Produce Accelerators

Posted by Derek
Aug 22 2005

My brother came up to Metigoshe from Fargo a couple of days ago. The weather has been windy the last few days, but today the breeze finally died down. We took the opportunity this evening to break out the Produce Accelerator and put some potatoes into low orbit. Some might call this an air cannon, others a spud gun. If you still don’t have a clue what I might be speaking of, there are a few sites (like Spudtech) that have some more details about the Accelerator’s brethren.

The cousins (hullo Karl, Neil, Evan) had a cannon here at the lake when they came up last year. After watching them fire it, I knew I had to have one. After much browsing of Google, where the above site among many others appeared before my eyes, the brother and I formulated an initial design. My Accelerator (our Accelerators; Brother’s isn’t here at present) is a simple pneumatic-powered muzzle-loaded cannon, in an over-and-under design. There are two main PVC parts - a pressure chamber, and the barrel - separated by a Rain Bird automatic lawn sprinkler valve rated for somewhere around 130psi. We originally had attempted to make the solenoid work with three 9v batteries in series, then three in series, in parallel with three more in series, but for some reason the 27vdc regardless of the resulting amperage we were feeding the poor thing wouldn’t allow it to actuate properly. It probably needed AC, in hindsight. After much trial and failure, we’ve resorted to just using the manual releases on the valve. Can slightly unscrew the solenoid or another little plastic peg to make it actuate.

The two PVC parts connect through the valve with 90 degree elbows, allowing the barrel and the pressure tank to be side by side (over-and-under). The pressure tank is a piece of 5 foot long by 3 inch diameter (90*pi square inches of volume, if my math is correct) schedule 40 PVC pipe, capped on one end with a pressure gauge and a tire valve for filling with air. The piping is labeled as ‘cellular core’ - we’ve had conflicting advice as to the safety of cellular core PVC tubing under pressure. “It’s schedule 40, so it’s fine” versus “cellular core is an explosion waiting to happen.” Eventually I’ll probably chuck the cellular crap and go with a nice piece of 6 inch pipe with an actual PSI rating. We normally fill the tank to about 105-110 PSI before firing.

The barrel is a 5 foot long by 2 inch diameter schedule 40 PVC, rated for somewhere above 160PSI. The Accelerator is downstairs and I’m feeling lazy, so “160+” is the closest I’m going to get for now. At a 2 inch diameter, we could probably extend the barrel another 5 feet to closer match the volume in the air tank. (v=Lpir^2; v=60pi1.5^2; 135*pi cubic inches on the air tank. since 1^2 = 1 we could use 135 inches (11.25 feet) of barrel to hold the same volume). As it stands, the produce of choice (normally potatoes, they’re cheap) is moving rather fast when it leaves the end of the barrel.

I’m going to have to take a stopwatch out tomorrow and see about getting some accurate timings on launch-to-impact. I’m curious to see what the hang time is of a vegetable fired near-vertical. I think the ones we did last year with this cannon were staying up for about 10 seconds. At 10 seconds, the peak of the produce’s flight should be at the 5 second mark. With acceleration from gravity at 9.8 meters per second per second (m/s/s), the velocity when the produce hits the lake should be 49m/s (or 109 miles per hour), and that should also roughly match the speed at which the produce leaves the barrel. That also places the maximum height around 122.5 meters (402 feet, or 134 yards). Whee!

So, the potato is accelerating to 49m/s over the five foot barrel; that’s 9.8 meters per second per foot… With a ten foot barrel, assuming we managed to keep a constant acceleration (a big ‘if’), we could in theory have 98m/s velocity at the end of the barrel. 219 MPH. Zoom.

My math probably sucks. Oh well. (edit - yes, some of it did, and it has been… corrrrrected. 1.5^2 is not 1.5)

Opie and Anthony (XM 202; ‘cringe humor’ not for the faint of heart or weak of stomach, XM has channel blocking options for a reason kids) are on vacation for a week. I don’t know how I’ll survive without my daily dosage of the Show That Is The Virus. Listening to Art Bell right now, on XM 165. Possibly on an AM station near you. Haven’t listened to his program much before, but the brother recommended it. Slightly amused so far.