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Experimental Copper Piston Cannon

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Experimental Copper Piston Cannon

Unread postAuthor: SpudBlaster15 » Mon Dec 21, 2009 2:01 am

This is a launcher that I built almost exactly one year ago. I wanted to try a new piston valve design, but I didn't get around to using it much, other than a few test shots performed last year around Christmas. This evening, I fired it for the first time in a year, and the power is quite impressive. At only 140PSIG, I was able to put a AA battery through 1/2" plywood with ease.

A few specifications...

Chamber: ~16" of 3/4" Type L Copper pipe
Barrel: 18" of 1/2" Type M Copper pipe (For AA batteries)
Valve porting: 0.625"
Piston: Machined 6061-T6 billet Aluminum w/o-ring seal

The valve design is unconventional but very effective. The piston is designed to completely seal off the pilot chamber during nearly the entirety of its travel.

To allow the chamber to fill, a hole slightly larger than the diameter of the piston o-ring was drilled in the inner piston 'sleeve', which corresponds to the position of the o-ring at the valve's closed position.

When air is pumped in, the pressure in the pilot chamber forces the piston to close and fully seat. At this point, the o-ring slips over the blind hole, and an air passage is created, which allows the chamber to fill.

When the launcher is fired, the piston needs to move back <1mm to completely seal off the pilot chamber.

Here's a diagram is you're having trouble visualizing it...

Image

The launcher:

Image

Pilot chamber and piston bumper:

Image

Piston:

Image
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Unread postAuthor: boyntonstu » Mon Dec 21, 2009 7:27 am

Nice design.

How far back does the piston travel?

How much air do you fell at the pilot exit?

Have you chronyed any shots?
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Unread postAuthor: psycix » Mon Dec 21, 2009 9:23 am

The piston is designed to completely seal off the pilot chamber during nearly the entirety of its travel.

During nearly the entirety of the travel, but certainly not during the entirety of the time!
I'd like to point out that there is quite some time between the pilot venting and the initial movement of the piston.
Especially pistons with a large porting compared to their diameter (faster opening!) will sit in that spot for a long time before popping open.


I think it performs better to make the floating O-ring act as a check valve by simply drilling a hole next to the groove.
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Unread postAuthor: jrrdw » Mon Dec 21, 2009 9:31 am

Wow 10 fittings with in 1 foot :shock: ! Was that a soldering nightmare? You should get some video up.
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Unread postAuthor: SpudBlaster15 » Mon Dec 21, 2009 3:17 pm

How far back does the piston travel?

How much air do you fell at the pilot exit?

Have you chronyed any shots?


Piston travel is about 1/2" IIRC.

The quantity of air exiting the pilot valve seems to be similar to the amount contained in the pilot chamber. The losses are essentially negligible.

I don't have a chrony, but I may do some pendulum tests in the near future.

During nearly the entirety of the travel, but certainly not during the entirety of the time!
I'd like to point out that there is quite some time between the pilot venting and the initial movement of the piston.
Especially pistons with a large porting compared to their diameter (faster opening!) will sit in that spot for a long time before popping open.


I think it performs better to make the floating O-ring act as a check valve by simply drilling a hole next to the groove.


Floating o-ring designs also have the issue of air loss during static piston position. Some loss through the pilot valve is unavoidable, no matter what design you use. I'm using a modified blowgun and a fairly small pilot volume, so the losses during the few milliseconds that the pilot is venting and the piston is static are insignificant.

This actually was originally a floating o-ring design, but the prototype valve did not seal as well as I wanted it to. This design works much better, at least for my particular valve and piston configuration.
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Unread postAuthor: Nwest82 » Mon Dec 21, 2009 5:17 pm

Cool, so is this a "QEV" style firing valve? I think "QEV" is what the valve is called? when All the air is dumped from the chamber for the one shot and then the chamber must be refiled before the next shot can be fired?. I take it this design is a muzzle-loader and not breech loader? I working on a project, talked about below, but first I want to build a pistol like this in .25cal or .38cal, depends on power output.
I just joined spudfiles.com as I am working on designing a .50cal rifle using the "Hybrid combustion" design,( I think that is what they are called)? They use propane or MAPP gas, a fan which usually runs all the time, (while in the gun is in use/being fired), and burst discs, etc. I still need to learn what MAPP gas is and how available it is, as well as what the "burst-discs" actually do are for. So far from about 10min. of reading I came up with that idea that the "Burst-discs" are simple ball valves which are somehow pe-charged with gas then they are opened up to release the gas into the chamber as a way to control how much gas goes into the "chamber" so a person doesnt overdo it and blow the chamber up. like it could if the gas were to be bleed direct from the tank to the chamber. Also instead of burst discs, I'm thinking on a using small pump design with 2-hoses with some type of check valves inside, instead of the "burst-discs" so when pumped it draws in without pulling air from the chamber, then when you push it forces the gas into the chamber but not back into the bottle, I'm thinking simple ball and spring valves one the opens under vacuum, and closes under pressure, (the one that will come from the bottle). Then the other check valve will be the same thing but installed into the line in a reverse order so it opens under pressure, but closes under vacuum.
I am a Airgun/powderburing pistol/rifle, wtc builder, but have yet to deal with "Combustion" Gun designs and what to lean more as I can see these being able to throw a .50cal at high FPS, mabey even past the sound barrier ;)
Anyway time to do some more studying.

Nathan
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Unread postAuthor: drac » Mon Dec 21, 2009 7:37 pm

No offense man, but you need to do a lot more research on hybrids. Not trying to be a jackass or anything, but you have basically no understanding of them except for part names, and that's not enough to build one. Your enthusiasm is great, but I would hate to see you get hurt.
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Unread postAuthor: Mr.Sandman » Mon Dec 21, 2009 8:00 pm

Uhh Nwest82, I suggest you look at the wiki becaue im sorry but you have no idea what you are talking about. A hybrid is basically a combustion made of a stronger material that uses mixes a certain multiplication of an atmospheric mix whixh is used in basic spudguns. A burst disk is just that, a disk that is used to hold pressure back until it is too high and literally the disk bursts, allowing the projectile to be fired. Once again take a look at the wiki and if you have any questions pm me or another member.
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Unread postAuthor: jrrdw » Mon Dec 21, 2009 8:21 pm

That's enough thread jacking, keep it on topic.
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Unread postAuthor: Nwest82 » Tue Dec 22, 2009 8:26 am

Sorry, I'll start my own thread as king questions then another when I do the build.
MR. Sandman: Thats why I stated that I have to do more studying, thinks for the tip. What I stated above was just what I gathered from 10min's of reading, I knew I was not "on-point", so to speak. I know about airguns and powderburners, I gunsmith and build both. But when it comes to spud guns its a whole different breed to me. I just remember watching a vidoe where a person opend up 3 "burst disc" then fired, I thought it was a way to bleed the gas into the chamber, but having a disc the "bursts" under pressure, (if pressure gets too high), makes more sense. Ok, well enough thread Jacking, Sorry", and thanks for the tip Sandman.
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