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Bore to Barell Ratio: Can a barrel be too short?

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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Bore to Barell Ratio: Can a barrel be too short?

Unread postAuthor: zerodivide » Mon Sep 04, 2006 9:17 am

Been using GGDT to model a 3" (about 1 meter actually, 39") long inline pneumatic. The sprinkler valve plus fittings take up almost 10" so that leaves me 29 inches to work with. To make it simple on materials I'm using the same 1.5" sched40 for both barrel and chamber. So I start with the usual 1.5:1 (but less than 6:1) rule of thumb. Pressure will be 50psi (60psi - 10%) because of pop-in schrader valve.

When I ran optimize to overall length it was telling me to use a 23" chamber with a 6" barrel. As far as barrels go, can it become too short to launch waterballoons/nerf/paintballs?

Is there some sort of ratio between bore and barrel length that you follow?




The instinctual ratio I made gave a 20' apogee with 100ft max range while the optimized one had 301' apogge with 120ft max range according to ballistic calculator.
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Attachments
Thisdoesntlookright.jpg
Optimized for 29" length, is that barrel length even going to let it fly straight?
ThisIsWhatIPlanned.jpg
This was my instinctual ratio.

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Unread postAuthor: noname » Mon Sep 04, 2006 11:32 am

If you have 29 inches, then make your barrel like 20" and your chamber 9". I made a little 1.5" gun a while ago which used a 5" long chamber, 14" long barrel, and a pop-in schrader valve which was safe at 120 psi. 50 psi isn't very strong.
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Unread postAuthor: boilingleadbath » Mon Sep 04, 2006 3:41 pm

Of course it's giving you garbage answears; take a good look at the values you'r feeding it.

(The valve stays open for 0 ms)
Garbage in, garbage out, ya know...

Acctualy use a valid model for a sprinkler valve (such available from the GGDT website), instead of a "generic" model.
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Last edited by boilingleadbath on Mon Sep 04, 2006 4:15 pm, edited 1 time in total.
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Unread postAuthor: LucyInTheSky » Mon Sep 04, 2006 3:51 pm

try 10ms opening time, dwell time 1000ms, Cv 6, seat diam, 1"
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Unread postAuthor: zerodivide » Mon Sep 04, 2006 6:28 pm

Cool! I knew I was modelling it wrong but was just too new at this to know my mistakes. Found the page where the Orbit Watermaster 1" was modelled and plugged the values in.

http://www.thehalls-in-bfe.com/GGDT/lib ... rbit1.html

I hope my values are more or less correct now. Incidentally, I've found that exchanging the barrel for the chamber at the same pressure has a marginal difference in fps. That is interesting.
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Attachments
9C20B.jpg
Following noname's suggestion of 9C:20B ... still at 50psi though
20C9B.jpg
Using the original short barrel configuration of 20C:9B @ 50psi on a 200gm nerf (guess)

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Unread postAuthor: boilingleadbath » Tue Sep 05, 2006 2:20 pm

Your flow values for the valve are in CVs, and the page calls for 23 PERCENT.

Go to options and change the flow values to efficiency.
(and for the record, 1.5" sch 40 pipe is 1.59" ID)
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Unread postAuthor: LucyInTheSky » Tue Sep 05, 2006 2:42 pm

Click where the valve info is and change it to "generic"
Then put in 6.1Cv, 1000ms dwell time, 12ms opening time.

You ammo cant be 200g!
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Unread postAuthor: zerodivide » Tue Sep 05, 2006 8:08 pm

@boilingleadbath
Thanks, trying it with your parameters now. Sorry, I don't really understand what CV is and how all these computations apply. Someday I hope to.

@LucyInTheSky
Also tried it your way with "Generic". The outcome seems less optimistic with this one but maybe its more accurate? I used 200 grams because I'll be firing flour, confetti, and maybe .12gram BBs out of it.

What and where should I read up to understand these parameters? My knowledge of physics and fluid dynamics has degenerated to barely understanding newton's law.

No plans of firing anything heavy and hard with my first design. For all I know the solvent welds are wrong and the endcap might pops out at 10psi ... hahaha. Thanks for putting up with me guys! Hopefully my mistakes will give someone else a clue later on.
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Attachments
Calcsasof9-5-2006.jpg
boilingleadbath's corrections
Calcsasof9-5-2006-Generic.jpg
LucyInTheSky variation

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Unread postAuthor: zerodivide » Sun Sep 10, 2006 6:35 am

Not sure what else I'm doing wrong now. The GGDT data suggested that a 9x1.5" chamber should be able to shoot out through a 20x1.5" barrel at about 70fps @ 50psi. The ballistics calculator estimated a 60+ foot apogee. Maybe I am losing pressure during removal of the bike pump from the schrader (hiss)? The watermaster + chamber didn't leak anymore. They passed a water and air test. I used tissue paper and a small empty shampoo bottle as a projectile but it only travelled 10 feet firing horizontally @ 60-70psi (before removal of pump connector). Maybe I should spray Pam unto the barrel? Then again I'm using the manual bleed screw to trigger. Maybe the above parameters are for a modded sprinkler?

If you have any enlightening comments on the questions above I'd like to hear it. Meanwhile I shall endeavor a brute force approach by making new pipes using the reverse ratio.
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Unread postAuthor: zerodivide » Sun Sep 10, 2006 6:55 am

More testing. Anyway, the chamber holds 60psi easily for 3 minutes. When I removed the pump there was a loud hiss. Then when I reconnected to get another reading there was another hiss. The gauge read 0psi. Just to be sure I pumped it again and there really was no pressure in there.

To test this, I filled it to 60psi and left the bike pump on. There was an earsplitting noise when I fired (not like the weak foomp from yesterday). Looks like I'm losing pressure removing the pump from the fill valve. Question is, how to prevent that? Should I use a fill valve extension with some sort of backcheck in the middle? Do they even make a backcheck that small?
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Unread postAuthor: schmanman » Sun Sep 10, 2006 7:03 am

put a ball valve on it in between the fill valve and the chamber. close the ball valve before you disconnect the bike pump.
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Unread postAuthor: zerodivide » Sun Sep 10, 2006 7:22 am

My schrader was a pop-in so its directly on the endcap. The chamber would have to be rebuilt to do this. But you've given me an idea! Maybe I can find a threaded ball valve that will hook up to the schrader and a extension/adapter that will let me connect the pump after the ball valve.
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Unread postAuthor: schmanman » Sun Sep 10, 2006 8:26 am

I doubt it. instead of building a new chamber, just drill and tap a 1/4 inch pipe nipple into the endcap next to the existing schrader valve, then put oon a 1/4 inch ballvalve, then put a 1/4 inch threaded shrader valve into the other end of the ballvalve. just dont use the existing fill valve.
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