Mushrooom Valve

A couple of quick questions

does it come back again?

how fast does it go? Roughly, because atmospheric pressure has a standard lapse rate so you can calculate the rough atmospheric pressure given an altitude

SO...you can take your ascent rate, calculate how much air you start with and at what altitude you effectively don't have significant air left and you have your danger zone. Now the condensation you worry about has a place within that danger zone, both in altitude, but more in exhaust rate from the box i.e. a function of speed of altitude change.

So how fast does it go through the danger zone, = how big a hole you need based on the velocity of outrushing air through the hole.

And the mushroom valve you refer to and can probably use, is nothing but a captive sheet of flexible material, allowed to exhaust in one direction. You can also find them on cargo (small) containers like Pelicans for air shipment.

No, it doesn't come back. It's part of the docking system. So the profile is this:

1) - Lift off - from atmospheric pressure to vacuum - full of soggy Florida air

2) - docking - starts in vacuum, then pressurized (the boxes are in the tunnel that goes between spacecraft) More than likely starting the process at subzero temperature.

3) - undocking - from atmospheric(-ish) pressure back to vacuum

4) jettison - past caring at this point.

Thanks!

I'm an admitted fool, but might I ask a couple of ignorant questions?

1. Is the box strong enough to hold a (near) vacuum before it leaves Mother Earth?

2. Since it's sealed, could you fill it with nice dry nitrogen instead of icky old air and fit it with a standard pressure-release valve? So that when it pops open there will be no condenstation?

No - the box is not air tight. The ideal solution would be a hermetic box filled with nitrogen, but we don't have the mass allowance for that.

Start with filling it with dry nitrogen...some of those silicon humidity absorbers should help dry things out. If you can obtain an inner humidity of 0% then there's not much that can condense as the box depressurizes. Solving the humidity problem beforehand is a lot better for the weight than dealing with a stronger box that would see some pressure buildup during a fast ascent with a slow air bleed.

Since this is getting launched the weight is going to really be at a premium so I'd see if there's a lighter way of plugging the hole in a way that will release the air during ascent. Does it have to remain completely sealed after depressurization? Personally I would just put a thin strip of metal (with a seal if you please) across the hole and rivet it on just one side. Kind of like a valve in an air compressor. It would deflect slightly if the guage pressure of the box increases and release some air. The more pressure, the more deflection and the more vented nitrogen.

The worst condition is if the box is called on to suddenly depressurize.

Under these conditions, a shock wave will form at the hole and the air velocity will be sonic. This makes it easy to calculate the hole required to give the maximum de-pressurization rate you can accept without condensation as the mass flow is density x hole area x speed of sound.

This gives a conservative result but should get you close enough to the vent hole size you need.

This makes it easy to calculate the hole required to give the maximum de-pressurization rate you can accept without condensation as the mass flow is density x hole area x speed of sound.

You wouldn't have an equation for this would you? I'm an electrical engineer.

Hi bhankiii

Mr Gee pointed out what I was about to suggest - why do you need an enclosure at all?

This way the problem would probably cease to exist.

A simple framework for support, if needed, would be lighter than a box; but preferably simply fasten the apparatus straight to the surroundings, although that would make jetisoning it difficult if you really don't want to bring it home.

In addition, can you encapsulate your electronics so condensation becomes irrelevant? you can then use a box with large vent holes (if a box is actually needed) and have no problem.

I can describe the simplified calculation process if you still need it.

Please advise.

Regards

Sceptic

I'm more or less electrical too, but that is a challenging set of environmentals - to my mind.

My first experience moving from aircraft to rockets I came up sharply against reliability numbers - they all ended at 113 seconds....

yeah - our slice of the reliability pie is 0.999988

Why seal the box?

I would not expect it to have to be rain tight or water proof in space.

Are you expecting explosive decompression?

Could you not use a nema 1 enclosure that would provide electrical safety from fingers etc, but provides no hermetic sealing?

What about using a dessicant in your box for takeoff. I suspect any air on the repressurizing will be from compressed bottles and be bone dry.

From an automotive electronics perspective we use Gore-Tex seals to achieve what you are after and maintain a liquid barrier. I'd suggest looking into this and contacting Bosch/Siemens for further information as they use them on all of their ECU's.

http://en.wikipedia.org/wiki/Gore-tex

In terms of flow rate, I would just work out worst case using previous suggestions and put in a safety factor.

Won't the NASA quality guys want to go over all these designs and calcs with a fine tooth comb anyway? Uncontrolled gas release (regardless of source or constituents) must be a very heavily criticised attribute of a space vessel component?

