Simple Tennis Ball Air Cannon

After reviewing several of the designs found on the Internet, we decided to try and design our own. The two main goals were to keep the cost low, and make it fairly easy to build. After a few brainstorming sessions we came up with an idea for a home made valve that is reasonably easy to build. I also thought of another single use valve that would be even simpler to build and cost very little. So we decided to build both designs and see how they both worked. The home made valve design is going to be called the "Pilot Valve Air Cannon". The single use valve design is called "Foil Valve Air Cannon".

The "Foil Valve Air Cannon" is extremely simple to build but not quite as easy to use when compared to other designs. As you can guess from the name the valve for this air cannon is simply a piece of aluminum foil. The diameter and thickness is such that the foil will break when a certain pressure is reached in the air tank. Other than simple design and low cost, an advantage to the foil valve is that it is probably as close to an instantaneous release of air pressure as you are going to get. The two main disadvantages are; it takes longer to reset for another shot, and it takes two people (or one person and an air compressor) to operate. With out an air compressor, you would pump up the air tank till it is about 80% of the pressure needed to break the foil valve. Then one person will aim the air cannon, while the second keeps pumping till the foil breaks and the ball launches.

There are three main components to any air cannon: the tank, the valve, and the barrel. Typically the tank on these home made air cannons is made from 3 or 4 inch PVC pipe. The barrel is made from 2 1/2" PVC. A special variety of PVC pipe known as SDR-21 is a better fit to tennis balls than standard PVC pipe. Standard PVC has a slightly thicker wall and is a pretty tight fit. Also note that the diameter of tennis balls can vary. It is my understanding from what I have read that a given brand of tennis ball will generally all be very close in diameter but another brand may be slightly smaller or larger. If you tennis ball doesn't fit as well as you like, look for another brand to see if the fit gets any better.

The valve is just a piece of foil, however we need a way to hold the foil in place between the air tank and barrel in a way that makes it easy to replace. The simplest device I have found for this purpose is a pipe union. They are made in all the standard materials that pipe is made from including PVC which is the cheapest. These pipe unions are made of 4 parts. The first part I will call the inlet that will be attached to the air tank. This part has a grove for the included O-Ring. It also has external threads on the opposite end. Then there is what I call the outlet. This part has a flat surface that seals against the O-Ring and a flange that mates with the final part, the retaining ring. The retaining ring slides over the outlet and then screws onto the external threads on the inlet. To turn this union into the foil valve, simply place a circle of foil between the inlet and outlet parts and screw the retaining ring down tight. It has been my experience that hand tight is enough to get a good seal. These pipe unions come in sizes from 1/4" up to 4". The 4" size is very expensive, over kill and will also require a pretty thick foil to hold the pressure. Remember the larger the diameter the lower the pressure the foil will break at. I would recommend using between 1" and 2", there is no point in having it any larger than the 2 1/2" barrel. For my initial testing I used a 1" union just to see how much pressure it took to break some standard foil from the kitchen. I found that you can stack as many as 4 sheets to increase the strength. You could probably stack more, but 4 is the most I have tried. Remember to monitor the pressure in your air tank and keep it well below the limit for the PVC you are using. It is not recommended to use PVC with air pressure because if it fails it will be an explosion of plastic shrapnel. In other words, use at your own risk, you have been warned.

The air tank is simply a piece of 3 or 4 inch PVC with a cap on one end and the right set of reducers/adapters to connect it to the pipe union. Once the tank cap is glued in place you will want to drill and tap a 1/8" NTP hole close to the end of the cap where the cap and PVC pipe have a tight fit. In this hole you will screw in your choice of fitting to get air into the tank. The simplest would be a Schrader valve like you find on a car tire. You may also want to drill and tap another hole for an air pressure gauge. Unless the pump and or compressor you are using has an accurate pressure gauge, I strongly recommend you attach an air pressure gauge to the tank. It may be just as easy to use a pipe tee so the pressure gauge and inlet valve can both be on the same port.

This is simply a piece of 2 1/2" PVC, preferably of the SDR-21 variety, cut to about 2 feet in length. From what I have read, 18" should be plenty, but I have not done enough experimenting to say for sure. Other than that you just need the right set of fittings/adapters to mate the bottom of the barrel to the foil valve outlet.

