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Reverse Optimization

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Reverse Optimization

Unread postAuthor: singularity » Fri Feb 22, 2008 8:01 pm

Thats right, I want to decrease preformance. Let me start form the beginning...

I met a guy a little while ago who had a big dog who loved to play fetch. But this guy had thrown out his back and need to have all this surgery and crap so now he cant throw very well. So eventually after talking for I offered to build him a cannon that would shoot tennis balls for him.

We talked a bit more and worked out some of the details and settled on a metered propane semi-advanced cannon with 2.5 inch barrel (of course he had no idea what i was talking about so I pretty much settled on that) I special ordered all the right parts (all pressure rated 4" fittings and 2.5" parts, none of that DWV crap you get at home depot) and solvent welded it together then realized "holy crap this is for a dog I don't need to be shooting tennis balls like several hundred feet"... (that just happened a few minutes ago)

So... I have two main concerns noise and distance, specs of the cannon are as follows...

chamber diameter - 4"
chamber volume - approx. 150 ci
barrel diameter - 2.5"
barrel length - 5 feet
fuel source - metered propane
chamber to barrel ratio - .8 to 1


To make the cannon less efficient I was thinking I should cut down the barrel to the 2 to 3 foot length but I knew that would make it louder, how much does a ported barrel cut down noise? Would lowering the propane to air ratio make it less powerful while at the same time ensure it fires EVERY shot? Or should I raise the ratio?

I have never tried to reverse optimize something before and I need some help. I would test it but the neighbors seem to dislike loud noises (i base this information on the fact that every time something loud goes off the cops show up)


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Unread postAuthor: ghostman01 » Fri Feb 22, 2008 8:33 pm

lol why didnt you just go for a pneumatic or some thihng much easier dont have to buy propane or anything just pump it up to say 20 psi or what ever and theres your reducing in power
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Unread postAuthor: boilingleadbath » Fri Feb 22, 2008 8:38 pm

Just build it really small, and with a .5:1 C:B ratio.

For just tossing stuff to a dog, I'd try a 12" chunk of 2.5" with an endcap on one end.

Fuel it with a syringe or something.

Don't even need a chamber fan in a chamber that small.
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Unread postAuthor: D_Hall » Fri Feb 22, 2008 9:59 pm

My vote....

Build a simply sprinkler valve-based pneumatic and use the pressure to throttle it.

Done.
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Unread postAuthor: Novacastrian » Fri Feb 22, 2008 10:59 pm

Here is one that i prepared earlier :P It launches a tennis ball around 50 feet, using butane from a syringe. It's about 2 feet long.
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Unread postAuthor: MrCrowley » Fri Feb 22, 2008 11:09 pm

Did he pay you to make it?

If he's done his back in, I doubt he wants to pump up a pneumatic, so that leaves him with CO2, which is overkill.

I'd say a foot long 3" chamber, with a 2' 2.5" barrel would be enough if it was just a spray and pray.

But if he paid alot for it, maybe you could have interhcnageable barrels so he can use it for personal recreation like the rest of us. :)
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Unread postAuthor: singularity » Sat Feb 23, 2008 12:10 am

I already built it, it the size of a standard cannon. It runs off propane so he doesn't have to worry about it not firing and because he can't use a hand pump (so pneumatic isn't even an option).

I built a basic combustion before, after about 25- 30 shots the electrodes were completely corroded. I built this one as a no maintenance needed type canon. I forgot about butane, how easy would it be to meter and would it screw up a standard air compressor reg?

Using like an 8% ratio of propane would decrease preformance right? would there be any negative effect to something like that (aside from excess CO production)?


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Unread postAuthor: jackssmirkingrevenge » Sat Feb 23, 2008 12:53 am

singularity wrote:Using like an 8% ratio of propane would decrease preformance right? would there be any negative effect to something like that (aside from excess CO production)


It would likely not ignite at 8%, better use a lower concentration, you could make it incrememntally lower until you get no ignition at all, then set it slightly above that.

I think the best idea here is to use a small chamber and a longer barrel that you normally would, thereby achieving low power but also good efficiency so low noise.
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Unread postAuthor: Pete Zaria » Sat Feb 23, 2008 12:59 am

You could create friction in the barrel. Consider building a sleeved barrel with an intentional gap in the inner pipe - then fill the gap with rubber or felt or etc... to provide friction and slow down the projectile.

You could decrease the chamber volume by adding a nice thick layer of epoxy to the inside...

You could make a "plug" for the end cap that would extend several inches into the chamber, reducing chamber volume...

Try playing with the propane mix. See how lean you can get it to reliably ignite. Lower explosive limit on propane is listed at 2.1%. See if you can get reliable ignition down around 2.5 or 2.7% and see what performance that yields.

Best solution in my opinion? Use whiffle balls. They don't fly that far, and I really doubt the dog will care what kind of ball it is.

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Unread postAuthor: Fnord » Sat Feb 23, 2008 9:36 am

Here's my suggestion:
Throw a ball valve on it.

You can open the valve a little to decrease the pressure exerted on the ball. It's adjustable, it won't be loud as hell, and it doesn't require pumping.

You may want to add a length of pipe on the valve if it makes noise.
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Unread postAuthor: Sticky_Tape » Sat Feb 23, 2008 10:10 am

Maybe put a union on it and make a sheet metal circle to fit and drill maybe 10 1/4 inch holes in it to restrict flow a bit?
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Unread postAuthor: psycix » Sat Feb 23, 2008 11:54 am

You should lower your chamber size.
It will become more quiet (duh) and less powerfull too
If you cant reduce the size of the chamber, then just put something in that takes up alot of volume :)
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Unread postAuthor: TurboSuper » Sat Feb 23, 2008 12:18 pm

I find the whole concept of a combustion a bit unwieldy for launching a ball for a dog. I'd just rig up some sort of spring piston pneumatic sort of deal.
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Unread postAuthor: mopherman » Sat Feb 23, 2008 1:11 pm

Turbosuper- a spring piston for a tennis ball barrel? lol

i would choke the flow down to about 3/4 or 1 inch. Then it would make a cool "fwooooosh" noise.
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Unread postAuthor: Jared Haehnel » Sat Feb 23, 2008 1:17 pm

I would suggest hacking off the barrel or at least drilling some holes in it. Second thought that would be a bad idea. Don't drill holes...

I think messing with the fuel-air ratio could make the cannon finicky. But the ball valve idea is pretty good. Going to have to try that some time...

If it was me I would have built a pneumatic it is easier to control the range by controlling how much pressure you put in the chamber. Probable if you just alter a few things about the design its not to late as long as you haven't started building. I wouldn't imagine he'd need any more then 30 or 40 psi... a portable 5 gallon air tank would be good for quite a few launches with a small chamber.

Jacking up your back sucks man I did that luckily I didn't have to have surgery. Hope it works out.
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Last edited by Jared Haehnel on Mon Feb 25, 2008 2:02 pm, edited 1 time in total.
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