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Basic Pneumatic Cannon Queetions

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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Basic Pneumatic Cannon Queetions

Unread postAuthor: BeaverRat » Mon May 24, 2010 8:05 pm

Ok - just to start it off, I did a search on these first... I am going to make a little transition to Pnuematics (they are quieter).

1. Does the C:B ratio matter on Pneumatics, if so, what is it?
2. Is there a proper ratio of barrel diameter to chamber diameter?
3. What is a good length for the pressure chamber (3/4th inch barrel)?
4. What else should I know before I make my first pneumatic? It will have a 3/4th inch barrel and will use a modded 1 inch sprinkler valve.

Thanks
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Unread postAuthor: 12332123 » Mon May 24, 2010 8:25 pm

Essentially, every choice you make is going to be a compromise; the greater the C:B ratio the greater the power, but also the greater the noise + impracticality and you face the law of diminishing returns - you're probably going to want it between 1:1 and 2:1 obviously depending on pressure.


Just download GGDT and play around with it until you get what you want.
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Unread postAuthor: Col. Kurtz » Mon May 24, 2010 8:31 pm

Pressure chamber should be whatever will best contain the total amount of air at the right pressure. If you're using a compressor, that pressure is 150-180 PSI, a manual pump may get you to 120 at the highest. So it depends on what kind of barrel to pressure tank ratio is, and on what you're ideal PSI is for the barrel and the object(s) you're firing. PVC is cheap, I'd just do a few basic small scales and see what ratio you like. GGDT also seems good, but, alas, I use a Mac, so I've never tried it.
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Unread postAuthor: Technician1002 » Mon May 24, 2010 9:31 pm

Lower pressure will most definately require larger chambers. The air in the chamber must be able to expand to completely replace the air in the barrel at a minimum. I like to have enough so as the projectile leaves the barrel, it was still under pressure and thus still accellerating.

Remember that expanding gas cools. This results in a pressure drop. If you want to adjust your power over a wide range, a larger chamber is desirable. If you have to hand pump your chamber in the field, a smaller chamber is desirable.
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Unread postAuthor: BeaverRat » Mon May 24, 2010 10:08 pm

I will be hand pumping it and using a 3/4th inch barrel. What size diameter pressure chamber should I use and how long should it be in relation to the barrel? Remmember, I am using a 1 inch sprinkler valve.

In other words, how should I build the best cannon possible with a 3/4 inch barrel? It has to be very powerful...
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Unread postAuthor: Technician1002 » Mon May 24, 2010 10:57 pm

For hand pumping, I recommend designing for the 60-100 PSI range. For that pressure range a 1:1 volume ratio works very well. My mouse musket ( a piston coaxial ) used that ratio with excellent results. I could launch 25 cent gumballs through 1/2 inch plywood. A sprinkler valve will yield slightly lower results.

On chamber shape, a short fat chamber performs better than a long skinny one. May I recommend a 2 inch diameter chamber necked to a 1.5 inch sprinkler valve necked to a 3/4 inch barrel? This gives the best performance ( you seem to want performance ) out of these components.

I'll let you do the math to find the length of 2 inch pipe with the same inside volume as your 3/4 inch barrel. This will make a nice compact launcher with higher strength (less leverage to break fittings at the valve.)

The larger diameters in the chamber and valve results in slower flow (higher volume) in these high pressure loss areas resulting in more pressure delivered to the projectile. Definitely mod the valve.
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Unread postAuthor: Lockednloaded » Mon May 24, 2010 11:18 pm

first off, pneumatics, though the idea is that they are quiet, can be much louder than your average combustion. My SV gun sounds somewhere between a .22 and a glock at 200 psi. If you are going for power and quietness, build as long a barrel as it is convenient to you, and go for maybe a 1:1 maximum if you want to keep the noise down.
I would further mod my SV if I were you if you want power. My super modded valve at 200 psi in a short 3/4" barrel packs a punch rivaling my piston cannon
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Unread postAuthor: BeaverRat » Tue May 25, 2010 12:14 am

I thought that sprinkler valves should not be brought above 125 PSI or 150 max?

