Author Topic: Open hydraulic schemes as implemented on Falcon9 grid fins  (Read 61399 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.

Offline 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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Offline 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.

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.

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.

Online 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).


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.

Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #20 on: 01/11/2015 03:20 am »
I'm still surprised the grid fins aren't electromechanically actuated.

Here's an interesting discussion of hydraulics vs. electromechanicals in aerospace applications:

http://www.mobilehydraulictips.com/end-hydraulics-aerospace-fast/

The big plus of hydraulics for F9 is that they already have the power source on board, ie compressed gas.
« Last Edit: 01/11/2015 03:21 am by Kabloona »

Offline CJ

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #21 on: 01/11/2015 03:25 am »
I can't imagine why "open" hydraulic systems for this sort of purpose would vent overboard? It makes no sense to me - and doesn't fit any definition of an open hydraulic system (circuit) I've ever heard. My best guess is that they aren't using pumps here; just a pressurized tank and one-way flow. There;s no reason to, and a lot of reasons not to, dump such fluid overboard.

Okay, I'll try to explain; if it's an open hydraulic system intended to provide nozzle control for a few minutes during ascent, it makes some sense to vent it overboard; it reduces mass, plus there's no harm (except you'll make a hell of a mess if you try to test it on the pad and don;t hook up drain lines).

However, in the case of the F9, there's no advantage to an overboard dump (its use is after stage sep) and plenty of pitfalls (at the least least, a slippery mess to clean up.) So, the logical choice, it seems to me, is to dump it into a lightweight tank (there's room for such tanks in that area, plus, they can be very light, much akin to an empty one gallon plastic milk jug, as there's no need to hiod pressure, just fluid.). So, why on earth would they vent it overboard when they can contain it all within the system at effectively zero added weight? In fact, the through-the-hull fitting may well outweigh the alternative of a capture bottle. (a capture bottle does not make it a closed system, so nothing they've said precludes it)


 

Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #22 on: 01/11/2015 03:45 am »
So, why on earth would they vent it overboard when they can contain it all within the system at effectively zero added weight?

There is not a single thing on a rocket that has zero weight, zero cost, zero assembly time, zero paperwork, zero anything. Every screw, nut, bolt, wire tie, etc, adds to all those categories.

So a different question is: Why would you spend money and time and add weight and complexity in order to catch spent hydraulic fluid that you can easily and safely just dump overboard? Since the system is at the top of a stage flying backwards, the slipstream is carrying the fluid up and away from the stage, so it won't be oozing down the sides. (And maybe they do catch it on F9R tests so it doesn't mess the vehicle/pad up in low-speed tests.)

SpaceX has an admirable design philosophy: KISS. Dumping spent hydraulics fluid is totally consistent with that philosophy.
« Last Edit: 01/11/2015 04:13 am by Kabloona »

Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #23 on: 01/11/2015 03:56 am »
Possible methods on the actuation method for the fins:

1) Single cylinder with torque arm. Downside is that the forces are unbalanced.
2) Dual Cylinder 'quadrant'. These can be seen on larger ships to turn the rudder to steer. The cylinders are cross connected so that an equal amount of fluid is required to go either way.
3) Rotary actuator. These would either be a rack & pinion design with hydraulic cylinders (most common) or an hydraulic vane motor (least likely option however) to push a central hub with a shaft.

Position control would likely be some sort of potentiometer mounted on the shaft extending out to the fin. This would provide feedback to the control system to adjust the hydraulic valve (likely a Moog servo valve, but could be a standard electrohydraulic valve to save on cost).

By using feedback, this becomes what is called 'closed loop' in electrical control systems. Without it, you wouldn't know the position of the actuator, hence 'open loop'. You can see how the terms of open & closed loop can become confusing.

Offline Hotblack Desiato

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #24 on: 01/11/2015 08:35 am »
for a regular rocket stage, what's the point of collecting the hydraulic liquid? a few minutes later, it hits the sea, and releases the liquids anyways. venting them during flight produces a very fine spray and dissolves it, whereas the tank containing the liquid would crack open and cause an oil film on the water surface.

for the F9R first stage, collecting the liquid might be a good idea, but so is using a environment-friendly liquid which doesn't harm anything. one thing that should be taken in consideration: they add 50% more liquid to the next attempt in february. so how much is the mass penalty of a closed system/low pressure plumbing to route the liquid back to the tank, instead of adding more liquid and a larger tank for it.

and by the way: the bigger environmental impact comes from unused RP1 in the stage. are there any numbers how much unused fuel remains in the stage?

Offline Excession

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #25 on: 01/11/2015 08:59 am »
You may have missed the extensive discussion we've had on this earlier in another thread.

I did, admittedly.

I just saw some interesting speculation on the SpaceX subreddit that, if they used RP-1 for the hydraulic fluid, they could just release it into the main fuel tank when done. Could add a bit of margin for that landing burn...

Offline CJ

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #26 on: 01/11/2015 09:13 am »
So, why on earth would they vent it overboard when they can contain it all within the system at effectively zero added weight?

There is not a single thing on a rocket that has zero weight, zero cost, zero assembly time, zero paperwork, zero anything. Every screw, nut, bolt, wire tie, etc, adds to all those categories.

So a different question is: Why would you spend money and time and add weight and complexity in order to catch spent hydraulic fluid that you can easily and safely just dump overboard? Since the system is at the top of a stage flying backwards, the slipstream is carrying the fluid up and away from the stage, so it won't be oozing down the sides. (And maybe they do catch it on F9R tests so it doesn't mess the vehicle/pad up in low-speed tests.)

SpaceX has an admirable design philosophy: KISS. Dumping spent hydraulics fluid is totally consistent with that philosophy.

Perhaps I'm wrong, but I don't see it that way. Either way of doing it (overboard, or a catch tank) adds weight and complexity. Overboard requires a vent fixture or port in the skin. A catch tank requires the tank. Both require some pluming. Which is simpler and lighter? You could literally use a couple of 2 liter soda bottles strapped into place for a catch tank - we're talking ounces, not pounds (they won't be pressurized). How much would putting a vent port in the skin cost, weight wise? It's weigh something - and my guess is on par with the tanks at best. Complexity? Putting a hole in the skin is not a trivial matter. 

I don't see any reason to worry about a couple of gallons of Hy fluid going into the water - it's in international waters, and I don't offhand think this would be a legal issue. What I do see as an issue is the mess the fluid would make on the F9; do they really want to have to have it all over the skin and getting into exposed parts? It just seems like an added headache to no gain to me. (and if they have to spend extra time cleaning the F9, that's money too). 

So, if I have to bet... I'll bet they are using a gas-press one way system that doesn't dump fluid overboard.       


Offline Mader Levap

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #27 on: 01/11/2015 10:35 am »
There is not a single thing on a rocket that has zero weight, zero cost, zero assembly time, zero paperwork, zero anything. Every screw, nut, bolt, wire tie, etc, adds to all those categories.
Dumping system also adds to these categories. This is non-argument.

You may claim that dumping system may be simpler than catch tank (in itself doubtful), but then you must have contingnency procedures in case of spill, testing on launch pad is more complicated (e.g. you have to have yet another drain pipe and THAT comes with another set of abovementioned categories), not to mention... oh wait, you mentioned that.

And maybe they do catch it on F9R tests so it doesn't mess the vehicle/pad up in low-speed tests.
Look, even you noticed already one problem with spilling fluid. Why they would catch it only in F9R-dev? Your assertion does not make any sense.

