Why use Helium

Why wouldn't hydrogen gas work?

Pressurizing the fuel side with hydrogen might work in theory, but there are several practical complications:

- Hydrogen embrittlement. Exposure to hydrogen damages many kinds of metal.
- Gases leak, especially gases with a low molecular weight, which is no big deal with helium, but can create an invisible fire with hydrogen.
- Mistakes happen, and accidentally pressurizing the LOX tank with GH2 would be a disaster.
- Using two pressurant gases instead of one adds complexity, which reduces operability, increases the number of things that can go wrong, and costs effort, time, and money.
- Everybody uses helium pressurants; hardly anyone uses H2. This would make it a "science project", which adds R&D costs, and most importantly wastes time.

Helium is expensive but not expensive enough to justify switching half the vehicle to hydrogen.

Why couldn't a flammable gas be used with the lox too? Is the cryogenic liquid going to ignite before it gets to the nozzle?

Are the alloys they're using susceptible to hydrogen embrittlement?

How about using compressed methane gas instead of helium? Even if it can only be used for the fuel side it would cut the usage of helium by half and it would add fuel to the rocket unlike helium.

There is a recent thread on Arocket which discusses why Helium is used. Basically, the benefits outweigh the costs.

what thread?

You will need to subscribe to arocket to see the archive -
http://exrocketry.net/mailman/listinfo/arocket

Then do a search - it's been the last two/three weeks when this has been discussed.

What is the operating pressure of the AA rocket motors if available? combustion chamber pressure determins most everything else about the fuel storage and delivery system.

I am amusing myself with liquid and hybrid rockets again so practical information is appreciated. I am consider using common CO2 as a pressurization gas for both the fuel and oxydizer.

GregW