Author Topic: Open hydraulic schemes as implemented on Falcon9 grid fins  (Read 48545 times)

Offline hrissan

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I just had a conversation with people who believe that the hydraulic fluid from the fins on Falcon9 is dumped overboard in the "open scheme".

My opinion was that it goes from the high pressure tank (accumulator, bladder tank, etc) iand ultimately ends in the low pressure (dump) tank, otherwise it would be hard to test fins without spilling very slippery fluid around. Dump tank should not weight that much in comparison to high-pressure tank.

Any expert here who knows how it actually works?

P.S. "Designed" a reusable system :) and attached the picture... Two bladder tanks, left one is highpressure connected to high pressure helium, right one low pressure, open to the atmosphere in flight, connected to the ground preflight.

Before launch the air is supplied to the right side from ground, and the fluid is pushed to the left tank through the check valve.

During flight the fluid is pushed by helium from left tank to the right tank moving fins...
« Last Edit: 01/10/2015 11:41 PM by hrissan »

Offline Kabloona

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Your design makes sense.

I don't claim to know whether they do in fact capture the fluid in a tank or just vent it overboard. I lean toward the idea of just dumping it overboard, though. It saves the weight, cost, and plumbing of the collection tank, with no major downside that I can see. The fluid could be dumped out a port near the forward end of the stage, and since the stage is traveling backwards, the slipstream would be carrying the fluid "up" and away from the stage, instead of spreading it down the sides where it might cause problems.

And it may be a clue that Elon mentioned in his tweet that the open system saves mass. The maximum mass savings is obtained by eliminating the collection tank and plumbing.

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

OTOH, we have seen F9R grid fin test videos without seeing hydraulic fluid spraying out, so you could be right.
« Last Edit: 01/11/2015 12:08 AM by Kabloona »

Offline Zach Swena

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How do we know that this is truely hydrolic fluid?  When the legs were announced, they said they were actuated by helium instead of hydrolic fluid to save weight.  Why wouldnt they just use that for the grid fins?

If they do use hydrolic fluid, they say they ran out so I would expect a vented expendable fluid system.

Online Robotbeat

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They said they ran out of hydraulic fluid. That means liquid (it's a trade term, like "brake fluid").

Pneumatics wouldn't work well for such an actuator since pneumatics are mushy. The legs just need to deploy (given their current working scheme), so that works fine for them.
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Online Robotbeat

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They dump the fluid. Saves weight and reduces complexity.


Other rockets which use open hydraulics:
1) Delta III
2) Conestoga


I'm certain there are many, many others. I believe the majority of hydraulic systems for rockets are open, though verifying that is going to be difficult.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Jim

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They dump the fluid. Saves weight and reduces complexity.

Other rockets which use open hydraulics:
1) Delta III
2) Conestoga


I'm certain there are many, many others. I believe the majority of hydraulic systems for rockets are open, though verifying that is going to be difficult.

Delta IV SRM's, Athena…..


Offline DanielW

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For the Thrust vector control on the main engines, I believe they use RP1 as the hydraulic fluid. Any ideas why this is not a good fit for the grid fins? I can think of.

1) long plumbing adds too much mass.
2) Insufficient tank pressure (needs to work when engines are not running so can't tap off the turbo pump.)
3) needs higher reliability since it will eventually be returning to populated areas.



« Last Edit: 01/11/2015 01:06 AM by DanielW »

Offline Kabloona

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They dump the fluid. Saves weight and reduces complexity.

That was my initial guess, but are you sure? Source?

Offline Excession

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They dump the fluid. Saves weight and reduces complexity.

That was my initial guess, but are you sure? Source?

Is Elon good enough?

Offline Lee Jay

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They dump the fluid. Saves weight and reduces complexity.

That was my initial guess, but are you sure? Source?

Is Elon good enough?

I don't know how they get away with that.  If we spill a pint of hydraulic fluid at work, the next thing is a full-up environmental remediation process.