Answer is simple: they did trade studies. I didn't, so it is just my bet: they used catch tank.
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Offline Remes

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #28 on: 01/11/2015 11:40 am »
What I do see as an issue is the mess the fluid would make on the F9; do they really want to have to have it all over the skin and getting into exposed parts? It just seems like an added headache to no gain to me. (and if they have to spend extra time cleaning the F9, that's money too).
My speculations:
1) They might have a hose to the lower end. This line can lightweight (even rubber), as there is no pressure on the release side. (Based on how fast the system acts, there might short pressure peaks, which would have to be flattened out, but from what I see from Grashopper videos, the pressure on the release side is most likely benign).
2) Rocket is anyway covered in cork, which might be replaced anyway

(And another thought: we didn't see the fins beeing tested on the launch pad. A possible explanation is a blow down/dump system.)
Other rockets which use open hydraulics:
1) Delta III
2) Conestoga
Delta IV SRM's, Athena…..
Ariane 5,
crosslinking here, as I didn't notice this thread yesterday:
https://forum.nasaspaceflight.com/index.php?topic=35853.msg1313843#msg1313843
« Last Edit: 01/11/2015 12:20 pm by Remes »

Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #29 on: 01/11/2015 12:47 pm »
Remember, the whole point of this system (goal) is reusability. They might be able to get away with dumping hydraulic fluid overboard during these few offshore tests, but what about Grasshopper? That testing is happening over ground.

I'm sure the teams at SpaceX have devised a great system to contain the fluid. It only makes sense, especially when you consider that the fins still need to operate while over the barge and they certainly wouldn't want a slick landing surface. And then there's the future landing pad, and you don't want oil all over that area.

One comment above was using RP-1 and to dump the excess back into the propellant tank. That's a great idea, however the returning fluid will be coming up against the pressure of the pressurant gas in the RP-1 tank. Now it can still work, and work better if they use a catch tank at first and then have some really small pump to feed it into the propellant tank at (or above) the pressurant pressure. A N2 (or He) operated diaphragm pump would work fine. As always though, we are adding mass: mass taking away from launch mass to orbit.

That would have to be a trade, and I'm sure that for the amount of fluid involved, it isn't worth it.  Now a cool thing is that if it were designed well, they could simply burn it by injecting it in to the fluid stream of the hot gasses exiting one of the engine nozzles (controlled from a valve after a catch tank, which would mass very little).

Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #30 on: 01/11/2015 01:11 pm »
I also need to ammend my earlier comment on the use of a servo valve (like Moog, or Parker) to operate the grid fins. These are the valves typically use in high performance applications, and are used throughout the aerospace industry. The problem however, is that they are very inefficient in operation.

If you think of an audio speaker, you are sending an electrical signal to it to move the cone in or out. In the same manner, the valve gets an electrical signal to push a pilot device back or forth to move a larger hydraulic spool in or out, which directs oil to ports to move an actuator one way or the other. But to get very fast response, that pilot device uses oil that constantly gets used up (creates a pressure drop across the valve), which drains the system.

So I think they may have opted to use a standard on/off or perhaps a proportional valve to deploy the grid fins (using a cylinder most likely), and another pair of valves to rotate the fins left & right. The valve would open, and the pressure would go directly to move the actuator. The faster the oil flows to the actuator, the faster the actuator moves. You still have an electric feedback mechanism which senses the position of the grid fin, and the system can respond to any error in that position to pulse the valve either direction to get it in the correct position. That would be the responsibility of a control system engineer.

In my profession, I'm one level below an enginner: a technologist in Fluid Power/Robotics, and a certified fluid power specialist (through IFPS).

Offline Herb Schaltegger


I just saw some interesting speculation on the SpaceX subreddit that, if they used RP-1 for the hydraulic fluid, they could just release it into the main fuel tank when done. Could add a bit of margin for that landing burn...

That would require a line from the interstage either through the LOX tank or along the outside of the fuselage and then through the skin and into the RP1 tank structure. Further, the RP1 is pressurized at least to some degree, which would require that the draining RP1 fluid from the interstage enter the tank at an even higher pressure ... Way too complex.
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Offline watermod

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #32 on: 01/11/2015 05:10 pm »
Starting to bring back memories of stories about the OTRAG rocket.
One of the things it's engineers were quote bragging about was that many of it's actuators were VW windshield wiper "motors" as they had the most real world use and analysis of any movement device.

Offline CJ

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #33 on: 01/11/2015 08:23 pm »
I'm a bit puzzled by a few of the comments in this thread, so in the interest of clarity, I thought it might be helpful to envision roughly how much hydraulic fluid we're talking about here? Is it a gallon or so? Or is it (to go to an absurd level) closer to 100?

If there's a lot of it, overboard dumping may make more sense. If it's in the range of a gallon or so, a small, very light, catch tank could be the simpler, cheaper, easier option. 

So, I'm asking; how much fluid volume do people have in mind? My own guess, somewhere between one and two gallons (based on the assumption that this is a pressure-fed one-way system, one actuator per grid fin, so 4 actuators.) 

Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #34 on: 01/11/2015 08:54 pm »
I'm a bit puzzled by a few of the comments in this thread, so in the interest of clarity, I thought it might be helpful to envision roughly how much hydraulic fluid we're talking about here? Is it a gallon or so? Or is it (to go to an absurd level) closer to 100?

If there's a lot of it, overboard dumping may make more sense. If it's in the range of a gallon or so, a small, very light, catch tank could be the simpler, cheaper, easier option. 

So, I'm asking; how much fluid volume do people have in mind? My own guess, somewhere between one and two gallons (based on the assumption that this is a pressure-fed one-way system, one actuator per grid fin, so 4 actuators.) 
For each of the grid fins to deploy (if hydraulic) would require perhaps a cylinder that is 3" bore x 1" rod x 6" stroke? That's 56.25 cu-in volume (piston end) x 4 cyl = 225 cu-in.
1 gal = 231 cu-in.

If the grid fins work independently, then a dual quadrant cylinder arrangement style (assume 2" bore, 1" rod, 6" stroke cylinders) would require 33 cu-in for each full turn. I'm not sure how many of these it would perform during its flight down, but if you assume 10 full turns: 33 x 4 quadrants x 10 turns = 1321 cu-in = 5.7 gal

So you are looking at a minimum of 7 gal of pressurized hydrulic fluid in that scenario & arrangement, which is peanuts.

Offline Razvan

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #35 on: 01/11/2015 09:04 pm »
I'm a bit puzzled by a few of the comments in this thread, so in the interest of clarity, I thought it might be helpful to envision roughly how much hydraulic fluid we're talking about here? Is it a gallon or so? Or is it (to go to an absurd level) closer to 100?

If there's a lot of it, overboard dumping may make more sense. If it's in the range of a gallon or so, a small, very light, catch tank could be the simpler, cheaper, easier option. 

So, I'm asking; how much fluid volume do people have in mind? My own guess, somewhere between one and two gallons (based on the assumption that this is a pressure-fed one-way system, one actuator per grid fin, so 4 actuators.)
I'm just guessing, 2 to 4 gallons it seems little, given the size of one gigantic fin of about 2m x 2m, I'd say max 10 gallon a fin...

Offline cscott

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #36 on: 01/11/2015 10:48 pm »
If the grid fins work independently, then a dual quadrant cylinder arrangement style (assume 2" bore, 1" rod, 6" stroke cylinders) would require 33 cu-in for each full turn. I'm not sure how many of these it would perform during its flight down, but if you assume 10 full turns: 33 x 4 quadrants x 10 turns = 1321 cu-in = 5.7 gal

And, again: this was the root of the issue: SpaceX modelling indicated that they would do no more than (say) 10 full turns on the way down, and instead they needed to do 11.  Modelling a control algorithm for hypersonic reentry is hard.  It's rather surprising their estimates were as close as they turned out to be.

Offline dglow

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #37 on: 01/12/2015 02:52 am »
If the grid fins work independently, then a dual quadrant cylinder arrangement style (assume 2" bore, 1" rod, 6" stroke cylinders) would require 33 cu-in for each full turn. I'm not sure how many of these it would perform during its flight down, but if you assume 10 full turns: 33 x 4 quadrants x 10 turns = 1321 cu-in = 5.7 gal

And, again: this was the root of the issue: SpaceX modelling indicated that they would do no more than (say) 10 full turns on the way down, and instead they needed to do 11.  Modelling a control algorithm for hypersonic reentry is hard.  It's rather surprising their estimates were as close as they turned out to be.

