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New Guy, want ideas for a pnematic tennis ball mortar

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: Blitz » Mon Aug 05, 2013 7:10 pm

Sometimes, yep! But often the fun for me is just throwing something together. :)
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Unread postAuthor: duane » Mon Aug 05, 2013 7:56 pm

dart guy wrote:You should have an air chamber connected to a ball valve, then you should have the ballvalve connected to a spring loaded trigger. This will release air quicker.


Dart,
I'm not sure that I understand you correctly.
Are you talking about using an air cylinder to quickly open a ball valve?

I've not had any experience with fast acting ball valves, but I have had plenty of experience with manually operated ball valves that we intentionally opened slowly, to very slowly pressurize something.

They do provide full flow when fully open, while still letting you slowly open them to gradually get up to full pressure/flow. But even as fast as you can operate them, they still open fairly slowly compared to some other types.

For FAST actuation, we've used poppet valves, diaphragm valves, spool valves, etc. Many of them are all way too expensive for my hobby budget. A simple sprinkler valve is IN my budget.

Granted, my experience has been in an industrial environment, where opening valves SMOOTHLY, sometimes thousands of times a day, without any sudden action is of a prime concern.

I don't think that is the case with spud gun valves. Seems like they need to open VERY fast. Admittedly, I'm out of my element here, but I'm trying to learn.

Educate me, please.

Thanks,

Duane 8) 8)
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Unread postAuthor: Technician1002 » Mon Aug 05, 2013 9:30 pm

Educate you.. Ok Most ball valves on a good day opened by hand open fully in the 100mS or slower range. A good piston opens fully in under 10 mS.

Flow when open is a very important factor which is based on what the air has to go around when the valve is open. A ball valve exceeds the flow of any sprinkler valve when open due to the straight through design, but as mentioned above it has a serious speed trade off. Which is better? depends on the size of the cannon, barrel length and projectile weight. Ball valve for large heavy projectiles that launch relatively slowly, and sprinkler valve for shorter barrels with lighter projectiles.

A home made piston valve, can provide excellet flow at reasonable speed and are the valve of choice for serious air cannon builders.
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Unread postAuthor: dewey-1 » Tue Aug 06, 2013 8:06 am

duane;
Have you considered using a 5 pound fire extinquisher as a chamber?
It is about 14.5 inches and 4 inch diameter.
http://sketchup.google.com/3dwarehouse/ ... revstart=0

A 3/4 inch QEV may actually be a better choice than a 1 inch sprinkler valve!

Welcome to the forum and by the way my name is Duane as well!
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Unread postAuthor: duane » Tue Aug 06, 2013 5:58 pm

Thanks for all of the encouragement and advise guys.

I've played with GGDT, last night with imaginary numbers, and this afternoon with some more realistic numbers that I was able to to track down. (I know they're not perfect numbers yet, but much better than I had guessed at last night)

I know that GGDT is not 100% accurate, but that's completely O.K. A computer model that's "only" 5-10% accurate beats the heck out of spending money and time experimenting.

It lets me change one variable at at time, and see the results, without spending a dime.

And it's totally busted some ideas that I thought that I "knew".

Thanks so much.

Duane
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Unread postAuthor: duane » Tue Aug 06, 2013 6:07 pm

dewey-1 wrote:duane;
Have you considered using a 5 pound fire extinquisher as a chamber?
It is about 14.5 inches and 4 inch diameter.
http://sketchup.google.com/3dwarehouse/ ... revstart=0

A 3/4 inch QEV may actually be a better choice than a 1 inch sprinkler valve!

Welcome to the forum and by the way my name is Duane as well!


No, I haven't consider using a fire extinguisher bottle.
But it does give me some ideas.

Thanks for the welcome.

Duane
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Unread postAuthor: duane » Thu Aug 08, 2013 6:47 pm

With you guy's advice, and after playing with GGDT a while, I've came up with a design for my tennis ball mortar.

Unfortunately, I have not found a local source for many of the components that I'll need, so I've got them ordered.

I'm sure to have missed a couple things, that I can hopefully find locally, but the parts on order come to less than $100.

The whole project looks to be at about $125.

Not bad for a project that'll keep me busy for months. Cheap entertainment, huh? 8)

Duane
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Unread postAuthor: Gun Freak » Thu Aug 08, 2013 7:27 pm

What sort of design did you decide on?
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Unread postAuthor: duane » Fri Aug 09, 2013 5:32 pm

Gun Freak,

I don't have an image to share yet, so I'll just have to try describing it...

(now is the time for you to put on your IMAGINATION glasses)

It's a barrel over chamber setup, using 2 1/2" SHD 40 PVC for both the barrel and the chamber, along with a couple SHD 40 PVC 90 degree fittings, etc.

The main chamber will be 60" long, then 2 90 degree fittings, a reducer, then a 1" Orbit sprinkler valve, another reducer (enlarger), and then a 2 1/2" barrel, roughly 55" long. (what's left from a 5' stick, after cutting off a couple stubs, for the fittings).

For GGDT estimation purposes, I'm using 70" for the total chamber length, and 55" for barrel length.

I'll mount it on a wooden frame, at "about" 45 degrees. It's supposed to be a tennis ball MORTAR, remember?
8)

My plans are to use 75 PSI CO2 as a gas source. I'm sure that we'll play with the pressure, and the angle, once it's built.

Thanks,

Duane
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Unread postAuthor: Blitz » Fri Aug 09, 2013 5:39 pm

Dang, that's a long chamber for 2.5" diameter pipe. That's a pretty tall mortar. :) Well, get started! You ought a be able to knock out that build in an hour or two, excluding.the mount.
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Unread postAuthor: duane » Fri Aug 09, 2013 6:20 pm

Blitz,

You're right. I'll model it again with GGDT before I build it. I've got 3-4 days before the Brown truck shows up.

Thanks,

Duane
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Unread postAuthor: Blitz » Fri Aug 09, 2013 6:49 pm

Yeah I'd see what effects shortening your chamber would have. That's an awfully large volume of air to fill up for what may be negligible results.

Also, don't forget that a tennis ball is inherently not a very effective projectile for distance, with the fuzzy covering and all. Its terminal velocity isnt too stellar so there's only so much you can do. :)

But I'll let someone who isn't dumb like I am have the final word.
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Unread postAuthor: duane » Fri Aug 09, 2013 7:34 pm

Blitz,

Just for fun, I scaled down the chamber length, and the barrel length by 50%. I had to increase the pressure by about 30% to get the same, or a little more muzzle velocity.

Hmmm.... I do want a Mortar, not a Howitzer. :D

I'll keep playing with the numbers.

Thanks for the inspiration.

Duane
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Unread postAuthor: Blitz » Fri Aug 09, 2013 7:45 pm

Did you try scaling down the chamber without changing.the barrel? :)
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Unread postAuthor: Gun Freak » Fri Aug 09, 2013 8:57 pm

Since you're working with a fixed amount of pipe, try getting the chamber as short as possible and barrel as long as possible. The barrel should, at the very least, be longer than the chamber, optimally a foot or two. Play with GGDT.
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