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Pumpkin Cannon Complete Success

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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Unread postAuthor: D_Hall » Fri Oct 30, 2009 11:08 am

SDdan wrote:The tinfoil idea in neat but I will be in a church parking lot and need all the control I can have.


In which case you can use a cascading burst disc design. 'Tis very similar to the already described burst disc design (ie, the "tinfoil idea") but instead of having...

Barrel / Foil / Chamber

You have....

Barrel / Foil / VERY Small Chamber / Foil / Chamber

At that point you fill the small chamber to (say) 3/4 of the pressure required to burst the foil. THen you fill the main chamber to full pressure.

To fire the gun, you can now either:
(a) Fill the small chamber to full pressure (can be done very quickly since it's such a small chamber).
(b) Vent the small chamber to zero.

Voila, a burst disc design that can be easily controlled.
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 30, 2009 11:15 am

in regards to the "oversize" chamber to barrel ratio. At lower pressure the best ratio shifts toward a larger chamber. Plug in the numbers in GGDT at several pressures. With the restrictive valve and it's loss, a 1:1 is about right for 25-35 PSI.


So it would appear... odd.
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Unread postAuthor: Technician1002 » Fri Oct 30, 2009 11:24 am

jackssmirkingrevenge wrote:
So it would appear... odd.


I've been playing with low pressures for an egg toss and the precision golf ball challenge. The ratios change drastically at low pressure. This is the primary reason for the work on sabots for golf balls. The expansion volume is too low to eject the sabot. In a GB barrel the expansion volume is too low to eject the ball. Low distance lobs are impossible without a loose ball in the barrel driven by a sabot or an insanely large chamber.
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Unread postAuthor: D_Hall » Fri Oct 30, 2009 12:30 pm

jackssmirkingrevenge wrote:So it would appear... odd.

Not really odd at all.

Look at it from a static force perspective.

First, lets assume that the lowest pressure one can get useful acceleration out of (in the barrel, NOT the chamber!) is 5 psig, or 20 psia.

If your chamber is say....
150 psig (165 psia), then your C:B ratio to get to 5 psig is 1:8.25.

By contrast, if your chamber is say...
30 psig (45 psia), then your C:B ratio to get to 5 psig is 1:2.5.

Take your chamber down to 20 psig (35 psia)? C:B is 1:1.75.

Note that this totally ignores thermodynamic losses and such, but it illustrates the point that the lower the operating pressure of your gun, the lower the C:B ratio is going to be.
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 30, 2009 12:33 pm

Granted, but in my defense I've never played with such pathetically low pressures :D
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Unread postAuthor: Technician1002 » Fri Oct 30, 2009 12:46 pm

jackssmirkingrevenge wrote:Granted, but in my defense I've never played with such pathetically low pressures :D


I presume you never tried to launch eggs intact or lob a golf ball on a flat on the ground target only 20 yards away. :D Look at all the fun you are missing.. You've done something if you have tossed a raw egg intact 100 yards. :D It isn't easy.
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 30, 2009 12:56 pm

Technician1002 wrote:You've done something if you have tossed a raw egg intact 100 yards. :D It isn't easy.


True... but I'm more of the "destroy an intact raw egg from 100 yards" school of thought, we occasionally have this challenge at the airgun range where it has to be done with your first shot, not that easy either ;)
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Unread postAuthor: D_Hall » Fri Oct 30, 2009 1:05 pm

Technician1002 wrote:I presume you never tried to launch eggs intact or lob a golf ball on a flat on the ground target only 20 yards away. :D Look at all the fun you are missing.. You've done something if you have tossed a raw egg intact 100 yards. :D It isn't easy.

I'm guessing there are severe restrictions on sabots?
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Unread postAuthor: Technician1002 » Fri Oct 30, 2009 2:08 pm

D_Hall wrote:
Technician1002 wrote:I presume you never tried to launch eggs intact or lob a golf ball on a flat on the ground target only 20 yards away. :D Look at all the fun you are missing.. You've done something if you have tossed a raw egg intact 100 yards. :D It isn't easy.

I'm guessing there are severe restrictions on sabots?


You are limited on the acceleration forces by the bubble in the egg. It places non uniform pressure inside the shell on launch. This limits practical launch pressures. Getting the range at low pressures is the challenge. You don't always get to pick the eggs for thick shell varieties. Sometimes they are provided at the contest to prevent cheating.
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Unread postAuthor: D_Hall » Fri Oct 30, 2009 2:11 pm

Can you inject water into the bubble? An insulin needle should do the trick and if you do it gently you won't even damage the egg's membrane.
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Unread postAuthor: Technician1002 » Fri Oct 30, 2009 2:37 pm

D_Hall wrote:Can you inject water into the bubble? An insulin needle should do the trick and if you do it gently you won't even damage the egg's membrane.


Any alterations to the egg is prohibited on most contests. This often means no paint or decorations other than a possible sharpie to id one at the landing site. Injection is not permitted in most contests. The egg and only the egg is permitted to make the trip.
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Unread postAuthor: SDdan » Sun Nov 01, 2009 11:12 am

Well 990ft Not bad at all 50 psi

Thanks Dan
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