I'm surprised by this. Either they didn't budget sufficient margin, which is shortsighted, or in fact they did, which means their estimates were off by far more than 10%.

Why not play it extra safe and double the amount of fluid until you know how much you don't need? Overbuild, then optimize the design.

Online LouScheffer

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #38 on: 01/12/2015 03:06 am »

And, again: this was the root of the issue: SpaceX modelling indicated that they would do no more than (say) 10 full turns on the way down, and instead they needed to do 11.  Modelling a control algorithm for hypersonic reentry is hard.  It's rather surprising their estimates were as close as they turned out to be.

I'm surprised by this. Either they didn't budget sufficient margin, which is shortsighted, or in fact they did, which means their estimates were off by far more than 10%.

Why not play it extra safe and double the amount of fluid until you know how much you don't need? Overbuild, then optimize the design.
On the assumption they are not idiots, I'm sure they did.  So their modeling showed they needed 6.6 turns, they designed for 10 (doubling stuff on a rocket is costly, so they added a 50% margin), and real life showed they need 11.

Offline Robotbeat

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #39 on: 01/12/2015 03:08 am »
They probably were off in their estimate by more than 10% and didn't think more margin was worth it (remember, more fluid would mean slightly less margin for engine-out, etc).
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Offline dglow

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #40 on: 01/12/2015 03:27 am »
Yes, agree – they weren't off by 10%, but they came up 10% short.

Offline cscott

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #41 on: 01/12/2015 05:18 am »
Yeah, when I wrote "no more than 10 turns" I meant that they did their error bars or whatever, and expected a mean of (say) 6 turns with < N% chance of needing more than 10 turns, for whatever N they felt comfortable with.

Offline NWade

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #42 on: 01/12/2015 04:16 pm »
Two thoughts/comments:
 
1) Talking about the fluid needed to support "turns" seems pretty speculative. Without knowing how the actuators work it is also quite feasible to assume that the fluid is expended just holding the fins in a specific position. So rather than being about "turns" its a simple time-function. Once they activate the fins they have x seconds of descent time per liter or cubic-meter of fluid (c'mon people, SI units - no gallons! *chuckle*).
One of Elon's tweets (a reply to a question) mentioned only having "4 minutes" of fin operation, afterall.
 
2) Comments about "how Grasshopper did it" are not applicable here. Grasshopper was specifically made as a development vehicle and we don't know how many components it actually shares with the F9 v1.1.  F9R-Dev1 is much closer to a "real" F9; but again we saw it get hacked and tweaked many times over its life and we cannot be sure that what they tested was exactly the infrastructure they used on the real F9 v1.1. 
 
This happens in aircraft design all the time - a new aircraft comes out and a prototype is built and flies for a year or two, then they build "conforming prototypes". Finally, they incorporate all the test results and changes into their final aircraft and that is what gets the OK (hopefully) from the FAA. This certified aircraft may have large differences from the original prototypes and further (smaller, usually internal) differences from the conforming prototypes. There's no reason this development path couldn't have been followed by the GH/F9R/F9 v1.1 series of vehicles.
 
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Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #43 on: 01/12/2015 04:25 pm »
Without knowing how the actuators work it is also quite feasible to assume that the fluid is expended just holding the fins in a specific position.

Not really. If you have a finite quantity of hydraulic fluid, there's no way you'd design a system like that. All the linear and rotary hydraulic actuators that I am aware of do not expend fluid in holding a position.

The speculation was not about how hydraulic actuators work, but about which type was used.

And Elon's reference to time was likely based on Monte-Carlo simulations by which they determined that the hydraulic fluid would last for about 4 minutes of controlled flight.

And yes, the speculation about "number of turns" is some people just pulling numbers out of a hat. But they do know how hydraulic actuators work.  ;)
« Last Edit: 01/12/2015 04:35 pm by Kabloona »

Offline MTom

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #44 on: 01/12/2015 08:23 pm »
Seeing the launch video I had a question about grid fins. This thread is the nearest about grid fins.

Belong these rods to the grid fins?

P.S. Maybe it helps also for this thread.

The whole video found here:
http://forum.nasaspaceflight.com/index.php?topic=35853.msg1314858#msg1314858
« Last Edit: 01/12/2015 08:24 pm by MTom »

Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #45 on: 01/12/2015 09:37 pm »
I had a schematic in the other thread, but it really belonged here.

Here's an updated version with what I believe to be the essential hardware, including proportional valves & safety valves.

The drain system is still the biggest unknown.

If it were a lead screw or rotary motion actuator, not much would change.

Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #46 on: 01/12/2015 09:38 pm »
Seeing the launch video I had a question about grid fins. This thread is the nearest about grid fins.

Belong these rods to the grid fins?

P.S. Maybe it helps also for this thread.

The whole video found here:
http://forum.nasaspaceflight.com/index.php?topic=35853.msg1314858#msg1314858


They look like inline geared actuators.
The top picture with the arrow pointing to the left looks like a sealed bearing with a shaft.

edit to add: to increase structural stability of the grid fins, it actually makes sense to carry the shaft bearing support across, likely stopping close tot he center, forming a cross pattern. It would help to stiffen the top of the first stage as well.
« Last Edit: 01/12/2015 09:41 pm by robertross »

Offline john smith 19

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #47 on: 01/12/2015 09:51 pm »
Not really. If you have a finite quantity of hydraulic fluid, there's no way you'd design a system like that. All the linear and rotary hydraulic actuators that I am aware of do not expend fluid in holding a position.

The speculation was not about how hydraulic actuators work, but about which type was used.

And Elon's reference to time was likely based on Monte-Carlo simulations by which they determined that the hydraulic fluid would last for about 4 minutes of controlled flight.

And yes, the speculation about "number of turns" is some people just pulling numbers out of a hat. But they do know how hydraulic actuators work.  ;)
I've come to this discussion late and I'm not clear where the "evidence" for this being a blow down hydraulic system.

The reason I ask is that in earlier days SpaceX made much of analyzing launch failures and explained how their architecture countered the major failure modes.

One of which was the use (in some LVs) of such systems and the way they could run out of actuator fluid.

Hence there interest in closed loop hydraulics (although I think they stayed with using RP1 as the "fluid")

AFAIk the SoA in actuator systems seems to be individual electrohydraulic units, replacing centralized pumps and networks of high pressure tubing with small separate packages to deliver high pressure fluid on demand.

This subject has come up before with respect to VTOL SSTO systems. The tradeoff has always between extra time costs weight, so the question is how much  time is needed, what range is needed?

Any system that can't draw on main engine power or tap the main propellant tanks (RP1 APU using atmospheric air perhaps?) will have severe  limits on maneuvering.  This would normally be countered by careful modelling and essentially choreographing the whole landing process very carefully.

Obviously the accuracy of that modelling becomes critical to avoid running out of charge/gas/fluid/whatever.

Somewhere along the way their modelling didn't work out.   :(

I've found it fascinating to watch the progress from people  saying "We can do this" to them discovering what it actually takes to do it.
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Offline Remes

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #48 on: 01/12/2015 11:28 pm »
Here's an updated version with what I believe to be the essential hardware, including proportional valves & safety valves.
I would put the pressure regulator behind the nitrogen valves, then the accumulators don't have to hold the full pressure of the nitrogen in the beginning.

Springs in the mode shown (fins retracted) would have there lowest force (as the springs are at their highest length). I wonder if we couldn't pressurize it from the beginning. To avoid the oil draining over the servovalve pilotstage there could be a very small switching valve, pressuring the cylinders into retracted position. Would be (I guess) lighter than springs.

I've come to this discussion late and I'm not clear where the "evidence" for this being a blow down hydraulic system.
Elon's comment about running out of oil (Twitter, after he was asked, how the fins worked).

Quote
Obviously the accuracy of that modelling becomes critical to avoid running out of charge/gas/fluid/whatever.

Somewhere along the way their modelling didn't work out.   :(
If we talk about modelling, we talk about modelling aerodynamics of 9 engine nozzles directed into the wind at super/trans/subsonic speeds. I think that's really a tough thing to model/predict (Edit:no steady flow state). It was a test flight, now they have their data.

