Pneumatic Musings
Tuesday, July 20, 2010 at 12:28PM
Bryan Erwine in Air Cannon, Guy Weekend, Make Movement, PVC, Personal

I'm lucky enough to belong to a group of friends who have a tradition of getting together at least once a year for a weekend of male bonding.  We do the typical things that guys do at these things, like drinking beer, playing sports, and acting like it's 1994 all over again.  Every year, somebody ups the ante in terms of activities, from purchasing enough gear to fill a sporting goods store to cooking a giant 3 lb. hamburger.

One of the specific things we love to do is shoot objects into the air and at each other. It started with a giant three person slingshot and water balloons. Last year, one of the Brians ( there are four of us with the same name) took things to another level with a PVC potato cannon that used aerosol hairspray as a propellant. Nothing beats watching a potato impact the side of a tree, its destruction punctuated with a loud and satisfying "SPLAT!". Even with all that fun, the engineer in me couldn't help but notice that the small barrel and uncontrollable hairspray explosion made it impossible to launch water balloons.

The Slingshot and Potato Cannon

With this in mind, I decided that my contribution to this year's Guy Weekend would be a pneumatic cannon.  The use of pressurized air as a propellant, along with a larger 3" barrel, meant that the amount of force could be precisely controlled, enabling me to launch larger and more delicate objects, such as the beloved water balloon.

Construction

I purchased a set of plans for an air powered water balloon launcher from myaircannons.com. The entire project took me about two weeks to complete and it cost me around $200.00 by the time I launched my first water balloon.  Don't be intimidated by that number, since much of that cost is wrapped up in mistakes and the purchase of hardware I will use on other projects.  I was very happy with the end results: I can launch a water balloon at least 150 feet and the list of items that I have propelled through the air includes potatoes, golf balls, tennis balls, T-shirts, stuffed animals, water bottles, and glow sticks.  Most importantly, it has put a smile on the face of just about everyone who has either seen or used the cannon, including my friends at Guy Weekend. 

Rather that give a detailed step-by-step account of the construction process, I will just offer up some lessons learned for anyone who wants to follow in my footsteps.

I would highly recommend the plans from myaircanons.com.  They're not perfect, but as somebody new to this type of project, the plans were invaluable in figuring out what needed to be done.  You can find free plans on sites like instructibles.com, but the myaircannons.com plans were easier to follow than anything else I found online.

My lack of knowledge about PVC ended up being costly.  While the plans explicitly call out for Schedule 40 pressure rated PVC pipe, I incorrectly assumed that all Schedule 40 pipe was pressure rated.  It is not, which is something I wish I knew before I purchased all of my fittings.  The FAQ at spudfiles.com does a great job explaining the differences between the two types of PVC pipe: NSF-DMV and NSF-PW.  NSF-DMV is used for waste water, while NSF-PW is pressure rated for use with potable water.  Sites like instructables.com and spudfiles.com are filled with wanrnings about the dangers of using NSF-DMV in air cannon construction.  Sufficiently scared about losing a finger in a horrible explosion, I decided to replace my DMV parts with new PW fittings.

While it was easy to find pressure rated PVC tubing locally, none of the hardware supply centers I visited carried pressure rated fittings for 2" and 3" PVC.  I ended up acquiring most of my NSF-PW fittings piecemeal from various online Mom and Pop hardware stores.  While the parts were cheap, the shipping and handling charges piled up quickly.  It wasn't until I had nearly everything that I found the McMaster-Carr Supply Company.  They were the only supplier to carry all the parts I needed. If I had ordered everything from McMaster-Carr, I would have saved at least $20 in shipping costs alone.

 

The heart of the cannon is a sprinkler system value, used to release air from the pressure chamber into the barrel. The plans specifically called for a Nelson Brand 7901 sprinkler valve, but at the time, I had a hard time finding that particular model at a reasonable price.  As an alternative, I found the Orbit 57461 Jar Top Valve which has the advantage of being cheap, easily modified, and available on Amazon (free 2 day shipping with my Amazon Prime membership). I had no problems using the Orbit value as a substitute, and following some online advice, ditched the solenoid mechanism for a manual trigger using a blowgun.  In theory, the manual trigger allows for a quicker and more powerful release of air and ditching the electronics simplified the overall construction process.  I used a great set of instructions from Hall Consulting to modify the valve and build a mechanical trigger.

It's been one month since I started launching things with my cannon, and I have been plagued by leaks involving the sprinkler valve.  The valve is connected to the cannon using threaded PVC nipples and the valve itself is held together with a hard plastic retainer ring which is threaded onto the body of the valve. As I have discovered, threads are the source of all leaks, even when teflon tape is used.  Two part plastic epoxy has become my best friend for plugging trouble spots, although it seems every time I fix one leak, another one takes its place.  The leaks haven't stopped me from using the cannon, but it does mean that I need to fire before the air pressure drops too low. 

The completed air cannon

Some miscellaneous construction points:

"Never bring a gun to a bazooka fight"

In Practice

I originally planned on using a small, portable $15 air compressor made by Slime to power the cannon.  It turned out to be underpowered, so I purchased a $50 Craftsman Rechargeable Air Compressor from Sears. Not only does it fill the air chamber very quickly, but since it runs off a battery, I'm not tethered to a car or wall outlet when I need to pressurize the cannon.

The key to launching balloons is to construct a sabot using two cups cut down to the diameter of the barrel. It's a great system, since nearly all of the air pressure is captured by the lower cup, which in turn, pushes the upper cup holding the balloon.  At first, I used styrofoam cups for my sabot system, but they were blown apart after each use.  I found that the clear plastic soft drink cups used at Sheetz gas stations were strong enough to withstand the forces generated by each firing. Also, a little bit of soapy water squirted into the barrel lubricates the cups and made a noticeable difference in the distance that objects travelled.

I found that 60 psi is the ideal air pressure for launching water balloons.  Anything higher will burst the balloons as they exit the barrel.  I have launched tennis and golf balls at higher pressures, but am hesitant go above 80 psi, the maximum recommended pressure for the sprinkler valve.  That being said, 80 psi is more than enough to launch most things a considerable distance.

Posing with my air cannon

Overall, this was a great project that everyone seems to enjoy.  Leaks aside, my only remaining problem is trying to figure out what the heck I am going to do for next year's Guy Weekend.

Article originally appeared on bryanerwine.net (http://www.bryanerwine.net/).
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