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Auto .22 Airgun using JSM's Poppet Valve (Will it work?)

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: killerbanjo » Wed Jul 21, 2010 4:07 pm

Well everything i make is coverd in hot glue, so i think i should continue the trend with epoxy. Yes i will follow in your epoxy coverd footsteps jackssmirkingrevenge!



P.S. Going away in 2 days to kentucky for a month so i wont be able to work on it until after :(
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Last edited by killerbanjo on Wed Jul 21, 2010 4:15 pm, edited 1 time in total.
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Unread postAuthor: Ragnarok » Wed Jul 21, 2010 4:13 pm

jackssmirkingrevenge wrote:Not that easy to model pop-off valve performance.

It's not that hard either. I do it occasionally.

Basically, a pop-off valve is an self-actuating chamber sealing piston valve with no pilot.

a) Work out the pop pressure (Spring force divided by pop-off valve seat area) and set it as the chamber pressure
b) Set the piston mass as the mass of the pop-off piston plus half the mass of the spring
c) use an arbitrarily small pilot diameter (GGDT gets funny if the number is too small, but set it as low as possible)
d) set the pilot volume set appropriately. A close enough value is ((piston area - seat area) * pop pressure)/spring constant) * piston area * 2.5

Okay... that's not exactly "easy", but it's possible, and is a close enough modelling.
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Unread postAuthor: jackssmirkingrevenge » Wed Jul 21, 2010 10:03 pm

Ihaveamonthtoreconsiderthedesign wrote:Yes i will follow in your epoxy coverd footsteps jackssmirkingrevenge!


Hope you don't get stuch, literally ;)

Ragnarok wrote:Okay... that's not exactly "easy", but it's possible, and is a close enough modelling.


... but how does that account for dwell time?
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Unread postAuthor: Ragnarok » Thu Jul 22, 2010 10:35 am

jackssmirkingrevenge wrote:... but how does that account for dwell time?

It's accounted for in the basic programming. That is, GGDT will model a piston valve re-closing.

To demonstrate - one curve:
Image
(Ignore the precise time scale, I just threw figures together to get a curve that looked about right.)

Although of course, if conditions are right, the projectile may well be gone before the valve reseals, making dwell time irrelevant from the perspective of modelling velocity (although not air use).
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 22, 2010 10:48 am

Interesting, I rarely trust GGDT for piston valve modelling and just go with a generic valve with arbitrary opening time and check for "best case scenario" performance.
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Unread postAuthor: killerbanjo » Mon Aug 02, 2010 11:45 pm

Does anyone know what pressure a 20oz co2 tank is when full? Thanks :D
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 03, 2010 12:07 am

800-1000 psi, depending on temperature
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Unread postAuthor: killerbanjo » Tue Aug 03, 2010 8:51 am

Thanks, i think co2 would be the easyest method of propoltion (Spelling :S) as its easy to get where i live
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Unread postAuthor: killerbanjo » Sun Aug 15, 2010 1:47 am

A question for JSR, you said you were getting 5ft/lbs at 100psi and a 36" 6mm barrel... if i went for a 36" barrel 5.5mm at 200psi do you think i could get 8ft/lbs or above? Also what construction techniques do you have for your barrels or do you just buy one, long tube.


Thanks :D


EDIT:


Also couldnt you just use a pre-made saftey valve in a chamber like this...


Image
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Unread postAuthor: jackssmirkingrevenge » Sun Aug 15, 2010 11:36 am

killerbanjo wrote:A question for JSR, you said you were getting 5ft/lbs at 100psi and a 36" 6mm barrel... if i went for a 36" barrel 5.5mm at 200psi do you think i could get 8ft/lbs or above?


I would say 8 ft/lbs is definitely possible :)

killerbanjo wrote:Also what construction techniques do you have for your barrels or do you just buy one, long tube.


One long brass tube, no real "construction" there ;)

killerbanjo wrote:Also couldnt you just use a pre-made saftey valve in a chamber like this...


Tried thathere, power was disappointing. The parameters aren't optimised for this sort of use.
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Unread postAuthor: killerbanjo » Sun Aug 15, 2010 10:24 pm

Hmm, thanks for the info, im getting back from holiday in a week so ill get to work Thats assuming i have cash in my bank :P (God damn dealextream.com and the-home-brew-shop.co.uk eating all my money :P).
How much air do you think would be wasted if i wen't the QEV method? Do you think it would be noticable or just a tiny bit? And one last thing, after looking at a few blow forward bolts and maybe your simple mag idea is a better ideap, after thinging about it, i think it would be awsomely ghetto to have an italian style smg strip mag :P.

PS: I think i might go for a blowgun trigger if i only use it to 200 psi
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Unread postAuthor: A-98 » Sun Aug 15, 2010 10:47 pm

Just my $.02, but HPA would be a better power source.

Oddly enough my recommendation isn't based in experience with HPA. However I've used CO2 extensively and from my experience you will most likely have problems with freezing and locking up, specifically if your tank is oriented horizontally.

Direct further HPA questions to JSM.
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Unread postAuthor: killerbanjo » Sun Aug 15, 2010 10:59 pm

Thanks for the tip however i plan on a slow firerate, 300 rpm or maybe even slower so i dont think i will have a problem but thanks for the reply
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Unread postAuthor: killerbanjo » Wed Aug 18, 2010 4:50 pm

Further brainfarts from me, why can you do the below? I would imagine that it would work, why wouldnt it? Oh and the tube running near the pop off is the air supply

Image


I also managed to find a gif of how the drozed worked and thought i would share it as i couldnt find it on the forum


Image
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Unread postAuthor: jackssmirkingrevenge » Thu Aug 19, 2010 2:52 am

killerbanjo wrote:Further brainfarts from me, why can you do the below? I would imagine that it would work, why wouldnt it? Oh and the tube running near the pop off is the air supply


Technically, but probably not worth the effort in terms of air saved to be honest.

killerbanjo wrote:I also managed to find a gif of how the drozed worked and thought i would share it as i couldnt find it on the forum


Didn't look hard enough ;)
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