Most of the PVC pipe and fittings are easiest to find at a local plumbing supply store. Be sure you get the PVC that is used for pressure and not the drain pipe that is not designed to be under any significant pressure, both are called "Schedule 40". While most of the parts can be found locally and often for a better price than places like McMaster.com, the 2 1/2" pipe, fittings and adapters are very hard to find unless you are lucky. The SDR-21 PVC is even harder to find in 2 1/2". I live in Oklahoma City and could not find any here, oddly enough though I did find some in a small town in Arkansas and had a friend that lives there mail me some. At the bottom I will include a parts list with part numbers from the McMaster.com web site to order all the parts you need.

This valve is a bit more complicated. It is made up of 5 main parts. A Piston that is about the same diameter as the inside of the air tank. A rubber cork with a center hole for mounting. A connecting rod to connect the piston and rubber cork. A guide to keep the rod in the center of the tank while the cork is open. A stopper plate with a hole the right size for the rubber cork to seal shut.

The piston is divides the air tank into to parts, a smaller part that is at the bottom and the main chamber at the top. With the air tank empty, air is pumped into the bottom chamber, this air will push the piston, which will in turn push the cork up into its hole. The seal between the piston and the wall of the tank is such that air can slowly pass from the smaller chamber to the larger. It is also designed so that air will not move past the piston the other way as easily. A weak spring or gravity may be needed to help the piston push the rubber cork into its hole. Once the tank has been pumped to the desired pressure, you are now ready to fire. To open the valve, air is quickly released from the smaller chamber. Since air can not rapidly move past the piston into the smaller chamber, the piston will be rapidly forced downward and pull the cork down and let the air out into the barrel. For this to work the piston needs to be larger than the cork. I don't have a definitive answer as to how much bigger but I would recommend having the piston at least twice as large as the cork.

I made the piston from 3/4" plywood. It could be made out of plastic or metal if you like. I used a 4" hole saw, which makes the wood about 3 13/16" diameter (4" Schedule 40 PVC has almost exactly a 4" ID). After the wood is sealed with thin epoxy, I glue a strip of rubber 1/16" thick and 2" wide all the way around the plywood disk. The rubber strip is positioned flush with the bottom of the piston and cut so that the ends are as close together as possible. This means that about 1 1/4" of the rubber strip is sticking above the piston. The theory here is that the air pressure will push this rubber tight against the wall of the PVC pipe and make a good seal when the air is pushing the piston down but still let air flow past the piston in the other direction. The 1/4" hole from the pilot drill in the hole saw is used to secure the piston to the guide rod.

The rubber cork was purchased from McMaster.com and tapers from 1.3" up to 1.6". it has a hole in the center that is just over 3/16". It isn't too difficult to force a 1/4" rod into that hole. Be sure that there is at least 1/2" of unthreaded part of the rod inside the rubber cork to make a good seal.

The connecting rod has 2 parts, a 1/4" diameter rod threaded at each end, and a tube that holds the piston and cork at the desired spacing. Add washers and nuts to secure the assembly together. Since this will be glued inside the air tank and not serviceable, I strongly recommend using lock tight and lock washers to be 110% sure the nuts don't come loose.

The guide is just another piece of 3/4" plywood cut so that it has a hold to guide the rod in the center and three legs to fit tight against the inside of the tank.

Unless whatever fitting you are using to adapt from the air tank to the barrel has the right size hole, you will need to glue some kind of stopper plate with a hole the correct size for the cork. This could also be made of wood. Just be sure you sand everything smooth and seal the wood good with epoxy. In my case I decided to use a piece of 1/2" thick acrylic, and had my brother machine a tapered hole in it to match the taper of the cork.

For the Pilot Valve Air Cannon, the Tank and Barrel are directly attached using whatever fittings/adapters are needed. For the tank you need an air inlet, pressure port and an air outlet. I recomend atleast two 1/8" NTP ports. One for the inlet and pressure gauge and one for the outlet. The outlet should be connected to some kind of valve such as an air gun with the nozle removed to allow air to escape as quickly as possible.