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Lack of Power

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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Lack of Power

Unread postAuthor: Dornep » Mon Aug 27, 2007 5:36 pm

I have a combustion which uses metered propane, BBQ sparker ignition with a single spark gap, and a 50mm chamber fan. The gun's chamber is made of a 17.5" long section of 2" SHC40. It uses a 2"x¾" reducer bushing with a ¾" end cap, and a 2"x½" reducer bushing for the barrel union. I have a ¾" barrel for shooting batteries and stuff, and a 1½" barrel for golf balls, persimmon's, spuds, etc. The 1½" barrel is 35" long and the ¾" barrel is 48" long.

I know the ¾" barrel is way short for a good C:B ratio, but the 1½" barrel is right on for a 0.8:1 ratio. My question is why does this thing shoot like a pea shooter? It looks impressive as hell but when I fire it, it's like... that's it? I fired a golf ball tightly wrapped in paper towel today, and when it fired it couldn't have traveled more then 100ft (guess-timating).

My question is would a 2" chamber reducing down to ½", and then back up to 1½" have any effect on power? Is my fuel mixture off? If you would like to see what I mean I can put up a video on youtube.

Edit** Pic's suck cause they're from a cell phone.
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Last edited by Dornep on Mon Aug 27, 2007 6:54 pm, edited 1 time in total.
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Unread postAuthor: turbohacker » Mon Aug 27, 2007 6:09 pm

I belive that your ristrictive setup is the main problom.

You should always have the reducer specific to each barrel so that you can have the optimum "valve" size.

I think that if you cut off the reducers and reduce your gun down to only 1.5 inch for the golf ball your performance will be much better. Im not sure on the fuel mixture-maybe a video? :)
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If it walks like a dog, sratches like a dog, and barks like a dog, than its probobly a...-TURTLE, it's a TURTLE!!! Cant you see?!!!

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Unread postAuthor: Dornep » Mon Aug 27, 2007 6:49 pm

I've rebuilt this gun once already because of a broken solder connection on the chamber fan. If I cut off the bushing again it will make the chamber even shorter and the meter will hang off the back, which it already does by about an inch.

If you guys are positive the crappy performance is coming from the reducer setup, I'm gonna tear it down and use the meter and fan control box on a 4" chambered cannon. It's a shame too cause this thing looks hella cool.
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Unread postAuthor: Fnord » Mon Aug 27, 2007 7:02 pm

I see no other reason for the lack of power other than the flow restriction.
Remeber, when you cut the diameter of the pipe in half, you reduce the flow by a factor of 4.
That pipe appears to be the color of sch80 pvc, did you paint it?
If it is just normal sch 40, it wouldn't cost all that much just to get a new piece of pipe and a couple fittings rather than cutting the current one down more.
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Unread postAuthor: Dornep » Mon Aug 27, 2007 7:22 pm

I painted the chamber and 3/4" barrel blue to match my truck's color. The only reason I built this cannon was to have a portable gun that I could break down and carry around. The work I put into the fan setup alone makes me hate to have to redo the whole thing a 3rd time.

All the PVC pipe and fittings I built this gun with, I got at work for free. I could just as easily get some 4" or 6" SCH40 or SCH80 for a chamber, and use this meter and fan control box. I noticed with my first cannon that people respond better when you pull out an 7ft long cannon compared to a tiny pop gun.
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Unread postAuthor: Novacastrian » Mon Aug 27, 2007 9:29 pm

Rebuild time, don't think about how much it sucks, just do it. :)
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Unread postAuthor: pyrogeek » Mon Aug 27, 2007 9:43 pm

Yes, go with a long, 4in diameter chamber. And put the reducers on the barrels. Anytime you restrict it below the diameter of the barrel, performance WILL suffer GREATLY!

The gun does look pretty cool though. I'd say you should keep it and stick a paintball barrel on it. Propane meters aren't that expensive, so just leave that one on. If anything just mount the regulator on your propane tank, so you can use it on multiple guns that use different pipes.

Or, this would be a great time to build a big piston pneumatic
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 27, 2007 11:36 pm

The easiest way to get no flow restriction whatsoever is to use the same tube for the barrel and chamber, with a simple stopper to make sure the projectile doesn't go too far into the chamber.
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Unread postAuthor: Dornep » Wed Aug 29, 2007 5:10 pm

Ok the 1/2" reducer is definitely the problem. I put a valve on the barrel union and left it closed, then attached a 1"x3/4" adapter to the rear of the gun with a 1" barrel exactly 8ft long. I dry fired it in the house and saw a flash travel down the barrel and heard a loud pop. My brother was outside and heard it also.

The only problem is that the chamber fan is in the very rear of the gun which is now the front of the gun. Will the pressure damage the fan? I have it super glued to the chamber wall and it's a very tight fit so I don't think it will come loose.
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Unread postAuthor: Novacastrian » Wed Aug 29, 2007 10:00 pm

The pressure will not damage the fan, however being at the front of the chamber it might get a little crisp after a while.
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Unread postAuthor: n3rdkw » Wed Aug 29, 2007 11:49 pm

Well, I don't have a gun that has a chamber fan, but from the dynamics of explosion, I do know that the position of the fan in the explosion does not affect the amount of damage it's going to take from the heat and fire. But I do think if you put it in the middle, the fan will not be affected by the expanding gas because if it's in the middle, the gas expansion won't push the fan blades.
I'm glad you found out the problem though.
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Unread postAuthor: SpudBlaster15 » Wed Aug 29, 2007 11:58 pm

There is a major difference in the forces the fan will experience when comparing front and rear chamber placements. Simply put, if the fan is positioned towards the front of the barrel, it will likely turn into a projectile.

The gases flow towards the area of low pressure (The front of the launcher) created when the projectile begins to move. If the fan is placed near the front of the chamber, it usually cannot withstand the force of the gas particles impacting it as they flow towards the barrel, and it will come out of the barrel in several pieces.

If the fan is positioned towards the rear of the launcher, it does not have to withstand the above mentioned forces, and it will survive the firing.
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Unread postAuthor: n3rdkw » Thu Aug 30, 2007 12:04 am

SpudBlaster15 wrote:There is a major difference in the forces the fan will experience when comparing front and rear chamber placements. Simply put, if the fan is positioned towards the front of the barrel, it will likely turn into a projectile.

The gases flow towards the area of low pressure (The front of the launcher) created when the projectile begins to move. If the fan is placed near the front of the chamber, it usually cannot withstand the force of the gas particles impacting it as they flow towards the barrel, and it will come out of the barrel in several pieces.

If the fan is positioned towards the rear of the launcher, it does not have to withstand the above mentioned forces, and it will survive the firing.




Well said, and uhmm... I guess you should ignore my comments
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