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GGDT lies

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 lies

Unread postAuthor: VooDooCC » Mon Jan 28, 2008 11:02 pm

So I finally had a day off work that I could test out my cannon without worrying about waking up the other people in the house. The gun is nothing special, I'll post pics when I get my hands on a digital camera again.

Anyhow, I tried using ggdt to figure out the best weight for the projectile, and according to GGDT and using the figures from the the archives it says that nothing will leave the barrel even though I was shooting 20 oz bottles of water about 70 to 80 yards.

the gun has a chamber 2 inch pvc and about 55 inches (including fittings) that I pressured up to 100psi released through a modded sprinkler to a barrel that is 3 inch pvc and 45 inches long
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Unread postAuthor: hi » Mon Jan 28, 2008 11:05 pm

you didnt input something correctly, my ggdt says that you would get about 150 FPS with it, which seems reasonable.
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Unread postAuthor: LikimysCrotchus5 » Mon Jan 28, 2008 11:12 pm

VooDooCC wrote:chamber 2 inch pvc and about 55 inches (including fittings) that I pressured up to 100psi released through a modded sprinkler to a barrel that is 3 inch pvc and 45 inches long


Your barrel is bigger than your chamber? that doesnt sound like that would get very good performance at all. You should change that.

Whats the size of the valve?

I wish i learned how to use ggdt :(
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Last edited by LikimysCrotchus5 on Mon Jan 28, 2008 11:41 pm, edited 1 time in total.
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Unread postAuthor: BrianMigs » Mon Jan 28, 2008 11:26 pm

I wish i learned how to use ggdt


I happen to be in that process right now. Dont worry, it just makes you depressed that your gun isnt as good as it could be, haha
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Unread postAuthor: SpudBlaster15 » Mon Jan 28, 2008 11:30 pm

LikimysCrotchus5 wrote:Your barrel is bigger than your chamber? that doesnt like that would get very good performance at all. You should change that.


I'll let DYI respond to that one...

I'm assuming that you did not enter the proper dimensions into the Chamber ID/OD box. This is a common mistake for first time users of GGDT. For a standard launcher, use an input of 0 for the "Inside diameter" box, and 2.067" in the "Outside Diameter" box.
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Unread postAuthor: VooDooCC » Mon Jan 28, 2008 11:34 pm

That was the other thing I was going to use GGDT to play around with, and I have heard so many different things about the right ratio. So I figured I would just 0.8 like Burnt Latke (I know there's is for combustion)

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What did I put in wrong?
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Unread postAuthor: jackssmirkingrevenge » Mon Jan 28, 2008 11:42 pm

"Inner diameter" should be zero as this is not a coaxial.

here's what I got:
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Unread postAuthor: Ragnarok » Mon Jan 28, 2008 11:49 pm

Yup, as Jack says, your inner diameter needs to be zero, and your outer diameter needs to be 2 inches.

It's a bit counter intuitive, but outer diameter is the internal diameter of the outer pipe in your chamber.

Inner diameter is the external diameter of any barrel that may or may not be running through the length of your chamber. As presumably, you don't have that, you need to set it to zero.
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Unread postAuthor: LikimysCrotchus5 » Mon Jan 28, 2008 11:50 pm

Well dont get me wrong SB15, i know that you can get better performance with that barrel to a certain extent, but it can be much better if you go with a larger chamber than you barrel and increase the size of the barrel somewhat.

You can get more muzzle energy (ft/lbs) from that setup with the current barrel compared to a smaller barrel with smaller ammo which will have less muzzle energy (ft/lbs) and more velocity (fps) for a fixed psi.

Bigger projectiles tend to have more muzzle energy (ft/lbs) and less velocity (fps), while smaller objects tend to have lower muzzle energy (ft/lbs), but a higher velocity (fps).
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Unread postAuthor: jackssmirkingrevenge » Mon Jan 28, 2008 11:56 pm

LikimysCrotchus5 wrote:Bigger projectiles tend to have more muzzle energy (ft/lbs) and less velocity (fps), while smaller objects tend to have lower muzzle energy (ft/lbs), but a higher velocity (fps).


Note that muzzle energy increases proportionally to the weight of the projectile but exponentially with velocity - this means that if you make a projectile twice as heavy for the same velocity, you'll get twice the muzzle energy, but if you throw it twice as fast for the same weight, you get four times the muzzle energy.
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Unread postAuthor: VooDooCC » Mon Jan 28, 2008 11:57 pm

Thank you JSS. It was the id problem and the valve data being wrong.
I feel like a dumbass now, and for the record GGDT does not lie. :oops:
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 29, 2008 12:00 am

VooDooCC wrote:Thank you JSS. It was the id problem and the valve data being wrong.


I don't know what valve you're using exactly so the valve data in my entry was generically guestimated so yours is probably correct, your main problem was the incorrect inner diameter.
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Unread postAuthor: LikimysCrotchus5 » Tue Jan 29, 2008 12:03 am

Yes i see what you are saying but it would take alot of psi to do that sometimes, and it also depends on what your shooting for, but thanks for enlightening me :D

I will have to consider this in my new cannon.
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Unread postAuthor: VooDooCC » Tue Jan 29, 2008 12:05 am

I'm still at the bottom of the learning curve, but I'm trying.

I see you guys talking about velocity and muzzle energy. I noticed that When I only filled the bottles about 3/4 of the way they seemed to fly farther, but that may have been the trajectory of them as well. So for distance should I be aiming more for velocity or more for muzzle energy? Or are the related and you can't increase one without increasing the other?
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Unread postAuthor: LikimysCrotchus5 » Tue Jan 29, 2008 12:08 am

Distance you want velocity, power and force is where you want muzzle energy.

so choose a smaller light weight projectile for distance.

Read my other post again and you will understand. You can get more of one than the other, it depends on the wieight of the projectile and size and how well it will fit down the barrel.
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