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ggdt HELP

A place to ask general spud cannon related questions.
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Unread postAuthor: FORE!!!! » Tue Jul 14, 2009 7:22 am

inonickname wrote:
FORE!!!! wrote:
inonickname wrote:It's very optimistic. I'll model it for you..

Be back in a second

It gives me 253 feet per second, though I don't have a perfectly accurate valve opening time figure.


is that what ur guns projectiles travel at? or is that what mine does?

and sorry frozebyte this new proggy is exciting,ill use one of theses(?) from now on lol


That's yours. GGDT tells me I get 955 feet per second at 156.3 ft/lbs. From a 5g projectile.. And then if I use hydrogen, 1449 feet per second at 360 ft/lbs.. Ahh, 400 psi with a super efficient piston valve ported b a 1/2" QEV..what Won't you do? ( next gun)

It will go up or down depending on how fast you open the valve. I put in 200ms as a starting point. Spring loading would definitely give moar.


955 fps omg i think uve just shat on me :( :idea:
is my 254fps respectable for a ball valve design?
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Unread postAuthor: jackssmirkingrevenge » Tue Jul 14, 2009 7:24 am

edit: nevermind
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Last edited by jackssmirkingrevenge on Tue Jul 14, 2009 7:25 am, edited 1 time in total.
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Unread postAuthor: frozebyte » Tue Jul 14, 2009 7:24 am

Sorry but how do you calculate for a coaxial? :(
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Unread postAuthor: FORE!!!! » Tue Jul 14, 2009 7:26 am

jackssmirkingrevenge wrote:edit: nevermind


never mind u beat me to it..
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Unread postAuthor: FORE!!!! » Tue Jul 14, 2009 7:36 am

anyway thanx for ur help guys i can sleep easy now :D
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Unread postAuthor: inonickname » Tue Jul 14, 2009 8:21 am

frozebyte wrote:Sorry but how do you calculate for a coaxial? :(


Barrel sealing piston valve, 45-55% flow co-efficient, seat diameter the same size as barrel, subtract barrel volume from chamber volume (you'll need to work out figures before putting it into ggdt), set it so you can define the volume, number of valves: 1, dead volume 0, initial position 0. Then set your pilot volume and vent size.

nothing wrong with those velocities. With a spring load for the ballvalve you could possibly pump another 50 feet per second in with much more respectable accuracy.
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Unread postAuthor: FORE!!!! » Tue Jul 14, 2009 9:01 am

inonickname wrote:
frozebyte wrote:Sorry but how do you calculate for a coaxial? :(


Barrel sealing piston valve, 45-55% flow co-efficient, seat diameter the same size as barrel, subtract barrel volume from chamber volume (you'll need to work out figures before putting it into ggdt), set it so you can define the volume, number of valves: 1, dead volume 0, initial position 0. Then set your pilot volume and vent size.

nothing wrong with those velocities. With a spring load for the ballvalve you could possibly pump another 50 feet per second in with much more respectable accuracy.


can u explane this spring load for a ball valve any links or pics???
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Unread postAuthor: inonickname » Tue Jul 14, 2009 9:16 am

FORE!!!! wrote:
inonickname wrote:
frozebyte wrote:Sorry but how do you calculate for a coaxial? :(


Barrel sealing piston valve, 45-55% flow co-efficient, seat diameter the same size as barrel, subtract barrel volume from chamber volume (you'll need to work out figures before putting it into ggdt), set it so you can define the volume, number of valves: 1, dead volume 0, initial position 0. Then set your pilot volume and vent size.

nothing wrong with those velocities. With a spring load for the ballvalve you could possibly pump another 50 feet per second in with much more respectable accuracy.


can u explane this spring load for a ball valve any links or pics???


It's essentially using a spring to open the ballvalve rather than by hand. The advantage is two-fold. It opens much quicker, resulting in better performance. Also, it increases accuracy because when you try to open a ballvalve quickly you will move the cannon, losing nearly all accuracy. The spring removes this problem.

All that's needed is a sear to hold the spring back, on release it will open the valve.

It's used on numerous french spud guns.
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Unread postAuthor: Technician1002 » Tue Jul 14, 2009 9:32 am

A quick note on using ball valves on smaller cannons.

A short barrel with a small projectile with a fast valve can complete the shot in time frames in the 10-20 ms range. A ball valve that is opened by hand can take typically 0.1 seconds to open or longer. The result is many launch velocities are less than 1/4 the speed a faster valve would provide.

The very slow opening valve severely limits flow and thus pressure on the projectile. This is why higher pressure, small projectile cannons benefit greatly from a faster valve.

Huge cannons with long heavy projectiles do fine with slower valves. The slower accelerating projectile and much longer barrel give the valve time to fully open early in the launch.
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Unread postAuthor: inonickname » Tue Jul 14, 2009 9:40 am

Yep, larger cannons with heavier projectiles lose out less with a ballvalve. If the barrel is uber long you will even get more power due to the high flow/low turbulence of a ballvalve. But don't be mistaken, that's a very very long barrel (think punkin chunkin, and at that size the valve needs to be operated by a pneumatic ram).
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Unread postAuthor: D_Hall » Fri Jul 24, 2009 10:14 pm

inonickname wrote:
frozebyte wrote:Sorry but how do you calculate for a coaxial? :(


Barrel sealing piston valve, 45-55% flow co-efficient, seat diameter the same size as barrel, subtract barrel volume from chamber volume (you'll need to work out figures before putting it into ggdt),


Uh.... No, you don't need to work out figures before putting them into GGDT. Coaxials is the reason the reservoir input fields include an inner diameter. Normally that value should be set to 0. But for coaxials it should be set to the OD of the barrel. And voila, GGDT will do the subtraction for you.
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