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The discussion over Falcon 9 failure and its hydraulic system involved many points over difficulties of pumping the liquid, reuse of the fluid as fuel, nomenclature etc.

What I missed was one crucial point: What's the benefit of using gas-pressurized hydraulic fluid instead of making the system pneumatic in the first place? It seems using gas pressurization beats all benefits of hydraulic over pneumatic - the incompressible fluid vs compressible gas - when the fluid can still back up and compress the pressurization gas.

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SF.
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    I don't understand your last sentence. – Hobbes Dec 21 '15 at 10:35
  • @Hobbes: One of most common problems with hydraulic systems is air entrainment - a serious problem thwarting efficiency of the system as the compressibility of gas creates a far greater play between hydraulically bound elements (you floor the brake pedal and the pressure can't overcome the strength of brake's return springs). In the gas-pressurized system gas entrainment is an inherent property of the system. – SF. Dec 21 '15 at 10:45
  • If you use a bladder to separate the gas from the hydraulic fluid, a gas-pressurized hydraulic system has no issues with gas bubbles. – Hobbes Dec 21 '15 at 12:28
  • @Hobbes: Except for that one huge bubble just past the bladder. There's nothing stopping the fluid from backing up and pushing the bladder back. – SF. Dec 21 '15 at 12:31
  • That is an issue if the object being moved by the hydraulic cylinder requires more force than the hydraulic system can supply. But that's a condition you have to avoid anyway. For the F9 system, they'd only run into this if one of the grid fins seized up. In normal operation, you only need to worry about air bubbles between the control valve and the hydraulic cylinder. – Hobbes Dec 21 '15 at 12:48
  • @Hobbes: Except if they used just gas, they would be transmitting exactly the same pressure, with about the same compressibility and no worry of running out of fluid. It's just that this pressurization system nullifies about all benefits of a hydraulic system over pneumatic. It strikes me as "Let's grow some carrots, sell them on the market and use the money to buy some celery". – SF. Dec 21 '15 at 12:54
  • The hydraulic systems in the shuttle had gas-pressurized accumulators in them which served several functions; one was to ensure that there was pressure at the pump inlets in free fall. So I don't think you can claim that gas pressurized hydraulic systems are inherently bad; these systems served well for 30 years. (They were closed systems of course). I'm baffled by your comment about the "same compressibility" though; surely you don't mean the hydraulic fluid and gas? – Organic Marble Dec 25 '15 at 03:58
  • @OrganicMarble: So the compressed gas was at the low-pressure end of the system, where it wouldn't hurt (unless it got to the high-pressure side). – SF. Dec 25 '15 at 07:13
  • And yes, I mean the same compressibility with pneumatic, and aerated hydraulic system with a considerable volume of entrained gas at equivalent pressure. Think of it: take a pure pneumatic system, a pneumatic system with a couple cm^3 of oil head before the piston, a pneumatic/hydraulic system with 50:50 proportions of gas and oil, and one where there's like 5% of gas, trapped at a random corner of the high-pressure side. They will ALL behave as pneumatic! Only if you reduce the amount of gas to a scarce few cm^3 the system starts behaving as hydraulic. – SF. Dec 25 '15 at 07:16

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The hydraulic fluid used to steer the fins is actually rocket fuel. After performing the function of steering the fins, the fuel drains into the main tanks to be burned by the engines.

This setup saves the weight of a hydraulic pump and dedicated fluid. Since the fins are useless at low airspeeds, i.e. at final landing, nitrogen (cold gas) thrusters are used instead at that stage. At this point, all the remaining hydraulic fluid can be dumped into the fuel tank, so the fuel budget is in fact constant no matter how much fin control was needed and no additional weight for a dedicated hydraulic fluid was needed.

Arguably, the fins could have been electrically controlled. However, that would require batteries and electric motors, which are more finicky than hydraulic motors.

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