Maybe they do use one of the bio fluids.

Offline Excession

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #10 on: 01/11/2015 01:39 AM »
Better for the environment than a rocket stage.  ;)

Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #11 on: 01/11/2015 01:48 AM »
They dump the fluid. Saves weight and reduces complexity.

That was my initial guess, but are you sure? Source?

Is Elon good enough?

You may have missed the extensive discussion we've had on this earlier in another thread. Some people have argued that an "open" hydraulic system can have a low-pressure downstream catchment tank which captures but does not recycle the hydraulic fluid. My view was that Elon meant the fluid was simply dumped overboard, as is commonly done on many aerospace systems.

Whatever the case, the word "open" is apparently ambiguous enough that some people believe there is a possibility that the system uses a low-pressure catchment tank. Which is why having confirmation from a source other than Elon's tweet would clarify the matter.
« Last Edit: 01/11/2015 01:51 AM by Kabloona »

Offline Space Ghost 1962

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #12 on: 01/11/2015 01:51 AM »
They dump the fluid. Saves weight and reduces complexity.

That was my initial guess, but are you sure? Source?

Is Elon good enough?

I don't know how they get away with that.  If we spill a pint of hydraulic fluid at work, the next thing is a full-up environmental remediation process.

Maybe they do use one of the bio fluids.

Toxic Substances Portal - Hydraulic Fluids

Issue is usually organophosphates - often additives. Concentration and substantial contact matters.

You can use a variety of common substances too. Pure mineral oil would be a likely one.

So not necessarily that its the hydraulic fluid itself, but an additive that allows for longer term use in a closed system, to decrease the breakdown of the working fluid. If its an open system, there's no reuse, thus breakdown doesn't matter, thus no need for additives.

Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #13 on: 01/11/2015 01:57 AM »
I don't know how they get away with that.  If we spill a pint of hydraulic fluid at work, the next thing is a full-up environmental remediation process.

Try spilling it at 100,000 feet at Mach 4 and I bet no one will notice.  ;)

Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #14 on: 01/11/2015 02:04 AM »
They dump the fluid. Saves weight and reduces complexity.

That was my initial guess, but are you sure? Source?

Is Elon good enough?

I don't know how they get away with that.  If we spill a pint of hydraulic fluid at work, the next thing is a full-up environmental remediation process.

Maybe they do use one of the bio fluids.

Hydraulic fluid could be anything, even water (though not likely due to many issues, including freezing).

Bio fluids would be a good choice, but they can run into trouble due to the cold (vegetable oils tend to turn into shortening at really cold temperatures).

Glycol is a great choice, as are certain High Water Content Fluids (HWCF).

Of course since the hydraulic fluid is at little to no pressure after being used, it could be stored on board in a catch bottle; we're likely not talking about that much mass, unless every kilogram counts for the landing.
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Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #15 on: 01/11/2015 02:06 AM »
cross-posting:

As to this talk of open vs. closed hydraulics:

There are actually many variants, so it's important to get the context correct.

Open 'loop' (hydraulics) is the fluid coming from a reservoir (typically vented to atmosphere, but it could be pressurized, typically with air for low pressures or Nitrogen for higher pressures and to keep the fluid 'dry') and is then delivered via a pumping device to deliver flow to the control valves and/or actuators. The returning fluid is fed back to the reservoir to be re-used.

Closed 'loop' (hydraulics) is the fluid kept in the loop, typically between a pump & actuator (typically a motor). It is only when the actuator is moved that the fluid in the loop moves within that loop. Of course typical hydraulic closed loop systems utilize a small pump to dump oil into the lower pressure of the two sides of the loop (after the work is done, from the motor for instance). And for what is allowed into the loop has to have an equal amount dumped out, and is usually handled by a valve, and is typically cooled & filtered. Properly designed, just the cooler itself could be used to hold the fluid required due to expansion & contraction.

