The Spudchucker II - A propane-powered ABS cannon

[sudpuzzer]

Hey all, I'd like to show you guys my second potato cannon (first attempt at propane), the Spudchucker II.

Features:
- Combustion type
- Propane fueled
- 4" x 26" dual layer combustion chamber
- 2" x 60" barrel
- 2" x 26" barrel
- 3" x 36" barrel
- 2" barrels are sharpened with barrel knives
- Aluminum cam-lock style barrel couplers
- 2 grip styles: shoulder fire and hip fire, complete with hockey tape grips
- Propane meter and regulator
- Magnetic propane tank holder
- 30kv gas stove igniter, 30kv wires, and dual spark gap
- Internal fan
- "Doomsday" safety switch

To start with, this is my second potato gun. The first used starter fluid and a piezoelectric BBQ lighter for ignition. I was unhappy with the consistency and reliability of that setup, so I set out to build a metered propane cannon to ensure a reliable shot every time.

The chamber is constructed of sections of 4" ABS pipe connected by female-female couplers. Since ABS is not my material of choice (Sched. 40 PVC in 4" is impossible to find in Canada), I figured doubling the wall thickness of the chamber was safer than going with straight pipe. With the pipe inside the couplers, the wall is double thick for the entire length of the chamber. Since the ABS is chemically welded, I feel that it is safe even though the chamber is made of several pieces rather than one piece.

I did some rough calculations for the meter pipe using P1V1=P2V2, targeting a 4.2% propane to air mixture, and a meter pipe pressure of 40 psi (the thinking here is that the tank will supply enough fuel until it goes below 40 psi, rather than requiring multiple injections for a higher pressure). In reality, the gun works best around 36-38 psi, depending on the outdoor temperature. My pipe length approximation was pretty close!

The electrodes, the fan contacts, and the injection pipe were the only holes cut in the chamber (through spots where it is double thick of course), and the whole design is meant to eliminate holes drilled into the chamber.

The long barrel was constructed using a chamber to barrel ration of 1.5:1, and the short barrel was made of left over scrap pipe (I call it the cricket gun, it packs a punch). The 3 inch barrel was made to shoot tennis balls and other large caliber ammo.

Here are some pics. Let me know what you guys think!

http://imgur.com/a/fHU8t#0

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[evanmcorleytv]

That was a bit more powerful than I expected. Fantastic looking cannon! Welcome to Spudfiles.

[CS]

Nice launcher. Honestly you would have been fine with the one layer of ABS, but hey I understand the fear of a launcher blowing up in your face. Thanks for the videos and welcome to SpudFiles.

[cowscankill]

I like it.
I'm looking to build my first combustion using a metered propane system, and this is pretty much what I'm looking for. Can I assume the first gauge is for checking tank pressure and the second is for checking the metered in pressure?

[jackssmirkingrevenge]

Depending on the size of your combustion, you can get away with a much simpler setup:

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[sudpuzzer]

The first gauge is just the pressure that the regulator is set at. The second gauge is the pressure in the meter pipe. If the regulator/meter pipe won't build 40 psi anymore, I'll have to change the propane cylinder, or do multiple injections until the bottle is empty I suppose.

[sudpuzzer]

Bit of a necropost, but I wanted to show you guys a little bit of an update on the Spudchucker,
First off, you will notice a nice new paint job. Was going for a bit of a military look, white stencil lettering and whatnot. This was done using Krylon Fusion sparypaint:





Secondly, I recently built a 3D printer, which has been perfect for making specialty parts:
Solidworks

Grip

Mount on the printer

In this shot, you can see the assembled grip. The red button is in place of the trigger, and closes the firing circuit on the spark generator.

Attached and wired! All it needs now is some paint.


Also, I took some semi-high-speed video of another dry fire test with a GoPro at 240fps. Still a little too slow a frame-rate to see much, but I did manage to snag a few shots from the footage:

(You can see the tape I had covering the barrel coupling flipped up and backwards from the blast)




The initial burst was followed by a small secondary burn. It seems to behave differently with sufficient back pressure, resulting in a complete burn..

Last section of my mega-post is some plastic ammo I have been working on:
The one on the printer is a a design similar to the compound rounds found on UltimateSpudGun.com. I have designed it to accept different top plates, and plan on making one from hardened steel in the future.


This is a plain round-nose slug, the bottom screws off and I filled it with pennies (not much use for them anymore!). I can't wait till I am done finals and it warms up so I can shoot some stuff with it. I will be sure to post videos for you guys.

Anyway, let me know what you all think!