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Pop-Off hybrid valve

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Pop-Off hybrid valve

Unread postAuthor: Lockednloaded » Sat Sep 18, 2010 3:37 pm

I have been inspired to convert my burst disk hybrid into a piston hybrid by awful galvanized unions that couldn't hold air to save their lives. I want a piston design that requires no o-rings because I do not have the facilities to make them. A pop-off hybrid valve pretty much perfectly fits the bill if I can make it correctly.

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It seems simple enough, but I know there is more than meets the eye here. To make it work efficiently I will have to tweak it and calibrate it, so does anyone have any calculations to calculate pop-off efficiency? Technician has also informed me that I will also need a bumper before the spring, any thought on that?
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Unread postAuthor: Lentamentalisk » Sat Sep 18, 2010 6:28 pm

The issue here is how easily the piston will crack if you use a spring. Even with just .75" of piston exposed, the compressed gas pre-ignition will be exerting somewhere in the 100lb range (assuming a highish mix). The entire point of a hybrid is that it bursts at or near the peak pressure, exerting several hundred psi on the projectile. If your piston cracks at less than that, its wasting power.

JSR has dealt with this (in pneumatic canons mind you) by using a gas spring. Certainly something for you to keep in mind. You could use a shock pump to pressurize the gas spring to 300psi on the 1" side of the piston, so it wouldn't crack till quite a bit higher than that, and when it did, it would fly open.
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Unread postAuthor: jackssmirkingrevenge » Sat Sep 18, 2010 7:56 pm

Lentamentalisk wrote:JSR has dealt with this (in pneumatic canons mind you) by using a gas spring. Certainly something for you to keep in mind. You could use a shock pump to pressurize the gas spring to 300psi on the 1" side of the piston, so it wouldn't crack till quite a bit higher than that, and when it did, it would fly open.


Have a look, certainly with a gas spring it's much easier to tweak.
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Unread postAuthor: saefroch » Sat Sep 18, 2010 8:21 pm

Just an idea to throw out there for you: A vented gas spring. Set it up to open at near peak pressure, but vent itself so once it starts to open, and stay open.
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Unread postAuthor: jackssmirkingrevenge » Sat Sep 18, 2010 8:29 pm

saefroch wrote:Just an idea to throw out there for you: A vented gas spring. Set it up to open at near peak pressure, but vent itself so once it starts to open, and stay open.


a pop-off valve with a pop-off valve, what a concept :D makes sense!
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Unread postAuthor: saefroch » Sat Sep 18, 2010 8:34 pm

:roll: Never thought of it that way... but that's the best piston valve design IMO, though I've never seen one built. :wink: :wink: :wink:
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Unread postAuthor: Lockednloaded » Sat Sep 18, 2010 11:48 pm

The pop-off plus pop-off would work, but having to refill the air spring every time slows down the ROF a considerable amount. The actual porting will be closer to 1/2" inch due to wall thickness, so ~ 100lb spring for 10x
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Unread postAuthor: SpudBlaster15 » Sat Sep 18, 2010 11:48 pm

saefroch wrote:...though I've never seen one built. :wink: :wink: :wink:


ORLY? :lol: :wink:
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 19, 2010 12:54 am

One thing your design doesn't have is a stepped piston, so on firing it won't really "pop" but likely fart somewhat....
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Unread postAuthor: Lockednloaded » Sun Sep 19, 2010 10:15 am

jackssmirkingrevenge wrote:One thing your design doesn't have is a stepped piston, so on firing it won't really "pop" but likely fart somewhat....


sorry, what exactly is a "stepped piston"?
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Unread postAuthor: saefroch » Sun Sep 19, 2010 10:38 am

LockednLoaded wrote: The pop-off plus pop-off would work, but having to refill the air spring every time slows down the ROF a considerable amount.


Hybrids are not known for having very high ROF anyway... the possible output increase would probably be worth it. :wink:
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Unread postAuthor: Lockednloaded » Sun Sep 19, 2010 11:09 am

People keep telling me that hybrids aren't "known for high ROF", but why not? What's wrong with having a decent ROF and hybrid power?
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Unread postAuthor: SpudBlaster15 » Sun Sep 19, 2010 1:18 pm

Lockednloaded wrote:What's wrong with having a decent ROF and hybrid power?


Nothing, it's just not typically done.

Hybrid ROF varies substantially from design to design. I was able to achieve a shot per minute with my burst disk designs, while my current piston hybrid can manage about 3, assuming the projectile doesn't require a complex loading process. However, some of the larger hybrids on this site have reload times in the tens of minutes.

What exactly is your definition of a "decent" ROF?
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 19, 2010 1:53 pm

Lockednloaded wrote:sorry, what exactly is a "stepped piston"?


I didn't realise the piston actually fits exactly in the tube where it travels, and therefore is indeed a stepped piston (ie one that exposes a greater area when it unseats therefore overwhelming the retaining spring and causing it to "pop")
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Unread postAuthor: saefroch » Sun Sep 19, 2010 2:30 pm

Lockednloaded wrote:What's wrong with having a decent ROF and hybrid power?


Nothing at all. One normally sees power decrease as ROF increases, and power is usually the reason one builds a hybrid. If it is possible to get a "decent ROF" without sacrificing much power, awesome.
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