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Coaxial Design

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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Coaxial Design

Unread postAuthor: d05 » Tue Dec 20, 2011 11:16 am

With the recent failure of my once-trusty decade-old cannon, I've taken to the internet seeking advice to improve. After some research, I'm digging the compact nature of a coaxial design. My question for everyone (or anyone able to accurately answer) is this: In regards to a coaxial design, is the pressure or energy generated to expel the potato any less with a coaxial design over the traditional barrel<->chamber. I can visualize how the combustion needing to rush to the back of the chamber, then forward through the barrel may lose energy compared to simply powering forward out the barrel, but I don't know the science to confirm or refute it. If this 'turbulence' is real, and is a problem, would a higher C:B ratio fix the problem? Thanks.
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Unread postAuthor: Gun Freak » Tue Dec 20, 2011 11:18 am

Coaxial combustion barrels generally don't run too far into the chamber in order to prevent excessive turbulence. In a pneumatic it does. But for a combustion I've never seen a barrel go too far in. Check out some of the advanced combustions, ie Mr Morph or the BL520.
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Unread postAuthor: mark.f » Tue Dec 20, 2011 11:59 am

Gun Freak, having the barrel at the very back makes it much easier to load...

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I don't believe turbulence can be "fixed" with a higher C:B ratio... you'd just be wasting fuel and making your cannon bulkier.

With a coaxial, you have to generally make your chamber wider to allow for enough chamber volume to maintain a decent ratio... so perhaps you should be looking into over/under configurations instead? For instance, if you wanted a 60x2.5" SDR-26 barrel for tennis balls/soda cans, a 4" chamber with the barrel all the way at the back would need to be 42" long, a 6" chamber would only need to be about 12" long, but fittings start to get a little expensive at that size (I usually tell people DWV is okay in a combustion cannon, but at those sizes, I would definitely recommend pressure rated fittings).

Best of luck either way.
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Unread postAuthor: d05 » Tue Dec 20, 2011 12:06 pm

Well, I did do some number crunching based on the following: I planned to use a 2" barrel, a 4" chamber, and wanted there to be at least 3" between the rear of the chamber and the end of the internalized barrel. Achieving a C:B ratio of 1.4:1, I developed a formula of y = .992x -8.506 where y is the length of the chamber and x is the length of the barrel (all in inches). The ratio ended up being so close that it seems like using my above requirements, the chamber will always be approximately 8.5 inches shorter than the barrel. I had planned on using the plain schedule 40 pipe I'd find in Home Depot / Lowe's, unless you think that would be a cause for concern - and to seal the barrel through the chamber I was going to sand out the inside of the reducer, slip it through and basically fill in area between the reducer and barrel with fiberglass resin.
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Unread postAuthor: mark.f » Tue Dec 20, 2011 12:12 pm

You'll be fine with solid core piping... just don't use cellular core, which is for the most part all those stores sell in 10' lengths.

Both stores sell shorter (2') lengths in solid core (pressure rated generally) sections for pipe fitting. If all you can find is cellular core in longer lengths, use the shorter lengths joined by a coupling.

Your fittings can be DWV, just make sure to clean, prime, and glue them carefully (DWV fittings have a smaller gluing surface). Of course, if you can find them, pressure rated fittings are much better.
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Unread postAuthor: motorfixer1 » Fri Dec 23, 2011 6:35 pm

The Home Depot by me sells 2",3", & 4" pvc in 10' sections all pressure rated.
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