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Comment by Robotbeat | Hacker News Reader

1) won’t condense on the rest of your spacecraft.

2) won’t corrode your thruster as much.

3) Chemical and fire safety aren’t concerns.

4) Is a fluid already so doesn’t need to be heated to flow.

Also, being monotomic makes it easier to model everything. A more complex molecule could have weird chemistry that ends up producing solid particles that impinge or maybe condense on your spacecraft (coating lenses, etc).

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None of these are really the primary reason why we use noble gasses. It has to do with the ionization energy of the gas, which tells us how much energy we have to spend per molecule to ionize the gas.

https://en.wikipedia.org/wiki/Ionization_energy

However noble gases don’t have the lowest ionization energy per unit molecular mass. Bismuth, Cesium, etc do significantly better on those metrics than even Xenon does, and Xenon is better than Krypton.

http://aerospace.mtu.edu/__reports/Conference_Proceedings/20...

EDIT: and iodine, ie the subject of this study, is also significantly better than Xenon in this metric: “ A possible alternative is iodine, which is much more abundant and cheaper than xenon and can be stored unpressurized as a solid. In addition, both atomic and diatomic iodine have a lower ionization threshold, and diatomic iodine has a relative mass that is almost twice that of xenon.”

How do you gasify it if you store it as a solid? Heat?

Iodine? Keep it cold and it stays solid. But it doesn’t liquify; it is one of the elements that can sublimate from solid to gas (not sure of the physical chem here, but the transition temp to liquid vs gas from solid is so close that it doesn’t have a distinct liquid phase(?)).

So I’m imagining a solid block of iodine (it is in cold space), and when the engine needs thrust, some current from the solar cells is passed through a heating element to drive sublimation + lots of complex rocket science that I do not know of.

If you are referring to the noble gases, I don’t think they are cooled to the point of solids; just high pressure liquids..

I designed iodine thrusters for a different company. Your description of how you store/flow the iodine is correct and it's no more complicated than that. It's actually an easy propellant to work with due to its easily controlled vapor pressure (through heat) - the main issue with designing these systems is dealing with how outrageously corrosive iodine is. It. Eats. Everything.

You can technically liquefy/boil iodine (I've done it a few times). It happens around 110C IIRC.

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