So, the SpaceX grid fins 'ran out of hydraulic fliud'. Huh?

[cvanc]

Is this some kind of open loop hydraulic system where the fluid is actually lost during operation? Never heard of such - is that even a thing?

 

[Peartree]

I didn't understand that either unless, somehow there is no onboard compressor but just one, pressurized, reservoir from which the actuators draw.

 

[markkoelsch]

All things considered I bet it runs off of a pressurized reservoir. Not a lot of room to hide compressors or hydraulics pumps I would think.

 

[mpitfield]

All things considered I bet it runs off of a pressurized reservoir. Not a lot of room to hide compressors or hydraulics pumps I would think.

That seems to make sense, less weight, less technology to maintain or go wrong, just need to add more fluid.

 

[Reinhard]

Is this some kind of open loop hydraulic system where the fluid is actually lost during operation? Never heard of such - is that even a thing?

The Merlin engine uses the fuel - put under pressure by the fuel pump - as hydraulic fluid. I'd guess, this is what is used for the grid fins too.
https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)

EDIT: On the other hand, this scheme requires running engines for hydraulic power, so this might not work for the grid fins.

Reinhard

 

[luke strawwalker]

The Merlin engine uses the fuel - put under pressure by the fuel pump - as hydraulic fluid. I'd guess, this is what is used for the grid fins too.
https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family)

EDIT: On the other hand, this scheme requires running engines for hydraulic power, so this might not work for the grid fins.

Reinhard

It would work if you stored the fuel under pressure in a hydraulic accumulator... basically a big pressure tank with a rubber bladder or piston in it and a compressible gas like nitrogen on the other side. The turbopumps pressurize the fuel, which then goes through a high pressure "bleed line" and check valve to fill the accumulators during flight (perhaps pre-filled and pre-pressurized before flight to ensure that the system is "charged" before liftoff). The check valve keeps the fluid in the pressurized reservoir until the system is needed. It would then have electrically actuated valves taking signals from the control system and directing fluid to the proper hydraulic actuators to move the control surfaces in the desired fashion.

The downside of such a system is that there is a finite amount of pressurized fluid in the system-- once it's exhausted, there is no more source of pressurized fluid to move the control surfaces, so they essentially "stick" in whatever position they were in when the fluid runs out (or pressurization runs out, whichever runs out first, usually fluid). Without the turbopumps creating additional pressure and feeding more fluid into the system to keep replenishing it between valve actuations, which are depleting the fluid supply, the system would run out of fluid and "quit".

It's possible something like this happened. The hydraulic system could be stand-alone as well, using pressurized accumulators to supply a finite supply of hydraulic fluid to the system without ANY engine pressurization or replenishment whatsoever... in either case, it looks like they underestimated the amount of fluid the system would need to complete the reentry and landing cycle, with sufficient margin for a factor of safety, since obviously running out makes for a bad day. They'll either have to increase the amount of pressurized fluid storage (larger or more accumulators) or reduce the flow rates required, or increase pressure (or a combination of these factors) to give them the required amount of hydraulic power needed to complete the mission, with a sufficient safety margin.

It's also possible that the guidance system was overcorrecting or sending overcompensating commands or something to the system, which caused it to expend the pressurized fluid at too high a rate-- in that case, they'll have to work out WHY the system used too much pressurized fluid and come up with a corrective strategy... shorter or less frequent control patterns, perhaps coupled with enlarged corrective surfaces (grid fins), or some combination of factors (perhaps along with an increase in pressurized fluid capacity). The design tradeoffs are multiple and interesting.

I'm actually rather surprised that they didn't use some sort of electrical linear actuators in lieu of hydraulic systems. Most aircraft are increasingly being designed to eliminate to the extent possible hydraulic controls and systems, because they are considerably heavier and more complex (and honestly, more failure prone in a lot of situations) than an all-electric (electromechanical) system using electrically operated linear actuators. Of course aircraft also have considerable electrical generating capacity drawing off the operating engine(s) and usually considerable battery backup, two things which might not be a good tradeoff mass-wise or space-wise on a space launch vehicle. Again, it comes back to design tradeoffs...

Interesting stuff to be sure! OL JR

 

[Tanarin]

Well Musk just clarified what happened. Basically they use an open system and they were off on their guesses by 10%.

Source: https://twitter.com/elonmusk/status/554023312033341440

 

[RAHagen]

But they hit the barge right? Doesn't that mean their guidance was fine and they just didn't slow down to a safe speed in time?

 

[mpitfield]

But they hit the barge right? Doesn't that mean their guidance was fine and they just didn't slow down to a safe speed in time?

This is where I am a bit confused. I thought the fins were for guidance as well as to help bring the rocket under control. If the rocket made it's way to the barge then what role did the lack of hydraulics play?

 

[georgegassaway]

RAHagen and mpitfield, I've replied to your questions in the SpaceX "attempt historic landing" thread:

https://www.rocketryforum.com/showt...-4-47-am-EST-(Jan-10th)&p=1407636#post1407636

- George Gassaway