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Large pneu. cannon help

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: Ragnarok » Wed Dec 26, 2007 8:03 pm

DYI wrote:2. Ye Olde Plasma Rifle; Turn the gas into a plasma by heating with extremely large electrical discharge in the chamber. According to one resident genius, could conceivably achieve velocity in excess of 7 km/s if aided by magnets to guide and concentrate the plasma. Not recommended for beginners (or anyone, really).

I like the fact I only mentioned the theory a few days ago, and it's now considered a semi-official way of breaking the sound barrier, and has been tagged with "Ye Olde".
But yes, certainly not recommended for beginners. Only complete nutters who (like to think they) know what they're doing should even vaguely consider the idea. Only total head cases masquerading as engineering students should come up with such an idea.

@The original poster, I'm not saying all that username:
Getting too obsessed with breaking with the sound barrier is a bad move. It's something I'd like to do, but if I were going to do it with any of my current launchers, it would probably be done it with a projectile so light it would do almost nothing on impact.

Still, I'm the hypocrite who's designing an entirely new propulsion system to use the 4th state of matter for propelling light projectiles past the sound barrier, so perhaps you should ignore me.
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Unread postAuthor: DYI » Wed Dec 26, 2007 8:34 pm

For the loudest noise you can make? I'm assuming that you actually want to fire something while making said noise, but the answer is kind of difficult. The only way to get more noise out of an optimised launcher is to sacrifice power, so it depends what kind of balance you want.

For noise, you need a wide bore, a suitably high porting valve, and excess chamber volume in a pneumatic. For a given barrel size, up to about a 4:1 C:B (although it varies based on pressure) you are gaining power by adding volume. Anything a whole lot over that is creating excess noise, but not doing a whole lot more.

So to answer your question, if you are building a pneumatic, and are limited by a 12' x 2" barrel, all you need to do is increase the chamber volume. Considering that the proposed chamber volume is already extremely excessive, you have no worries there. The next part is, if you really want noise, I'm assuming you want more of a "BANG" than a "WHOOSH", in which case you want either a piston valve, or a burst disk valve (which both have potential to open in sub-millisecond times), and not a ball valve which will be comparatively slow.

In your cannon, the pressure drop by the end of the barrel at 150 psi will only be ~30 psi, while in an efficient launcher it would be 130 or more.
So, take the noise a blowgun makes at 120 psi. Then multiply the opening time by 50, and the surface area of the escaping mass of air by ~250, and you have your expected noise. In all likelihood, louder at close range than anything you have ever heard. Adding a muzzle break could also possibly increase noise, as well as the more beneficial effects of reducing recoil and increasing accuracy if built properly.

And thanks for your comments on my intelligence. Although I'm not a genius by any means, I try my best to help on here. I'm 15 years old, and employed as a cashier at a local pharmacy, so I don't really make my own living. I'm planning on going into engineering, hopefully.

@Rag: Attaching "ye olde" to the plasma rifle just seemed to fit, considering that it's absurdly futuristic technology. Lets hope you can build it in some form eventually, or I may have to try it in a few years...
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Unread postAuthor: Ragnarok » Wed Dec 26, 2007 9:27 pm

DYI wrote:@Rag: Attaching "ye olde" to the plasma rifle just seemed to fit, considering that it's absurdly futuristic technology. Lets hope you can build it in some form eventually, or I may have to try it in a few years...

Well, if I had told myself a couple months back I would be trying to design plasma rifles now, I'd probably have laughed, then... probably started to design one, if I've got my number right.
I actually surprise myself sometimes, and that's not always a bad thing.

Ye olde concept should see the light of day sometime, and hopefully it will all work exactly as it's supposed to. I think my electronics will need a bit of a brush up before that though, they're a touch rusty.

Now the weird thing is, Firefox's spell checker registered "Ye" fine, but "olde" is apparently wrong. And, laughably, so is the word "Firefox".
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 26, 2007 10:12 pm

DYI wrote:and employed as a cashier at a local pharmacy, so I don't really make my own living. I'm planning on going into engineering, hopefully.


dude! free syringes :D
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Unread postAuthor: DYI » Wed Dec 26, 2007 10:22 pm

dude! free syringes


You'd love that, wouldn't you Jack? :P
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 26, 2007 10:24 pm

I work in a pharmaceutical lab, I'm sorted for supplies ;)
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Unread postAuthor: tansit234 » Wed Dec 26, 2007 10:36 pm

Without a doubt ball valves are strong compared to much else out there and are easy to use. If you wanted to get fancy with it you could attach an air ram to the handle and the ram's trigger valve could be fed from the cannon's tank or a compressor hose. It will open faster than you can open it for sure. The Mythbusters 2000psi batmobile hook cannon used a ball valve and air ram to open it. Only thing is you need to adjust the ram's travel to not break the valve with the right lever length or slotted connections for slack.
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Unread postAuthor: jackssmirkingrevenge » Wed Dec 26, 2007 10:42 pm

tansit234 wrote:The Mythbusters 2000psi batmobile hook cannon used a ball valve and air ram to open it.


If you have 2000 psi on tap, even a manual ball valve will be enough, especially if used in conjunction with a burst disk.
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Unread postAuthor: brogdenlaxmiddie » Thu Dec 27, 2007 9:21 am

I think I will go with creating a piston valve, they seem not so hard to make and I like the idea of them. Also, they have been recommended many many many times to use. You guys are awsome!
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Unread postAuthor: DYI » Thu Dec 27, 2007 10:34 am

PVC piston valves are very simple to build even with the most basic tools. For the simplest versions, all part modification you need to do is dremel the center stop out of a bushing.
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Unread postAuthor: brogdenlaxmiddie » Thu Dec 27, 2007 2:08 pm

what do you guys recommend for building the stopper like thing inside of the valve?
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Unread postAuthor: Fnord » Thu Dec 27, 2007 3:13 pm

You mean the piston, or the thing that stops the piston when it comes back?

Pistons can usually be made from lightly sanded PVC end caps with a layer of neoprene rubber on them to seal the barrel.

For a bumper, I think most people use either tennis balls cut in half or really tough foam rubber.
You should try to make sure the piston only travels back about 0.5 - 0.75 inches for a 2" barrel. This will reduce stresses on the back of the valve and still give maximum flow.
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