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Timing a Piston Valve Hybrid Cannon and Barrel Material

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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Timing a Piston Valve Hybrid Cannon and Barrel Material

Unread postAuthor: TenTerabyte » Sun Aug 18, 2019 7:50 pm

I have built a few pneumatic cannons before and I am planning to make a hybrid cannon. I would rather use a piston valve than a burst disc. I want to do a 5-10x propane, but I don't know how to control the piston valve. I have searched on the forums but I am probably missing something big. Do they use normal piston valves or are they controlled by positive pressure? Also, would 1.5 inch metal conduit work for a 10x barrel. I am trying to shoot eggs with 3D printed sabots and i cant have a seam on the inside.
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Re: Timing a Piston Valve Hybrid Cannon and Barrel Material

Unread postAuthor: Moonbogg » Mon Aug 19, 2019 7:56 pm

A hybrid piston's opening pressure is determined by how much air pressure you pump behind it. I'm more interested in how you are matching the shape of that egg so perfectly that it won't break upon firing. The sabot has to match the contour of the egg perfectly, right? Otherwise it breaks upon firing for sure.
Wait a minute...you mentioned timing the piston valve. They don't work like that. Are you familiar with the concept of a common spud gun piston valve? I used to think they were timed too. Wow that was a long time ago. Look in the cannon data base and study them.
They have air pressure behind them which keeps them closed. There is a small surface area exposed to chamber pressure, and when it builds upon firing, the force balances out on the front and back of the piston causing it to open. Once it cracks open, the whole front face of the piston is exposed to the chamber pressure and it flies open, allowing all the pressure to escape down the barrel.
There are some inefficiencies, such as leakage past the sealing face as the pressure builds right before opening. I'm not sure how much that hurts performance though as I haven't experimented that critically. They seem to work well enough as they are.
So again, you're launching an egg at over 1000 psi. Interesting.
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Re: Timing a Piston Valve Hybrid Cannon and Barrel Material

Unread postAuthor: TenTerabyte » Mon Aug 19, 2019 9:43 pm

I am thinking of a sabot with foam to hold it on about half of it's surface area evenly and also a 10ft barrel to have a slower acceleration. I would also carefully pick out eggs to fit the sabot. We have too many duck eggs and they are a lot stronger than chicken eggs so we can fire those. Now that you mentioned that cracking the valve open with the pressure is what opens the piston, that makes a lot more sense.
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Re: Timing a Piston Valve Hybrid Cannon and Barrel Material

Unread postAuthor: jackssmirkingrevenge » Tue Aug 20, 2019 7:31 am

Further to what Moonbogg said, many hybrid piston designs dump the area behind the piston as soon as it starts moving, allowing it to snap open much quicker.
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