Hi. My idea after reading all the posts thus far is to have two valves with fair sized openings. One valve is an inlet and one out. These valves could be simple non return valves (spring loaded flap type with the spring not too strong so that with any pressure change they will open) This will have a dust free controlled environment. Inside the box you place a separate compartment with moisture absorbing substance (like those small bags you get in packaging) So at the time of decompression when the air goes out it will be dry air and any condensation that might result will be less and if this happens there is some moisture absorbing for peace of mind.

Given the general and specific requirements for this application, I recommend a re-engineering of this, these, boxes.

In a household environment with electrical amateurs of all ages running around putting all your wiring in boxes makes some sense.

In the environment you refer to they only make some sense. Not enough to justify the weight, cumbersome access, and poor serviceability.

Wire harnesses should prove to be an improvement on all these parameters and most importantly have an advantage in Kaizen, the art of making it impossible to get it wrong.

Apollo 13 almost never came home due to this basic wiring design flaw. No reason to make that mistake again eh?

If you have to go with a box adapt a plastic automotive box, lighter, better serviceability and enough of an enclosure to keep out mice and most moron's. The only downside I can see would brittle material at heavy sub-zero. The Charpy Impact, Notched Low Temp test is only good to -30C probably above the range you would anticipate.

If you don't have time, or permission to re-engineer the boxes look at a custom fold for the corners of the box that will leave a large cross-section open with no straight path to the goodies inside. That approach should serve to keep the rain out and let the air flow.

Best wishes,

Mr. Gee

A mushroom valve is one that will stay shut under pressure on the top (upper surface) and so the pressure will act to close it and keep it tight. You need pressure control valve this is like a tapered valve which opens slowly and backed up by a spring which will determine at what pressure it will open - eg a needle valve. As it has an open and closed direction two mounted in opposite directions could be used to release the pressure slowly or quickly depending on the taper. The speed of release would depend upon the taper and the spring loading.

A simpler way would be to use the valves from car or truck tire - pick your size - put one each way for going up and coming down. These are available for bicycles tyres to earth movers. These are one-way pressure valves.

It's been an interesting read looking at all the ideas for how to do this but I can't shake the feeling that we are re-inventing the wheel. This has to have been done before. There has to be somebody out there who isn't "competition" who knows how to do this. I'd call NASA first. I know people at ITT space systems, but might they be "competition?" Somebody on this blog must have more contacts in the satellite business....

Bhankiii, my suggestion would be to make the box out of perforated aluminum sheet and dispense with the gaskets completely. It will save even more weight and will still give you a decent faraday cage. The alternative would be standard vent plug. Is there another reason why you want a solid box? Washdown? Particle ingress?

BTW, I'm sure you've thought of this, but some electronic components can't handle vacuum...

I suggest you use a short length of flat rubber tubing with some curl in it...like an

eustachian tube. The device would look something like a childs party noise maker that uncurls when they blow into it and when the pressure has equalized recoils. The curl would not be a full curl but more like the shape of a "J". Any excess pressure would be belched out as the tube streightens and swells open.

Another idea I've seen used, if it doesn't interfere operationally, electrically, etc, is to use expanding foam to minimize any open volume and, subsequently atmosphere, in the box. Ventilation holes in the box would then allow any remaining atmosphere/gasses to dissipate to vacuum.

Hooker

Encapsulate the electronics and open up the enclosure with large holes. Forget about the gasket. The loss of metal where the holes are and loss of the gasket will make up for the weight of encapsulating the boards.

How about potting the entire assembly in epoxy? We use potted switches in all kinds of harsh environments.

lg uses a mushroom valve on their front loading washing machines

front loading washing machines - That's 2 count for the washing machine. Maybe the answer is there.Does anybody have a washing machine as per my post or the diagram with a better view to probe further?

From my perspective, you have two primary options. First, the mission profile should tell you how long the box may be stored in the humid FL environment prior to launch. Knowing this and the maximum leak rate will permit you to calculate the amount of desiccant required to keep the internal dew point below the minimum environment temperature. The second option is to provide a lightweight hermetic (or pseudo-hermetic) seal. This may not be as complcated as you think. We do it all the time. It can be a thin sheet metal cover or even a metallized plastic membrane. The idea is to keep the moisture leak rate below the level that will lower the internal dew point below the environment temperature. In my experience with missile electronics packaging, valves will not be permitted due to reliability concerns. Valves are commonly used in packaging marine and other electronic systems, but I have never heard of them being allowed in high reliability electronics.

Before shipping the box tape a couple of small desiccant bags and seal the entirety with a red vapor barrier. Imprint with "REMOVE BEFORE FLIGHT" over all visible surfaces. Add checklist item to pre-launch list to remove seal wrap and desiccant bags.

Let the box breathe naturally on the lift-off and docking operations. In space condensation should not be an issue and after that you don't care.

