Newbie First Cannon - Good but need input

[dolphman69]

Hey guys,

Newbie here. Just built my first simple air cannon. I call it the "Scorpion", based on the PVC bipod and other scorpio-aesthetics. You hold it like an RPG to fire.

It's muzzle loaded, for now (something I would like to change), and the WHITE PVC is just structural, none of it is glued in (waiting until I finish the design). So no air travels down white PVC at all. The pneumatic power comes from the 100psi fire extinguisher connected via 1/4 vinyl hose (rated at 250psi) connected to a 1/2 PVC irrigation pipe (Sch80) I believe. I had to bore out the fire extinguisher's "barrel" to a 1/4 inch in diameter in order to get more air out of it.

I then used a quarter inch brass barb and 1/4-1/2 brass coupling to go from the hose to the 1/2 inch Sch80 pipe barrel. That, so far, is the only PVC on this whole contraption that funnels air.

In other words, anything connected has a 250 psi rating or above. The fire extinguisher itself is rated to 200, but I haven't charged it that high to be safe.

I thought the fire extinguisher (modded to have a shrader valve) would be a nice way to fire the gun, using its handles as a built in trigger. But I don't like the fact that, with one squeeze, all the air is dumped.

QUESTIONS:
What kind of simple piston design would increase velocity and add more power?
Would this piston design allow me to retain some air pressure in the fire extinguisher for subsequent shots? (in other words, multi-shot capability)
How could I breech load this or perhaps create a magazine fed/semi-auto capability?

I was also thinking it might be cool to shoot 1/4 steel ball bearings (slingshot ammo) out of this, in case of a zombie apocalypse. Perhaps a BBMG?, but I would want it to shoot a higher caliber round than just the standard .177. However, the AA batteries are pretty cool. With some duct tape, they seal the barrel quite well, allowing them to be shot out at a decent velocity even with the simple setup I have now.

Any ideas on how to improve this with the above design goals above would be MUCH APPRECIATED! At the end of the day, anything simple that could add power and create a multi-shot capability would be great. Creating a multi-ammo capability is probably a long shot, and I'm happy to stick with the AAs for now.

Thanks everyone.
-dolphman


One more thing - the ultimate goal would be to make this thing as self-contained as possible, in other words; have some kind of portable air compressor/pump capability. Not sure how many pumps it would take to get the fire extinguisher up to 100psi. But the goal is to not have to have an air compressor be the only way to fill it.

[killerbanjo]

Nice looking gun mate, dont get more simple than that! Once built a gun similar with a drinks bottle and a garden hose thing, the one with a trigger.

Check out what jack did hereto achive semi automatic

[dolphman69]

Thanks Killerbanjo, gonna investigate this more. Looks like a great solution.

This is gonna sound real newb and stupid, but just so I understand simple piston mechanics correctly:

If air from the fire extinguisher (about 100psi) enters a simple chamber with a simple piston that is driven forward by the air pressure and self seals against the barrel, won't that STOP any further air from being expelled out? I assume then a spring would push the piston back once I let up off the extinguisher's handle.

My only concern would overpressure develop within this piston chamber? The chamber would be rated to a high psi, but say for instance I kept the trigger squeezed well after the piston fired forward. Without any air leaks, would pressure build, or would it just stay at 100 psi (whatever the pressure inside the extinguisher is currently)....

Not that I would build it this simply, I guess I'm just trying to figure out some basic physics and not endanger myself needlessly. I hear there are safety valves that blow when PSI gets too high. Is that something that would need to be installed? Or is this an unfounded fear and I misunderstand the physics?

[Mr.Tallahassee]

The pressure would actually stay at approximately 100PSI. As long as the parts are rated high enough then you'll be fine. As to the spring pushing it back, unless you can bleed the pressure off then it'll stay sealed. I'm not sure what you are trying to do though. Could you enlighten us on you plans?

[dolphman69]

Thanks Mr. Tallahassee.


I guess what I am hoping to do is to create a simple piston that'll push enough air volume to increase the velocity of the AA batteries (or other projectiles, though it may make more sense just to stay with the AA ammo for now), while not bleeding/emptying out all the air in the extinguisher.

I was assuming then, that the piston would close the distance pretty quickly with the barrel, seal against it, and use only a minimum of the 100psi air in the chamber(extinguisher). Then, I take my fingers off the trigger to stop the air flow coming in from the extinguisher. I'm hoping there's still enough air left for several more shots.

as to the spring pushing it back, unless you can bleed the pressure off then it'll stay sealed.

If I just release the trigger on the extinguisher, won't the lack of pressure coming from the extinguisher into the piston allow the spring to push the piston back, away from the barrel? Or will there be air trapped in there from the extinguisher's previous burst? Is that why I'd need a bleed valve?

