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piston hybrid, relief 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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Unread postAuthor: bigbob12345 » Fri Apr 04, 2008 10:03 am

also i am going to use a really tight spring behind the piston to hold it there until the pressure is at about 400psi.


where are you going to get a spring that strong?
I doubt they even exist

edit:Ha I think I just answered my own question
let me guess, Larda :D
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Unread postAuthor: Jared Haehnel » Fri Apr 04, 2008 10:07 am

You'd want to be careful if your combustion say reach 455psi it would be enough to open the piston briefly but would give you really lame performance...

But if the pressure reached 1000 PSI you'd be fine however you'd still have about 400 PSI in the chamber after the projectile left the barrel. That pressure alone would be enough for another shot so you'd have to have some way to fire another shot or to release the pressure in a safe manner.

Edit:....They exist.... just not at your general hardware store...automotive maybe...
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Unread postAuthor: bigbob12345 » Fri Apr 04, 2008 10:11 am

I would not say something like that is avaible at even an automotive store, the internet would be your second best bet
first best bet being larda's place :D
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Unread postAuthor: SpudFarm » Fri Apr 04, 2008 10:18 am

haha larda probably have one but we work with engines here and i can get variator springs for snowmbiles in different size that fits in eachother..

a 10x has 1000psi peak (about)

if i get a ball valve at the "pilot" side i probably are going to use it but i thought of a catching mechanisem (?) so it stopps

and the pointlessness of the pilot is that the piston will not open when the spring is there that is why i use a so tight spring..
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Unread postAuthor: Jared Haehnel » Fri Apr 04, 2008 10:22 am

I had thought of a similar idea not so long ago... a spring loaded catch would do nicely....Have you looked at Fnords piston hybrid...
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Unread postAuthor: SpudFarm » Fri Apr 04, 2008 10:30 am

yes and i have had some pm'ing with him about it...
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Unread postAuthor: sniperjosh » Sat Apr 05, 2008 7:19 am

Jared Haehnel wrote: But if the pressure reached 1000 PSI you'd be fine however you'd still have about 400 PSI in the chamber after the projectile left the barrel. That pressure alone would be enough for another shot so you'd have to have some way to fire another shot or to release the pressure in a safe manner.


Pretty sure after the combustion is complete the gases will cool and the pressure will drop.

I don't think a heavy duty spring is going to give the same benefits as associated with higher burst pressure in burst disc hybrids. The spring would reduce the pistons movement/speed a lot and the valve will not open fully, inhibiting the flow to the barrel.
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Unread postAuthor: Jared Haehnel » Sat Apr 05, 2008 7:26 am

In principle it would work the same a a piton valve on a pneumatic... Once the valve opened fully it would only a little less efficient then a burst disk...

Your right the gases would cool but I don't think they cool as fast as you might think they will. After the gun fires and the piston seals off the barrel the gas its going to be hot and gradually cool off. Even after saying that the pressure in the chamber will likely remain high above 1 atm even after the gases have cooled off completely because he started above 1 atm...
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Unread postAuthor: sniperjosh » Sat Apr 05, 2008 8:07 am

Thats what I'm saying, the piston will not open fully. Maybe with a Longer area behind the piston with a long, progressive spring It will achieve the same flow. Still going to have a slower opening time VS no spring, and I can't see the multi-shot idea working.

Yes, the pressure remaining should equal the pressure prior to combustion (in a closed chamber). In the closed chamber testing I've done, the gases cool very quickly, all you see is the pressure gauge spike and drop back to original pressure, almost as if you just flicked the needle with your finger. The valve will open and vent some of the gases, so i would say the remaining pressure would actually be less than initial pressure.
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Unread postAuthor: Jared Haehnel » Sat Apr 05, 2008 8:24 am

1000 PSI is a lot of pressure think about the blow back on a rifle the force of the expanding gas is more then enough to cycle the action.....

We are talking about moving the piston back a very little distance to achieve maximum flow... if its a two inch chamber it only has to move back about an inch or so before maximum flow is achieved...

You right its not as instantaneous and is far inferior to a burst disk but it does offer a way to adjust with reliability at what pressure the piston opens at....a looser spring the lower the pressure the piston opens at the tighter the spring the higher pressure the piston opens at...

If you have a super tight object the object could also act as an burst disk and piston would only be a means to ensure that combustion has reach far enough to produce a desire velocity

As for the gas it would be best if he had someway to keep the piston open after the shot to let out all remaining gases...
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