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Bolt action piston build

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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Bolt action piston build

Unread postAuthor: daccel » Sat Sep 05, 2009 10:56 pm

Ok, at the moment this is running as a pneumatic, because I couldn't find a piezo and didn't feel like running out to the store, but it's so close to functioning as a hybrid now that the valve and chamber adapter fitting are done I had to share!

I'll work on a sketch soon to better illustrate it, for now pictures will have to suffice. Fires 1/4" bb's, piston has 5/16" porting. Unknown chamber vol, final one still TBD.

Fueling is still a challenge I have to overcome, because I want the chamber to refill with the movement of the bolt that loads the next round.

Here it is operating as a regular piston pneumatic at only 175 psi (have a leak). The pop can picture is the result, didn't penetrate the first side, just went through the hole then dented the second side. Considering the pilot volume is probably around the same as the chamber through a schrader, and the grease hose chamber has an 1/8" restriction, not bad?

[youtube]http://www.youtube.com/watch?v=tieH6x-dtsY[/youtube]

Really excited that the valve functions flawlessly so far, after all that effort. Stay tuned as the rest of the cannon catches up!
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Unread postAuthor: inonickname » Sat Sep 05, 2009 11:45 pm

Awesome. How long do you think the piston will stay open for during firing? I'm trying to decide on a design for maximum performance with minimum upkeep.

Nice work
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Unread postAuthor: theBOOM » Sun Sep 06, 2009 1:19 am

Good work Daccel! im glad your back into spudding my man ;)
Can´t wait for you to test it as a hybrid
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Unread postAuthor: daccel » Sun Sep 06, 2009 11:06 am

Thanks guys. Just a correction, the piston has a 3/8" port, but then the hole into the breech tube is 1/4". Might port the latter out a bit and smooth out each side of the hole for better flow.

I'm not sure how long it will stay open. But I suppose one could roughly calculate that. The piston is .375" on the small end and .5" on the big end, which gives an area difference of 1:2.3. So I'd have to measure the piston travel and air spring volume to calculate this accurately, but take the air spring pressure at max piston travel, and multiply that number by 2.3 and I should get the chamber pressure at which it will close. Of course more pressure would hopefully escape in the time it takes the piston to overcome inertia and friction and reseal.

Ideally you'd make your piston with an even smaller difference, I think I chose those simply because of the o-rings I could get my hands on.

What do you mean by upkeep, low maintenance?
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