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Suggestions for my next long distance potato planter

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: Technician1002 » Thu May 30, 2013 11:08 am

When you go to a large fast valve, the need for high pressure is greatly reduced. When I mentored a team in a T shirt launcher competition, we had a 100 PSI limit. We built big believing that being small and then falling short of the goal is worse than building big and can reduce pressure if the goal is met. The team was launching shirts off the back wall of the upper deck with less than 1/2 the allowed pressure. They used a 2 inch valve and a 3 inch barrel.

Advice.. Go big on the valve and everything between the chamber and barrel. Don't feed a 2 inch barrel with a 1 inch valve.
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Unread postAuthor: izod » Thu May 30, 2013 3:38 pm

Looking at the costs of a good sized dust collector valve i think i'm going to have to look more into the custom built piston valve.

I'm guessing that the same is applied to the piston valve, the larger the valve opening the lower the psi i need..?
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Unread postAuthor: jackssmirkingrevenge » Thu May 30, 2013 4:11 pm

Technician1002 wrote:When you go to a large fast valve, the need for high pressure is greatly reduced.


I would disagree. Pressure has a very direct influence on power. In my younger days I used to think I could match the power of commercial PCP rifles with low pressure just by having an extremely fast valve, but at the end of the day you can't compare 150 psi with 3000 psi.
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Unread postAuthor: Technician1002 » Fri May 31, 2013 9:22 am

I think the creation by D Hall speaks volumes on the subject. For a 2X hybrid, it has plenty of power. In terms of PSI, it is not very high relatively speaking.

In my cannon, there is a roll off on performance gains with pressure increases above 80 PSI. It may be due to SOS somewhere in the system.
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Unread postAuthor: jackssmirkingrevenge » Fri May 31, 2013 12:30 pm

Technician1002 wrote:I think the creation by D Hall speaks volumes on the subject.


Here's a little GGDT simulation for a given launcher, a mid sized coaxial pneumatic.

In the first scenario, it's running at 50 psi with a valve opening time of just 1ms, which gives us a muzzle energy of 204 ft lbs.

Doubling the pressure to 100 psi, and dramatically slowing down the valve to 50ms gives us a muzzle energy of 251 ft lbs, almost 25% more power.

Granted it's not very efficient, less than half that of the launcher at low pressure, but if you're seeking power, moar pressure is usually a good answer :)

That being said, you should aim to optimise all your parameters, so a faster valve is still a very good thing.
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Unread postAuthor: izod » Fri May 31, 2013 1:07 pm

so in a nutshell
big hole + big air + big speed = big distance
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Unread postAuthor: jackssmirkingrevenge » Fri May 31, 2013 2:00 pm

izod wrote:so in a nutshell
big hole + big air + big speed = big distance


Lets look at the various parameters:

barrel length: the longer the barrel, the longer the air will accelerate the projectile. This happens to a certain level though, depending on your chamber size. It is possible to have a barrel so long that after a point the projectile actually starts to decelerate.

chamber size: the bigger the chamber, the more air there is to push the projectile, but again there's an upper limit - once the projectile leaves the muzzle, adding more air is futile.

pressure: Force = pressure x area, so the bigger the pressure in the barrel, the bigger the force on the projectile, so more pressure always equals more power.

valve speed: the faster the valve opens, the faster pressure builds in the barrel, so more force

valve flow: same as above, the bigger the flow the faster the pressure buildup.

You mentioned distance, if you're shooting potatoes then more power is not necessarily better, if the potato disintegrates in the barrel then you're screwed.

If you just want to go further, then optimising your projectile is one of the best things you can do independent of your launcher.

A long thread but worth a read if you really want to push the limits: http://www.spudfiles.com/forums/spudgun ... 22491.html
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Unread postAuthor: Technician1002 » Fri May 31, 2013 11:56 pm

Spuds tend to tumble in flight. They will often change direction while in flight. Fun to watch a long distance shy shot weave and bob. This tumble in flight tends to limit the overall distance to about 150-250 yards max. High pressure has limited gains after a point.
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