Airbag inflation system

[ENGIFAR]

Currently designing a system that requires inflating multiple airbags during end of flight recovery. The current approach being taken is to use stored gas within a high pressure cylinder and a solenoid valve to control flow. A pressure switch would monitor the pressure level and activate the solenoids when required.

I am aware that this approach introduces many challenges that must be overcome. I will list a couple that I am aware of below.

-possible leakage through solenoid valves during high accel flight maneuvers.
-working with high pressure and temperature differentials pose multiple concerns. Air from cylinder (~5000 psi) enter[pre][/pre]ing bags will be extremely cold. Gas heating will cause expansion during descent. Changing atmospheric pressure (inflation anywhere from 10k ft to sea level and maintaining pressure throughout descent) presents a moving gauge pressure target (~3 psig, bags passively vent at ~7 psig bags must also fillin a reasonable amount of time <15 seconds)
-initial extremely high flow rate that declines as vessel empties
-decrease in temperature gas is filled at will cause a decrease in available pressure at altitude in cold environment. (Soaks at up to 40k ft long enough to reach equilibrium)
-safety issues filling cylinders to high pressures
-any residual gas in airbags will expand at altitude and possibly cause premature inflation (very bad-worst case failure, emphasizes need for zero leakage solenoid.

Basically, my question is does anyone has any experience with a similar system? Would another approach be more feasible? (gas generation, blower, other ideas?) Any tips to overcome the challenges with a stored gas system or any complications that I am overlooking? Suggestions for a solenoid valve that won't leak (N2) and can withstand the high pressure for an extended amount of time? As a cherry on top of these hurdles, the system must be low cost both initially and per flight.

Thanks in advance for anything you can offer. This is not my area of expertise and information is hard to come by regarding an application such as this.

 

[MikeHalloran]

High pressure gas cylinders also tend to be heavy, as does high pressure plumbing.

How about a pyrotechnic gas generator, with a premeasured charge per bag, and an electrical igniter?
You don't care if the gas is not N2, because there's only enough propellant in one bag to inflate that one bag to whatever design pressure you like.



Mike Halloran
Pembroke Pines, FL, USA

 

[ENGIFAR]

Thanks for the reply Mike. I have pursued the pyrotechnic approach. Unfortunately the price I have been quoted is astronomical and a nonstarter. Lots of NRE in addition to high refurb cost. If price were not an issue I think this design is a no brainer. However, complications also arise from the high inflation altitude and maintaining pressure throughout the descent. From my understanding gas will need to be constantly produced to account for increasing atmospheric pressure with decreasing altitude and excess gas will need to be produced and vented out. The vehicle could be in descent for upwards of 10 minutes. Hence the NRE cost. I don't believe anyone will have something OTS but I may be wrong. If you know of a product I'd love to hear about it.

 

[Compositepro]

Are these air bags used as landing cushions or drag chutes? Why are they inflated at high altitudes? Liquified gas can be stored at lower pressure and higher density. Cushioning air bags are designed to vent during impact so that they do not bounce like a basketball. This sounds like a student post.

 

[VEBill]

If you need air bags to cushion landing, then inflate them just before impact. There were several Mars landers that famously used airbags to cushion impact.

If the air bags are required on the way down, then use air pumps to keep them full. I can only presume that an inflator pump can be less mass than air tanks.

Alternatively, route combustion exhaust as your filler gas. Huge volumes from small amounts of fuel.

Also, look at the air amplifiers used to inflate emergency exit slides on aircraft. According to one report, they get about ten times the air as is in the tank.

What scale is this? Size will drive different solution concepts due to scaling laws.

 

[MikeHalloran]

REFURB? Hell, throw 'em away after every deployment.

That should push the quantity numbers up enough to get the unit cost down, and you can spread out the NRE over a much larger population.









Mike Halloran
Pembroke Pines, FL, USA

 

[ENGIFAR]

Thanks for the replies.

They are landing airbags. Not a student just not my area of expertise as I stated. As for scale, looking to inflate 2 bags ~35 cu ft each. Storage space is not really an issue. I have a large compartment to work with and calcs show I have enough margin for stored gas with a specd out OTS tank. Liquified gas is going down the gas generator route and the cost is a not viable.

Customer wants to keep the system all analog and no channels are available for any type of altitude sensing device. I have pushed for this but not gotten anywhere. When I said low cost I mean extremely low cost. The system must use the same signal from chute deployment.

VE1BLL-some very interesting ideas. I like the idea of not needing to store or generate gas. It's not going to space so the vehicle is completely surrounded by gas. I have looked into the idea of using a blower but my colleagues informed me that they are inefficient and won't meet the fill time requirement. They could however be used as a top off method.

Not too concerned about mass either. I have a budget of 100 lbs and nowhere near that. Gas storage tanks are about 8 pounds if I remember correctly (composite wrapped aluminum) of course lighter is always better but cost is my primary driver (and functionality of course)

I've also looked into aspirations (air amplifiers) but it would require a large inlet area in the free stream-a luxury I am not able to obtain.

I'm going to put some thought into your other ideas as well.

 

[ENGIFAR]

Mike-I like the thought process. Unfortunately, I'm only on contract to build a few initial test assets with the possibility of much larger quantities down the line. It's a huge gamble and one my employer will most likely not even consider.

 

[KENAT]

Hang on, don't you just need an oversized tank, a differential pressure monitor (i.e. comparing the pressure in the air bag system to external pressure) and a few valves with controller. Tank doesn't have to be heavy, though both light and cheap are conflicting requirements.

Military applications have moved away from pyrotechnic charges for this type of thing toward pressurized air systems for various reasons including the toxic byproducts produced by typical chemistry of such charges as I recall.