There might be some way to control the venting by monitoring the outside pressure & via (transducer) feedback release pressure incrementally. I know that you already have the technology in the basic blood pressure unit. A cuff is inflated... then a monitor listens for pulses. (If there are no pulses detected the unit will increase the pressure to a limit & a 'time out' will occur if there is a leak or the some other extenuating/movement?maybe circumstance). At any rate the unit will on 'hearing' the pulse release pressure until no sound is heard/detected. Another way is to create compartments/diaphragms within the pressurized area & vent them as per outside pressure detection/feedback. Engineers at Welch Allyn might help with these tasks. Have fun & good luck. Carlos Oh! Another way might be to create a diaphragm for the outer body which will expand or contract sufficiently to resolve interior pressure trouble. One more way would be to create a tube with a gasket behind which a spring is situated. As the pressure inside increases the gasket & spring are acted upon & this movement exposes an opening for venting... it comes back down as per spring tension/pressure interplay. A diaphragm with a pin valve assy. might work OK too.

"The choices that you make might be mistakes but it's never too late to turn around." Tony Lang - musician

One other issue. Mushroom valves (flapper valves, check valves are all the same) only work in one direction. this is incompatible with your pressurization scheme since you will be repressurizing after exposure to vacuum. you'd have to put two of them in from opposite directions to allow the box to repress. That is why venting to the surrounding atmosphere is the best option.

Really good one Rorscgach. Till date never knew these things existed.i think since aerospace where one does the impossible expecting possible, he should first create a test environment in his company lab and test on your advice. Looks viable as per their Pdf catalog description.

Last night i was watching National Geography and they were showing the mars-craft and how it got stuck in the martian sand. They then had no options but to create an environment similar and experimented over 2 months before coming up with a solution.

He'd need to do similar to assure that his impossible is possible.

I think what you are looking for is also called an umbrella valve. Vernay Laboratories makes a wide assortment of these valves. They open under pre-determined pressures. If you put one or more facing into the cabinet, and the same facing out, you will get a controlled release of pressure.

Dennis

Thanks for all your input. Here are my responses:

We can't do away with the enclosure, or fill it with holes or use a wire mesh, or plastic, because of EMI considerations, (we have to meet MIL-STD-461D and E), and structural considerations (14g at liftoff).

We can't fill the box with dry nitrogen because the box is not air tight. We can't fill it with a dessicant because sitting on the pad in Florida for a month would saturate it. And once the box is cold and in a vacuum, repressurizing the tunnel will cause condensation in the box much too quickly for a dessicant to remove.

The electronics will be covered with a conformal coating - that doesn't mean that we still don't care if we get water or ice in the box.

Sure, we're talking to the NASA community as well. I just enjoy hearing from the guys on this forum, because I never know what to expect.

It's looks like the preferred solution is going to be something like a little pipe perpendicular to the wall of the enclosure - it's better than a hole for EMI.

What I still don't know is - how big to make the hole?

The problem isn't that condesation forming with too fast of a volume change, condensation will occur if the velocity is too fast. What is the volume of the box?

My WAG* (nothing SWAG** about it) would be using 1/4" tubes, 1/2" long. Maybe one on each exposed face?

* Wild A$&*#d Guess

** Scientific Wild A$&*#d Guess

That's pretty close to what my NASA expert sent me this morning, so I guess you get the prize.

My box looks like this, about 12 inches tall. So I guess the volume is about 1/3 cubic feet.

If you can take up as much volume as possible, with either potting, or dessicant perhaps, then the risk of icing goes down. here is a thought, instead of making it pressure tight, if you can merely seal it and fill it with silicone oil then the whole venting/icing/humidity issue goes away. Silicon oil is not compressible.

What we used to do with electronics for ROV's was we filled the case with oil and mounted a diaphragm in one side of the case so that the pressure would equalize without water contamination.

You wrote the box would be re-pressurized. quicky expanding air cools so if the expansion happens too quick this will also be a problem. I would use nothing less than 1/4 inch as a hole for your 1/3 cubic foot volume.

Hi bhankii,

Lots of answers so far. I don't know how to calculate the size of the hole, but it seems to me the gaskets are not needed, and may be a liability. How about a rubber diaphragm about 1/2 inch diameter, possibly with a pin hole in it. It would blow out if pressure change was too quick. The pin hole would stretch if the change was slow. This would work both ways. Also, if the box was heated (with a resistor) before the launch, the humidity would be driven out. Then you wouldn't have any condensation issues.

S

A 1/2 inch inch rubber hole will cause us to fail our EMI requirements. Having the system powered on before launch is an integrated mission operations nightmare. I know - we're a rules-y bunch here. And warm air holds more moisture, doesn't it? Unless you go very very warm.

