Complete Plans

Pete Zaria · Posted: 09/03/2006 0:24 AM Post subject:

These are the plans I've used for my combustion gun. This is my own design, but I've borrowed bits and pieces from many other cannons. I've built one just like it for a friend, who loves his. Once it's built, it will fire very consistently.

The objective for this cannon was to build a very reliable combustion gun, on a tight-ish budget. So there's no regulator, and many of the parts can probably be scavenged. I know you could do a nicer job with higher-priced parts, but that wasn't the objective here.

All pieces are Sch 40 ABS. Cellular core will work, but solid is prefered. Mine is mostly cellular core, I've fired several hundred spuds with no problems or indications of wear.

Before I start, let me say that I added the rear assembly and pressure gauge as afterthoughts after the cannon was built. If you're building from scratch, it would be better to put the pressure gauge between the two ball valves instead of on the hose, but it works fine as-is. The rear assembly could be solvent-welded directly to the chamber, without male or female adapters, but its kind of nice to have it removeable.

Here you go:

A few tips.

When solvent welding pieces together, roughen up both surfaces that are going to contact each other with some sandpaper (200 grit or less). Apply the glue liberally, and twist the pieces as you push them together. Push them down *really* hard, until you're sure they're in as far as they'll go. Wipe up the glue that overflows out of the joint. If the glue overflows the joint a little bit, you know you're using enough glue.

To make the handle, take a piece of 1.5" ABS about 5-6" long, and use a hack saw or file to round one end to match the contour of the 4" combustion chamber. Press it up to the chamber and see how closely it fits, and file it until you get it close as you can. Then use superglue or epoxy (ABS glue won't hold) to glue the handle to the chamber. The end cap on the bottom of the handle is optional, but looks good. Obviously, the BBQ sparker is put through a hole in the 1.5" pipe. I don't remember the correct size drill bit. The wires are run out of a small (1/4th") hole in the back of the handle (directly behind the sparker), which go to two spark gaps made of steel bolts. Use washers and nuts on the steel bolts, on both the inside and outside of the chamber wall (no need for a nut on the outside, because of the screw head). The spark gaps are wired in series. If you can't figure out the wiring you probably shouldn't be building a spudgun, but go here if you need help: http://www.advancedspuds.com/multiple.htm Use nice thick wire.

The propane meter is the only complicated part, but it should be pretty self-explanatory from the diagram. Use air compressor hose for the hosing. If you can't find compressor hose, you can use the coil-up type retractable garden hose. If all else fails, clear half inch tubing works, but I've had the clear half inch tubing burst when exposed to pressure for more than 5 minutes or so at a time.
Use 3 wraps of teflon tape on every connection for the propane meter, including where the meter goes into the chamber. Before you install the meter system onto the chamber, open the first valve (leave the second one closed) and the valve from the propane tank, and submerge the whole propane meter assembly underwater in the sink or bath tub to check for leaks. If there are any bubbles, use more teflon tape and screw the fittings in tighter.
Make sure you tighten down the hose clamps as far as possible. Use a big screwdriver and twist the hell out of it.

The propane meter system is strapped to the chamber using two pieces of water heater strap, to keep the propane meter from moving if the cannon is dropped.

Make sure you tap the propane meter into the chamber where there are two layers of ABS, meaning where a fitting and the chamber pipe overlap. If you don't have a tap, drill a hole that's a bit too small, and then screw the 90 degree elbow into the pipe using a wrench. Try to go in as straight as you can, as far as the elbow will allow.

Benzomatic propane cylinders fit perfectly in 3" ABS, so I mounted an appropriately sized piece of 3" ABS on the other side of the chamber, which holds the propane tank.

The chamber fan is simply epoxied to the chamber wall. The wires run through a very small hole, again drilled through two layers of ABS. Fill the space around the wires with epoxy or superglue. The wires go to a small project box ($3 at radioshack) with two 9v batteries (wired in series, not parallel) and a momentary switch.

FIRING THE CANNON

1. Load projectile into barrel, almost all the way to the chamber (1-2" from chamber). Measure out the right length on a shovel handle or small pipe (for a ramrod) and mark it with duct tape for future reference.
2. Put on end cap or close ball valve (if you're using the rear assembly).
3. Open valve from torch head and first ball valve (valve closer to propane tank). Wait for correct pressure (38 PSI for propane, 48 PSI for MAPP. I recommend propane, its cheaper and the difference is very small).
4. When the meter pipe is up to pressure, turn off the valve from the torch and close the first ball valve.
5. Open the second ball valve to vent the propane in the meter pipe into the chamber.
6. Hold down the fan button for 3-5 seconds to mix gas.
7. Aim and fire.
8. Vent chamber before repeating. If using a cleanout cap, remove the cap and use the fan for 6-10 sec. If you're using the rear assembly, just open the ball valve and run the fan for 10 sec.

Don't leave pressure in the meter pipe hose for very long, unless you're using compressor hose, which can take the pressure.

Hope someone gets good use out of this Smile

Peace,
Pete Zaria.

EDIT: These plans are posted for educational purposes ONLY. I am not influencing anyone to build this device, nor do I endorse such. If you build this device, YOU are responsible for anything you do with it, or anything that happens to you or your property because of it. I assume absolutely zero legal or moral responsibility by distributing these plans.

- There, now my lawyer will be satisfied if anyone blows themselves up with these plans and decides to sue me.

But seriously, this is pretty safe. ABS rips clean and doesn't shrapnel even if it does fail. However, don't tempt fate by using projectiles with a REALLY tight seal. My general rule is, if I can't push the projectile down the barrel without having to use excessive force or my bodyweight, it shouldn't be used as a projectile.
Don't leave the gun in the sun, period. ABS is pretty weak when it's hot. And give it at least 30 seconds between shots.

Peace,
Pete Zaria.