If you are planning on this system being reusable pyrotechnics can be especially problematic. Conventional cartridges 'gum up the works' and the system needs cleaning. They were working on cleaner burning cartridges 15 or so years ago but not sure what became of it.

Or, if you go pyrotechnic one main charge and X number of small 'top up' charges fired off as required might be technically feasible.

A bit unclear on the requirement to vent excess gas if you're only going 'down' but might be a good safety precaution.

10k ft to seal level might be low enough for initial pressurized gas and a sustainer pump but you'd have to do the mass study of the extra mass for the pump system V slightly over-sizing the gas tank. For Spherical tank the increase in mass to increase in volume ratio should be pretty favorable i.e ^3 or something unless my tired brain is on the fritz. If the pump can be parasitic off some other drive or even turbine driven from air flow or some such might change the math. You'll really only be from 10k to sea level, not 15k ft to 5k ft mountain plateau or some such?

Suggest you talk to the folks that make ejector racks &/or the pneumatic systems of 'single use guided aircraft' if you catch my drift.

As stated, there's also the Mars Rover or whichever one it was that used similar system.

(I'd think twice about talking to Takata but that may just be me.)

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[ENGIFAR]

Kenat-That was my original thought process that it wouldn't be too difficult to implement the stored gas approach. Discussions with certain individuals have steered me in other directions but I haven't heard anyone say that its not possible-just problematic from experiences. Some of these individuals have an agenda of course (selling gas generators).

Current gas generator systems have moved away from the toxic gas when it presents a concern with a freon replacement (in space, there is no one around to complain). Regardless, 500k of NRE is just not feasible.

The cleaning of the tanks went into the high refurb cost. Need to send them all back to the supplier before flying again. Expensive but still cheaper than purchasing new units.

On second thought, there are probably multitudes of Takata airbags laying around at highly discounted rates!

Thanks for the input I will look into your suggestions tomorrow.

 

[3DDave]

Look at the inflatable rafts. There may be some inspiration there.

If using CO2 then make sure there is a heater or at least some insulation to keep them warm. They don't need to be hot, just not frozen. Basically, you get a liquid at 1000 psi, so the inflation load isn't as high as with N2, more like a gas generator without the fuss. You could use 2 of them. One that will inflate to desired pressure at sea level, with the bag maximum desired pressure limited by a pressure relief valve and then a second tank with a regulator that maintains a minimum desired pressure after the first one is done.

You can add aspirators into the line from the gas source. The problem is ensuring enough pressure to overcome the dynamic pressure during descent, so the amplification won't be as much as under static conditions, for example

https://www.youtube.com/watch?v=jeRVGp54pak

 

[rb1957]

you're pumping atmospheric air into these bags ? novel

why not a parachute ?

another day in paradise, or is paradise one day closer ?

 

[ENGIFAR]

Airbags are in addition to chutes to further reduce landing impact.

 

[VEBill]

Here's a company down the coast from me.

http://stelia-northamerica.com/contact/

(formerly Composites Atlantic).

I had a plant tour once. I saw them making spherical high pressure tanks for the Space Shuttle. Aluminum sphere about 40cm (?), wrapped up with carbon fibre I think.

Offered here in case you need another vendor, or wish to take advantage of currency exchange rates.

--

The air amplifier used on the aircraft Emergency Exit Slides didn't look that big.

The air amplifier could be fed with high pressure, high speed gas from a flexible line from the bottle, and thus the air amplifier could (conceptually) be installed on any surface of the air bag, and thus pulling in the large volume of air from ambient atmosphere.

Aircraft Emergency Exit Slides do seem to be a fairly close match to what you're doing. Many of your requirements roughly match those of slides.

Good luck.

 

[Sparweb]

The CO2 cartridges on inflatable life vests fit in the palm of your hand. A cartridge that would fill your 35 CF bag would be larger, but you can get CO2 cartridges in many shapes and sizes.
Could you install multiple cartridges, then build a mechanism to fire them in stages? Each stage can be triggered by a pressure gauge that senses when the bag pressure drops below the threshold.
You don't need pyrotechnics to rupture the seal on these types of cartridges, a sharp pin will do if it's pushed with enough force or leverage. The little mechanism that fires the CO2 cartridges on a life vest is also very small and doesn't require much force on the pull cable. One solenoid could be equipped to activate the first for the big discharge, then activate later if top-up charges are needed.

STF

 

[VEBill]

Backing up a bit, some other concepts...

Russian space capsules, which return to land on land (as opposed to water splashdown), have a similar requirement to cushion the last few feet. They use little rockets.

Another approach used in similar cases, to protect occupants, is collapsing seats. I've seen video of helicopter crash test where this was used. The seating is designed to give under crash loading, to provide a more comfortable crash.

Another approach would be deployment of one or more crushable structures, perhaps made from expanded or crinkled aluminum. Could be designed to be quite spindly, thus not require as much volume.

If the goal is to reduce the peak deacceleration at impact, there are quite a few different approaches.

Presented just in case the feasibility of the airbags slips away from you.

Good luck.

 

[pyro]

Unless there is a mission requirement for it, why is the parachute being deployed at high altitude? On ejection seats we delay the main parachute deployment but use either a ribbon or aeroconical drogue for stability and reduction to a moderate descent rate. The timing of the main chute deployment is when the aircrew is in thicker, warmer air and the risk of running out of emergency O2 is minimized. For your case, a simple manometer could act on a shear wire (basic altimeter design) and spring-loaded firing pin that would impact a small primer/cartridge to activate the chute and airbags. The AV-8B parachute spreader gun does precisely that, striking what amounts to a shotgun shell.