But another question: Do we have evidence, that loosing the fins is the cause to the crash? It is a valid assumption, but it could also be unrelated.
« Last Edit: 01/12/2015 11:42 pm by Remes »

Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #49 on: 01/12/2015 11:29 pm »
I've come to this discussion late and I'm not clear where the "evidence" for this being a blow down hydraulic system.

Elon tweeted it was an "open" system and they ran out of fluid.

Quote
The reason I ask is that in earlier days SpaceX made much of analyzing launch failures and explained how their architecture countered the major failure modes.

One of which was the use (in some LVs) of such systems and the way they could run out of actuator fluid.

Yes, and they avoided that problem by using a fueldraulic system for the TVC actuators. But in this case, the grid fins were an "add-on" package on the wrong side of the fuel tank, and they evidently chose a standalone fluid tank.

Quote
AFAIk the SoA in actuator systems seems to be individual electrohydraulic units, replacing centralized pumps and networks of high pressure tubing with small separate packages to deliver high pressure fluid on demand.

Yes, there's a good summary here:
http://www.mobilehydraulictips.com/end-hydraulics-aerospace-fast/

Quote
Any system that can't draw on main engine power or tap the main propellant tanks (RP1 APU using atmospheric air perhaps?) will have severe  limits on maneuvering.  This would normally be countered by careful modelling and essentially choreographing the whole landing process very carefully.

Obviously the accuracy of that modelling becomes critical to avoid running out of charge/gas/fluid/whatever.

Somewhere along the way their modelling didn't work out.   :(

That's what test flights are for. Won't happen again.  ;)
« Last Edit: 01/12/2015 11:34 pm by Kabloona »

Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #50 on: 01/12/2015 11:41 pm »
Here's an updated version with what I believe to be the essential hardware, including proportional valves & safety valves.
I would put the pressure regulator behind the nitrogen valves, then the accumulators don't have to hold the full pressure of the nitrogen in the beginning.

Springs in the mode shown (fins retracted) would have there lowest force (as the springs are at their highest length). I wonder if we couldn't pressurize it from the beginning. To avoid the oil draining over the servovalve pilotstage there could be a very small switching valve, pressuring the cylinders into retracted position. Would be (I guess) lighter than springs.

The springs are shown with the fins in the horizontal position - retracted or deployed. Actuating the cylinders in either direction causes the fin mechnism to rotate outside the tank.

As I had noted earlier on servovalves: they are inherently inefficient. I would suspect they use proportional directional valves instead (though possibly 2-stage, it doesn't really matter for this notional concept).

Offline Remes

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #51 on: 01/13/2015 12:01 am »
The springs are shown with the fins in the horizontal position - retracted or deployed. Actuating the cylinders in either direction causes the fin mechnism to rotate outside the tank.
I was never good in reading mechanics. I thought if two cylinders extract, then fin goes out, if cylinders move differentialy rotation occurs. I guess I just wait for the 3D-Animation. :D

Quote
As I had noted earlier on servovalves: they are inherently inefficient. I would suspect they use proportional directional valves instead (though possibly 2-stage, it doesn't really matter for this notional concept).

www.mylesgroupcompanies.com/moog_pdfs/Moog%2030%20Series%20Catalog.pdf

It says on page 11 that leakage is <4% of rated flow. Might be worth taking a proportional valve, albeit I never saw it used in aerospace applications. I'm thinking of this grid fin at super sonic speeds, where flow changes very rapidly and the control system must act very fast in order to compensate disturbances. A servovalve has an insane dynamic, which is not found (afaik) in other valve types.

Offline robertross

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #52 on: 01/13/2015 12:54 am »
The springs are shown with the fins in the horizontal position - retracted or deployed. Actuating the cylinders in either direction causes the fin mechnism to rotate outside the tank.
I was never good in reading mechanics. I thought if two cylinders extract, then fin goes out, if cylinders move differentialy rotation occurs. I guess I just wait for the 3D-Animation. :D
I did not show the deploy mechanism in that diagram, on the rotation left & right. And I just realized I should have centered the cylinder rods.
So don't think about the deploy mechanism, just how the grid fin rotates about a central shaft. It only turns perhaps 45 degrees in each direction. So based on my diagram (and imagine each pair 1/2 way out): if you put oil into the bottom of the cylinder on the left, it extends out AND it feeds oil into the rod side of the cylinder at the right, pushing it down. Both cylinders are moving, and that in turn rotates the lever arm about the central pivot. When the cylinder atthe left has fully extended out, the one on the right has fully retracted in.

Quote
Quote
As I had noted earlier on servovalves: they are inherently inefficient. I would suspect they use proportional directional valves instead (though possibly 2-stage, it doesn't really matter for this notional concept).

www.mylesgroupcompanies.com/moog_pdfs/Moog%2030%20Series%20Catalog.pdf

It says on page 11 that leakage is <4% of rated flow. Might be worth taking a proportional valve, albeit I never saw it used in aerospace applications. I'm thinking of this grid fin at super sonic speeds, where flow changes very rapidly and the control system must act very fast in order to compensate disturbances. A servovalve has an insane dynamic, which is not found (afaik) in other valve types.
I suppose I should have added an on/off solenoid valve inline of the fluid stream after the pressure reducing valve, to enable the flow of oil.
The problem is: once the fins deploy, that leakage is continuous. You are discharging up to 4% of the rated oil flow whether you use the fins or not.

I do agree that servovalves are second to none in performance of frequency response, but there are some phenominal proportional valves made these days with much less leakage. Parker makes an industrial proportional valve that has servo valve performance that operates much like the voice coil of a speaker. They actually use that in their promotional material.

Offline llanitedave

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #53 on: 01/13/2015 02:04 am »
Seeing the launch video I had a question about grid fins. This thread is the nearest about grid fins.

Belong these rods to the grid fins?

P.S. Maybe it helps also for this thread.

The whole video found here:
http://forum.nasaspaceflight.com/index.php?topic=35853.msg1314858#msg1314858

Here's a similar shot from CRS-4.  It doesn't show as much because the camera is pointed more at an angle towards the nozzle, but I don't see the structures on that one that you see on CRS-5.  So I do think those must be new additions for the grid fins.

*edit:  Found the CRS-3 video of the same event.  Better angle, and the interstage is definitely bare compared to CRS-5.
« Last Edit: 01/13/2015 02:18 am by llanitedave »
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Offline john smith 19

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #54 on: 01/13/2015 09:57 am »
Elon's comment about running out of oil (Twitter, after he was asked, how the fins worked).
Noted. That is about as authoritative as it comes.  I really must join Twitter at some point.
Quote
If we talk about modelling, we talk about modelling aerodynamics of 9 engine nozzles directed into the wind at super/trans/subsonic speeds. I think that's really a tough thing to model/predict (Edit:no steady flow state). It was a test flight, now they have their data.
Interesting that no one bought that point up before the flight. Although I had thought the fins deployed below M1. Still a very tough modelling problem, but a bit simpler.
Quote
But another question: Do we have evidence, that loosing the fins is the cause to the crash? It is a valid assumption, but it could also be unrelated.
Excellent question. Correlation yes, causation?

Elon tweeted it was an "open" system and they ran out of fluid.
Noted
Quote
Yes, and they avoided that problem by using a fueldraulic system for the TVC actuators. But in this case, the grid fins were an "add-on" package on the wrong side of the fuel tank, and they evidently chose a standalone fluid tank.
Powering large, high speed, high accuracy actuators (even for a short time) is probably one of the most demanding mechanical design problems around. It also looks like a very  late addition to the first stage design, presumably thrown up by the Grasshopper tests, which is why you'd do them, but otherwise unexpected.
Quote
Yes, there's a good summary here:
http://www.mobilehydraulictips.com/end-hydraulics-aerospace-fast/
Yes and It's where I drew my precis from.  Sadly those aircraft systems exchange high pressure (4000psi for aircraft?) tubing for  a ready electrical supply.

