I love potato guns. They were one of the first things I learned to build that did a specific, powerful thing. The sharp surprise of watching an object crack out into the sky made me feel simultaneously powerful and very, very small. They were a big hobby of mine all through high school, and are kind of nostalgic for that reason. These days though, things are different. I live in the middle of a city with nowhere to responsibly lob things, and I just can't get excited about guns in the same way I could as a teenager.
This spring I was in the BROS show “Gründlehämmer,” and the final scene involved setting off several air cannons on my body, courtesy of Blood Director Randall Ziman:
The effect worked beautifully, and it got my wheels turning again. Confetti cannons are all about fun and crowds and friendship rather than power and destruction. I have made a couple since, but I'm still a bit rusty. A couple weeks ago, I had a pile of 1” and 1/2” PVC and fittings from an old centaur costume, and took a Sunday to try something out.
I wanted to see if I could build a small cannon with the parts I already had, and without using a commercially available valve. There is kind of a size limit on valves that you can buy for a comfortable amount of money (1” solenoid valves for automatic sprinkler systems, or large ball valves) and beyond that you basically need to build them yourself (though I would love for someone to correct me on this statement!) There is also a tradeoff between flow and opening speed. Sprinkler valves open fairly quickly but restrict the air flowing through them. Ball valves open slowly (and with a little elbow grease) but can present an opening as wide as the fitting. A large enough piston valve can do both. I hadn't built one since high school and could definitely use some practice with a miniature version.
The design was a basic piston valve:
The body of the valve is a tee fitting, with input and exhaust valves on one end, a smaller output tube at the opposite end, and an air tank coming off the side. A moving piston can move forward to seal off the output or slide back to block the exhaust and reveal the output. The piston is not a perfect seal against the valve body, but very close.
By my estimate a cross section of the space between the 1” body and the 1/2” output (really 1.03” ID and .84” OD, nominal plumbing measurements are all over the place) has an area of about .31 square inches, equivalent to a .62” hole (somebody correct my numbers if I'm missing something), so the narrowest point is the output itself.
When air comes in through the input, pressure behind the piston becomes much higher than in the rest of the system, pushing the piston forward to seal the output tube. Air slowly sneaks around the piston and fills the tank until both sides are at equal pressure. Because the head of the piston is partially blocked by the output tube, more force is exerted on the rear. This force holds the seal.
Opening the exhaust empties the space behind the piston. The pressure in the tank pushes the piston back, revealing the output. Air in the tank escapes through the output and does whatever work you need it to do. Some air does escape around the piston and through the valve, but not much.
The build hit a few bumps, but it mostly came out how I expected.
I ended up having everything I needed. Fitting the output tube into the tee required removing the lip from a reducing bushing.
A NOTE ON SAFETY: This design is intended as a test, not to be used for long periods or at high pressure. Cutting a fitting short like this immediately invalidates the manufacturer's pressure ratings. For my purposes this was good enough, but it would have been much safer to dremel out the rim from the inside and reinforce the joint with epoxy. As it is, PVC is not rated to be used with compressed gases. It becomes brittle in cold temperatures, and gases get very cold when they expand rapidly. It is important not to let a PVC system get terribly cold, either from the weather or from operating, and to note that pressure ratings are intended for water and may not apply in these cases.
I wanted to double check the volume between the pipes, so I did a little displacement thing.
The piston is a shaved-down cork, and I beveled the end of the output tube to help it bite into the piston.
I had a bunch of schrader valves laying around from Gründlehämmer, so that was no problem. I dry fit everything and blew into the holes to make sure the piston was moving around. For the sake of budget, I built a simple trigger mechanism using a cork to plug a hole, and a pin to hold in the cork. It works ok enough for testing, but should be replaced with a ball valve down the road.
It works! I spent a good two hours just shooting things around the yard and annoying the neighbors with my compressor.
Ultimately I am pretty happy with how this turned out. For a larger-scale project I would want to rethink the piston itself, and work out a better exhaust valve, but it's at least a start.
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