Right now, even if I quickly let go after firing, I still manage to dump most of the air.

I guess I'm looking for the most effective and efficient way of getting the most velocity out of the cannon without dumping all the air in one shot. From the little I've ascertained from these forums and other sites, I'm going to have to invent some kind piston/valve combo. All of this would be assembled on the top of the gun where the vinyl hose currently inserts in the Sch80 PVC barrel. So I got some work left to do.

The problem is, unlike a lot of other pneumatics, I'm using an extinguisher, so it changes the air flow dynamics a little. I'd like to keep it, and find a way to make it all work better.

Killerbanjo sent me a link to a cool valve thing, I'm just a newbie and some of the more physics/tech stuff is taking a while to sink in.

Just trying to get my head wrapped around it all, if you will. Thanks for the help so far.

[jackssmirkingrevenge]

As already suggested, if you want efficient and powerful semi-auto, look into a quick exhaust valve and slide valve combo.

It's cheap and works brilliantly for very little hassle, and best of all it conserves air, because firing empties the secondary chamber but blocks off the primary chamber (which would be the fire extinguisher in your case).

This is the logic behind QEV and slide valve, the former works just like any piston valve and the slide valve is just one form of different types of three way valves that can be used in this situation.

From another post:

Basically something like this:



When the valve is in the forward position, it allows air to flow from the main supply to the QEV and fill the pilot and firing chamber.

When the valve is in the rear position, two things happen:

- main supply is cut off

- QEV pilot chamber is open to the atmosphere, emptying it and firing the launcher

The valve goes back to the forward position (you need to add a spring or do it manually) and the cycle can be repeated.

This is the concept explained in slightly clearer terms:



The slide valve is basically a three way valve.

A three way valve has three ports, which we will call A, B and C

There are two possible paths, AB (with C blocked off) and AC (with B blocked off)

A is connected to the pilot area of the piston valve, QEV in our case

B is connected to your fire extinguisher

C is simply left open to the atmosphere.

At rest, the three way is in position AB. This means your reservoir is filling the launcher chamber.

When the trigger is pulled, the valve switches to position AC. This means the reservoir is blocked off, and is no longer filling the pilot and chamber, while the pilot is exhausted to the atmosphere, which fires the launcher.

Once the trigger is released, the valve goes back to AB, refilles the pilot and chamber and the cycle can be repeated.

[dolphman69]

Phenomenal! I love it. Thanks for the very in depth explanation. It's perfect for my research needs. I'll go about finding a way to implement this.

Thanks again everyone.

[dolphman69]

I just realized that although this is a great setup, it doesn't quite work with my setup, due to the fact that the extinguisher does not supply a continuous flow of air to fill up any reservoir. In fact, it is the reservoir, and air is "triggered" out of it by squeezing the handles.

I could modify this setup a bit, perhaps add a separate blowgun trigger to release the air in the pilot. I would be squeezing two triggers though to fire the gun.

-Rich

[jackssmirkingrevenge]

This seems to be a trivial objection, nothing a ziptie or some wire holding the fire extinuisher handle open won't solve easily.

[dolphman69]

Ahhhh yes. I see. Duh. My mind was still fixated on the fact that the extinguisher was specifically selected for its natural trigger capacity. I guess it's still safer holding air than PVC.

[dolphman69]

Has anyone seen a good idea for loading AA batteries into a spud gun? I would like the gun to eventually be magazine fed the AAs. I wonder if there isn't any good magazine ideas out there.

Bolt action would work too, but isn't preferred if possible.
-Rich

[jackssmirkingrevenge]

I get the feeling that deep down, you just want to make this: http://www.spudfiles.com/forums/my-full ... 23527.html

[dolphman69]

Hah! That's funny. Yeah, you're right, it is pretty cool. Although he has to lug that compressor around, and I couldn't afford to waste that much ammo.

I will however, ask him about how he made those magazines.

Thanks all!

[dolphman69]

Talked to the full auto guy, got some tips on the magazine. So that's good.

I had another question, anyone on this forum ever attempt to make a shotgun cannon? I have some bbs/ball bearings lying around, and was just curious if anyone had attempted this.

I'm assuming the PVC barrel wouldn't be up to snuff on this. How about a copper pipe?

[killerbanjo]

When i made a "shotgun" the problem i had was you have to have an airtight wad for a pneumatic, or power is pathetic unless your going high pressure. I used HDPE rod and filed an oring grove into it for a small cylinder and although it worked it was a pain in the ass if you lost the plastic bit or it defomed too much.

The other way it could be done is by loading several projectiles into a normal barrel, you would not get much of a spread though unless you made the end of a barrel like a blunderbuss.