Anyway... thanks guys, those responses really helped me and I already have a design working out.

One question, my local hardware store only carries a max sprinkler valve size of 1 inch. Is that too small, if so where can I find a 1.5 inch model?

Thanks
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Unread postAuthor: Technician1002 » Tue May 25, 2010 12:47 am

One inch is fine. Going larger than the 3/4 inch barrel is a good idea. 1.5 inch is better if you can find it.

For pressure, he is hand pumping it. Most hand pumped stuff is in the 60-100 PSI range except very small chamber stuff that is pumped with a shock pump. A sprinkler valve is not used in those designs.
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Unread postAuthor: BeaverRat » Tue May 25, 2010 8:39 am

Ok, One last question...

I will also be building a cannon with a 2 inch diameter barrel for spuds. What size diameter pressure chamber should I use for this, 4 inches diameter 12 inches long?

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Unread postAuthor: Technician1002 » Tue May 25, 2010 10:08 am

2 inch is a little large for most spuds. They won't make an airtight seal unless you find bakers. 1.5 inch is the most common size for spuds. 3 or 4 inch works well. Depending on the design pressure, again, go by volume.

The volume of a cylinder is the area the cross section times length.

Area of a circle is pi times radius squared.

Example. 2 inch pipe has a 1 inch radius. 1 inch squared is one inch. 1 inch times pi = 3.14 square inches. If it is a foot long, that would be 3.14 times 12. The answer will be in cubic inches.

Find the voulme of your barrel and then calculate the chamber volume to give you your desired volume ratio.

edit; Played with the math. A 3/4 inch barrel 36 inches long has a volume of about 16 cubic inches. For a 1:1 ratio a 2 inch chamber will only need to be about 5 inches long. As you can see instead of using 3/4 inch pipe a yard long, using 2 inch pipe less than a foot long would make a nice compact launcher. All my recent launchers use a short fat chamber. The chamber on mine are over double the valve seat diameter. This ensures the chamber can be less than 1/4 the barrel length and still have the same volume.
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Last edited by Technician1002 on Tue May 25, 2010 11:39 am, edited 3 times in total.
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Unread postAuthor: saefroch » Tue May 25, 2010 10:11 am

pi*r^2*l = V

You'd need the ID of the chamber, ID of the barrel, and the length of the barrel, follow suggestions above. Don't forget that the chamber volume isn't just the pipe you use for the chamber, it extends a little beyond that.

Hand pumping can reach higher pressures than 100 psi, I've reached at least 200 with a nice aluminum bike pump. Just saying, though hand pumps are somewhat whimpy, they can handle pressures over 150 psi. Just be careful with going past the end of the gauge.
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Unread postAuthor: BeaverRat » Tue May 25, 2010 9:53 pm

Ok, here is what I have...

2 inch pressure chamber (about 9 inches long)
1 inch Sprinkler Valve
3/4th inch barrel (60 inches long!)
I have a C:B ratio of 1:1
I modded the Sprinkler Valve with a air gun...

I believe that I am good to go! Im looking forward to trying it tommorow when the glue dries.

Here is a question - I didn't mod the actual air gun except for taking out the nozzle. Will I still see a performance gain over just a standard unmodded sprinkler valve?
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Unread postAuthor: Crna Legija » Tue May 25, 2010 10:31 pm

yea the pilot volume on sprinkler valves are small so it will be plenty,but if you have lots of hose connecting it thats not good
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Unread postAuthor: Technician1002 » Tue May 25, 2010 11:47 pm

Start testing at about 40 PSI and work up to 100. I think you will like it.

1/4 inch ball valves make excellent triggers too.

Eagerly waiting photos in the cannon showcase. :)

1:1 works best at pressure above 20 PSI. Lower pressure will require a larger chamber if you decide to toss eggs. ;-) I've gotten some raw eggs out about 200 feet unbroken until they landed.

Edit, When this is working your first mod may be to add other screw on barrel sizes. It widens your ammo choices. My little 1 inch marshmallow cannon has barrel sizes from 1/2 inch to 1.5 inch and with an adaptor, a tennis ball and t shrt barrel.
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