Potato Howitzer

Boom! The classic potato gun harnesses the combustion of flammable vapor. Show us your combustion spud gun and discuss fuels, ratios, safety, ignition systems, tools, and more.
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Tue May 12, 2020 10:16 am

I just finished building a wheeled potato "howitzer" out of spare parts. It works pretty well, although I need to take it somewhere a bit more rural for testing once it's legit to travel around again.

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Youtube vid:
https://youtu.be/ocxYe_j6nOw

It's got the standard 2" barrel & 4" chamber (I was hoping to go bigger, as I have some bigger PVC, but this looked the best with the wheels and other parts I had). Fuel is propane and igniter is one of those AA battery BBQ sparkers. I'm really pleased with the elevation crank, I can dial it in to be pretty accurate on small targets. The barrel is a pass-through design so it can be loaded from the breech.

I'm planning to do a few upgrades like a recoil spring system and maybe a fuel metering system (thanks to someone who posted a comment on my video and directed me here!). I also just ordered some cheap stun guns to see how those compare to the BBQ lighter.

I've been building spud guns for a long time and have a few others on my YT page, including some silly attachments for my handheld cannon (drone net launcher, grappling hook launcher, etc).
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mark.f
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Wed May 13, 2020 1:34 pm

Nice! I'm very impressed by all the "errata" like the homemade scrap elevation adjustment and frame. I can tell a lot of persistence was required to get all of that to fit together, good job!

Few suggestions:
1.) As far as the recoil springs, they probably won't do much with light projectiles like lime peels and potatoes either. If you get some light enough springs I'd probably add a bumper at either end of the travel as well since springs aren't shocks. ;)

2.) Stay away from oxygen in your fueling system. Moonbogg had the best suggestion over on Youtube, you're leaving a lot of performance on the table just by not fueling accurately (and not having a way to agitate fuel to get it to diffuse completely). If you wanted to upgrade on the cheap look into syringe fueling (JSR's video below).



Fitting a chamber fan into that chamber would be a little tricky. I'd probably look into a distribution "bar" along the length of the chamber with some small ports drilled along its length to help the fuel diffuse upon injection.

3.) A spark is pretty much a spark (unless you want a lot of sparks and therefore need a higher voltage), the electric BBQ igniter is a great option. Unless you're looking to minaturize things you could stick with it pretty much forever. Even doing things like adding safeties + remoting the firing switch can be done just by wiring some switches between one end of the battery and its contact in the igniter.

Good job again, glad to hear from some people who discovered the hobby on their own and then found the forums.

P.S. during the introduction it looked like you were sitting in front of some DOM tubing and I was wondering why you were deigning to build with PVC. :P
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Wed May 13, 2020 2:17 pm

Ha, the tubing at the beginning is plastic (not sure what sort actually). All of that stuff came from a work clean-up where someone left a bunch of junk behind. There was some metal tubing left behind as well, which I grabbed, but I don't own a welder (it's on the list sooner or later). I was thinking of using the larger diameter stuff for a cannon but didn't have large enough size adapters on hand. Plus it looked silly on the wheel set that I scrounged.

The electronic lighter is great so far! I've been using the old clicky red-button igniters for years (or the self-lighting propane torches jammed directly into a chamber). Those always seem to fail after a while from the combination of heat and pressure, and never seem to have a consistent spark. On a few prior cannons I've done spark plugs with the electrodes bent wider to get a bigger spark, that seems to work OK if they're placed correctly in the chamber.
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Wed May 27, 2020 2:14 am

Awesome, can't wait to see it in action... 4th of July? Can blank it off in the back yard? 8-)
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