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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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GGDT interpret performance?

Unread postAuthor: metalmeltr » Tue Dec 21, 2010 11:05 am

This is the result of a recent attempt of trying to model my next marshmallow gun.

chamber 1x6"
barrel 1/2" PVC-length between 30-36-not yet determined
solenoid controlled 1" sprinkler valve

What does the barrel choking flow output in GGDT mean?




is there anyway the uploader can be changed-PC Guy- to accept gdt files?
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Last edited by metalmeltr on Tue Dec 21, 2010 11:24 am, edited 1 time in total.

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Re: GGDT interpret performance?

Unread postAuthor: jackssmirkingrevenge » Tue Dec 21, 2010 11:18 am

metalmeltr wrote:What does the barrel choking flow output in GGDT mean?


It means the valve has more flow than the barrel will allow, basically a good thing :)

Ok what do i have to do to post the GGDT results?


Image

... then paste the image into MS Paint or similar image editing program.

It would be good if we could share GGDT files though...
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Unread postAuthor: metalmeltr » Tue Dec 21, 2010 11:27 am

Thanks jack,
I think i figured it out about the same time you replied.

How accurate are these result for using the original solenoid control?
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 21, 2010 11:35 am

A couple of things to fix:

- unless it's a coaxial, "outer diam" should be the inner diameter of the chamber, and "inner diam" should be zero.

- your projectile diameter is 0.108" less than the barrel inner diameter, is this actually the case? If so, you need a smaller diameter barrel or bigger diameter marshmallows ;)
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Unread postAuthor: Ragnarok » Tue Dec 21, 2010 11:52 am

jackssmirkingrevenge wrote:unless it's a coaxial, "outer diam" should be the inner diameter of the chamber, and "inner diam" should be zero.

My recommendation is to download this file and paste it into the root GGDT language folder. When it asks if you want to replace the old one, click yes.

This renames Inner and Outer diameter to less ambiguous names. (And shouldn't break anything if you do it right. If you want to go back to the old version, there's a backup of the normal "Main.txt" file in the English language subfolder.)
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: Technician1002 » Tue Dec 21, 2010 12:07 pm

I used to worry about Barrel choking flow until I understood it meant the valve provided very little pressure drop into the barrel. Now most of my launchers provide that error message.

One item to be cautious of is an excessively long barrel. With a long barrel, even though the projectile is still accelerating at the muzzle, the muzzle velocity may be reduced.

When you get the barrel choking the flow error, try plugging in shorter barrels and pay attention to the final velocity, not the fact is is still accelerating as it exits.

I know this is counter intuitive as the natural assumption is to lengthen the barrel so you accelerate the projectile even more before it exits, but this often results in lower overall acceleration and a lower muzzle velocity.

I found this to be the case when we built the competition t shirt cannon. We started with a long barrel and trimmed shorter, and as we did, the muzzle velocity increased.

In your posted trace of GGDT, you have a large drop in acceleration near the muzzle. Try a shorter barrel and see if the muzzle velocity increases. Another suggestion is to go for a larger diameter chamber. Getting marshmallows well over 800 FPS is possible. I dented a car door with a marshmallow.

When properly trimmed, the projectile will show good acceleration for most of the barrel travel and a gentle roll off near the muzzle.

We trimmed a barrel for a 4 inch foam ball launch. This is a test shot at 60 PSI. At the 100 PSI regulation limit there was less drop off in acceleration.
Image

GGDT for the above launch. The barrel is still a little long, but that was kept as a compromise for launching projectiles heaver than a foam ball.
Image
I tweaked the GGDT valve details to match the actual measured performance. This is how I found the valve efficiency.
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Last edited by Technician1002 on Tue Dec 21, 2010 12:25 pm, edited 2 times in total.
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Unread postAuthor: metalmeltr » Tue Dec 21, 2010 12:16 pm

jackssmirkingrevenge wrote:A couple of things to fix:

- unless it's a coaxial, "outer diam" should be the inner diameter of the chamber, and "inner diam" should be zero.

- your projectile diameter is 0.108" less than the barrel inner diameter, is this actually the case? If so, you need a smaller diameter barrel or bigger diameter marshmallows ;)

thats what that means? ok i always wondered why the outer diamiter even mattered

.5 is an estimate


Edit If the marshmallows fit tighter in the barrel we could have much faster muzzle velocities-no real suprize but they are significant gains.
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Unread postAuthor: Technician1002 » Tue Dec 21, 2010 12:47 pm

What kind of performance are you looking to reach? Is this a plinker or more serious? You are still showing relatively slow marshmallows. Competition level is about double that and above.

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

TESTING 1 inch valve with marshmallows (961 mph!!!)

From this page;
http://www.honkernet.net/AHS/departmental/Robotics_engineering/09-intel-blazer_tech-challenge.htm
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Unread postAuthor: omniscient » Tue Dec 21, 2010 12:58 pm

Why...
1.) The ridiculously small C:B ratio?
2.) Is your piston mass entered as 22 oz. ?
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Unread postAuthor: metalmeltr » Tue Dec 21, 2010 1:06 pm

I could easily build a higher performance cannon, this for marshmallow gun wars similar to paintballing, although speed of sound would be cool this is being used against human targets. Nerf darts will also be used as ammo, my gun last year ran 200psi and it was too much.

Last year's gun-http://www.spudfiles.com/forums/my-air-cannon-t20662.html
it had the pvc chamber when used last year
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Unread postAuthor: Technician1002 » Tue Dec 21, 2010 1:10 pm

OK.. Limiting the speed to paintball speeds is a good call. The small chamber will ensure rapid reloads in the field.
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Unread postAuthor: metalmeltr » Tue Dec 21, 2010 1:11 pm

omniscient wrote:Why...
1.) The ridiculously small C:B ratio?
2.) Is your piston mass entered as 22 oz. ?

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

I used this to model the valve
i now see it said 22 gms not ozs

the small chamber is so it can quickly be filled by an integral electric pump similar to this-http://www.spudfiles.com/forums/my-first-cannon-ever-with-on-board-compressor-t21937.html
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Unread postAuthor: jimmy101 » Tue Dec 21, 2010 6:16 pm

I believe in GGDT you can set the graph's x-axis to be the ammo's position in the barrel then set the y-axis to velocity. Give GGDT a barrel longer than you think you need and it should give your a decent prediction of the barrel length versus performance trade-off.
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Unread postAuthor: metalmeltr » Tue Dec 21, 2010 6:23 pm

Well I'm designing for two factors

Tacticallity-apparently thats not a word- basically a short gun for easy maneuvering-room clearing

Pneumatic performance-less important with short ranges and light projectiles

Just looked at last GGDT run-wont work for me-40" barrel-looking at about 25"
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Unread postAuthor: jackssmirkingrevenge » Tue Dec 21, 2010 6:32 pm

metalmeltr wrote:Tacticallity-apparently thats not a word- basically a short gun for easy maneuvering-room clearing


With a marsmallow launcher :D Wouldn't you be better off with a shotgun or submachinegun style launcher for that purpose?
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