Microwave electrothermal thruster

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

1. The propellant gas is first inputted tangentially into the MET through the nozzle, which allows the plasma to form.
2. By doing so tangentially, there will be a vortex flow (circular flow)  in the system, which creates a cool environment for the plasma to be stabilized.
3. In order for the plasma to ignite at low levels of electromagnetic power to create thrust, it must be at a low pressure; however, if the plasma has already been ignited, it will be able to survive in the high pressures.
4. The free-flowing plasma is heated and released through the nozzle, thereby creating thrust.

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:

1. The microwave electrical field causes the electrons to speed up, which then causes them to have collisions with the molecules and atoms inside the plasma.
2. Through the collisions, there is a transfer of energy to the atoms and molecules in the plasma.
3. The energy is then converted into heat energy by having inelastic collisions.[3]

Thrust is the force that is applied on the rocket caused by when the propellant is released. The formula for thrust is given as:

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

Where thrust is given as ${\displaystyle \tau }$ in Newtons(N), ${\displaystyle {\dot {m}}}$ as mass flow rate in kilograms/second(kg/s), ${\displaystyle u_{e}}$ as exhaust velocity in meters/second(m/s), ${\displaystyle p_{e}}$ as exit pressure, ${\displaystyle p_{a}}$ as atmospheric pressure, and ${\displaystyle A_{e}}$ as nozzle exit area in meters^2(m^2).

Specific impulse is how efficiently the fuel of the MET is used to create thrust. The formula for specific impulse is given as:

${\displaystyle I_{sp}=\tau /{\dot {m}}g}$

Where ${\displaystyle I_{sp}}$ is given as specific impulse, ${\displaystyle \tau }$ as thrust in N, ${\displaystyle {\dot {m}}}$ as mass flow rate in kg/s, and ${\displaystyle g}$ as the gravitational acceleration of the earth.

When applying the conservation of momentum law, the relationship between mass of propellant and initial mass of the spacecraft can be shown as:

${\displaystyle M_{p}/M_{i}=1-e^{-\Delta v/I_{sp}g}}$

Where ${\displaystyle M_{p}}$ is given as propellant mass, ${\displaystyle M_{i}}$ as initial spacecraft mass, ${\displaystyle \Delta v}$ as change in velocity, ${\displaystyle I_{sp}}$ is as specific impulse, and ${\displaystyle g}$ as earth’s gravity.[4]

The MET’s main purpose is spacecraft propulsion. The energy that is created is meant to be converted into kinetic energy, which will produce thrust in space. Some tasks include orbit raising and stationkeeping. 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. This includes the maintenance of satellites at certain positions.[5]

This is one of the more recent applications of a microwave electrothermal thruster created in August 2020. This invention used the functions of a MET to create a precise control system. When the MET changes the energy from electromagnetic waves to propellant, it allows for the small impulses of the MET to give control over the satellite.[2]

This invention is pertaining to the MET adaption for space electrothermal propulsion. In order to control the altitude of a satellite/spacecraft and for primary propulsion, the tunable frequency MET was provided. Instead of a magnetron (microwave generating device), there were alternative constructional features which included using generators and semiconductors. 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 can be found in many different parts of the cosmos.[3]

In general, electrothermal thrusters have the lowest efficiency among most other electric propulsion systems. MET ranks lower than most electrostatic thrusters such as ion thrusters.[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).[7]