Which reminds me. Do F9 stages run off batteries or do they have some kind of on board generation? AFAIK the whole GNC runs on batteries but once you've got legs and  fins being deployed (and actively moved in some cases) is that a still the best option?
Quote
That's what test flights are for. Won't happen again.  ;)
Well probably not.  :(

I think one of the subtle ways that things have gotten a lot better in aerospace design is the amount of data that can be collected and stored during a single test flight.

High density data storage, high accuracy data acquisition, high data rate (at shortish range) comms, along with assorted image analysis methods mean that a huge amount of results can mined from what is quite a short actual event.

When you compare this with (for example) the 40+ test launches of the Titan 1 the progress in this area is quite extraordinary.  :)

I suppose I should have added an on/off solenoid valve inline of the fluid stream after the pressure reducing valve, to enable the flow of oil.
The problem is: once the fins deploy, that leakage is continuous. You are discharging up to 4% of the rated oil flow whether you use the fins or not.
This should be an obvious observation but if all the piping is rated to full hydraulic pressure why not put a simple (fast) on/off valve on the  drain, so fluid is lost only when they system is activated and gas kept out?
Quote
I do agree that servovalves are second to none in performance of frequency response, but there are some phenominal proportional valves made these days with much less leakage. Parker makes an industrial proportional valve that has servo valve performance that operates much like the voice coil of a speaker. They actually use that in their promotional material.
From memory proportional valves are a lot more expensive than simple on/off types.

My favorite hack in this area was the Aerojet tank pressurization system for the Kistler stage. IIRC they use 4 on/off valves with different flows to in effect implement an electro-pneumatic  DAC. The flows (IIRC) were sized to the tank emptying rates so the scale was pretty non linear.
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Offline pagheca

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #55 on: 01/13/2015 11:00 am »
I have a few additional questions:

(1) when the fluid ends, do the grids go to a "parking" position (e.g. is spring loaded or just driven there by the aerodynamic friction) or not? This is relevant because if yes, the engine can still try to correct the trajectory (at least at high speed), if not it may create a problem. I'm afraid the answer is NO, and that's may have played a role in the accident.

(2) when the ACS find a problem with sensors or actuators, does it trigger an operation mode where it is trying to miss the target to save the ASDS or not? I"m quite surprised they didn't protected the equipment on the bridge from a crash.
« Last Edit: 01/13/2015 11:00 am by pagheca »

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #56 on: 01/13/2015 11:27 am »
I suppose I should have added an on/off solenoid valve inline of the fluid stream after the pressure reducing valve, to enable the flow of oil.
The problem is: once the fins deploy, that leakage is continuous. You are discharging up to 4% of the rated oil flow whether you use the fins or not.
This should be an obvious observation but if all the piping is rated to full hydraulic pressure why not put a simple (fast) on/off valve on the  drain, so fluid is lost only when they system is activated and gas kept out?
[/quote]
Because the valves are rarely rated to full pressure on the tank port. If it were, it would be a massive valve. In addition, the solenoid tube has a slug of metal inside that rides on a film of oil and that slug has to displace the oil in front of it to move.
Quote
Quote
I do agree that servovalves are second to none in performance of frequency response, but there are some phenominal proportional valves made these days with much less leakage. Parker makes an industrial proportional valve that has servo valve performance that operates much like the voice coil of a speaker. They actually use that in their promotional material.
From memory proportional valves are a lot more expensive than simple on/off types.

My favorite hack in this area was the Aerojet tank pressurization system for the Kistler stage. IIRC they use 4 on/off valves with different flows to in effect implement an electro-pneumatic  DAC. The flows (IIRC) were sized to the tank emptying rates so the scale was pretty non linear.
A proportional valve might be 4x the cost of an on/off valve, but a servo valve can be more than 5x the price of a proportional valve. And there is no way you can get good control response (and accuracy) from an on/off system like you can with a proportional or servo system. If you overshoot your mark, you have to activate the opposite valve, and hunt back and forth, because the flow of oil is constant to the actuator in an on/off system. IN a proportional or servo system, your control logic determines how much power to send to the valve, which sends a specific flow rate of oil to the actuator.

The response time for the valves will be dictated by how fast the grid fins need to move to compensate for a vehicle's movements. That there determines the overall cost of the system: response time. That's also a great benefit of returning the first stage: all those servo actuators & control computers used to gimbal the engines (not to mention the engines themselves).

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #57 on: 01/13/2015 11:30 am »
I have a few additional questions:

(1) when the fluid ends, do the grids go to a "parking" position (e.g. is spring loaded or just driven there by the aerodynamic friction) or not? This is relevant because if yes, the engine can still try to correct the trajectory (at least at high speed), if not it may create a problem. I'm afraid the answer is NO, and that's may have played a role in the accident.

(2) when the ACS find a problem with sensors or actuators, does it trigger an operation mode where it is trying to miss the target to save the ASDS or not? I"m quite surprised they didn't protected the equipment on the bridge from a crash.

1) In my conceptual design, yes: loss of fluid pressure returns ALL the grid fins to a horizontal position, so you have equal drag on the descent. You wouldn't need to park the fins close up against the body though, but they could be designed that way.

2) I wouldn't know, but that should be part of their FTS in my mind.

Offline JamesH

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #58 on: 01/13/2015 01:31 pm »
According to interview with Musk, the fins ran out of fluid with a minute to go. Quite a long time...

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #59 on: 01/13/2015 01:48 pm »
launch vehicles use batteries to power all electrical systems.

Offline cscott

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #60 on: 01/13/2015 01:51 pm »
I don't think we need to speculate about whether lack of fluid pressure got the fins "stuck"---a full minute of flight with no roll control authority means that residual roll is bound to have built up.  Spinning stage centrifuges the fuel/ox away from the inlet, so the engine shuts down.  Result: "hard" landing.

I do think it is interesting that the onboard control didn't try to vector the stage away from the barge, especially if they had a fluid pressure sensor and knew roll was building up.  But maybe the software was still hopeful they could land the thing before roll built up too much.  By the time the engine shuts down, of course, the opportunity to divert has past.

But SpaceX may well have welcomed the near-miss and barge damage, in exchange for being able to recover more pieces of the stage.  Not clear that (in hindsight) a divert would have been "better" for them.
« Last Edit: 01/13/2015 01:52 pm by cscott »

Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #61 on: 01/13/2015 03:21 pm »
I do think it is interesting that the onboard control didn't try to vector the stage away from the barge, especially if they had a fluid pressure sensor and knew roll was building up.

Your roll scenario is entirely plausible.

But do we know for a fact that it didn't try to divert? Chris' article seems to suggest it may have, but his wording is unfortunately vague:

Quote
The first stage made a valiant effort to hit its mark. However, the return was deemed to be too fast for a secure landing, resulting in the loss of the stage.

Maybe I'm reading too much into it, but the word "deemed" seems to suggest a judgment call made by the stage and a consequent attempt to avoid an un"secure" landing by diverting.

Otherwise, why not just say the stage came in too fast and crashed off target? There's no "deeming" in that scenario.
« Last Edit: 01/13/2015 03:26 pm by Kabloona »

Online Chris Bergin

I never said anything to imply anything.  You are reading way too much into that.
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Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #63 on: 01/13/2015 03:38 pm »
I never said anything to imply anything.  You are reading way too much into that.

Thanks, Chris. So you didn't mean to imply the stage may have attempted to divert. Sorry for putting words in your mouth.  ;)

Online Chris Bergin

I never said anything to imply anything.  You are reading way too much into that.

Thanks, Chris. So you didn't mean to imply the stage may have attempted to divert. Sorry for putting words in your mouth.  ;)

No probelmo. You asked, and alerted me to the question - that's perfect in my eyes.

Will be scared of using the word "deemed" again, however! ;D
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Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #65 on: 01/13/2015 03:43 pm »
Ah, words, those slippery buggers... ;)

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #66 on: 01/13/2015 03:45 pm »
Chris, can you tell us:

(1) is "too fast" meaning... coming down too fast? (someone pointed out this is not obvious).

