A Relatively Harmless Weapon

When we were preteens, someone in our neighborhood showed us how to make a tennis-ball cannon. It consisted of cutting off the top and bottom of about 6 cans of beer and then taping them together. For the seventh can—the cannon’s base and reaction chamber—we left the bottom intact and cut off only a circular part of the can’s top so it could act as a support for a tennis ball. About an inch from the bottom we also poked about a 1 cm- hole and then taped the base to the rest of the can assembly. Through the hole, we poured in a bit of gasoline, slid the ball in, shook the cannon, waited and then ignited it with a match.

The first time we fired the cannon, we were in the fields behind our street. Its range astonished us as the ball almost inadvertently struck a woman on a 2nd floor balcony, hundreds of feet away. She was putting out her laundry, and she hollered at us when the ball bounced off the brick wall behind her. A week later, just before we tried it again, an inquisitive adult by the name of Eduardo laughed in disbelief when we told him what we expected to happen. The cannon was almost in a vertical position when we fired it. Eduardo’s jaw dropped almost as low as his clavicle. The ball had gone so high that we momentarily lost sight of it.

After hearing the story, my students were exploding with enthusiasm to build their own. In the early years of my teaching career, beer can-diameters had already been made incompatible with the size of tennis balls. But the biggest problem was that it would have been irresponsible to use gasoline around students. The heat of combustion for its safer substitute, ethanol, was no match for that of gasoline. Launching it became a hit and miss affair, and we did not initially get the bang or the range promised by my recollection.

Understanding the theory helped us out in the long run. Before ignition, some alcohol molecules are in the vapour phase. Along with the air molecules in the base of the cannon, they exert pressure on the ball. But that pressure is not significantly different from the air pressure exerted on the other side of the ball, facing the cannon barrel’s exit.

After igniting the alcohol, we have a different matter. If there’s sufficient oxygen, the alcohol can optimally react with oxygen gas in a 1 to 3 ratio to yield carbon dioxide and water in a 2 to 3 ratio. The total potential energy of the oxygen and alcohol lie mostly within the bond energies of oxygen and alcohol’s intramolecular bonds. That sum exceeds that of the intramolecular bonds of the products of combustion, which is why the excess energy is released.

The sudden release of heat excites the molecules of the CO2, H2O and of the part of the air that did not react. As these molecules pick up speed in a confined volume, they exert far more force per unit area on the ball. The pressure deforms the ball, and as it regains its shape, some of the energy becomes the ball’s kinetic energy, accelerating it out of the cannon. But the gas-ball collisions are not elastic. Some of the energy bunches up air molecules, forming waves. In other words, some of the energy ends up as a whistling or thunderous sound, depending on how much heat was released. With an unsealed ignition-hole and reliance on tape, some exhaust gases escape, wasting even more energy.

One year I was in my brother-in-law’s steel workshop when I noticed some spare metal cylinders. I got him to weld one to a base, told him where I wanted the hole, and suddenly we had a much more solid tennis ball cannon. When I brought it to school, to get more of the alcohol vapours to ignite, to approach a more stoichiometric ratio, we decided to use a portable oxygen tank to inject gas into the chamber. To prevent it from escaping, we sealed the hole with plasticine through which we had inserted a firecracker fuse.

The firing of the cannon was spectacular, judging from the exploding sound and voices after the fuse went off. The ball zoomed across the parking lot. No one actually saw the ball in flight, but we found it because some of us heard it hit a tree that intercepted its trajectory. Here it is in action:

There’s more than enough muzzle velocity for the ball to detach someone’s retina, so safety goggles are a must. But that danger aside, it would be nice if gun users would some day replace their bullets and guns with alcohol-cannon projects.