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Over Under Size Questions

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Would you rather see 2" or 3" U-Bends ( Aesthetically )

2" U-Bend
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3" U-Bend
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Over Under Size Questions

Unread postAuthor: Taelryn » Fri Jul 24, 2009 12:55 am

Hello all,

I'm new to posting, but I've been lurking in the forums for years. I've wanted to make a propane fueled tennis ball cannon for years now. I've owned a spray n pray ( 3 x 18 chamber 1.5 x 60 barrel) for nearly 10 years.

I've looked at burnt latke, forums here, pretty much everything explaining how to do it while getting construction ideas.

I've taken a liking to the over under styles mainly this one:

http://www.spudfiles.com/forums/recently-completed-t12102.html

I however wanted a metered system much like:

http://www.spudfiles.com/forums/triple-thunder-noise-cannons-t15211.html

Here was my idea:

4 x 24 clear chamber
2" U-bend
2 1/2 x 48-72 barrel

I expected the C:B ratio to be around 1.2-1.5:1

My concerns lie with mainly aesthetic points. After the chamber, I wanted to have a 4 x 2 reducer that connects to a 2" u-bend. Then 2 x 2.5 reducer and then the 2.5 barrel.

Would anyone recommend a 3" u-bend, so that I would be able to breech load the tennis balls. I hear that they are slightly difficult to ram down the barrels. I ask this for the easy load option, but also because I haven't seen many examples of a 3" u-bend .

Also I would be painting the whole thing in the end anyway. (Thinking gloss electric green) aesthetically

Any other comments welcomed

p.s. - Finding 2.5" pvc besides grey conduit is becoming quite the pain (Not even including 2.5" fittings) :(
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Unread postAuthor: colty-bear » Fri Jul 24, 2009 1:18 am

why use a u bend just use 2 90o elbows it looks much better
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U Bend PVC

Unread postAuthor: Technician1002 » Fri Jul 24, 2009 1:25 am

U bend.. You bend.. OK cheap joke, but seriously, done properly, it is very possible to bend PVC by heating it until it behaves like a rubber garden hose and letting it cool into the new shape. Here is an example. This is a test bend of 3/4 inch pvc to make Christmas decorations.
Image
To keep the bend from going flat, either fill it with sand or an inflated section of surgical tubing. Without pressure of some kind, it tends to kink when bent sharply. Only moderate pressure is required. Higher pressure may distort the PVC.
Image
This bend was done by heating the area over a propane torch. Distance was kept far to prevent burning it. Use constant fairly rapid motion to heat it all the way around and prevent overheating any spot.

Do not heat the end where you will glue it to a fitting. You don't want any distortion at the joint to weaken the glue joint.

Other things can be done with heating PVC. Here is an example of crowning a barrel to make it easier to stuff in t shirts.
Image
This barrel will be used to launch t shirts at a concert on August 8.
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Unread postAuthor: mark.f » Fri Jul 24, 2009 5:40 am

Uhh... you cannot bend or find u-bends to make a gun as compact as the Blue Flame.

Note how close the two street 90's bring the chamber and barrel together.
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Unread postAuthor: Moonbogg » Fri Jul 24, 2009 10:16 am

What is it with people not using barrel supports? Won't the recoil crack the pipe eventually?
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Unread postAuthor: jimmy101 » Fri Jul 24, 2009 12:48 pm

Moonbogg wrote:What is it with people not using barrel supports? Won't the recoil crack the pipe eventually?

Yep. The photos of the "BlueFlame" suggest a pretty unsafe design.

An "over-n-under" should always have adequate barrel supports.
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Unread postAuthor: mark.f » Fri Jul 24, 2009 12:48 pm

Yes, not supporting the barrel will create a lot of stress at the elbows.
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Unread postAuthor: Taelryn » Fri Jul 24, 2009 2:46 pm

I do plan on having supports similar to Starman's cannons. With the size of my planned chamber and barrel I will definitely need it.
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Unread postAuthor: sgort87 » Sun Jun 13, 2010 5:54 pm

Pardon me for kicking up an old thread, but I just wanted to quickly address something about the Blue Flame support issue.

I decided against a support because of the way it's designed to be held. Since it's supported by one hand on the barrel and the other on the chamber grip, the recoil will never produce significant stress on the bend.

So far, I've had my Blue Flame 3 for 2 years now, and the threaded elbows show no signs of stressing. I've given it all kinds of abuse by carrying it just by the barrel or just by the chamber, but still no problems. The Blue Flame 2 before it used solvent welded joints and also showed no stress. The Blue Flame 1... Well, the chamber cracked before anything else could happen to it.

I'd say barrel supports are not needed for small/medium cannons designed to be held by the barrel and chamber with a reasonable projectile weight. The bonus of this is fast barrel-swapping and breech cutting for any size barrel. And that's a bigger bonus than many may assume.
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