Pneumaticaly Triggerd Burst Disk Shoulder Mount Design

[noob of noobs]

Recently, I discovered a design for a pneumatically triggered burst disk system on the wiki. It looks pretty good, and could supply a lot of power. I also realized that I have almost no large power based cannons, just small bore rifle stuff. So I decided to make a pneumatically triggered burst disk (a concept I’ll just refer to as PTBS) large bore cannon. I decided to put in a shoulder mounted format, so after lots of planning and doodling in inappropriate times, I drew up this. It will be made out of PVC to reduce costs, and should provide some decent power behind a PVC “rocket” projectile and various other 2” ammo.
But I do have a couple questions:
-A) Is this a practical design?
-B) What would you recommend as a good burst disk? I’ve heard aluminum foil works well, but do you know about how much pressure they fail at?
-C) Will a blowgun pilot the valve well?
-D) What lengths do you recommend I make the chambers and barrel? I’m looking for compactness, but would prefer power.
-E) Is a double barrel design viable?
-F) Any other good ideas for ammo?


Thanks for your time, I appreciate all comments
P.S. Sorry about the crappy image quality... Also, all pipes are 2" schedule 40 PVC.

[noob of noobs]

Uh... any input?

[mega_swordman]

I cannot seem to understand the significance of the ball valve and blowgun. If you could explain it to me (I also don't understand the picture comment), it would be appreciated.
Otherwise, it looks like the hose from the union to the handle is a bit long. Reducing it would make it more efficient.

[noob of noobs]

Okay, so you open the ball valve while the cannon is being pressurized. Then, once half of the pressure the burst disks can reach is acheived, the ball valve is shut off. Then, the main chamber is further filled to pressures exceeding the burst disk. But since there's a resovoir of air on the other side of the disk (between the two unions) the difference in pressure is not enough to burst the disk. Then, the air in between the unions is released through the blowgun. Thus, there's less pressure in the resovoir, and there's a higher pressure difference between the two sides of the first burst disk. It breaks, and then the second one breaks too, thus releasing air into the barrel.
Here's the link that shows how the system works http://www.spudfiles.com/spud_wiki/inde ... burst_disk

[mega_swordman]

That sounds like it could work, but would be a bit troublesome to fill. I say go ahead and try it.

A) I think it could work in theory, but it will be a bit of work replacing the burst disks and pressuring. However, I believe it is good enough to attempt.
E) I wouldn't. Just stick with one barrel
F) Potatoes perhaps?

[noob of noobs]

Thanks. Potatos are nice, but they don't always fit into 2" pipe. But hey, it's been a while since I shot an actual spud!

[SpudFarm]

that is going to work well but i think a blowgun will use some time to emty the second chamber mabe you can use a little QEV that should do the trick if you can get one

[noob of noobs]

I assume that by the second chamber you mean the one between the unions. A QEV would be nice, but they're a big expensive. I might just use a spring loaded ball valve if the blowgun doesn't work well...

[SpudUke5]

I dont think you have to worry about the emptying of the chamber between the unions. It should work very well even if you use a ball valve, i dont think it really matters in this case.

Since all spudguns work on principles of pressure equalization, then this should work with no problem, as long as the material can be broken with the differences of pressure. So when you empty that area between the unions, it causes a very large pressure decrease, and since high pressure tends to move to low pressure areas, then the first burst disk is ruptured, then following the second until the pressure is equal in the chamber and the area around it.

So it should definately work.

[SpudFarm]

but i like a shot the split second you hit the trigger and a ball vale would do that with zero problem but a blowgun and a QEV alows him to get a better trigger system more practical to trigger and he will be more steady on firing but a ball valve needs a little force to open and then the gun is pointing another direction and accuracy is getting a step further down the lather.

sorry if i don`t explain that so simple

[Pilgrimman]

I have one that is almost the same, but small bore (3/4"). The design will work perfectly, as I have already built it! (Amazingly enough for me, It worked perfectly 1st try, and I have never had so much as one problem from it! I'll post a pic as soon as I get a chance.

Interesting fact, the GGDT simulation of said gun says that the pilot volume I have, (5 in.^3) actually increases performance by 2 fps! (Cool, but not earth-shaking )

Edit: Here: http://www.spudfiles.com/forums/viewtop ... tml#151122

[SpudFarm]

the 2 fps part may be the gasses accelerating into the second burst disk and build a DDT like effect i know it has a name but i cant remember the name of this effect.

[Pilgrimman]

Might be shock heating, but I don't know...

[noob of noobs]

Sweet. I don't see how DDT could occur in a pneumatic, but I suppose it could happen if you get enough pressure...
I'm thinking of a 3' barrel and have the chambers be 2' to 2.5' Does anyone object with aluminum foil disks? I also might try some PVC rockets for ammo.

[Pilgrimman]

I personally think foil is awesome for burst disks, so I'd use it. The only snag might be the amount of foil needed since your cannon is large bore. (My 3/4" gun bursts one layer of fouil at ~20 psi.) Yours would probably be less even than that.)