The materials required are:
- PVC or Plexiglas (1-1.75m in length) must have an inner diameter of 1.5"
- Ball Valve (inner diameter of 0.5" is appropriate)
- Plexiglas rod to make vacuum pump fitting
- Pressure Gauge (of any variety, and need not be too accurate)
- Vacuum Pump
1. Depending on the vacuum pump that you will be using a fitting must be made that will allow the pump to be attached and create a tight seal so that the air can be evacuated from the tube. The fitting should enter the tube at least halfway across the interior to keep the ping-pong ball in front of the fitting. The picture below shows the fitting that I built as well as how it is attached to the cannon. A hole is drilled straight through so that air can pass through. The ball valve can be purchased at any local hardware store. This fitting is only a sample and anything that fulfills the purpose is sufficient. The pressure gauge is just one we had sitting around and was attached by tapping threads into the side of the tube. Thread tape was used at the attachment points of the vacuum pump attachment and the pressure gauge to ensure that a proper seal is achieved.
2. In order to fire both ends must be sealed. The vacuum attachment end shown above is sealed by placing a piece of 3" wide packing tape and pressing around the edge to seal the end then the excess was pushed down to stick to the side of the tube. A second piece of tape is placed on top of that to prevent pieces of tape from flying into the tube when it is fired. The muzzle end is sealed with a piece of aluminum foil that was smeared with vacuum grease and placed on the end. Then a piece of packing tape was placed on top of the aluminum foil to strengthen the foil. Below is a picture of the muzzle end.
3. The cannon is fired by first attaching the vacuum pump and turning it on. Wait until the pressure reaches at most 10% of atmospheric and then close the ball valve (the ball valve is closed so that air that comes in when the seal is broken is not pulled out by the vacuum pump.) Once the ball valve is closed a nail or something else with a point is used to quickly pop the tape. We generally fire at a pop can to avoid dangerous projectiles and to show the damage that can be done with a ping-pong ball going at a high velocity. I built two cannons, one with a length of 0.95m and the other with a length of 1.65m. Below are pictures of cans that we fired at, the first was shot with the short cannon, the second with the long cannon. The difference in damage done is due to the higher kinetic energy of the ping-pong ball from the longer cannon.
WARNING: The cannon is quite loud when fired and if you are going to hold the muzzle end to aim it at the can safety glasses should be worn because pieces of aluminum foil do have a tendency to come off.