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spottedgeckgo
04-12-2015, 08:11 PM
Normally, I consider myself the physics guy, but I'm coming up short on my research. I've seen various types of gasses suggested for fuel to ionize and make these engines work. H and He because they are so abundant, and Ar or Kr are the current ones being utilized in testing. Is it generally going to be better or more efficient to use a heavier gas?

My thoughts on this, is that you can store a heavier amount of Ar, for a fixed size of tank, than you can H or He, therefore your fuel weight is higher for the same sized tank and your Delta-V numbers go up. But it takes more energy to get the heavier atoms moving as well. Would an argument against liquid helium or compressed hydrogen mainly revolve around the tank size required to store enough of it for something like interplanetary travel? Or is there something here that I'm missing entirely? Since H and He are so light, will they produce higher exhaust velocities for the same amount of electrical energy put in to the system? Thanks guys.

Muppster
04-12-2015, 09:16 PM
Let me google that for you. VASIMR propellant (http://www.adastrarocket.com/aarc/faq1).

King Neptune
04-12-2015, 10:55 PM
I had never thought about it before, but an inert gas would be preferable because wouldn't react with anything, and hyrogen passes through tank walls, so one would need to take extra to cover the loss. The exhaust velocity times mass is what one would be interested in, and a given energy input would produce the same exhaust energy regardless of anything else. Argon and Krypton would have less velocity but more mass compared with Helium.

The only advantages would be in storage. As you mentioned, a given volume of Ar or Kr has more mass than an equal volume of He, and it would required less energy to compress the heavier gas, I think.

I would think that any relatively unreactive gas would be O.K., but the mas per volume gives an advantage to Ar and Kr.

spottedgeckgo
04-13-2015, 03:27 AM
Thanks King Neptune. Yea, that made sense in my head, but I wasn't sure if maybe those gasses would present problems of their own. Nitrogen is another that comes to mind, as it can be stored in liquid form, but I think Ar and Kr would still allow you to store more mass easily. Nitrogen also being fairly abundant in the solar system gives it so potential advantages too.

Muppster, I was trying to search the AD homepage earlier and didn't pull that page while I was fishing around. Thank you for the link. But that further compounds the question. What advantage would a lighter gas have on Isp and Delta-V I wonder.

King Neptune
04-13-2015, 03:57 AM
Thanks King Neptune. Yea, that made sense in my head, but I wasn't sure if maybe those gasses would present problems of their own. Nitrogen is another that comes to mind, as it can be stored in liquid form, but I think Ar and Kr would still allow you to store more mass easily. Nitrogen also being fairly abundant in the solar system gives it so potential advantages too.


I was thinking about nitrogen while I was writing that. It is common in space, and N2 is stable and not very reactive. I've never had anything to do with ionizing any, but I would think that breaking the bond would leave both atoms ionized.

It's something to think about.