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Could someone...?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: SquishY » Sun Oct 29, 2006 6:15 pm

run a GGDT on what I'm going to be building in the very near future? 4"x24" chamber, 2.5"x6' tennis ball barrel, and a 2.5" barrel sealing piston with 2" porting and a 1" ballvalve for a pilot.
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Unread postAuthor: SpudStuff » Sun Oct 29, 2006 6:46 pm

I'll do it. In just a minute.
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Unread postAuthor: SquishY » Mon Oct 30, 2006 2:49 am

Thx man, I'm still learning the GGDT. I know I'm kind of late posting this seeing as we've already talked for quite a time and looked the file over. Having what I want put in the GGDT and sent to me also helped me to learn because I had no idea which values were right or even where some went.
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Unread postAuthor: POS » Tue Oct 31, 2006 8:16 am

long minutes ...
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Unread postAuthor: joannaardway » Tue Oct 31, 2006 2:02 pm

Ok. I've done it instead.

With a tennis ball, the figures you gave, and 100 psi @ 70F, you'll get about 530-550 ft/sec with about 600 ft lb of energy depending on exact valve efficency.

And, I've even fed those figures into my range calculator for you. You'll get about 120 yards of range. GGDT is more optimistic with a 150 yard prediction, but I'd trust my figure further.
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Unread postAuthor: SquishY » Tue Oct 31, 2006 2:22 pm

Thanks Joann, I didn't bother to use the external ballistics on GGDT. Would 100% efficiency be possible or would the highest be closer to 80% with it going 4"-2"-2.5". OH and is that 120 yards going for distance or just a slight upward trajectory to actually aim at something?
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Unread postAuthor: joannaardway » Tue Oct 31, 2006 3:48 pm

That 120 yards is at about a 30 to 40 degree launch angle for maximum range. Anyway between those 2 angles would get you about that range.

Barrel sealer efficency is typically in the 40%-50% range, which is actually very high when you consider that pipe is only about 65% efficent. (Doesn't quite make sense, but...)

Don't ask me what 100% effiency represents - I can tell you that it would be impossible to do though.

To help you in future, piston valves are usually between 35% to 50% efficent.
Chamber sealers: 35%-45%
Barrel sealers: 40%-50%
Co-axial: 45%-50% (I believe)

Sprinkler valves usually end up about 20%-25% efficent, and burst discs are about 60% efficent.

Hope I've been helpful.
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Unread postAuthor: POS » Tue Oct 31, 2006 4:53 pm

I don't dare to ask the coef. of a bal valve. Will be rather low I guess ? Around 10 to 15 ?
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Unread postAuthor: SquishY » Tue Oct 31, 2006 8:35 pm

I think it would be safe to assume about that, unless you get into those really technical ones like on pumpkin chuckers that use rams to open and close the valve.
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Unread postAuthor: joannaardway » Wed Nov 01, 2006 5:08 am

Ball valves have a maximum Cv of about 60-65%, but they have strange flow-effects while they open, and they open slowly.

To get a rough GGDT, I use a flow figure of about 40%-50% and an open time of about 150 to 200 ms, unless it's opened with a spring/ram/etc.
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Unread postAuthor: rixth » Sun Nov 05, 2006 1:17 am

I beg to differ.

I have a ball valve cannon, the valve is 2.12" ID, same as my barrel.

With a 10-15% Cv, it predicts 112m for a 50g lemon at 40psi. I have measured a firing and got JUST short of 200m. Lets say that it rolled for 40m after impact.

With GGDT, I have to put the Cv at 50% to get 157m range (with a 150ms opening time).

I might get a big flow because the ID of the valve is big?
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