What about a 1/2" porous sintered brass vent like those commonly used on gearboxes. Since they look "solid" from an EMI standpoint, you should be able to have as many as you like/have room for. Of course it is all about the volume you have to vent. If you can minimize the volume of air and ensure that the vent opening is large enough to prevent the velocity from being too high, then there will be very little self-refrigeration. you can't really have too large an opening (or rather, too many small openings below 1/4 wavelength which is why perforated metal if sized correctly will NOT fail you for EMI. trust me on this. I've been there, I've done that. I've got the stained t-shirt too.) for this.

I like this one. At least from an appearance it is neater than the 1/4" tubes that I suggested, but mass may or may not be the driving factor. I also like your pointing out that velocity of air, not volume of air could cause the "cooling"/condensation potential.

A possible issue with sintered metal or mesh is how fast it might cool off during lift-off, which may or may not cause "dew" to form and frost up. Not the same issue as expansion cooling, but could be similar in the problems it might create.

Again, this might be an issue that goes away if area of sintered metal/mesh is large enough. Anyway, break out the scales and see which idea weighs in best.

Another idea might be to build a mockup and see if you can test it in a hyperbaric or environmental test chamber. There are a few hyperbaric chambers around the country that can test for various changes in environmental pressure.

I don't know how to calculate the size of the hole.

I've a following formula I use in determining steam leakage. Can this help ?

W= 24.24 x Pa x D² (Napier's equation)

W = leakage rate lbs/hr

Pa = the absolute pressure drop across the orifice in pounds per square inch absolute(psia)

D = diameter of leaking orifice in inches

(in steam we do like this:

Steam operating pressure: 130 psig, Diameter hole: 0.25 inches

Absolute pressure: 130+14.7 = 144.7 psia

W= 24,24 x 144.7 x (0.25)² = 210.5 lbs/hr)

Thanks - I'll look up this equation and see what I can learn from it.

What I am reading is a lot of guessing. There is no substitute for R&D. What I mean is, you have to set up a decompression chamber and test your theories on the ground. As far as the size of the hole, start very small and work your way up until you get the results you want. As far as the atmosphere in Florida, use a baggie to keep the air out until that is no longer an option. Use wax in your hole that will be blown out under decompression to help keep the box sealed until it is in space. I think the air that enters the box upon recompression of the surrounding environment while in space will be well controlled and not resembling that found at Daytona Beach. If the baggie and/or wax are not an option then you must use Florida air in your testing. There is no substitute for knowing for sure what will happen in the real world, even if it is out of this world.

he should first create a test environment in his company lab and test on your advice -

as I said in #32. and agree with you on -

What I mean is, you have to set up a decompression chamber and test your theories on the ground.

The following web site has lots of calculations for many areas. It may be of use:

http://calculatoredge.com/

S

From skim read of the question and thread real fast, I'd say two vents of cone shape that react to pressure for expansion regardless of temperature. - As an experiment. one for in the box, and the other for out the box. Real quick skim read to produce that. Material for stable leaf shutter type valve may be carbon fiber, or stranded ceramic. Decompression chamber tests recommended.

Your statement of the problem makes it sound like venting the moist FL air after launch, is your only objective. I propose that your objective should be to eliminate the possibility of water condensing on critical surfaces during any portion of the life cycle of the product. This means either eliminating the possibility of condensing conditions prior to launch, or designing the electronics to survive condensing cycles. The latter is a difficult but not impossible task. If it were my problem, I would concentrate on moisture sealing techniques to prevent the ingress of moisture into the package. This can be accomplished by sealing the package with materials with low water vapor transmission properties, and the use of hydrophilic surfaces inside the box so condensation will form preferentially on those surfaces.

Hi bhankiii:

Sorry to be slow getting back to you on this.

Is air in the airlock to be dumped or reclaimed into the vehicle?

This affects the rate of decompression as reclaiming the air (which I suspect is what will be done) is slower than just dumping it into space.

Based on a very rough calc, assuming 1 sec for decompression, I arrived at a vent area requiring a 3/8" hole. Slower decompression rate will allow a smaller hole.

Your actual problem is to keep the inside of your box in rough pressure equilibrium with the airlock so as not to unduly pressurize your box.

If you have a baffled lid (for em shielding)with, say, 1/16" gap intermittently along it, this equilibrium should be readily established.

So make your box with a poor fitting lid and you shouldn't have a problem.

Good luck

Regards

Sceptic

Thanks, I don't know whether the air will be vented or reclaimed, or a combination of both. But we'd like to plan for the worst case.

Worst case would be a Russian Progress on manual control crashing into it.... or some other kind of Russian space junk....

Hopefully we won't run into any Russians on our trip to the moon - maybe some Chinese or an Indian or two.