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Is helium used as a hydraulic fluid to power the grid fins? Or for engine gimbaling?

What are the pros and cons of using helium instead of hydraulic fluids?

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    $\begingroup$ Do you mean "hydraulic" where it says "hydrolock"? $\endgroup$ – Organic Marble Jan 3 at 19:41
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    $\begingroup$ Assumes facts not in evidence. I haven't seen any sources that claim He is used as a working fluid, and it would be a spectacularly bad choice because you'd have to cool the entire system to 4 K. $\endgroup$ – Hobbes Jan 3 at 20:22
  • $\begingroup$ I don't know whether it is in F9, but it could be used in pneumatic actuators as a gas. Many propulsion system valves in the shuttle were pneumatically actuated by high pressure helium gas. $\endgroup$ – Organic Marble Jan 3 at 20:25
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    $\begingroup$ @OrganicMarble They do use Helium to expand the landing legs I believe. $\endgroup$ – geoffc Jan 3 at 20:38
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    $\begingroup$ The reason why propellant is used in the engines for hydraulic fluid is that propellant is available in the engine anyway. The reason why it's not used for the grid fins is that those are on the opposite end of the rocket from the propellant tank, and it is easier to put fluid tanks at the top of the rocket than run plumbing through the entire rocket. $\endgroup$ – Jörg W Mittag Jan 4 at 0:17
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It was thought, initially that the grid fins used RP1 propellant as a hydraulic fluid, when it was using an open system (The fluid is ejected after use until you run out, which they did). After a landing failure, they changed to what they call a "Closed System" where the system reuses the hydraulic fluid, but I cannot find a reference to that actually being the case.

It makes sense in an open system, as it would not need yet another working fluid in the Ground Support Equipment. In a closed system, would also make sense to switch to a proper commercial hydraulic fluid.

Helium is used to pressurize the fuel tanks, also slightly to inert the tanks since Helium won't burn.

As the fuel or oxidizer is used up, injecting helium keeps the stage from collapsing. (Imagine a plastic soda pop bottle, suck out some to drink, with a seal around the mouth and the bottle collapses. This would be considered terrible in a booster).

The landing legs are opened by pressurized helium.

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    $\begingroup$ Do you have any references to back up your assertions? $\endgroup$ – Organic Marble Jan 3 at 20:27
  • $\begingroup$ The only thing in this comment that isn't (to my knowledge) confirmed is RP1 as hydraulic working fluid. Unfortunately this was all pieced together from tweets and miscellaneous sources so I don't have time to dig up sources yet. Far as I know actual grid fin operation is guesswork, other than it being hydraulic, starting as an open system, switching to closed at some point. Likely power sources include He or N2, and working fluid could be any of a variety of aerospace grade fluids or RP1. $\endgroup$ – Saiboogu Jan 3 at 20:52
  • $\begingroup$ Thanks, the first sentence was especially what I would have liked to have confirmed. $\endgroup$ – Organic Marble Jan 3 at 21:03
  • $\begingroup$ References in related questions: space.stackexchange.com/questions/7771/… and for the engine gimbal system: space.stackexchange.com/questions/32234/… $\endgroup$ – Hobbes Jan 3 at 21:39
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    $\begingroup$ Source on going closed (he mentions it in a few random tweets - this isn't the first, but works) twitter.com/elonmusk/status/878823434268033025?lang=en $\endgroup$ – Saiboogu Jan 4 at 14:52
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Helium is a gas at any reasonable temperature, so a system using helium would be pneumatic.

Pneumatic systems are less suited for exact position control than hydraulics: because gases are compressible, it's difficult to predict the piston position from a given input. In hydraulics there's an exact relationship so you have good position control, and stability under load.

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