Microwave electrothermal thruster, also known as MET, is a propulsion device that converts microwave energy (a type of electromagnetic radiation) into thermal (or heat) energy. These thrusters are predominantly used in spacecraft propulsion, more specifically to adjust the spacecraft’s position and orbit. A MET sustains and ignites a plasma in a propellant gas. This creates a heated propellant gas which in turn changes into thrust due to the expansion of the gas going through the nozzle. 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.

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.

The resonant cavity is the round overlapping section waveguide that is shorted by the two endplates. The cavity is near the separation plate. 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. The antenna is used to input the microwave power. Although most of the power is absorbed by the plasma, some of it is reflected. Another part of the MET is the plasma. In some cases, plasma is also referred to as the fourth state of matter. The plasma is the main portion of the MET. It is created inside of the system by heating the propellant and is exhausted to generate thrust. The last part of the MET is the dielectric separation plate. This piece of the MET allows both parts of the cavity to be controlled at various pressures.

In order for the MET to create thrust, it must go through a 4 step process of converting electrical energy into heat energy.

During this process, the antenna section is held at atmospheric pressure to ensure that there is no plasma formation close to the antenna. It also ensures that the separation plates are not held at two significantly different pressures, which would put stress upon the two plates.

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. The formula for thrust is given as:

${\textstyle \tau ={\dot {m}}u_{e}+(p_{e}+p_{a})A_{e}}$