Kids, Do Try This at Home
We built a backyard cannon, and you can, too
Photo: Shayne Lynn
A WHIFF OF GRAPESHOT (or of dirty laundry) should suffice to daunt any invaders of Paul Wallich’s verdant Vermont domain.
Whoosh Boom Splat is a book for people who get along with the staff at the local hardware store. When I went to find parts for the T-shirt cannon on page 120, I got the new kid at the store. He was assigned the job of helping me as a way of learning the store’s inventory. We spent the better part of an hour finding the right combination of reducers, couplers, valves, and PVC pipes to match the functional specs laid down by author William Gurstelle.
The book (published by Three Rivers Press, New York, in 2007) contains 10 projects for people who want to create interesting but potentially dangerous gadgets from PVC pipe and other readily available supplies. At the safer end of the spectrum are a blowgun for miniature marshmallows and a balloon-powered slingshot. For the more daring, there’s a steam cannon that requires injecting water into a red-hot pipe fitting, a hair spray and Taserpowered potato shooter, and a pulse-jet demonstrator, which involves setting off several hundred controlled explosions inside a glass jar in the course of a few seconds. All 10 projects are reasonably safe as long as the reader observes the proper precautions.
I chose the cannon because it involved neither combustion nor uncomfortably high voltage. Instead, it simply requires a compressed-air reservoir and barrel, both made of PVC pipe, and a valve for delivering compressed air from the reservoir to the barrel quickly on command. Ancillary parts link those pieces, control the valve, and get compressed air into the reservoir in the first place.
The young clerk finally declared defeat when we reached the penultimate item on my list: a retaining nut for a 1/8-inch NPT air-tank valve (better known to most of us as the filler valve for an automobile or bicycle tire). One of the older hands took pity on both of us, picked up a different version of the valve, and walked right to another drawer, where he pulled out a slim metal torus that screwed onto the valve’s tapered threads. ”I knew I had some half-inch pipe-thread nuts around here,” he said modestly.
The solenoid-operated sprinkler valve was the only fluid-flow item my hardware store couldn’t supply. Nor could my local outlet of the home center that Gurstelle suggests, but an equivalent was readily available online. Then it was off to RadioShack for the electrical parts: a couple of switches and enough 9-volt batteries to feed a 24-V solenoid. I decided to forgo a ready-made project box in favor of a case from my junk shelf. For all I know, the Ethernet-to-AppleTalk adaptor it once housed still works perfectly, but no equipment anywhere in the civilized world needs that particular translation anymore.
Assembling a project from Whoosh Boom Splat is mostly a matter of following an amalgam of instructions, but in every project there will be a moment or two of improvising very, very carefully whenever the instructions or your materials fall short. Along the way I found myself meditating on engineering practice. Pretty much all of the parts in the T-shirt cannon, like those in the book’s other projects, are the wrong ones for the job, as far as conventional notions of design elegance go. PVC plumbing pipe was never meant as a compressed-air reservoir, diaphragm-style solenoid valves are horrendously unsuited to rapid airflow, and the less said about using 27-V dc to drive a solenoid whose manufacturer calls for 24â''V ac, the better.
But it works. And the parts or their equivalents are all readily available throughout the industrialized world. The devices can be assembled by anyone willing to study the instructions. They’re even corrosion-resistant. Talk about appropriate technology!
Implicit in Gurstelle’s design—and explicit in his text—is an appropriate respect for devices that can potentially do serious damage to life, limb, and property. At the cannon’s maximum design pressure of 5 atmospheres, the back-of-the-envelope range for a 1â''kilogram projectile is on the order of 100 meters. So the two-switch firing design, for example (flip the cover off a toggle switch and turn it on to activate, push the momentary-contact switch mounted on the other side of the box to fire) is a healthy reminder that the cannon is serious as well as fun. The extensive derating of the pressure vessel similarly melds cautious forethought and sobering reminder. In more common use, the PVC pipe is designed to be twice as strong as it needs to be to contain a pressure of 8 atmospheres.
Actual assembly is simple but tedious: glue the PVC components together into subassemblies, wait for the glue to set, glue the subassemblies together. Apply Teflon sealing tape to everything that has threads, screw it all into the right places. The one finicky bit with my project was fitting the reservoir-filling valve; as its metal pipe threads tapped their way into the hole in the PVC end cap, I had to back the valve out every few turns to remove the swarf.
When the cannon was finished—I used only a few pieces of duct tape to hold the two barrels together and attach the control box to the rest—it was off to the closet for a test shirt. I chose my oldest, rattiest work shirt, rolled it up, stuffed it down the barrel, and then pressurized the tank to a mere 1.36 atmospheres with a bicycle pump. WHOOMP! Out came the shirt, fluttering to the ground on the other side of the front yard. At 2.72 atmospheres: WHOOMP! again.
I could have played with that toy all afternoon if my shirt hadn’t gotten stuck 10 meters up in a tree.
About the Author
PAUL WALLICH is a science writer who lives in Montpelier, Vt.