SpudFiles

Hello i am new to this site and have a few questions.
I am building a air cannon for my sons school project and to be used at home after.I am currently waiting on the sprinkler valve.I planned on using 2.5 sdr21 at 24'' long.I need to know what size air chamber to use.I am using 4 inch.I have researched c/b ratios and now my head is spinning.So has anyone built one and what size do i use.I see american air cannons has plans but they want 20.00 which i really dont want to put any more money into it.Has anyone bought these plans and would like to share.I also have 20 feet of tennis ball size pipe .I had to buy 20 feet.If anyone would like some and would pay shipping i could probably ship it.
Thanks

simply download GGDT and model your cannon in there and you can mess around with the chamber. http://www.thehalls-in-bfe.com/GGDT/ i'd do it for you but idk what sprinlker valve you have

turner wrote:simply download GGDT and model your cannon in there and you can mess around with the chamber. http://www.thehalls-in-bfe.com/GGDT/ i'd do it for you but idk what sprinlker valve you have

i second this. also keep in mind you pump method bike pump = a lot of work.

ill take the pipe.

Plus, simply look around the Pneumatic Cannon Showcase, there are hundreds of examples to copy.

Also, please stay away from DWV Fittings. It is not pressure rated and could result in injury. Make sure it has NSF-PW and a pressure rating on it. As you are using a 4" chamber, Home Depot etc will not have the reducer to go from that down to your barrel size. You need a reducing bushing, something you might have to purchase from a plumbing store. Do not use a Bell Reducer.

DWV Fittings look like this:

jmadden91 wrote:Make sure it has NSF-PW and a pressure rating on it.

Actually it's; make sure it has NSF-PW or a pressure rating on it. You don't need both. Some pipe has NSF-DWV but has a rating printed on it. That's fine. Some pvc fittings just have NSF-PW on it and no rating, also fine.

So if it doesn't have a rating or NSF-PW, it's probably not pressure rated.

@OP,
I'd help you with GGDT inputs for the sprinkler valve but apparently the Spudtech archive is down and that's where I found the inputs. Maybe check the GGDT website.

edit: I wouldn't expect great performance with a sprinkler valve by the way. Probably less than 200fps and a distance around 200'. A 1" sprinkler valve simply doesn't have enough flow.

I actually am in the process of finishing my tennis ball coax. I used 54 inch 2.5 inch pipe for the barrel and i had a 3 foot 4 inch chamber. This is for a coax though. Do you have any idea of what you want it to look like? Over under? Straight? Coax?

Oh and the performance difference between a 12" and 24" 4" chamber is pretty small, you may as well go with a 12" chamber and save yourself the hassle.

RabidDuck wrote:I actually am in the process of finishing my tennis ball coax. I used 54 inch 2.5 inch pipe for the barrel and i had a 3 foot 4 inch chamber. This is for a coax though. Do you have any idea of what you want it to look like? Over under? Straight? Coax?

tonnor, have a look at RabidDuck's cannon. Construction is a bit more involved but this will certainly give you the edge in terms of performance. A much simpler design that will give you even better performance would be a burst disk design conceptually along these lines, basically the chamber and barrel are separated by a length of frangible material (foil, tracing paper etc). The design I linked to uses a sharpened projectile to burst the disk but for tennis balls you can pump it until the disk bursts, giving you a full flow valve with opening time of practically zero By using foil disks you can increase the number of disks until you reach the maximum burst pressure you can achieve with your pump.

Thanks for the info.I read in this post not to use a bell reducer what is the reason for this?Correct me if i am wrong but a bell reducer would go from 4" to 2'' sort of like a funnel.Is this correct?I have all the parts except the 4" cap and the reducer so i can still buy the bushing instead.I plan on using a 24" 2.5 barrel then a 1'' rain bird sprinkler then 2" elbow then another 2'' elbow then a 4 inch air chamber approx 16 '' .Do you believe this will work pretty well?I am lookiing for maybe 150 to 200.All the fitting i bought i pretty much cant return because the company has a 25 percent restocking fee but he was the only on to carry 2.5 sdr21.Again i will try not to be a pain in the a** with the questions but i dont want to spend the time/money on this and end up with a cannon that doesnt work well.Thanks

tonnor wrote:A bell reducer would go from 4" to 2'' sort of like a funnel. Is this correct?

Yep thats correct. The reason being almost 100% of the time, bell reducers are not rated for pressure.

The guys are right about the 1" sprinkler limiting performance, they just don't have the flow capacity to squeeze all the air from the 4" section through the 1" Valve, to launch through a 2.5" Barrel. Its will be a bit of a bottle neck.

A piston valve (see wiki) will greatly improve performance however if you want to keep things simple I would suggest a larger 2" Sprinkler Valve, just to reduce the bottle necking effect a bit, which will in turn, improve performance.

Before you start buying parts and spending money i would draw up a plan. It helped me a ton when i knew what i needed and had a picture in front of me to see what i had to do. I found that Google SketchUp was easy and fast to use. A lot of people just use paint though.