Random Electric Thruster Question

[Nori]

Been searching for a answer to this but can't seem to find it...

For the theoretically possible electric thrusters (Hall, VASMIR, Magnetoplasmadynamic) why do they use the propellent that they use?

For instance, most of the "Ion" engines actually in use seem to use Xenon gas. I imagine that is used over Argon because of higher densities? But if that is the case why no use Krypton?

Then you have VASMIR which they have been testing with Argon. Is that because it is cheaper to test with Argon? Or could it use Xenon?

Magnetoplasmadynamic is supposed to use Hydrogen, but I believe it could be used with Argon and or Xenon. Not sure what the advantages/disadvantages of each is.

[MoparGamer]

Xenon is the preferred propellant for ion propulsion of spacecraft because of its low ionization potential per atomic weight, and its ability to be stored as a liquid at near room temperature (under high pressure) yet be easily converted back into a gas to feed the engine. The inert nature of xenon makes it environmentally friendly and less corrosive to an ion engine than other fuels such as mercury or caesium. Xenon was first used for satellite ion engines during the 1970s.^[165] It was later employed as a propellant for JPL'sDeep Space 1 probe, Europe's SMART-1 spacecraft^[20] and for the three ion propulsion engines on NASA's Dawn Spacecraft.^[166]

Source: http://en.wikipedia.org/wiki/Xenon

Not sure about your other questions (To lazy to look)

[Ralathon]

The trick is with the ionization. Before you can use an atom in an ion engine you have to ionize it and that takes power. Power that you'd rather use to accelerate your atom.

Xenon is convenient because it is very easy to ionize, pretty heavy and very easy to handle.

[christok]

There are three types of electric propulsion:

- Electrothermal is a normal rocket except the propellant is heated electrically instead of chemically.

- Electrostatic, AKA ion engines, accelerate electrically charged ions by means of electrostatic fields.

- Electromagnetic engines, AKA magnetoplasma engines, accelerate a plasma using the interactions between electric currents and magnetic fields.

Besides the important points MoparGamer and Ralathon raised about xenon for ion engines, these general principles apply:

- You generally want the propellant to have the lowest possible molecular mass because you get higher exhaust velocity. This favours hydrogen and to a lesser extent helium and is the reason why some electric and nuclear designs use those fluids.

- HOWEVER electrostatic thrusters require high voltage/low current electrical systems for high propellant particle masses and low voltage/high current systems for low particle masses. This strongly favours high atomic masses to the extent that there has been work on droplet-propellant ion engines to further increase mass. Xenon wins hands down over hydrogen or helium in this case.

- Low density makes for bulky and heavy tanks.

- At least in thermal engines you want the propellant to have as few internal degrees of freedom as possible because these suck out some of the energy that would otherwise increase speed. This favours noble gasses such as helium and xenon.

- Cost. The ideal propellant is not the one with the highest delta-v per unit mass, it's the one with the highest delta-v per dollar spent. (The latter may tend to be the same as the former for especially later stages in the mission.) This favours, for example, hydrogen over helium.

- Toxicity/corrosiveness/fire hazard. You want to make it as easy as possible to produce, store and launch the spacecraft and associated propellants. This tends not to be such a big problem with electric thrusters but favours noble gasses over hydrogen.

Some thrusters, such as the (electrothermal) resistojet, can in principle run on almost any fluid. Don't we all dream of ISRU...

[Idobox]

resistojet is lame, arcjet is so much better for electrothermal, and it is not much more difficult to make. It is actually widely used in attitude keeping for small satellites.

[christok]

resistojet is lame, arcjet is so much better for electrothermal, and it is not much more difficult to make. It is actually widely used in attitude keeping for small satellites.

And arcjet is lame because it wears out quickly. Anyway, I mentioned resistojet simply as an example of one electric thruster that can use a wide variety of propellants. The main reason for using an arcjet is that it produces much more thrust than most electrical thrusters, which is not equally important for all applications.