Big Stuff

[Jimmy K]

I am making a kinda large bore (maybe 4 or 5 inch by about 10 feet) trailer mounted pneumatic cannon with computer controlled tilt and pan capabilities. With a barrel size this big, what kind of air supply should I use/get? I was thinking maybe an old 60 gallon air compressor tank with one of these valves (http://www.spudtech.com/store/index.php ... 7cb3c6b574) at maybe 100 psi. Would that be sufficient? Is volume or pressure more important in this case?

http://www.spudtech.com/store/index.php ... 7cb3c6b574

Edited by jrrdw, made link active/clickable.

[Technician1002]

The chamber size is fully dependant on what pressure and final performance you want. For low pressure so you don't explode pumpkins or soft mellons you will want a larger chamber than if you are trying to simulate cannon fire with a low volume high pressure chamber.

What is your intended use?

GGDT (Gas Gun Design Tool) may help with the design to find the minimum chamber size for the accelleration you need for your application.

[Jimmy K]

I'm going for range for this cannon. My tract of land is about a half mile long so I'd like it to go at least that far, but be able to go further if I took it to a larger farm. So you're saying low pressure high volume? Do you think a 10 foot barrel would be too short?

[Technician1002]

GGDT which I mentioned has a balistics calculator so you can get an idea of the projectile range with a set of cannon parameters. The projectile will have a huge influence on the distance.

For example, I launch marshmallows a couple hundred feet. I launch golf balls out of sight.

Trying to match a barrel to an unknown projectile, pressure, valve, etc is pretty much not possible.

Is this a pumpkin cannon?

[Jimmy K]

Yep, pumpkins, and maybe some squash, zukes, or watermelons, depending on shape. However, I might downgrade and use bottles, depending on cost.

[Ragnarok]

I was thinking maybe an old 60 gallon air compressor tank with one of these valves at maybe 100 psi.

You'd actually be better off with the Mauler valve from LaunchPotatoes - and I don't say that because they're a site sponsor. I say that because the Mauler is both more powerful and cheaper (not that it really sounds like this is a tight budget project).

And actually, on the note of valves - for a 4 or 5" diameter barrel, and a chamber of that size, I'd really recommend dual valves (or if you felt like it, triple valves could be a laugh) - only having a single 2" valve will quite dramatically limit the airflow and thus performance. It'd be a waste to put all of the rest of the effort in and not get the full potential out of such a project.

[Fnord]

(ninja'd by rag...)

From 1:1 - 2:1 chamber to barrel is typical for a pneumatic. after 4:1 the returns really start getting low.

2" porting is kinda small for a 4" barrel. If you don't want to make your own try to find one on ebay or localy.


long liink

Pay attention to what he said. You get 2, one is damaged. You might be able to fix it.

[Jimmy K]

How would 2 or more valves be set up to go off simultaneously?

And would a steel or aluminum barrel be better than PVC for something like this? I can get 10 foot sections of 4" sch 40 aluminum pipe for $180

[Ragnarok]

How would 2 or more valves be set up to go off simultaneously?

Just plumb them into the same trigger mechanism. They might not be perfectly synchronous, but because of the physics of the way piston valves work, once one has opened, the others will follow very quickly.

And would a steel or aluminum barrel be better than PVC for something like this?

Yes. The increased stiffness alone would be a huge advantage. Aluminium is a pretty good barrel material as things go.

[Fnord]

With aluminum you could crank up the pressure to what those big valves can handle. With pvc I wouldn't go past 120 or so. Even sch 10 aluminum should be fine for a barrel.

[Jimmy K]

These people also sell 0.125" thick aluminum pipe. Seemed way to thin to me, but its incredibly cheap. Is this a plausible option or should I just stick with the sch 40?

[dewey-1]

These people also sell 0.125" thick aluminum pipe. Seemed way to thin to me, but its incredibly cheap. Is this a plausible option or should I just stick with the sch 40?

SCH10 aluminum pipe is nominal .125" wall.
Actual wall is .120"

See McMaster Carr

[Technician1002]

Depending on how long you want to wait for a chamber to pump up, a smaller chamber may do the job. I have a 4 inch barrel for my 700 cubic inch chamber. With lightweight projectiles, I can exceed 500 FPS at 100 PSI.

[Jimmy K]

For a chamber I was thinking of an old 60 gallon vertical air compressor tank. So you're saying that would be too big? I want power and it just seems like a 30 gallon tank wouldn't do it for me. Am I wrong?

[Ragnarok]

I want power and it just seems like a 30 gallon tank wouldn't do it for me. Am I wrong?

Well, a 10 foot long 4" diameter barrel will have volume of around 25 litres (you'll have to excuse my using metric - I can't think in US gallons).
A 30 US gallon tank is 113.5 litres. So that's a chamber to barrel ratio of about 4.5:1.

I doubt there's a launcher out there which can actually make effective use of that kind of C:B ratio. Doubling to a 60 gallon chamber from there would just make it take twice as long to fill, with no discernible improvement in velocity.

And as you're going to be using a sub calibre valve, this goes even more so. There just won't be the airflow to take advantage of a larger chamber.
As I have said on previous occasions, it takes a really good valve for there to be much point in more than a 1:1 ratio, and even with such a valve, once you've gone past 2:1, you really are in the realm of diminishing returns.

If anything, I'd suggest that you looked for a smaller chamber still. Even 15 or 20 gallons would be more than enough. Performance will be near identical and amongst the benefits, it'd take less time to fill and would cut down on noise levels.

And on the note of that second one, even if you're one of those types that "want big boom", this thing will still be teeth-looseningly loud.
Not quite as so as if you were to use a bigger chamber, but is it really worth increasing fill time just for the sake of making the thing more unpleasantly loud?