(2) did you get this bit of info confirmed from a particular source?
« Last Edit: 01/13/2015 03:45 pm by pagheca »

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #67 on: 01/13/2015 04:00 pm »
Chris, can you tell us:

(1) is "too fast" meaning... coming down too fast? (someone pointed out this is not obvious).

Do you mean as opposed to going sideways too fast?

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #68 on: 01/13/2015 04:10 pm »
Chris, can you tell us:

(1) is "too fast" meaning... coming down too fast? (someone pointed out this is not obvious).

Do you mean as opposed to going sideways too fast?

Yes. It's a discussion going on on the ASDS thread.

p.s. I'm writing over a a phone and cannot check easily if were you pointing that out.
« Last Edit: 01/13/2015 04:10 pm by pagheca »

Offline Mongo62

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #69 on: 01/14/2015 04:23 pm »
Elon elaborated on the grid fin issue at a recent Q & A, where he states that SpaceX needs "to make sure our hydraulic actuators don't run out of fluid and go hard over".

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #70 on: 01/14/2015 05:05 pm »
Elon elaborated on the grid fin issue at a recent Q & A, where he states that SpaceX needs "to make sure our hydraulic actuators don't run out of fluid and go hard over".

 :o  Ooh - that sounds like the fins might get stuck fully actuated one direction or the the other, perhaps even forced to one extreme by the lack of fluid.  Double un-good!

Offline john smith 19

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #71 on: 01/14/2015 06:21 pm »
Because the valves are rarely rated to full pressure on the tank port. If it were, it would be a massive valve. In addition, the solenoid tube has a slug of metal inside that rides on a film of oil and that slug has to displace the oil in front of it to move.
I don't think I've made myself clear. If I understand correctly the drain is a low pressure pipe when the actuators are not in motion. What I had in mind was a fast acting, low pressure valve to retain fluid. In principle the system lifetime would then be restricted to the number of actuator movements. Obviously to be worthwhile it would depend on how frequent and wide a ranging the individual motions are. 
Quote
I do agree that servovalves are second to none in performance of frequency response, but there are some phenominal proportional valves made these days with much less leakage. Parker makes an industrial proportional valve that has servo valve performance that operates much like the voice coil of a speaker. They actually use that in their promotional material.

A proportional valve might be 4x the cost of an on/off valve, but a servo valve can be more than 5x the price of a proportional valve.
Handy to know. BTW It's quite interesting how these systems do have some resemblances to those used in pressurization systems, both for main stage and in the hypergolic systems used on satellites and space probes.
Quote
And there is no way you can get good control response (and accuracy) from an on/off system like you can with a proportional or servo system. If you overshoot your mark, you have to activate the opposite valve, and hunt back and forth, because the flow of oil is constant to the actuator in an on/off system. IN a proportional or servo system, your control logic determines how much power to send to the valve, which sends a specific flow rate of oil to the actuator.

The response time for the valves will be dictated by how fast the grid fins need to move to compensate for a vehicle's movements. That there determines the overall cost of the system: response time. That's also a great benefit of returning the first stage: all those servo actuators & control computers used to gimbal the engines (not to mention the engines themselves).
Yes response time pretty much drives this application. I will note that SpaceX's computer architecture does mean they can insert higher performance CPU's (I think they normally run 20MHz+ ARM's when ULA is running something like 1MHz 1750A's) which can cope with algorithms that incorporate more complex actuator dynamics and support more channels of them.

BTW the Aerojet paper I mentioned was from the AIAA J. Guidance,  Contro and Dynamics, Vol 23 #4 July-August 2000."Multiple On-off valve control for a launch vehicle tank pressurization system"
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Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #72 on: 01/14/2015 06:49 pm »
Elon elaborated on the grid fin issue at a recent Q & A, where he states that SpaceX needs "to make sure our hydraulic actuators don't run out of fluid and go hard over".

"...which is kind of what happened," Musk added.

Offline Patchouli

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #73 on: 01/16/2015 01:42 am »
I had a schematic in the other thread, but it really belonged here.

Here's an updated version with what I believe to be the essential hardware, including proportional valves & safety valves.

The drain system is still the biggest unknown.

If it were a lead screw or rotary motion actuator, not much would change.
I think it might drain into a tank but I believe it's open simply because Spacex is using nitrogen or helium bottles already present in F9 to run the hydraulics vs dealing with the added complexity an APU.

To get it from the drain tank back to the pressure tank would require either require a pump which means more batteries or an APU or enough lines and valves for the drain tank and source tanks to be switched which would introduce more failure modes.
Plus it would take time to relive pressure in one tank and bring the other up to pressure.

An interesting thought I wonder if something similar to Xcor's piston pumps could be a dirt cheap APU if the ever decide to go with a closed loop hydraulic system on one of their rockets.
« Last Edit: 01/16/2015 01:53 am by Patchouli »

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #74 on: 01/17/2015 09:40 am »
An interesting thought I wonder if something similar to Xcor's piston pumps could be a dirt cheap APU if the ever decide to go with a closed loop hydraulic system on one of their rockets.
There is also Flometrics (if they're still in business) positive displacement pump, which is not from a competitor.

Tanks of high pressure gas may sound a bit primitive but engineering a light weight, compact high power system (even for a short time) is a very tough task. The one's I'm aware of for aircraft use are small(ish) gas turbines, air spun propeller driven pumps or (on the F16) a system using MonoMethylHydrazine (like that on the Shuttle, but sealed for one shot use). Obviously that's not an option for SpaceX (and I don't think any player who wants keep their costs down and carry humans with minimum trouble should go that way).   
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Offline guckyfan

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #75 on: 01/17/2015 08:39 pm »
They already have nitrogen in the interstage for their cold gas thrusters. They will need a lot less of that now they have grid fins. So probably they don't need to add any gas to drive their hydraulics. Only the hydraulic fluid.


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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #76 on: 01/18/2015 08:24 pm »
I don't think we need to speculate about whether lack of fluid pressure got the fins "stuck"---a full minute of flight with no roll control authority means that residual roll is bound to have built up.  Spinning stage centrifuges the fuel/ox away from the inlet, so the engine shuts down.  Result: "hard" landing.

Elon says the fins were hard over so yes they were stuck.  There are different ways that they can be designed, and the way it was done as seen by the evidence we have before us is that opposite pairs were designed to get stuck in the same plane when actuating fluid went missing.  Simple FMEA driven decision to avoid the possibility of spin up.

I would put the pressure regulator behind the nitrogen valves, then the accumulators don't have to hold the full pressure of the nitrogen in the beginning.

- Not likely that the liquid fluid is in an unpressurized (or low pressurized) tank initially and gas pressure is applied shortly before fluid pressure is needed.  Reasons: a) no benefit to the tank from the standpoint of time exposure, time doesn’t count unless you’re dealing with material creep (which isn’t the case), b) no benefit in number of pressure cycles tank will see (one per flight either way) c) a closed valve will be more reliably successful in leaklessly closing against a liquid (the “hydraulic fluid”, whatever it is) than a gas (presumably N, but even more the case if He), d) energy in the form of gas P*V would be lost at the moment that you opened the valve to pressurize the hydraulic fluid tank in flight, e) The gas pressure tank size would need to be increased (vs. pre-pressurizing the volume above the fluid.

Also, there would seem to be some probability that there would be pistons (if cylindrical tanks) or bladders (if spherical tanks) separating the gas volume from the liquid volume so that the gas didn’t escape through the actuators in the early going before air drag has a chance to orient gravity relative to the stage.

