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GGDT issues

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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GGDT issues

Unread postAuthor: Pilgrimman » Wed Aug 01, 2007 3:09 pm

I'm having trouble with the GGDT. I've built my first coaxial piston gun. The only difference is that the barrel only goes in about halfway into the pipe. This is the reason for some of the following specs*. I have trimmed down the specs for my gun in the program to pessimistic proportions, but it still returns muzzle velocities of ~950 fps, which I highly doubt to be true. The specs are as follows:

Gas: Air
Temperature: 80 F
Pressure: 100 psig
Outer Diameter: 2"* (The chamber is 48", but I subtracted 8" to account for the barrel)
Inner Diameter: 0"* (See above)
Length: 40" (Here's where I made the subtraction)
Volume: 125.664 in3
Valve Type: Barrel Sealing Piston
# of Valves: 1
Flow Coefficient: 20%
Seat Diameter: 1.5"
Dead Volume: 1 in3
Piston Diameter: 1.75"
Piston Mass: 10 oz.
Vent Diameter: 1"
Pilot Volume: 20 in3
Bore: .5"
Length: 48"
Friction: 1 psi
Mass: 1 g (It's a SMALL paper towel wad)
Diameter: .5" (It does seal well)
Initial Position: 1"

These specs are highly pessimistic, as the flow coefficient I plugged in is less than that of an unmodified sprinkler valve. Also, the pilot volume I plugged in is obscene. It's really a lot less, but I was still getting velocities of ~1200 fps when the pilot was less. I honestly have no idea what is causing these weird readouts, but it would be cool if it did shoot 900+ fps! Any help is appreciated! The software is v4.4.
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Unread postAuthor: noname » Wed Aug 01, 2007 3:22 pm

Inner diameter should be the outer diameter of your barrel.
So it would be
Outer diameter: 2"
Inner diameter: .84"
Chamber length: 48"
Barrel length: 48"

I'll edit this post to explain that adding a value for the inner diameter is just what I've learned from other people over the years.
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Unread postAuthor: clide » Wed Aug 01, 2007 3:27 pm

Why is your seat diameter 1.5" and your piston diameter 1.75" if you have a coaxial with a 2" chamber and .5" barrel?

Other than that a 1 g projectile will go extremely fast out of any decent gun and GGDT has questionable behavior at high speeds. I wouldn't be surprised at all if it was indeed shooting close to the speed of sound, but it is a bit pointless because a 1 g projectile that is .5" in diameter will slow down so fast and fly so bad that it is pretty much worthless. Model it with a more realistic projectile and you will get more realistic results.
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Unread postAuthor: Pilgrimman » Wed Aug 01, 2007 4:51 pm

@ clide

It's my first piston gun, so I wasn't too terribly wottied about efficiency. The fit is probably better than that in actuality (I guessed, within reason, see the thread below), but I just wanted a piston that would get the job done. However, you were very right to question the seat diameter. It was a typo. The seat is only a little more than .5". I have shot it with a small rock, and at only 30 psig, it FLIES, so it's obviously doing something right! Thanks for the help though! Maybe someday I'll get a chronograph... :cry:

http://www.spudfiles.com/forums/easy-piston-for-noobs-t9669.html
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Unread postAuthor: clide » Wed Aug 01, 2007 6:04 pm

Here is what I get, with some more reasonable variables. I'm assuming you used 1/2" sch40...
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