Electrothermal Propulsion Explained

Arcjet Propulsion System

Electrothermal propulsion systems are one of the most fun to study because they range from simple concepts to complicated concepts. I chose to draw a diagram and explain my personal favorite electrothermal propulsion system, the arcjet. The arcjet is a very useful electric propulsion system in that it has a lot of potential to be powerful and it is very simple in concept. Like before I will explain the system in simple terms first and then a more advanced explanation along with some design notes.

Simple Conceptual Explanation:

I have labeled the main operating components of the system:

1. Cathode - Electrode
2. Anode - Opposite electrode
3. Insulator with inlets - Non-conducting material

An arcjet is a simple device to understand especially if you just read about the electromagnetic and electrostatic propulsion systems. First a gas flows into the chamber and surrounds the cathode. The cathode and anode have a large potential difference and are set very close to each other which produces a very strong electric field. This electric field is so large that, like the other two propulsion systems, breaks down the gas by bombarding the gas with fast moving electrons that are shooting across from anode to cathode and on their way across collide with gas molecules and give them a lot of energy per unit mass. The high energy density causes the molecules to lose electrons and ionize. Here is where it differs from other systems. The ionized molecules are very energized and get very hot. The chamber that they are being ionized in - between the cathode and anode - is very small. This high heat and small space creates a lot of pressure and it's this pressure that propels the gas outside the chamber and past the nozzle to create thrust. It is very similar to solid rocket boosters in that it is the release pressure created by heat that provides thrust. Except in the case of the SRB (solid rocket booster), the heat is caused by an initial heat source and a continuous chemical reaction and in the case of the arcjet it is the ionization of molecules by the large electric field that provides the heat.

More Quick Explanation (for those that know about basic electric and magnetic principles):

The diagram drawn about shows a cross sectional view of the arcjet propulsion system. The main operating components are numbered:

1. Cathode
2. Anode
3. Insulator (with inlets)

An arcjet is part of the electrothermal propulsion system. It uses a strong electric field between an outer anode and inner cathode to ionize an inlet gas in order to create high energy density through heat and use the heat and small chamber between the electrodes to create high pressure. The high pressure then expels the gas out the nozzle.

Design Notes:

1. Honestly the arcjet is very reliable and requires little design considerations as far as the component configuration in order to operate correctly. When initially starting the gas flow and turning on the power source, the electric arc will most likely occur at the parts of the cathode that are pointed and/or close to the anode, but only milliseconds after this will it balance itself out to a point were the arc passes through the nozzle and outward towards the inner walls as shown below. Most will draw it this way if you were to do a search of arcjet pictures on the web, and it is misleading for those that don't understand electrodynamics and are wanting to understand electric arcs. If there was no gas flow into the chamber continuously then the arc would not look like this. This arc configuration only happens because the gases are flowing out of the chamber at high rates (high mass flow rate). This high mass flow rate is what carries the molecules that are ionized out of the chamber. The arc raps back around because it is still trying to return to the anode on a path of least resistance. So you can imagine that the more power supplied to the arcjet the more bellowing out the electric arc will do.

I try my best to make the information provided as accurate as possible. If you spot a mistake or disagree with what I have wrote please let me know. Thank You.

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