SpudFiles

Greetings, I'm new to this forums, I found it while looking on the internet spud cannon designs. I joined and I've found it very useful. Also I'd like to introduce a new model I've been think of and will begin in a few days, I have designed other pneumatic spud cannons before but until now im getting into the combustion designs

The idea came when a friend built a combustion cannon for airsoft like the ones you see on youtube. The I came up with an idea to avoid having to open a cap filling with spray closing and igniting, in other words, make it more realistic. The I came up with this...

The image includes the 4 phases of operation.

1) Load the cannon with the ammo, I have not designed it yet but the idea is fitting a balloon inside of the shell with a propane/oxygen mix. The bullet/potato will be fit to the shell but loose enough to release from the shell when fired.

2) The black tube, will be then pushed forward and secured by using a system similar to a bolt action rifle, the small black arrow will be 2 nail/screws that will make the electric arc, they will also be used to puncture the balloon inside the shell, releasing the mix. The purpose of the blue tube will be to avoid the shell from going further into the barrel, it will be wide enough to allow the bullet to go through but not biog enough for the shell

3) the cannon is fired as in any normal combustion cannon, the bullet flies out and the shell remains. The idea is making those pvc tubes be tight enough to avoid leaks and pressure loses

4) The black tube is pulled back, and the shell taken out ready for the next round.

I still have to design the electrical wiring for the spark. The images were quickly made in powerpoint but it will be made as tight as possible as mentioned above. I just wanted to share this design since I have not seen similar design on internet (I have not checked well here though).

I'll be posting images as soon as the development team assembles

You would still need some sort of metering system to fill the balloon. Also, it would be difficult to get very much fuel behind the projectile since it's confined to such a small space. I would think the power of such a design would be limited.

Personally, I think you would be better off with an off-board fuel metering system. Instead of a balloon, you would have a small piece of pipe which injects a precicely measured amount of propane into the chamber via a quick disconnect valve. You could also build multiple meters for multiple shots before having to re-fill them if you want a faster firing rate. If you incorperate a chamber fan, you will end up with a very reliable and powerful cannon.

welcome to the forum

A note on cartridges, with a simple combustion for decent power its volume is going to have to be much bigger than that. Some of my own efforts can be found here.

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K thanks for the advice, you are rigt maybe a balloon is too small for a moderate power, to meter the mix I thought on using 2 small pressurised tanks (propane and oxygen) and try different mixtures and volumes. Thanks for the advice, and that cartridge is pretty cool indeed I will try that

I thought on using 2 small pressurised tanks (propane and oxygen) and try different mixtures and volumes.

Correct me if I'm wrong, but I wouldn't try using a pure oxygen/propane fuel mix if that is what you are refering to. It is dangerous considering PVC's tendency to fail when submitted to intense pressure spikes beyond those created by a simple atmospheric air/propane mix.

JSR's idea is pretty cool though. Much better than using a balloon if you're looking for a cartridge design. Since it is ridgid and has a fixed volume, you would only need to add a set amount of propane rather than trying to add air then propane into the balloon.

Yeah I've been doing research on mixtures and you are right about pressure in the PVC, I will use a normal air/propane.

Also, how would be using butane instead of propane? does it has a significant difference?

considering propane expands faster than butane,id stick with propane

edit: try looking for mapp gas ,thats even better

Cris wrote:Also, how would be using butane instead of propane? does it has a significant difference?

I use butane in my combustions, choice of propellant does make a difference but is not as important as having the right volume and mixture.

Plus butane comes in handy aerosol cans can be used with the tiny fill valves from disposable lighters

Still couldn't be used by itself in this mind you, you'd overfill it instantly. Still need a buffer of sorts.

Cris wrote:Also, how would be using butane instead of propane? does it has a significant difference?

No significant difference in performance. If the fuel is metered correctly you probably won't be able to tell the difference even with a shooting chrony.

There are a couple of small differences, not related to performance, between butane and propane. You can get a cheapo disposable lighter for $1 and fuel a typical sized cannon many times before the lighter is empty. For perhaps $5 for a propane cylinder plus what another $15 for a torch head you get enough fuel for thousands of shots with propane and a useable torch as a bonus. So, depends on your budget.

Propane is usable down to below 32F. Butane's pressure drops too much when it is cold and gets to be a problem. The boiling point of butane is 31F so you are probably limited to temperatures of above 40F. (Most spudders avoid firing plastic cannons when it is that cold anyway so it isn't usually a problem.)

Learn more about combustion spudgun fuels at the SpudWiki fuels page.

MAPP is a better, not a lot, but a bit. Latke saw a 12% increase in muzzle velocity with MAPP versus propane. Shooting spuds, the shot to shot variability is probably 5 to 10%. So, whether it is worth the cost is pretty subjective. Cost is higher than with propane or butane. For perhaps $60 you can get a cheapo MAPP/O2 torch setup. That's enough fuel for thousands of shots and a MAPP/O2 torch suitable for small brazing and welding jobs.