This project is one I did about 2 years ago, but it was a lot of fun to build (and shoot) so I decided to post a build log for it.
At the time I was working with the 14 & 15 year olds in our Church's Boy Scouts program. They had heard rumors of some of the stuff I used to do in high school, and they wanted to experience some of the fun I had. Among the many "fun" and usually dangerous things I did was build Spud Guns (also known as Potato Cannons). I instructed them on how to build their own basic combustion cannons, and they had a blast (literally) doing it.
To ramp up their excitement for the project, I decided to build a more advanced version. Although it works on the same principles, mine is much more complicated compared to the basic combustion cannons that I showed them how to build.
I built a circuit that allows a fan to blow inside the chamber and circulate the fuel/air mixture. This assists with combustion and also makes a handy tool to expel the used up propellant out of the cannon once fired. To the right, you see the ignition system consisting of an electronic grill ignitor and a dual spark gap inside the chamber. Sounds like a Tazer when the trigger is depressed.
The cannon itself was constructed of ABS SCH40 pipe. The barrel is 6ft long x 2in wide. It is fixed, but I have made ones in the past with interchangeable barrels to shoot other objects such as marbles, racket balls, even whole rolls of toilet paper. Below is the same picture from above, but with some labels for the different components involved in the cannon's construction
It features a metered propane injection system to accurately allow me to get just the right amount of propellant into the chamber. I found that around 60 psi is the perfect mixture for this particular cannon. The ABS already comes black in color, so I used some blue spray paint to spruce up a few of the chambers and add some color.
I will be posting a video of me loading and firing off the cannon. Stay tuned, and thanks for visiting.