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New guy saying hello

A place to ask general spud cannon related questions.
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New guy saying hello

Unread postAuthor: Lee in Texas » Tue Aug 29, 2017 11:13 pm

I found this site by doing a search for "air cannon forum". I recently built my first; a bait launcher that uses compressed air. I'm a plumber and it is made entirely of plumbing parts. I'm here to learn more, though. Things I want to learn are what to use for a bigger air reservoir; one that has a larger opening than the 3/4" NPT on mine. I also want to make a large diameter fast-acting valve, to empty the entire reservoir as fast as possible. Of course the goals are for more power.

Image

The air reservoir is a thermal expansion tank for a water heater. There is a pressure gauge, so it can launch to repeatable distances. Valve is modified 1" sprinkler. The PVC pipe nipple that threads the barrel into the copper fitting got bent after this pic was taken. I have since acquired 1 1/2" copper and PVC fitting that are much stronger and allow the barrel to be removed without tools. It will make transport much less of a headache.
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Re: New guy saying hello

Unread postAuthor: jackssmirkingrevenge » Wed Aug 30, 2017 8:01 am

Welcome to the forum!

Pretty good for a first build, though the first and most obvious way to increase power is to install the valve just before the barrel. Your design has a lot of "dead space", volume where the air from the valve has time to expand and lose energy before it can start pushing the projectile.

Your chamber is probably far too big so most of the air is wasted, what pressure are you running at?
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Re: New guy saying hello

Unread postAuthor: Lee in Texas » Wed Aug 30, 2017 1:51 pm

Hey wow. Great information. In my first thread.

I only went up to 60psi for some test shots. I was using a bicycle pump and its gauge only went that high. I didn't know if I would damage the pump. I made a test projectile that weighed the same as a 6" "ice bullet" that would contain frozen bait. At 60psi, into a strong headwind, it traveled 73 yards. I'm sure it has more in it.

I'll rebuild it soon, reconfigured to move the valve close to the barrel. Thanks for the tip. That's the reason I signed up. BTW I plan to use nitrogen tanks to fill the reservoir. I own 2 of them and refills are cheap.

Thanks again. Lee
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Re: New guy saying hello

Unread postAuthor: jackssmirkingrevenge » Thu Aug 31, 2017 5:52 am

You should be able to double that pressure within the limits of the fittings you're using which will give you a lot more power. Roughly twice as much, actually ;)

Consider using a 2 inch sprinkler valve that will give you much more flow compared to your current setup.

For an air chamber, a section of PVC pipe roughly the same volume as the barrel will be more than adequate and not have the same flow restrictions.

For the ultimate performance without too much extra budget, consider a burst disk setup:

Image

Your chamber and and barrel are separated by a union in which you have a disk of frangible material (like for example aluminum foil) sealing off the chamber. In order to fire, you pressurize the chamber until the pressure become too great for the disk to contain it causing it to rupture. The burst disk is the ultimate "valve", with virtually zero opening time and maximum flow. The disadvantage of this setup is a small delay between starting to feed air and the valve bursting but for a bait cannon this should not be an issue.

Also don't neglect projectile design, a simple cylinder will not travel as far as a projectile made with some attention to aerodynamics. Here's a possibility, assuming you're freezing it, a round nosed projectile with a cardboard "tail" that will stabilize it by drag:

baitprojectile.jpg


Also make it Pykrete style by mixing in some wood pulp to make it more likely to survive the acceleration forces in the barrel.
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Re: New guy saying hello

Unread postAuthor: Lee in Texas » Fri Sep 01, 2017 12:06 am

Whoa. Great ideas. I did not know that sprinkler valves were any bigger than 1". (I'm a plumber but I have never worked for a company that works on sprinklers. Most people don't want to pay what plumbers charge). Great idea on projectile design, also.

I have to confess ignorance with the burst disc idea. I don't understand how it would be better than a fast acting valve very close to the barrel.
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Re: New guy saying hello

Unread postAuthor: Lee in Texas » Fri Sep 01, 2017 3:09 am

I did some more searching and saw your article on burst discs. Fascinating reading. I'm really anxious to build a pneumatic Paris Gun, now.
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Re: New guy saying hello

Unread postAuthor: jackssmirkingrevenge » Tue Sep 05, 2017 6:03 am

Lee in Texas wrote:I'm really anxious to build a pneumatic Paris Gun, now.


This book is one of my prized possessions :)
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Re: New guy saying hello

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Re: New guy saying hello

Unread postAuthor: Lee in Texas » Tue Sep 05, 2017 11:12 pm

Wow. I had never heard of that book. I need to find a copy for myself.
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Re: New guy saying hello

Unread postAuthor: jackssmirkingrevenge » Thu Sep 07, 2017 6:51 am

Prized for two reasons, aside from the subject matter it is the work of the man himself.
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Re: New guy saying hello

Unread postAuthor: Air2theBrain » Mon Nov 13, 2017 10:48 pm

Thats an awesome bait launcher, what a great idea well done 8)
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