Two Simple Tennis Ball Air Cannon
(aka Antenna Launcher) Designs
by KE5GIX and KE5GIY
Why Build a Tennis Ball Air Cannon
As any Armature Radio operator knows, the higher your antenna is the better. In an emergency, trees are a handy place to hang an antenna. The trick is how do you get your antenna up the tree. Several solutions exist, however it seems the Tennis Ball Air Cannon is one of the easiest and safest methods. The way it works is you attach a line of some kind to the tennis ball, push the tennis ball to the bottom of the cannon barrel and launch it over a tree limb. You can then hoist your antenna up the tree. In most cases you will need to first hoist up a small rope or other line, as the light weight line the tennis ball can carry, isn't strong enough to lift an antenna and the feed line up the tree.
My dad, KE5GIY, thought we should look into building one of the air cannon designs that are plentiful on the Internet. After reviewing several, 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 design that uses a home made valve that is reasonably easy to build. I, KE5GIX, also thought of another very inexpensive, single use, valve that would be even simpler to build, and reduce the over all cost. We decided to build both designs and see how they compare. The home made valve design we'll call a "Pilot Valve Air Cannon". The single use valve design we'll call a "Foil Valve Air Cannon".
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. This idea is not new, there are single use, pressure relief valves that work the same. The main difference is they are designed to be a safety device and are made to very high precision. For our foil valve, we will simply use standard aluminum foil available from your grocery store or McMaster.com. 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 possible. 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. In the case of our tennis ball air cannons, we also need some kind of reel that will allow the string to unreel very fast. Some people use a standard fishing reel. Others simply unwind the string in a circle on the ground, or in a bucket. I liked the what the guys at
www.akbeng.com used on their CSV19 antenna launcher, a reel that is used for bow fishing called a "Zip Reel™" that can be found at Saunders Archery - USA. Not only does it do a good job at letting out the line fast, it also just looks cool. Typically the tank on these home made air cannons is made from 3 or 4 inch PVC pipe. For the valve you need something that will open very fast, and let the air flow with very little restriction. A popular choice is often a solenoid sprinkler valve. Some use it as is, and have use a battery to open the valve, others modify it so you don't need electricity. 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 very tight fit. Also note, the diameter of tennis balls can vary. It is my understanding that tennis balls of a given brand will generally all be very close in diameter, but another brand may be slightly smaller or larger. If your tennis ball doesn't fit very well, look for another brand.
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 for low pressure of say 30 psi or less. However, higher pressures require the pipe union to be very tight.
If you have a compressor or some other means to fill the tank rapidly, the leaking at higher pressure isn't a problem because it leaks relatively slow. If you are using some kind of a hand pump or a slow air compressor, the leaking at higher pressure may be a problem. I am going to experiment with various lubricants on the threads of the pipe union to see if I can make it easier to get it tight enough to not leak at higher pressure. Pipe unions come in sizes from 1/4" up to 4". The 4" size is very expensive, over kill, and will also require relatively thick foil. 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 the prototype I am using a 2" pipe union, using that size union and standard aluminum foil (not the heavy duty foil) from the kitchen, one layer of foil breaks at about 10 PSI, two layers break at 20 PSI, three at 30. . . I have used as many as four layers at a time. I have also used a very heavy duty foil from McMaster.com that breaks at about 60 PSI with only one layer. 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's 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. If you want to be really safe, have the money, and don't mind a much hevier air cannon, you can build it from metal pipe.
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. If you don't want to drill and tap for the pipe fitting, you can use reducers to get down to the size needed for the inlet valve.
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.
The reel mount is made of a 1/4" thick piece of plywood, glued to a block of wood with half of a 2 7/8" Dia. circle cut out. That block of wood has a groove cut in the side that is glued to the plywood. It is held to the barrel with a zip tie though the groove and around the barrel. (I'm not going to take the time to draw a zip tie in 3D :)
Most of the PVC pipe and fittings are easy 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. To make things more confusing someone decided they should both be called "Schedule 40", so be sure what you get has a pressure ratting marked on it. 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.