Now the rocket would have to have a closed 'system' (so no fluid escapes), but it also must have either a pump driven by some means to deliver this flow, or it has an accumulator to store the fluid under pressure (think of a water pump & tank on a well system in a house). The tank (accumulator) has to have a pressurant gas (Nitrogen typically for hydraulics to prevent explosion due to hydrocarbons present) at a pressure just under that required to operate the system's actuators. Now if there is no pump in the system (external turbopump or similar driven off a shaft or via hot gases, or an electric pump with a battery), you have to rely totally on the size of the accumulator (tank) to supply that fluid. This fluid typically passes through a pressure reducing valve to drop the pressure to somthing more usable, plus it allows a higher charge pressure in the accumulator to lengthen the amount of time to operate the hydraulics. Once the pressure drops off below the setting of the reducing valve & the minimum that is required to run the actuators, they cease to work effectively.

Although it adds weight, the simplest way to boost the amount of time available to operate hydraulics in this manner (if it is indeed how the system works) is to add more accumulators, or make the existing one(s) larger. Increasing the pressure in the accumulator is possible (gas side) but it requires a stronger tank, and likely a new pressure reducing valve that can handle the higher initial pressure.

Now they can also have the accumulators charged with Helium, and with the propellant tanks also using this gas, if they had an auxilliary bottle on board, they could supplement it with that.
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Offline Lee Jay

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #16 on: 01/11/2015 02:23 AM »
They dump the fluid. Saves weight and reduces complexity.

That was my initial guess, but are you sure? Source?

Is Elon good enough?

I don't know how they get away with that.  If we spill a pint of hydraulic fluid at work, the next thing is a full-up environmental remediation process.

Maybe they do use one of the bio fluids.

Toxic Substances Portal - Hydraulic Fluids

Issue is usually organophosphates - often additives. Concentration and substantial contact matters.

You can use a variety of common substances too. Pure mineral oil would be a likely one.

So not necessarily that its the hydraulic fluid itself, but an additive that allows for longer term use in a closed system, to decrease the breakdown of the working fluid. If its an open system, there's no reuse, thus breakdown doesn't matter, thus no need for additives.

One major use for those additives is reducing the amount of air that is dissolved into the fluid.  That could be a problem with using RP1.

Offline LouScheffer

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #17 on: 01/11/2015 02:35 AM »

I don't know how they get away with that.  If we spill a pint of hydraulic fluid at work, the next thing is a full-up environmental remediation process.

Maybe they do use one of the bio fluids.
You can get hydraulic fluid and machine oils that are rated as safe for human consumption.  These are normally used for lubricating and powering machines that make food-grade products, such as the plastic that is used to wrap meat in the supermarket, or the plastic film under the cap of a pill bottle.

Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #18 on: 01/11/2015 02:46 AM »
The main reason mineral-based hydraulic fluids are toxic are the heavy metals that are added (zinc, among others) to aid in anti-foaming, anti-oxidation, viscosity improver...;hence the term 'additives'.

If the system is essentially one-time use, or controlled (temperature, ETC) you could easily get away with an hydraulic fluid that is environmentally 'sensitive'. It's not 100% biodegradable, but does meet many environmental regulations. Our company uses a product called Environ (made by Petro Canada).

Remembering those who made the ultimate sacrifice for our rights & freedoms, and for those injured, visible or otherwise, in that fight.

Offline Lee Jay

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #19 on: 01/11/2015 03:13 AM »
The main reason mineral-based hydraulic fluids are toxic are the heavy metals that are added (zinc, among others) to aid in anti-foaming, anti-oxidation, viscosity improver...;hence the term 'additives'.

If the system is essentially one-time use, or controlled (temperature, ETC) you could easily get away with an hydraulic fluid that is environmentally 'sensitive'. It's not 100% biodegradable, but does meet many environmental regulations. Our company uses a product called Environ (made by Petro Canada).

I'm not sure the interstage in space and through hypersonic entry while in a rocket plume is all that controlled.

I'm still surprised the grid fins aren't electromechanically actuated.

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