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Weak 4" Bore Cannon

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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Weak 4" Bore Cannon

Unread postAuthor: Dornep » Tue Sep 25, 2007 8:22 pm

I've built a 4" bore cannon using an 8ft section of SDR-26. It has an on board propane meter made of a 12" x 1/2" galvanized steel nipple, and an 80mm fan attached to the cleanout cap (dwv oh my!) The propane meter holds 75ml of water.

For testing I'm using 3" end caps and plastic folgers coffee cans as ammo. I want to put this cannon in the showcase after it's painted and firing like I think it should be.

Currently the cannon is pathetically weak. When fired the projectile exits the barrel followed by a large flame and a whoooomp sound. I'm injecting 75psi of propane into the chamber which is 44" long including the cleanout cap. The barrel is 53" long. According to the spudtool this should equal a 0.8 to 1.0 C:B ratio. I've messed around with the chamber length and amount of propane injected. I shortened the chamber by about 10-12 inches and injected 60PSI of propane which seemed to give better results but still not as powerful as my other combustions.

Is it even possible to get a cannon made with the same diameter barrel and chamber to be powerful? If I'm using the wrong C:B or amount of propane, what would be the best setup I could go with to get the most performance out of this beast?


On a side note this type of gun allows an ultra easy, bazooka style reloading process, since my projectiles are perfectly airtight when wrapped with duct tape.

To fire the gun I unscrew the cap shove in a coffee can and set the depth with the ram rod or my arm. I then screw in the cap, switch the fan on, and inject the propane. Then whooomp. The great thing is.. to vent the chamber all you have to do is slide in another coffee can and it pushes the spent air out of the chamber and down the barrel in front of the coffee can. With a two man team.. One to carry the ammo and reload, and one man to carry the cannon you could get a fairly high rate of fire.

I will get some pic's and maybe video tomorrow. Any tips would be much appreciated.
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Unread postAuthor: iPaintball » Tue Sep 25, 2007 8:28 pm

Your meter might be off. That's tho only thing I could think of.
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Unread postAuthor: Dornep » Tue Sep 25, 2007 9:01 pm

According to Harvel, 4" SDR-26 has an ID of 4.134. Which should be 436.81 cubic inches of volume for my 44" chamber. I just checked Burnt Latkes fuel tool and it says I need 60PSI of propane in my 75ml meter pipe. I guess the difference in inner diameter of SCH40 and SDR-26 is enough to make a weak cannon.

I will try this mix tomorrow and see how it does. Hopefully I will have it on the showcase in a few days. :D
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Unread postAuthor: MikeNice » Wed Sep 26, 2007 2:17 am

If you're running a C:B of .8 to 1 thats going to be a huge part of your problem. Not that the magical 1.5:1 is perfect but you need to have less barrel. The weak shot and "whoooomp" sound you are getting is because your fuel / air mixture is finishing combustion before your projectile is leaving the barrel or close to it. I know you're getting a large flame coming out of the barrel, but it is being sucked out of the barrel by the projectile. The projectile is moving fast enough for inertia to finish getting it out, but the combustion process at that point is no longer generating pressure. Pressure behind your projectile is dropping before it is getting out of the barrel. Why? Either too much barrel or not enough chamber. Take your pick, the answer is the same, you need to fix your C:B ratio and get it closer to 1.5:1.

With your current .8:1 either your chamber needs to be almost twice as big or your barrel half as big. I would shorten the barrel till you get closer to the right ratio.

A 44"x4" chamber is massive and this could lead to several issues. 1. is mixing the fuel / air properly 2. is making sure there is enough of it around your ignition posts. 3. is the number of ignition posts and what you are using to push the gaps.

What you want is for pressure to increase all the way down the barrel until the projectile is out. This is going to happen with a proper C:B ratio, proper fuel mixture and good ignition. You've got to have all 3.
I still don't think 1.5:1 is the perfect numbers but it's going to work a lot better than the .8:1 you are currently using. I would focus my efforts on getting the C:B ratio closer to 1.5:1 and getting the chamber fan out of the cap and into the chamber if it were my gun.

Good luck with the beast.
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Unread postAuthor: Fnord » Wed Sep 26, 2007 9:52 am

Are you using any kinda of wadding at all, or just duct tape?
Was it really hot outside when you attemped to fire?
How long do you let it sit between fueling and firing, and does your fan have enough space around it to circulate air properly?

Try putting 35-40 psi of fuel in the meter and letting the fan mix it for 10sec or so.
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Unread postAuthor: Dornep » Wed Sep 26, 2007 11:10 am

EDIT**

I think I have it just about figured out now... Tried shortening the chamber volume by a few inches and injected 60PSI like I did the other day it actually seemed powerful this time around.

I fired a Bernzomatic propane bottle wadded with the coffee can. I shot it almost vertically towards the sky and it was an awesome sight... It seemed weird to be able to see the propane tank get smaller and smaller as it went up. Gave a real impression of power.. I will do some more tests this weekend and decide on a paint scheme... I'm also going to see if I can make some finned PVC rockets for it.











@: MikeNice

This thing is super basic, I have 1 homemade spark plug with a gap of about 1/4 inch. It's located right next to the propane injection point and is powered by a BBQ ignitor. This is thin pipe so I wanted to keep all the fittings on the cleanout cap area so they would be in two layers of PVC.

I know that long chambers don't perform as well as short fat ones, but this setup is all I have to work with. The only thing I bought for this cannon was the cleanout cap. All the other parts were salvaged from other cannons.

I also know that my fuel mixing setup is pathetic for a chamber this size. I could install two or even three fans in the chamber, but I would like to keep everything in the chamber removable so I can vent it the same way as I described in my first post. If it comes down to it I will install two fans and more spark gaps and use the fans to vent the chamber instead.

When I inject the propane I do it slowly since the injection point is right in front of the chamber fan, which is blowing towards the barrel. I figured this would mix the propane and air better since it should be spreading the propane through the chamber more evenly. I also do 30 second count before I fire, and you can tell the difference in power. If I inject the propane and start clicking the sparker immediately the coffee can barely exits the barrel.

@: Fnord

#1 The coffee can uses duct tape and a plastic grocery bag. I put the can in the grocery bag and then stuff the excess bag into the can and then put the lid on the can. It gives a very good seal.

#2 The tempature was around 75-80 degrees fahrenheit

#3 I usually let it mix for about 30 seconds. No the fan is in a really shitty spot.

And I will try your suggestions and see what happens. Thanks for the help guys.
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