This catapult-style tennis ball launcher is my first attempt. Ip’m sure theren’s an engineering principle that covers this kind of machine, but I am uninformed, so bear with me. The shredder motor winds the elastic band until it gains enough potential energy to start the arm moving. Once the arm starts, it accelerates quickly and slams into the forward stop, throwing the ball. I have fitted the arm with a second, weaker elastic band which tensions as the arm reaches the forward stop. When the direction of the motor is reversed, the tension is released from the main elastic band, allowing the second band to return the arm to its resting position.
I built this launcher for a friend of mine after he helped me machine some custom aluminum frames for the 2005 ASME Regional Design Contest. It features a 2.5 ID pvc pipe barrel (perfect for tennis balls) and a 4 ID pvc chamber. The valve is a 2 diaphragm sprinkler valve. I modified it to allow for remote pneumatic actuation rather than using the solenoid provided. I did this by removing the solenoid and covering the opening with epoxy. I then tapped into the chamber above the diaphragm and ran some .170 ID polyethylene tubing to a push-button valve located midway down the launcher. The valve has the lowest working pressure at 150 psi, although I have only tested the launcher up to 125 psi.
Science can be incredibly fun. Learning about rockets and how they work is fascinating stuff. Being able to learn by doing is even more rewarding and can teach a young person the basics of science through hands on experimenting. Rocket science is one such field that benefits from a hands on approach. This article will teach you about how to build a fundamental rocket device by building a tennis ball launcher.
Pull the specialized Forward Kinetic Motion Inhibitor (commonly called a bungee cord) back and clip it on a sight or bayonet mount. Without the Inhibitor attached to the rifle, the whole launcher will be shot off along with the tennis ball. If you are using the Longitudinal Elongation Adaptor for a long muzzle, the Forward Kinetic Motion Inhibitor can be lengthened by loosening the Metallic Base Compression Ring (the hose clamp) and pulling the Inhibitor out to the next predetermined setting (the second knot). If you don't need this second setting, the extra length of the Inhibitor can be trimmed off with scissors or a knife.