Now changing subjects to the overall design of the hydraulic system, I don't see what much of the rampant speculation here and earlier on the barge thread is necessary for.  A fair portion of what's being argued is answered by Elon's words.  The rest we won't know for sure but given what is known and physics I think the major likely chunks of the system are fairly obvious-
- Fluid is stored in pre-pressurized accumulators or tanks with gas pressure applied
- Gas is likely nitrogen since its already at the top of the stack for the thrusters and as previously stated less nitrogen will be required by the thrusters for grid fin flights. (edit, I see this was stated immediately above as I wrote this)
- Many of the usual constraints on hydraulic fluid don’t apply (anti-foaming, high temp, etc.) RP-1 is a plausible substitute
- Above all I see a compelling reason that (as previously proposed here, on reddit, and on the barge thread but with little traction) RP-1 is used and that its then spewed into the fuel tank.  It all makes sense except for the difficulty of plumbing it there past the oxygen tank.  It does little good from a weight saving perspective to drop it overboard at that point as 98% (a made up number) of the DV energy has already been put into it by then.  Depleted of its pressure energy it still has an much greater combustion energy to give to the effort.  I’m not going to get into the math but I don’t think it would be hard to make an energy based calculation for saving the fuel by ducting it back into the fuel tank which is only at a few tens of psi.  If the speculation about the  volume of hydraulic fluid being consumed was correct above in the 5-10 gallon range and if the environmental impact filing for landing at SLC-13 was correct in the remaining fuel quantity (IIRC, 15 gallons, though that seems a small fraction of a second of Merlin run time) then RP-1 as hydraulic fluid fuel makes sense.  Also, IIRC, the Merlin thrust vector actuators are hydraulic and use high pressure RP-1 from the turbopumps (then presumably flowing back to the tank?) as hydraulic fluid so I think there is SpaceX specific precedent on this method of operation.

Again, not saying that's how it works but given what we know I think its vastly most likely.

Less likely and moving into the more speculative area I'm guessing that the grid fins are deployed via a simple mechanical system based on the motion of the interstage away from the second stage and stay in the 90 degree position until folded in preparation for re-launch (not sure if I missed anything, do we have any indication that the fins ever fold back toward the stowed position in operation?)

Mark
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Offline MP99



I don't think we need to speculate about whether lack of fluid pressure got the fins "stuck"---a full minute of flight with no roll control authority means that residual roll is bound to have built up.  Spinning stage centrifuges the fuel/ox away from the inlet, so the engine shuts down.  Result: "hard" landing.

Elon says the fins were hard over so yes they were stuck.  There are different ways that they can be designed, and the way it was done as seen by the evidence we have before us is that opposite pairs were designed to get stuck in the same plane when actuating fluid went missing.  Simple FMEA driven decision to avoid the possibility of spin up.

My suggestion was that adjacent fins be designed to hard over in opposite directions, which means that opposing fins would cause the stage to spin up. However this would be counteracted by the other two torquing in the opposite direction.

Cheers, Martin

Offline Remes

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #78 on: 01/19/2015 01:07 pm »
I'm guessing that the grid fins are deployed via a simple mechanical system based on the motion of the interstage away from the second stage and stay in the 90 degree position until folded in preparation for re-launch (not sure if I missed anything, do we have any indication that the fins ever fold back toward the stowed position in operation?)

at 2:03
every fin seems to be controlled in both axis.

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #79 on: 01/20/2015 03:18 am »
every fin seems to be controlled in both axis.

They certainly do.  Your evidence is quite convincing.
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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #80 on: 01/20/2015 08:19 am »
I think too they have 2 degrees of freedom (tilting on the rotation axis, and up/down). I also think that they are interconnected, so there is only one valve or mechanism controlling all of them. Something like wires or rails.
« Last Edit: 01/20/2015 08:28 am by pagheca »

Offline cambrianera

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #81 on: 01/20/2015 09:26 am »
I think too they have 2 degrees of freedom (tilting on the rotation axis, and up/down). I also think that they are interconnected, so there is only one valve or mechanism controlling all of them. Something like wires or rails.

Not likely.
To keep it simple, it's better to have two actuator for two degrees of freedom.
This is also the strong point of hydraulics, actuators are light, cheap and simple (compared to electromechanic).
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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #82 on: 01/20/2015 09:34 am »
I think too they have 2 degrees of freedom (tilting on the rotation axis, and up/down). I also think that they are interconnected, so there is only one valve or mechanism controlling all of them. Something like wires or rails.

Not likely.
To keep it simple, it's better to have two actuator for two degrees of freedom.
This is also the strong point of hydraulics, actuators are light, cheap and simple (compared to electromechanic).

Right, I've not been clear enough:

I suggest that a single mechanism could have been used to control the four grid fins through a system of wires or common rails, so that the movement of the 4 grid fins is coordinated at all time.
« Last Edit: 01/20/2015 09:42 am by pagheca »

Offline cambrianera

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #83 on: 01/20/2015 10:27 am »
I suggest that a single mechanism could have been used to control the four grid fins through a system of wires or common rails, so that the movement of the 4 grid fins is coordinated at all time.

Still not likely.
It's difficult to implement modifications on wires, links and mechanical connections, your coordination laws will be "set in stone".
It's better to have separated actuators (eight for four grid fins) as simple and direct as possible.
Coordinations and movement link to be done by SW (control system).
As ever, IMHO.
Oh to be young again. . .

Offline pagheca

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #84 on: 01/20/2015 11:07 am »
Ok, let me elaborate a little bit on this suggestion...

If the 4 fins are interconnected, they could be used to

(1) counteract spinning,
(2) change the deceleration rate
(3) act as the feathers of an arrow, to realign the longitudinal axis and damp vertical oscillations.

However, they could not be used to control the attitude in a more general way. This should be totally deferred to the engine gimbals and throttle.

IMHO a system with 4 grid fins interconnected, for example, by a rotating crown could be as solid, light and easy to design as a system with 4 independent servos. It would limit the possibility of a fin stuck in a certain position, lack or coordination, etc., removing some failure modes. Also, the algorithm controlling the fins and the gimbals would be much simple to parametrize, as they would serve almost independent functions and as the output parameters would be 2 rather than 8 (for the fins only).

Note I'm not campaigning for this model. I just proposed it here as meat for the discussion. It came to my mind as I couldn't find a single image or movie where at least 2 fins are not in the very same attitude. In particular, I noticed that in this flight:



after 2:03 you can see a fast movement probably due to the shock of the landing, where the two visible fins jump in perfect coordination, like for a vibration.

Actually, this also suggested to me that the fins mechanism is designed to use the air pressure as a spring, so that when the deceleration is from the bottom with no air resistance they naturally tend to fold down.

Again, I'm not particularly attached to this idea and if someone have seen the fins in an uncoordinated attitude, please post the image or video here so my question would be withdrawed.
« Last Edit: 01/20/2015 11:22 am by pagheca »

Offline Zardar

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #85 on: 01/20/2015 11:40 am »
Ok, let me elaborate a little bit on this suggestion...

If the 4 fins are interconnected, they could be used to

(1) counteract spinning,
(2) change the deceleration rate
(3) act as the feathers of an arrow, to realign the longitudinal axis and damp vertical oscillations.


Your 3 modes are valid, however, it won't work for the other, probably most important mode, which is to tilt (or steer) the stage, so it falls at an angle through the air, therefore moving its target impact point sideways. (or counteracting wind motion)
This is what's needed to reach the landing pad or barge, given the uncertainties of the precise co-ordinate of atmospheric entry point, the horizontal components of the stages motion at that entry, and also prevailing atmospheric conditions.
To visualise that, the opposite fins will need to be set to be in the same plane as each other, as opposed to counter to each other (which would just impart (or reduce) spin and drag as you describe above)

Offline cambrianera

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #86 on: 01/20/2015 11:58 am »

IMHO a system with 4 grid fins interconnected, for example, by a rotating crown could be as solid, light and easy to design as a system with 4 independent servos.

It is a very good solution for a mass produced device.
Not for a continuously morphing quasi-experimental vehicle.
Oh to be young again. . .

Offline pagheca

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #87 on: 01/20/2015 01:48 pm »
The grid-fins are independently controlled. See Twitter below. Probably for the reason Zardar pointed out.

« Last Edit: 01/20/2015 02:05 pm by pagheca »

Offline JamesH

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #88 on: 06/23/2015 10:27 am »
Apologises for resurrecting this thread, but was there ever any confirmation on what was used as fluid, and if RP1, was it vented back in to the main tank or overboard?

