SpudFiles

So a buddy and myself each have our own combustion based PVC potato cannons. The specs are pretty simple. We have the 2 inch barrel with the 3 inch combustion chamber.

We are having a competition in the coming weeks. As a result, I will be building a new one that will ideally go 200+ yards. I have heard the 2in to 4in ratio is ideal, but I wanted some expert advice.

Also I was hoping to get some extra tips and advice on how to squeeze out extra performance. I currently use Aquanet hairspray, but if anyone can suggest anything else or tips I'd greatly appreciate it.

Finally, I heard that if you load the potato at the top of the barrel, proceed to spray the aerosol into the combustion chamber, and THEN push the potato down the barrel that there is a more positive result due to the compressed air. Is this true?

I am really looking to blow my friend away because he is talking a lot of smack so any suggestions are welcome. Thanks.

Well if you are really up to it you can make a advanced combustion cannon which increases performance by using proper C:B ratios, metered propane, chamber fan, multiple spark gaps, and stun gun ignition.

If you follow these requirements, you should have a great performing cannon. I guarantee you would win your competition you're in if and only if you are the only one with this kind of cannon and no one uses a hybrid.

You will find that performance of a advanced combustion is very optimal compared to a spray n pray (what you have now).

Ok, few starting pointers.

Get a good ratio. Ratios are related to VOLUME, not diameter.
For a given chamber volume, the optimal barrel has a volume between 25% to 30% bigger than the chamber.
For a given barrel volume, it's a bit different, but a slightly larger chamber will add a little more power. A chamber with the same volume as your barrel will do nicely.

Use a proper fuel. Sprays, to put it simply, suck - if you have to use one, use starting fluid. Hairspray is about as bad a choice as you can make.
A propane metering system will add power, consistency, and badass.

Chamber fan - this is just a computer fan inside the chamber. Helps mix the fuel creating a faster burn, which gives you more combustion pressure.
I'm going to make this clear - DO NOT use a brushed motor based fan. These can ignite your mix by mistake. Some people use them, I wouldn't touch them with a bargepole.

Your potato loading trick is half true, but you won't successfully manage it like that. Compressed gas mixes in combustions (normally known as Hybrid cannons) are exceptionally powerful.
However, ramming the tater down later won't help. You'll more likely loose power because gas'll leak past the projectile, and the spud probably won't sit at the base of the barrel properly.

Take a good look around http://www.burntlatke.com/
Very good site talking about most of the things I've mentioned, but there are a couple of pages that talk about 1.5:1 ratios. Ignore those - their later research is much more accurate, and finds it's closer to half that.

I'm afraid I wasn't clear. Although I would someday love to use advanced metering systems and propane, I'm afraid that will not be the case for this competition. Specifically I am looking for advice on the "spray and pray" model. Also does starter fluid come in a spray? Thanks

If you can't use metered fuel, at least use a fan. They are cheap, easy to rig up, and, even in a spray'n'pray, they can increase performance by up to 30% in some cases. Axe works as an OK spray'n'pray fuel, as does starting fluid. The higher percentage of actual fuel in your spray, the more beneficial it will be. Look for ingredients like butane, propane, isobutane, etc... A propane propelled can is necessary (i.e. not WD40, which is CO2 propelled).

This competition sounds a lot like punkin' chunkin'; technological advancement is not allowed, so the only real solution is to build bigger.

OldSpice60 wrote:Also does starter fluid come in a spray?

Yes it does. It's largely ether, which as a fuel is pretty good - basically pure fuel, no fragrances or stuff to gunk up the electrodes.
If you're using a spray, a slightly larger chamber will help, so go for a 1:1 ratio.

Other than the metering, all the other tips I mentioned should be applicable.
I forgot to mention multiple ignition, but whether that is such a good thing is rather up for debate these days.

OldSpice60 wrote:I'm afraid I wasn't clear. Although I would someday love to use advanced metering systems and propane, I'm afraid that will not be the case for this competition. Specifically I am looking for advice on the "spray and pray" model. Also does starter fluid come in a spray? Thanks

Yes, you were clear. To keep it fair you will be useing the same fuel even. Keep your eletrodes clean is a good tip. Pratice counting how many seconds/or just the amount of spray time, change it up to find out when it's shooting best and remember to use that amount when competeing. Also pratice best shooting angle, 45 degrees is said to be best. Good luck and keep us posted. Video's of a winner would be great.

jrrdw wrote:Also practice best shooting angle, 45 degrees is said to be best.

Far from it. It's closer to 30-35^o with a spud when drag is accounted for.
There's not a huge difference, but throwing some generic figures at the updated LRC spreadsheet I'm finishing up (mostly done, will be released to the public when I've sorted some bugs and handled a few other things), I reckon you'll gain a valuable ~10% range if you fire at 32^o rather than 45^o, which only applies when drag isn't accounted for.

Ragnarok wrote:

jrrdw wrote:Also practice best shooting angle, 45 degrees is said to be best.

Far from it. It's closer to 30-35^o with a spud when drag is accounted for.
There's not a huge difference, but throwing some generic figures at the updated LRC spreadsheet I'm finishing up (mostly done, will be released to the public when I've sorted some bugs and handled a few other things), I reckon you'll gain a valuable ~10% range if you fire at 32^o rather than 45^o, which only applies when drag isn't accounted for.

Is/does your spreed sheet work anything like this. Someone gave this link out during a talk about the GGDT.

The spreadsheet is many times more powerful than that.

For one thing, I cannot see how that calculator can possibly accurately account for air resistance without knowing the projectile diameter and drag co-efficient at a minimum.
So, I have to say it can't.

Oldspise The fan is the next step up in performance, for mixing and venting.W/fan you could easily out shoot him in RPM ,rounds per minute.

Like others have posted, the best/easiest thing is to install a chamber fan. The fan should be running when fueling, firing and venting the gun. You can use a fan with brushes since many folks have without any problems. Many other folks have tried to ignite spud guns using a brushed fan and have never gotten it to work. Just follow good gun handling procedures and keep the loaded weapon pointed in a safe direction at all times. (Which of course you were going to do anyway.)

Double bevel the barrel, that is, sharpen both the inside and outside of the muzzle. Most of the bevel should be on the outside.

Go with fan, then metered propane i would say.
I have both in my combustion and the performance is phenomenal. Definitely invest in it if you can
Also i'm surprised no one mentioned this or maybe they did and i just didn't see it, but i'd at least use pressure rated pipe, maybe not pressure rated fittings, but pressure rated pipe.

If you weren't going with a spray n pray type thing i would immediately suggest a hybrid, they are so much more powerful. But you're not after that.

My gun can easily go 200+ yards, more like 500 i think. Good luck OldSpice

Thanks to all for the fan ideas. Since I'm new to this site, does anyone know of a link to a thread with basic instructions on a basic fan? And what is the fan that I should not use for fear of sparks?

Just go to a computer store or if you have an old computer, get an 80mm computer fan, they're the most common fans in computers, just get the standard black one, should only be a few dollars if that.