Flutterby™! : I'm baaaack 2001-04-10 18:02:32+00

I'm baaaack

2001-04-10 18:02:32+00 by Dan Lyke 7 comments

Whew. Back from flyover country. Longer notes and pictures later. Had an awesome experience with a potato cannon, blowing holes in plywood and getting nearly 9 second flight times which implies 300+ feet of altitude, with a little engineering we could probably endanger low flying airplanes with errant spuds. Nice to get together with the family and be reminded of old traditions and events. And reminders of why I live in California, despite all the down-sides.

[Andy, Tyler and Dan with potato cannon]

For entertainment purposes only, you do this at the risk of your own damned flesh! I guess there's only one picture of the first one in my account of last Thanksgiving's Sierra trip.

The two I've been involved in building have been schedule 40 plastic pipe, the first was ABS, the second PVC.

You can see from the image on the right, the PVC one we built last weekend, that there's a barbecue lighter plugged into the bottom of the 4" diameter, 1' to 1.5' chamber (we didn't know we'd be muzzle loading and made it short enough to get an arm in to rear-load it), and that feeds into about a 4' or 5' 1.5" diameter barrel.

If you've got small seed potatoes, a 1.5" barrel is about right, 2" will work with a cheap bag of commercial potatoes and should be large enough to hold a golf ball [gulp], and the larger surface area behind the potato might be advantageous.

We put the lighter out the back end on this one because Matt had a piece of 4" PVC lying around and it was cheaper that way, in the other one I used a (more expensive) pre-molded piece that had a 2" side junction and stuck the lighter through a hole in the end cap there, that was easier to make ad-hock mounts for various firing angles because the lighter wasn't digging into the ground.

It's fairly obvious looking at the picture what you're building, I'd look through your scrap bin and then head down to the hardware store and figure the cheapest way to put together something like this.

The hole for the lighter should be as tight as possible because there will be some hot blowback through it. Nothing terribly damaging, but if there's too much of a gap the person firing will feel it. If you can find a lighter with a round shaft rather than a square one that's easier to match.

The firing procedure is: Coat the inside of the tip of the barrel liberally with oil (ie: canola or whatever), you only need to do this once every couple of shots. Cram the potato down with a broom handle 'til it's just about at the chamber, stuffing it in there will strip the potato down to a slug that snugly fits inside the barrel. We've got a screw cap on the base end, unscrew that, spray AquaNet

or a similar butane or propane propellant hairspray for about 3 seconds, screw the cap back on, aim, and click the barbecue lighter. Be careful of what's down-range, you can get a quarter mile out of these things.

Have an extra lighter handy, the shock and/or the residue from the hairspray tends to do something to the spark gap that makes it need some adjusting after a few shots to continue to light.

I haven't done any engineering or burst tests, although so far I haven't seen any indications of impending failure (a firing does tend to wedge that screw cap on pretty hard). Short of a pressure transducer, I'd build my next one with a full 8' or 10' barrel and try trimming it down if I wasn't getting at least 8 second flight times at high angles, and I'd also try a larger chamber, because longer sprays don't seem to be producing bigger shots so it's probably about optimal for a 3 second load.

I'd be careful with the larger chamber, that should probably be done with a system that'd let you do burst testing, 'cause I've no idea how close we are to the limits of schedule 40 plastic. My gut tells me we can take it a little further, but I think I'd set up a pure propane feed and a remote ignition system (maybe a surplus automobile coil and spark plug) so that I could both get a reproducible load and test destructively from behind protection.

Sorry about the lack of action pictures, I forgot my camera when we went out to the farm for firing so I'm waiting for my sister and parents to send pictures they took. Last weekend:

Oh yeah, on our last shot Matt (my youngest sister's husband) tried some starting ether, we couldn't get it to light, so he opened the chamber back up and did a final hairspray load, the shot went off with an extremely sharp report and some interesting muzzle flash. My guess is that the remaining ether exploded somewhere in that firing. On the basis of how loud and sharp that report was I'd be extremely wary of ether as a propellant.

Thanks Sounds like fun...Might try one when I have my next vacation at my parents farm in Ohio

I think I might consider using schedule 80 and would go for CPVC if possible. It's stable at higher temperatures. Don't try this with ABS. ABS is combustible. If you use lighter fluid (naptha) for the propellant, go easy on it.

Actually, heat-wise the oil we used to lubricate the threads browned, but oil inside the combustion chamber was still clear, so I assume that the browned oil was from compression on the threads and the flash inside the chamber was over so quickly that there just wasn't that much total heat.

Kinda like those experiments gone awry where ya lose your eyebrows and keep your skin.