Offline Jarnis

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #89 on: 06/23/2015 12:20 pm »
Apologises for resurrecting this thread, but was there ever any confirmation on what was used as fluid, and if RP1, was it vented back in to the main tank or overboard?

Not RP1, and vented overboard. They had separate fluid tank for the fins, with no recovery because the fins are needed only for a short time (a fluid recovery system that would pump it back to a tank would weigh extra, not worth it vs. having a tank big enough to just have enough while dumping used fluid overboard)


Offline JamesH

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #90 on: 06/23/2015 02:27 pm »
Sorry, I was a bit unclear, I meant venting back in the main RP-1 fuel tank to be used as fuel, not in to a holding tank.

But if it was definitely vented overboard that's a moot point, and there is no point in using RP-1 either.

Thanks.

Offline Jarnis

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #91 on: 06/23/2015 02:38 pm »
Sorry, I was a bit unclear, I meant venting back in the main RP-1 fuel tank to be used as fuel, not in to a holding tank.

But if it was definitely vented overboard that's a moot point, and there is no point in using RP-1 either.

Thanks.

This was all covered when the first barge landing went pear-shaped due to running out of hydraulic fluid. Separate reservoir at the interstage, where the fins are. Piping it from there to the fuel tank would mean a long extra pipe past/through the liquid oxygen tank - completely impractical. No idea what they used as fluid, but almost certainly not RP1.
« Last Edit: 06/23/2015 02:39 pm by Jarnis »

Offline Mongo62

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #92 on: 06/23/2015 03:05 pm »
Not RP1, and vented overboard. They had separate fluid tank for the fins, with no recovery because the fins are needed only for a short time (a fluid recovery system that would pump it back to a tank would weigh extra, not worth it vs. having a tank big enough to just have enough while dumping used fluid overboard)

Do we have confirmation that it was vented overboard? I thought that the consensus here was that the used fluid would most likely have been dumped into a lightweight low-pressure container (something like an empty 2L pop bottle) near the fins, rather than expending the engineering resources to design the modifications to the interstage to allow for dumping the working fluid overboard.

Offline Kabloona

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #93 on: 06/23/2015 03:47 pm »
Not RP1, and vented overboard. They had separate fluid tank for the fins, with no recovery because the fins are needed only for a short time (a fluid recovery system that would pump it back to a tank would weigh extra, not worth it vs. having a tank big enough to just have enough while dumping used fluid overboard)

Do we have confirmation that it was vented overboard? I thought that the consensus here was that the used fluid would most likely have been dumped into a lightweight low-pressure container (something like an empty 2L pop bottle) near the fins, rather than expending the engineering resources to design the modifications to the interstage to allow for dumping the working fluid overboard.

I don't recall we ever got definitive word. But I don't believe there was ever "consensus" here either that the spent fluid was captured. Overboard dump of hydraulic fluid is common aerospace practice (see Conestoga rocket TVC system) and "modifications" to the interstage for such a system would be minimal. The outlet would be just a tube that vents spent fluid into the atmosphere. It's about the simplest plumbing system you can design on a rocket.
« Last Edit: 06/23/2015 03:49 pm by Kabloona »

Offline CJ

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #94 on: 06/23/2015 09:42 pm »

I don't recall we ever got definitive word. But I don't believe there was ever "consensus" here either that the spent fluid was captured. Overboard dump of hydraulic fluid is common aerospace practice (see Conestoga rocket TVC system) and "modifications" to the interstage for such a system would be minimal. The outlet would be just a tube that vents spent fluid into the atmosphere. It's about the simplest plumbing system you can design on a rocket.

IMHO, we can assume (always a dangerous thing) that the hydraulic system on the Grasshopper 2 for its grid fins would operate the same way as what was used on the ASDS landing attempts. Hydraulic fluid dumping from the interstage would leave some visible marks, and I don't recall seeing any (though that does not exclude them being there). Also, dumping into a holding bottle may well have a lower mass cost at launch than overboard dumping (which requires a hole in the skin). the dump could be as simple as the aforementioned 2 liter soda bottle (I doubt they'd use that, but something light, for sure). 

My guess, we won't know for sure until we see detailed photos of a recovered F9 on the ASDS. (Any fluid on the skin is going to be very apparent due to the soot). It's one of the things I''ve long been planning to look for.

 

Offline CyndyC

The preceding should not be the last statement here, especially after Kabloona redirected the big fin discussion from ASDS to this thread. The hydraulic fluid is dumped overboard. The SpaceX Bible at Spaceflight101 says so, so believe that until a SpaceX employee or an authorized spokesperson says differently.
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Offline Remes

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #96 on: 03/11/2016 09:45 pm »
I would put the pressure regulator behind the nitrogen valves, then the accumulators don't have to hold the full pressure of the nitrogen in the beginning.

- Not likely that the liquid fluid is in an unpressurized (or low pressurized) tank initially and gas pressure is applied shortly before fluid pressure is needed.  Reasons: a) no benefit to the tank from the standpoint of time exposure, time doesn’t count unless you’re dealing with material creep (which isn’t the case), b) no benefit in number of pressure cycles tank will see (one per flight either way) c) a closed valve will be more reliably successful in leaklessly closing against a liquid (the “hydraulic fluid”, whatever it is) than a gas (presumably N, but even more the case if He), d) energy in the form of gas P*V would be lost at the moment that you opened the valve to pressurize the hydraulic fluid tank in flight, e) The gas pressure tank size would need to be increased (vs. pre-pressurizing the volume above the fluid.
Everything right, but not one is related to my point.
The helium is e.g. 300 bar (atmospheres), your working pressure for the hydraulic system is 100 bar. You don't want to put your hydraulic fluid reservoir under 300 bar. Therefore : 300 bar helium bottle, pressure regulator down to 100 bar, reservoir, plumbing, valve, actuator.

I would add one point to your list: testing the system before start requires pressurizing the hydraulic fluid. (Must not be real movement, in case the hold mechanism is non reversible. Observing the pressures in the actuators might be enough).

Quote
Also, there would seem to be some probability that there would be pistons (if cylindrical tanks) or bladders (if spherical tanks) separating the gas volume from the liquid volume so that the gas didn’t escape through the actuators in the early going before air drag has a chance to orient gravity relative to the stage.
I would assume spherical tanks with bladders. Less weight. Also there are already spherical tanks already in use.

Offline OxCartMark

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #97 on: 03/16/2016 07:31 pm »
5 Pages of discussing and I'm not convinced we know anything about how SpaceX has done this.  We don't know the fluid.  We don't know if the fluid is collected locally, goes into the fuel tank, or sprays willie nilly about into the breeze.  We don't know much about the pressurized supply, whether there are separate fluid supplies for each fin or one central one, whether the gas is He or N2, whether the gas bottle and the fluid bottle are separate or unified, whether there's a piston or not between the gas and fluid.  We have failed in this.  Perhaps we add this to Chris' list of questions to ask.  Perhaps someone here needs to call SpaceX and get me some answers.

And separately, now that we know that the grid fin system works and gets the stage consistently very close to the landing zone I am ready to re-design the system to be lighter.  How about a ram air turbine that deploys into the airstream after re-entry?  The only problems (and they are huge) is that a) you'd need to cut out a significant portion of the interstage cylinder structure (all load carrying) to allow it to exit, and b) there is nowhere convenient to put it other than at the center of the interstage because the Mvac nozzle is so large in diameter. 
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Offline Chris_Pi

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Re: Open hydraulic schemes as implemented on Falcon9 grid fins
« Reply #98 on: 03/17/2016 03:57 am »
To add to the list of potential problems,

Can a single ram air turbine design produce useful power over the huge speed range (Terminal velocity up to probably very supersonic) and still be reasonably sized?

As far as placement goes, It could be put in the interstage above the S1 engine bell. Swing the whole thing out to clear the interior. Trouble is besides being one more thing to do just before separation it's absolutely got to move or the S1 bell is trapped in the interstage. Or gets a big dent in one side as the center pusher shoves it past. Bad day either way.

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