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GGDT/HGDT Request Thread

A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 29, 2011 5:42 am

nathanhd123 wrote:I have just found this page on my QEV. Can anyone make sense of it?


There isn't really any relevant data on that page. What are you trying to do though, calculate the performance of an existing launcher or predict the performance of a future design?

I am trying to get some data to put into GGDT as I do not have a spanner big enough to open my QEV.


No vice in the garage to hold the body?
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Unread postAuthor: nathanhd123 » Mon Aug 29, 2011 6:46 am

I am trying to calculate the performance of an existing launcher. Unfortunately I don't have a vice - although someone is giving me an old one soon. Do you have some generic data for a 3/4" QEV?
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 29, 2011 6:54 am

nathanhd123 wrote:Do you have some generic data for a 3/4" QEV?


This should be a reasonable approximation:

valve type - generic
# of valves - 1
Flow Coef. - 5
Seat Diam - 0.75 in
Open Time - 20 ms
Dwell Time - 999 ms
Dead Vol - 1 in3

For some hard data, using a microphone and an audio program that can be downloaded for free, adecent homemade chronograph can be made.
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Unread postAuthor: nathanhd123 » Mon Aug 29, 2011 7:17 am

How do you go about calculating the projectile friction?
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Unread postAuthor: MrCrowley » Mon Aug 29, 2011 7:19 am

nathanhd123 wrote:How do you go about calculating the projectile friction?

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

GGDT wrote:Projectile -> Friction
Most projectiles will experience some friction in the barrel. As the reader may imagine, this variable defines frictional forces in the simulation. However, it should be noted that this value is NOT the coefficient of friction that many with physics training may expect to see. Such data can be difficult or impossible to obtain. Instead, GGDT uses a more practical approach to friction; that of equivelent pressure. Place your projectile in your barrel and blow begin to pressurize the breech. At what pressure does the projectile first begin to move at? Whatever that pressure is, put it in the blank! (Note: Human lungs can produce about 2 psi. If you can use the barrel like a blowgun and get the projectile to move....)


In other words, just leave it as is. Shouldn't make much difference.
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 30, 2011 1:55 am

GGDT instructions wrote:At what pressure does the projectile first begin to move at? Whatever that pressure is, put it in the blank!


This raises an interesting question, maybe D_Hall can weigh in on the matter - does GGDT assume this friction only until the projectile starts to move, or along the whole time the projectile is in the barrel? If it's the former case, it could be used to simulate the addition of a detent to the system...
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GGDT?

Unread postAuthor: Atattack » Wed Aug 31, 2011 5:46 pm

I am designing a water balloon launcher, and am trying to calculate the optimum C:B Ratio for a gun with 72" length. I am using a 4" aluminum pipe for chamber and a 3" aluminum pipe for barrel. I am using a 1.5" solenoid valve with a CV of 29.

I am just surprised by the c:b ratio recommended. I am not sure if I entered info correctly, especially in the valve section. Any help is appreciated. Thanks

Working on getting it pasted in forum, thought I did but can't get it to upload
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GGDT?

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Unread postAuthor: sco77 » Wed Aug 31, 2011 6:03 pm

I hate to hijack thread but I also hate to create duplicate threads...

Why does GGDT give low FPS result with a faster valve open time? I was thinking maybe it meant how long the valve is open, but that's cant be the case either since setting it high also kills FPS.
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Unread postAuthor: Technician1002 » Wed Aug 31, 2011 6:23 pm

The problem with the low FPS is the dwell time. Try again with a dwell time of at least 20ms. It is simulating a hammer valve which opens for 2.5ms and is closed again due to no dwell time. The air in the barrel continues to expand and the power rapidly drops off early.
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Unread postAuthor: Atattack » Wed Aug 31, 2011 6:27 pm

I ran a calculation using a burst disk of 1.5" and a CV of 29 and got a totally different suggestion. Said to use a 21" 264cc Chamber and a 51" 360cc barrel. Totally confused.
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Unread postAuthor: Technician1002 » Wed Aug 31, 2011 7:08 pm

Dwell time on a burst disk is > one second. Once open, it remains open. :D
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Unread postAuthor: Atattack » Wed Aug 31, 2011 7:14 pm

So when I change the dwell time, it says "ms" for the units of measure, guessing milliseconds. So wouldn't a solenoid valve remain open until the gas vents the chamber so wouldn't it be the same as a burst disk for this?
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Unread postAuthor: MrCrowley » Wed Aug 31, 2011 8:44 pm

@Atattack,

Your topic has been merged with the GGDT/HGDT Request Thread. Please continue your discussion here.

Thanks :)
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C:B Ratio for 3" Barrel & Chamber

Unread postAuthor: Atattack » Wed Aug 31, 2011 9:21 pm

After running some calculations on GGDT it calculated that the optimum barrel configuration for a gun with a 3" barrel and chamber would be a 24" chamber and a 48" barrel @ 125psi. I thought that the general consensus was the opposite for pneumatic guns.
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Unread postAuthor: Gun Freak » Wed Aug 31, 2011 9:25 pm

GGDT doesn't optimize the lengths by performance. Rather by efficiency. At a certain point, a larger chamber starts to give back not as much gain, and this is where GGDT cuts it off. Making the chamber larger, however, will still give you much better performance.

This should have gone in the GGDT thread I think.
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