Servo - December 2015

of power out of the cannon that would cause pumpkin carnage upon impact, and the first incarnation of our chunker just wasn’t getting there. The worst damage we could do to a hollowed out mini-pumpkin was to leave a crack in the orange gourd’s wall. One thing that we noticed during testing is that each shot did not deplete all of the pressure in the chamber — activating the electric valve would only result in a drop of about 20 PSI or so. This meant that we were leaving a lot of pressure on the table.

There had to be a better way! Settling for low impact shots would have been as cynical as that scrooge, Frank Cross firing Eliot Loudermilk and giving out towels as Christmas presents.

Another problem with the first version of the cannon that hampered our initial testing was that one of the joints sprung a leak. We were able to patch it by slathering on PVC cement and pulling some pressure out of the chamber to suck in the cement. However, it was just another indicator that improvements could be made. We were eventually able to splatter some pumpkins with our Mark II chunker, but diving into that now would be jumping the gun like putting up Christmas decorations the day after Halloween.

It’s Beginning to Look a Lot Like Chunk-mas

Leaping into design upgrades without looking at the physics first would be like putting on the big red suit without reading the Santa Clause. The core concept behind a pneumatic cannon is really very simple: Compressed air is stored in a chamber. When the valve at the opening of the chamber opens, the compressed air expands and pushes out the projectile. Our simplified model assumed that the pressurized air in the chamber would flow seamlessly into the barrel once the valve opened. That, however, didn’t account for something that was seriously limiting the power of our chunker: pressure drop over the valve.

The electric valve we used in the first incarnation of our pneumatic cannon was an electric sprinkler valve. The tube diameter was only one inch — already a big drop from the four inch diameter tubing we used for the chamber and the barrel. Only so much air can flow through the smaller diameter tube. When that air reaches the wider tubing used for the barrel, it expands to fill the larger volume, losing some pressure in the process. Like the Grinch’s heart, our valve tubing was at least two sizes too small.

The design of the electric sprinkler valve compounded the problem. Electric sprinkler valves like the one we were using employ a diaphragm valve design. In a diaphragm valve, the path through the valve is opened and closed by a diaphragm usually made of rubber or plastic. The diaphragm is connected to a plunger that raises and lowers the diaphragm over a weir. The weir is a partial wall that minimizes the distance that the diaphragm has to travel to open or close the flow path. Minimizing the required travel of the diaphragm makes the design efficient from an actuation point of view, but diaphragm valves are as miserly with airflow as Ebenezer Scrooge.

There is a precedent for modifying sprinkler valves for use in air cannons, but many of the suggested modifications are focused on increasing the speed of the