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A potato gun powered by compressed air
This summer I really didn't have time to work on my other project, the hovercraft, so I decided to do a small project on the side. I have wanted to try my hand at building a compressed air powered potato gun for a long time. Most of the knowledge that I used to plan and build this came from searches on the Internet, and my own prior experiences. For people who would like to attempt a project like this, I would suggest using caution since there is a very real danger for explosive failure, and serious injury or worse. You must be sure and use Schedule 40 PVC that is pressure rated (it will say on the pipe). I also wrapped the holding tank in a cloth to catch PVC fragments in the event of a failure. That being said, building an air cannon is not very complex. I was ready to fire about 48 hours after going to the hardware store.
I used a very simple in line design for my air cannon. The basic idea is that a large volume of air is compressed behind a valve. When the valve is opened the air rushes into a long section of pipe that acts as a barrel. This rapidly expanding air will force out whatever is in the barrel. A 2 foot section of 4 inch diameter pipe forms the air holding tank. The pipe diameter is reduced from 4 inches to 1 inch where T connectors attach the pressure gauge and inflator valve. The inflator valve is simple a 1/4 inch quick release male coupler. This allows me to easily and securely attach to my air compressor. Also the connector is threaded so that it can mate with the PVC pipe. A 1 inch electric sprinkler valve allows the air to be quickly released from the holding tank. The sprinkler valve is threaded. This allows for different sized barrels to be interchanged very easily. As of right now I have a 1 inch x 9 foot barrel and a 2 inch x 5 foot barrel.
Firing the cannon is fairly simple. Air is compressed in the holding tank. My weakest pressure rating is 125 psi on the valve, so I limit the psi to around 100 psi. To open the valve a current must be passed through the solenoid. To do this I have wired a model railroad transformer to the solenoid through a pair of buttons. Both buttons must be pressed to fire, which gives some added safety.
The farthest measured shot I have made is just over 200 yards (Some unmeasured shots have gone farther than this). This was with the 2 inch barrel at an angle of 15 degrees. Those of you who know a little physics realize that the best angle for maximum distance is 45 degrees. From this data I estimate that the cannon's muzzel velocity is around 135 MPH for a small potato. I also estimate that idealy the cannon should be able to fire about 400 yards at 45 degrees (if my math is right). However, that is from running the numbers without respect to air resistance so we'll have to wait on the actual data.