic forces (Lorentz forces, J x B) in the plasma discharge to accelerate the charged propellant. Since both the current and the magnetic field are usually generated by the plasma discharge, MPD thrusters tend to operate at very high powers to generate sufficient force for high specific impulse operation, and thereby also generate high thrust compared to the other technologies described above.

There are many more types of electric propellers in development. The two main types of electric thrusters used are Ion Thrusters and Hall Thrusters. It is speculated that these types of thrusters would be able to successfully carry out a manned Mars mission and a trip to farther planets like Jupiter, Saturn, or Uranus.

This article was written by Sanskar Arora, IEEE-SPS, VIT Vellore