A large percentage of mechanically-inclined kids attempt at some point to build a potato cannon. I was enticed by the idea in 2012 when I was a junior in high school, and I thought that I had enough mechanical prowess to build a good one. Looking back at how it turned out back then, I see that even with a simple design I made many mistakes. However, I did learn from those mistakes as well as my college education and have made fitting adjustments to the chamber volume and barrel length and geometry that should greatly improve projectile velocity and accuracy.
I greatly dislike the firing mechanism that I chose to implement back then, and I plan to model and 3D-print an attachment that will function much better than my previous handle design ever did. The original mechanism included a simple knob made from PVC tubing and held on by PVC glue which housed a grill igniter firing button whose spark wire protruded into the chamber. I have different tools at my disposal now than I did when I first began constructing the cannon, such as 3D modeling and 3D printing, and such tools make a great deal of difference in the quality and appeal of the final product.
As I often used to do, I started out believing the idea that the bigger everything was, the more power I could get out of it, so I selected a larger-than-normal 4-inch internal diameter, 2-foot-long combustion chamber. I found later after doing some research in that area that with the selected diameter, a barrel of about half of that length was optimal. The barrel that I originally selected also happened to be twice the ideal length to achieve an optimal volume, so I recently cut that in half as well. With the original long barrel, I intended to get better accuracy, but I didn’t have a great understanding of the friction that would be introduced, thus lowering the power. I now have a much better feel for the effects of friction between various materials and know that I chose poorly back then.
I look forward to implementing my new design ideas, both modeling and assembling them, but first would like to obtain a fused-deposition-modeling 3D printer to save on printing costs and to help with quick prototyping. I will also be experimenting with new propellants as in the past, all I ever tried was hairspray which left a film behind in the chamber, interfering with the ignition circuit. Butane or propane should suffice, but it will be more fun to test the many available options and quantify the results to select the best candidate. The scientific method is always the best route when it comes to optimizing one’s use of physics and chemistry.