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Possible Piston Valve Hybrid Design

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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Possible Piston Valve Hybrid Design

Unread postAuthor: frocksie » Thu Apr 02, 2009 7:27 pm

I was just thinking about making a piston valved hybrid when I came up with this idea, and I was looking for some imput on the design. I didn't include the ignition system, as that is no problem. My main question has to do with the piston valve setup and the "pop off" valve that would vent the pilot area.

I was thinking about making it out of this pipe. My question regarding that pipe is whether or not the pipe will rust to the point where the o-rings will cease to seal.

If there are any question, feel free to ask. Also, the drawing is to scale, and should be actual size.

Thanks.
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Unread postAuthor: Mitchza89 » Thu Apr 02, 2009 7:54 pm

I personaly have never built a hybrid but I reckon the best thing to do would be a cheap proof of concept. Get some SCH80 pvc pressure pipe and some couplings to sleeve over them like in your pic and just start off cheap and small.

Buying all that steel, planning over and over again with putting all the work into cutting/welding/machining and then finding out it's not going to work will be a real bummer.

Best of luck and I hope it turns out well for you mate,

Mitch.
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Unread postAuthor: frocksie » Thu Apr 02, 2009 8:44 pm

Thanks Mitch. I was really wondering about the mechanics of the piston valve, as I am not sure that I want to build a prototype out of PVC. I think that it should work, but I just want to be sure before I buy the pipe. Thanks.
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Unread postAuthor: Mitchza89 » Thu Apr 02, 2009 8:46 pm

Piston valve in what application? Hybrid or Pnuematic?
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Unread postAuthor: jook13 » Thu Apr 02, 2009 8:51 pm

I am finishing the production of a piston valve and have test fired it. Mine is more like a traditional barrel sealing valve, but similar to this design. I dont see the need for a popoff valve. I imagine the ammo to be out of the barrel before it even pops open, but that is just a guess and I may be wrong. I just allowed for more piston travel and had the air behind the piston act as an air spring. After a bit of testing, it works great.

So is your plan to fill the fuel and air both from behind the piston?
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Unread postAuthor: mark.f » Thu Apr 02, 2009 8:53 pm

As for the tubing rusting, if you lubricate the piston and track it rides in with some sort of water-repellent grease, you shouldn't have problems with rust.
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Unread postAuthor: frocksie » Thu Apr 02, 2009 10:07 pm

Mitch: This would be a hybrid piston valve.
Jook: I wasn't sure if the pop off would be necessary, as I wasn't quite sure as to the opening time with and without it. I do think that only an air spring would be okay on second thought, and I will allow for extra travel as you did. I was going to fill the air spring first, and then fill the chamber on the other side of the piston, the valve for which I did not include in my diagram.
Mark: Thanks. Would the chamber rust significantly if not coated?

Also, does anyone know whether or not it would be safe for me to weld this together myself, or if I should take it to a machine shop for a professional job.
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