Parts and Sources
Schrader Valve -- same as used on car tires (Reservoir Inlet)
Cap 3" (Reservoir Cap)
PVC Pipe 3" x 5' (Reservoir Body)
Coupling 3" (Reservoir End)
Reducer Bushing 3" M Socket - 2" F ntp (Reservoir to Valve)
Nipple 2" (Reservoir to Valve)
Union 2" ntp (Foil Valve)
Reducer 2.5" F socket - 2" M ntp (Valve to Barrel)
PVC Pipe 2.5" x 5' (Barrel)
Heavy Duty Aluminum Foil (Foil Valve)
Zip Reel™ (bow fishing reel) #1212
Spectra® Fiber Fishing Line, 50lb test, .013" DIA. - 300 Yards 38-416-932-02
Bass Pro Shops
There are other miscellaneous parts such as wood and screws to make a mount to attach the reel to barrel of the air cannon, some way to attach the line to the tennis ball, something to add weight to the tennis ball, and of course tennis balls. Most of the PVC parts can be found a local industrial plumbing outlets, probably at a lower price, especially the pipe it's self. Home stores are NOT a good source, as they normally only have drain pipe in these sizes, drain pipe will not hold the pressure needed. For the prototypes I am working on, I used 4" PVC for the air tank. However due to the price of 4" vs 3" fittings and the extra difficulty of gluing 4" PVC, I will be using 3" PVC on any future projects. To make up for the reduced diameter of the smaller pipe, simply increase the length by 77%. As for the size of the tank, I have read that having the same volume of air in the tank as there is in the barrel, is a good rule of thumb. However, I have not done enough testing to confirm this. One thing that can be done is to make your tank significantly larger than it needs to be, then fill it with water to reduce the air volume and test it to see at what point it starts to reduce the performance significantly. I'm not sure but I suspect the pressure you use will have an effect on the ideal size of the air tank.
I have finished building the Foil Valve Air Cannon and done some preliminary testing. The first test was to launch a tennis ball with out any extra weight or line attached. The height that can be achieved with pressure as low as 30 PSI is very impressive. I would guess that the heights are over 500 feet. Once the line is attached and weight is added to the tennis ball, the height is reduced significantly. I have recently obtained an
F1 Chronograph that is normally used to measure the speed of bullets from a hand gun or rifle. It has a range of 30 to 7000 fps. I have only had time to do one test fire using it. For the foil, I used a .002" thick heat treated aluminum foil from McMaster.com. That foil breaks at about 60 PSI. The measured speed was 250mph. That is one fast tennis ball. I have only done one test with a weighted ball and the line attached. The ball was weighted down to a total of 5 oz and I used about 80 PSI. About 190 feet of line came off the reel with the ball launched vertically. I hope to do some tests with different weight tennis balls, a longer barrel, and possibly a larger air tank. I will add more to this page when I have done more testing. The one thing that is a bit of a pain with this air cannon, is replacing the foil valve. In order to keep a good seal on the air tank, the pipe union needs to be screwed down tight. Due to the large size of the pipe union it is a bit awkward and difficult to get wrenches on tight. I am thinking about making a wood ring with a long handle out of plywood and epoxy it to the pipe union to make it easier to tighten and loosen the pipe union.
Pilot Valve Air Cannon
This design has the advantage that it is much easier to reset. Just pump it up and you are ready to fire. One person can easily operate this design. The basic principal of this home made valve design is the same as you will find in the solenoid valves used by most people for this type of air cannon.
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.
How It Works
The piston 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 direction 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, from the larger to the smaller chamber, the piston will be rapidly forced downward and pull the cork down and let the air out through 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 in diameter 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 that is inside a tube that holds the piston and cork at the desired spacing. Add washers and nuts to secure the assembly together. The nuts will hold the piston and rubber cork tight against the tube. 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 rod inside the tube is only one way it could be done. If you have access to a lathe you could make the whole thing out of one metal rod. Or you could cut the head off of two bolts and weld them inside of a tube.
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.
Tank and Barrel
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 recommend at least 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 nozzle removed to allow air to escape as quickly as possible.
If you want to contact me, I can be reached using my call sign at arrl.net. Note my call ends in X not Y.