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Comment by motoboi | Hacker News Reader

Not only is pressurized gas used to avoid the tanks from crumpling from negative pressure, they cannot sustain themselves without internal pressure.

This gas need to be as inert as possible, hence helium. Another noble gas could in theory be used, but I think price is the reason.

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Not just price, but density is the reason. The next noble gas up is neon, which is quite scarce and more expensive than helium, and denser anyways. After that it's argon, which is cheap and plentiful, but about ten times denser, so it's not nearly as good as helium.

Nitrogen would be the obvious choice but it dissolves in liquid oxygen, making usage harder. Soyuz however does use nitrogen.

Gases at the same pressure and temperature have the same molecular density, so lowering molecular weight translates directly into lower total weight of pressurant gas needed.

Alternately, if the propellants allow, it's possible to use those in gaseous form for pressurization (by passing some of it through a heat exchanger). This increases the weight of pressurant gas wasted but it saves the weight of the Helium system, and the complexity, so ultimately it's a big win. Unfortunately, this doesn't work with kerosene so the Falcon 9 needs a Helium system.

Could they use oxygen gas for pressurization, since the helium tank in a Falcon 9 seems to be inside the liquid oxygen tank? Or would it be too dangerous?

You can use oxygen gas for pressurization of oxygen tanks, but it introduces a new set of problems, since you've got to get a stable pressure of oxygen gas inside a container with a huge volume of sub-cooled liquid oxygen, and you have to have a high-pressure tank in there anyways to pre-load pressurized oxygen gas, so that you can get it into the tank fast enough. Also it means you've got reactive oxygen gas all around, so you have to redesign things for that.

However, this kind of oxygen-gas pressurization is exactly what's planned for SpaceX's next-generation oxygen-methane rockets.

The Space Shuttle used autogenous pressurization for its main engines / external tank, and the Delta IV is eventually planned to do so as well. And as you mentioned SpaceX plans to use the same system with their next generation of rockets. It is tricky to get working correctly and requires a larger up front engineering effort, but the payoff in simplicity of operations is significant. Additionally, it provides gaseous propellants that can be used for other purposes, such as attitude control thrusters.

Yes, it's great if you can manage it. But helium is so pleasingly inert and cold, I can see why it's not always the choice.

Too dangerous, it's not a good idea to combine pressurized high temperature oxygen and kerosene. Also, I think the Oxygen would dissolve in the kerosene, which would complicate the combustion process and make it less predictable.

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