Microwave electrothermal thruster, also known as MET, is a propulsion device that converts microwave energy[1] (a type of electromagnetic radiation) into thermal (or heat) energy.
A MET sustains and ignites a plasma in a propellant gas.
A MET’s heating feature is like one of an arc-jet (another propulsion device); however, due to the free-floating plasma, there are no problems with the erosion of metal electrodes, and therefore the MET is more efficient.
[2] The MET contains key features and parts that contribute to its efficiency.
The parts include: two endplates (nozzle and antenna), plasma, and a dielectric separation plate.
[1] The resonant cavity is the round overlapping section waveguide that is shorted by the two endplates.
There are two end plates inside the MET: the nozzle and the antenna.
The nozzle’s function is to convert the gaseous plasma into thrust.
In some cases, plasma is also referred to as the fourth state of matter.
It is created inside of the system by heating the propellant and is exhausted to generate thrust.
This piece of the MET allows both parts of the cavity to be controlled at various pressures.
The physical process for what takes place on a molecular level can also be explained in the following manner: Thrust is the force that is applied on the rocket caused by when the propellant is released.
Specific impulse is how efficiently the fuel of the MET is used to create thrust.
[4] The MET’s main purpose is spacecraft propulsion.
Orbit raising is changing the orbit of a ship using propulsion systems, while stationkeeping is maintaining a spacecraft’s position in relation to other spacecraft.
[5] In August 2020, a control system based on the MET was devised.
When the MET changes the energy from electromagnetic waves to propellant, it allows for the small impulses of the MET to provide autonomous control over the satellite.
In order to control the altitude of a satellite/spacecraft and for primary propulsion, the tunable frequency MET was provided.
This made it more efficient allowing the thruster to operate at two separate frequencies.
[6] Relative to other electrothermal thrusters, the MET ranks higher than resistojets and some claim that they may be able to achieve similar performance to arc-jets.
This is based on the supposition that the MET provides higher specific impulse, or in simpler terms more thrust for the amount of fuel.
Another advantage is that because microwaves can be collected and fed directly into the thrust chamber, the MET is extremely compatible with space transport.
Finally, the MET can be run on water vapor as a propellant, which is abundant throughout the cosmos.
[3] In general, electrothermal thrusters have the lowest efficiency among most other electric propulsion systems.
[3] Another disadvantage is that the MET has relatively low thrust compared to chemical rocket engines (although this is true for electric propulsion in general).