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According to many news articles, the crash of the Falcon 9 during landing on 1/10/2015 was due to running out of hydraulic fluid for the steerable hypersonic grid fins. This was seemingly confirmed the Musk's tweet "Upcoming flight already has 50% more hydraulic fluid, so should have plenty of margin for landing attempt next month."

Why, oh why, do the fins consume hydraulic fluid?? Hydraulic fluid is normally used in a closed system that only consumes the fluid via leaks, right? Are the leaks due to temperature changes so bad, and so unsolvable, that the only solution is to just have more in reserve?

Craig McQueen
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pbristow
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1 Answers1

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Normal hydraulic systems are closed. When hydraulic fluid is squeezed out of a cylinder, it returns to a holding tank ready to be pressurized again by the pump.
SpaceX have confirmed the Falcon 9 uses an open system instead:

Hydraulics are usually closed, but that adds mass vs short acting open systems. F9 fins only work for 4 mins. We were ~10% off.

In a rocket, you don't want to run a pump (because pumps consume a lot of power, and their drive system is heavy). Instead, you use a tank of e.g. nitrogen to pressurize the fluid. This means you can't return the used fluid to the pressurized reservoir (you'd have to use a pump to get the fluid to a higher pressure than the reservoir, which defeats the point). So in this case, the used fluid can't be reused.

There are exceptions. The Shuttle SRBs used a hydrazine-powered power unit to drive a hydraulic pump for nozzle control. But hydrazine is something you want to avoid if you can (it's nasty stuff). There's a tradeoff between a simple (but limited-life) nitrogen pressurization system, or a (more complex but doesn't run out of hydraulic fluid) hydrazine power unit.

Hobbes
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    The first Delta III launch failed because it ran out of hydraulic fluid ~70 seconds after launch. The engine nozzles were limit cycle gimbaling. – Organic Marble Jan 11 '15 at 15:55
  • Ahh, so obvious! I didn't know an "open system" exists in the hydraulic world, but it makes perfect sense. Of course, calling it "hydraulic fluid" seems like a misleading name. In almost any other field the term would be "propellant". But I digress :) Thanks! – pbristow Jan 11 '15 at 22:14
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    Could STS SRBs not have used hydrogen peroxide instead of hydrazine? Equally nasty? Lower energy density? – Anthony X Jan 12 '15 at 02:55
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    @Anthony, good point. Can you create this as a new question? It deserves more than a comment. – Hobbes Jan 12 '15 at 07:23
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    @pbarranis: Why would you call the fluid that drives pistons to control the rocket's tail fins "propellant"? – slebetman Jan 13 '15 at 06:47
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    @slebetman It's definitely a potAto/potaato situation. If they're using compressed gas, like nitrogen, to move the fins directly then just like in an aerosol can, it seems to me to be a propellant. If the compressed gas is pushing a liquid to move the fin, then I guess it's reasonable to say the gas is the propellant and the liquid is a hydraulic fluid. I work in software tho... our industry has no legs to stand on while complaining about misnomers in other industries :) – pbristow Jan 15 '15 at 14:49
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    @pbarranis from what's been described, the nitrogen provides pressure, but the working fluid is a liquid (i.e. hydraulic, not pneumatic) – Nick T May 27 '15 at 14:20
  • @NickT You are absolutely right - thank you for explaining why the terminology seemed broken in my head. Reason: broken head, not terminology :) – pbristow May 27 '15 at 18:34
  • Personally, I wonder why not a simpler solution based on a couple of valves: two tanks, one as source of the fluid, the other for return fluid. A pair of valves decides which of the tanks sources the fluid and which collects it, and another pair pressurizes the source tank and depressurizes the return tank. This could actually be driven from a single actuator for the four valves. – SF. Dec 21 '15 at 03:21