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A few questions on chamber length and pistons

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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A few questions on chamber length and pistons

Unread postAuthor: jmccalip » Sun Apr 12, 2009 8:13 pm

I am building a golf ball cannon, and I currently have(not glued):

10ft of 4 " pressure rated PVC
10ft of 1.5" SDR 21 PVC sleeved in 2" shd 80(barrel)
And the appropriate fittings to make a 3" piston valve.



What would be the ideal chamber/barrel length for both portability and power? I was thinking a chamber and barrel length of 6ft, but I'm not sure, and I don't want to waste the expensive pipe.

Also, how long should the body(not including gaskets) of the piston itself be?


Thanks
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Re: A few questions on chamber length and pistons

Unread postAuthor: Technician1002 » Sun Apr 12, 2009 8:28 pm

jmccalip wrote:I am building a golf ball cannon, and I currently have(not glued):

10ft of 4 " pressure rated PVC
10ft of 1.5" SDR 21 PVC sleeved in 2" shd 80(barrel)
And the appropriate fittings to make a 3" piston valve.



What would be the ideal chamber/barrel length for both portability and power? I was thinking a chamber and barrel length of 6ft, but I'm not sure, and I don't want to waste the expensive pipe.

Also, how long should the body(not including gaskets) of the piston itself be?

Thanks


Portability depends on what you consider portable.

There is a great tool called Gas Gun Design tool that would help with finding sizes of barrel and chamber. Plug in the values and see what works best in your size limitations.

The piston needs to have enough travel to move back about 1/2 the diameter of your barrel at the minimum. It should be long enough to be stiff and not bend, break or bind and hold your sliding surface seals. Shorter is lighter. Lighter is better. Too light is too weak. What pressure are you planning on using?

***EDIT***
Sched 80 is heavy. Think about balance in holding it.
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Unread postAuthor: jmeyer1022 » Sun Apr 12, 2009 8:34 pm

I'd go with a 2 1/2-3' ft Chamber and 6' barrel at most. My 4' barrel on my 2" piston cannon weighs a load so to keep weight down I'd suggest keeping it low.
This will give you great power with portability. My cannon is 2' x 3" chamber and a 4 ft sdr 21 sleeved sch 80 barrel. Without the barrel on and facing the cannon to the floor( which would be about 2' up due to the chamber) It will shoot itself about 6" off the ground at 80 PSI.

It's pretty powerful and very easily portable, just unscrew the barrel and it's easy to carry anywhere. Hope this helps.
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Unread postAuthor: jmccalip » Sun Apr 12, 2009 9:51 pm

Thanks guys!


I don't think I want to go above 60 PSI, I don't want to take any extra risk on this thing blowing up. Would a longer chamber with a lower psi(50-60) be the same as a shorter chamber at a higher psi(80-100)?


I don't plan on making this a cannon you can hold in your hands, I just want to be able to transport it easily.
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Unread postAuthor: Technician1002 » Sun Apr 12, 2009 11:05 pm

jmccalip wrote:Thanks guys!


I don't think I want to go above 60 PSI, I don't want to take any extra risk on this thing blowing up. Would a longer chamber with a lower psi(50-60) be the same as a shorter chamber at a higher psi(80-100)?


I don't plan on making this a cannon you can hold in your hands, I just want to be able to transport it easily.


Lower pressure requires larger chambers as the air expands less, more volume is required.

For example a chamber at 6 bar has a volume that will expand to 6X it's volume with released. A chamber at 3 bar will expand to 3X it's volume. One of my launchers, a shirt launcher, with many screw on barrels does very well at 25 PSI feeding a 3 inch barrel 6 feet long. The chamber was originally sized to feed a 4 inch barrel launching foam Poof balls.

Have you tried Gas Gun Design Tool?

For transportability, does this mean able to carry or fit in your car. I had to cut all my barrels to a maximum length of 8 feet to fit in my sedan.
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Unread postAuthor: jackssmirkingrevenge » Sun Apr 12, 2009 11:55 pm

jmccalip wrote:Would a longer chamber with a lower psi(50-60) be the same as a shorter chamber at a higher psi(80-100)?


For a given mass of air, higher pressure is always better.

Just to give you an idea, I had modelled a virtual launcher on GGDT, using the same quantity of air but with ever decreasing chamber size (same diamter but shorter length) and therefore increasing pressure, and here's what I got:

20 inch chamber at 50 psi - 406 feet per second

10 inch chamber at 100 psi - 537 feet per second

5 inch chamber at 200 psi - 684 feet per second

2.5 inch chamber at 400 psi - 830 feet per second

1.25 inch chamber at 800 psi - 958 feet per second

0.625 inch chamber at 1600 psi - 1006 feet per second
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Oops.. Accidental bumb.

Unread postAuthor: Technician1002 » Tue Apr 27, 2010 12:07 am

I like your numbers. I need to put together a fridge compressor for my Marshmallow cannon. In theory I can go to 300 PSI with it. With the 7 foot long TB barrel it should do a nice job. :D

OOPS sorry for the bump.. My bad. Posted from the link here on a new thread.
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Unread postAuthor: jackssmirkingrevenge » Tue Apr 27, 2010 12:10 am

Image

Once you go HPA you never go back ;)
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Unread postAuthor: SpudBlaster15 » Tue Apr 27, 2010 12:24 am

jackssmirkingrevenge wrote:Once you go HPA you never go back ;)


Until you go hybrid, then ETG, then high explosives, then... :roll: :wink:
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Unread postAuthor: starman » Tue Apr 27, 2010 5:34 am

SpudBlaster15 wrote:
jackssmirkingrevenge wrote:Once you go HPA you never go back ;)


Until you go hybrid, then ETG, then high explosives, then... :roll: :wink:


...then nuclear.... :wink:
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Unread postAuthor: SpudBlaster15 » Tue Apr 27, 2010 5:42 am

I would have said ETCG, but that works too. :D
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