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GGDT interpret performance?

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: metalmeltr » Tue Dec 21, 2010 6:46 pm

Sub machine-cool-difficult to implement
Shotgun-may build-long reload time

I also need a good sidearm
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Unread postAuthor: metalmeltr » Tue Dec 21, 2010 10:26 pm

After some on the fly designing-zip ties another cannon and a "stock" made of 3/4" PVC- I think i have some design changes that will affect the GGDT model.
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The stock resembles this, I am going to try to mount the compressor in a piece of 3" pipe in between the top and bottom of the stock
stock.JPG (15.67 KiB) Viewed 282 times

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Unread postAuthor: metalmeltr » Wed Dec 22, 2010 9:35 am

Another run of GGDT results one illegal in Australia, good thing I don't live there,

25" barrel and pressures higher than i will typically run.

GGDT figure coming soon.
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Unread postAuthor: metalmeltr » Wed Dec 22, 2010 3:23 pm

Built a breach loading barrel from 1/2" PVC. Wish i had the camera to take a picture of this thing, but i guess an explanation will have to do.

I cut away the top half of the barrel in the breach area, then after fighting to try to get 3/4" PVC to fit over the 1/2 I gave up an remember that pop bottles fit over 1/2" PVC to i cut the neck off of two of them and epoxied them in to the ends of a piece of 1" PVC.
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Unread postAuthor: velocity3x » Wed Dec 22, 2010 3:53 pm

Technician1002 wrote:The school I worked with clocked a marshmallow at 1409 FPS using a 1 inch sprinkler valve.


Tech,
Would you happen to know (through either trial or guess) the maximum G force that a marshmallow can withstand during launch?
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Unread postAuthor: Technician1002 » Wed Dec 22, 2010 6:36 pm

That I don't know. Even denting car doors with them leaves them in one piece and only slightly misshapen. Marshmallows are pretty tough. 8) Nothing I have that is legal damages them on launch.
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Unread postAuthor: velocity3x » Wed Dec 22, 2010 6:50 pm

Have you ever compared their actual performance to the performance predicted by GGDT?
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Unread postAuthor: Technician1002 » Wed Dec 22, 2010 7:02 pm

Yes.. GGDT is reasonably close for ball park answers below 1,000 FPS, but tends to be optimistic when using the older version. I have not played with the new version enough to know. If you see a rise in speed, then a roll off in acceleration and then a stronger acceleration again (S curve) the second high acceleration is most likely in error in GGDT in the transonic ranges.

Actually getting a marshmallow above SOS with compressed air is easier in GGDT than it is in real life.

GGDT does not model shock waves in the barrel from plumbing joints that impact real performance. Sonic flow in real plumbing is hard to do.

In this thread you can compare my actual performance with a 4 inch foam ball against it's model in GGDT. The two graphs almost match exactly. This is how I found the CV value for the QDV. It is based on the amount of air that had to go through the 2 inch valve to fill the 4 inch barrel at the proper rate to launch the ball.

Plugging in lower CV values into GGDT will not reach the measured muzzle velocity.

http://www.spudfiles.com/forums/ggdt-and-in-tank-quick-dump-valve-performance-t18090.html

At speeds above about 900 FPS GGDT tends to have larger errors.
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Unread postAuthor: metalmeltr » Thu Dec 23, 2010 1:25 am

Rethinking this design closer to bullpup, i am going to have to buy the valve and compressor to start mocking this thing up.
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