sprinkler valve distance

[Atrocity]

hi, im a little new to all this, i just recently finished my pnuematic cannon and i was test firing it the other day and i wasn't getting the distances i was hoping on achieving. It's an over under cannon with a sprinkler valve activated by a blowgun. The chamber is 2 ft long and 2" thick with a 5ft barrel that is also 2" thick. The valve is 3/4 inch which goes all the way from the chamber to the barrel, you think this is my problem? would going to 1 inch make a significant difference? Im filling to 100psi and im getting about 90 or so yards. Also whenever i load a tater down the barrel its hard to get down there from some pressure and somtimes it just bounces back out, i was thinking of making a small hole at the base of the barrel to let air out when loading. I'll post pics later. Thanks in advance

[maverik94]

I believe that the problem is that you need to make your barrel smaller than your chamber. I have a sprinkler valve rifle that has a 1 1/4" & 1" chamber and a 1/2" barrel, and with 60 psi, I can fire a wooden dowel 100+ yards. The chamber size is not so important, it is more the Chamber-barrel (C:B) ratio. The bigger the barrel is, the bigger the chamber should be. I would suggest at least a 2:1 ratio, meaning the chamber is at least 2 times the diameter of the barrel. For example, with a 1/2" barrel, use at least a 1" chamber. Also, for that, you should use a 1" sprinkler valve. I hope this helped! Have fun! Good Luck and ALWAYS use pressure rated pipe!!!!!!!!!!!!

[King_TaTer]

Yeah I agree with maverick. It seems a little strange that you had a 2 ft long chamber and a 5 ft long barrel of the same diameter. But as you said your new and it's not too bad a thing. What would be bad is if you didn't use pressure rated fittings and/or pipe. If your able to, post a picture of the gun and we could tell you if what you used is pressure rated or not.

Anyways sounds good for a first. Hope too see more from you.

[Technician1002]

The chamber is 2 ft long and 2" thick with a 5ft barrel that is also 2" thick. The valve is 3/4 inch which goes all the way from the chamber to the barrel, you think this is my problem? would going to 1 inch make a significant difference?

Two math things to look at is the maximum air speed through several places. When you double the diameter of a circle (barrel) over a restriction, the area is 1/4, so for the same pressure on the other side, the flow through the restriction needs to be 4X the speed you will get in the barrel.

A 3/4 inch valve has close to 1/2 the cross section of the 1 inch. Do the math to find the cross section area of your restriction. If you can find a 1.5 or 2 inch, you will be impressed.

[D_Hall]

Agree with Tech. A 3/4" valve just isn't up to pushing 2" diameter projectiles of significant mass.

[Atrocity]

Thanks for all the help everyone. tech so what you're saying is that my barrel to restriction ratio should be 2:1? I'll probably just downsize the barrel for golf balls. Now i know what to improve for my next project, thanks.

Heres some pics of my cannon, im pretty sure it's pressure rated:

[sniper hero]

your barrel is too long (or thick)
try ggdt to check for the optimal barrel lenght

you might want to keep the barrel the same and make the chamber bigger:D More POWER

[King_TaTer]

Yup your good. It's all pressure rated assuming those are threaded nipples, bushings, and st. elbows. Which from what I can make out they are. If you want a quick increase in power I would saw off the end cap put a coupler in its place and extend the chamber at least 3 feet or more. Then weld a new end cap on and swap the schrader. This is just one of your many options, this is probably one of the easiest. Good luck

[jimmy101]

Agree with Tech. A 3/4" valve just isn't up to pushing 2" diameter projectiles of significant mass.

Shouldn't that be of "low mass"? As the mass of the ammo increases (or the friction) doesn't the efficiency (flow, opening time etc.) of the valve become less important?

[maverik94]

yeah, I would just put a 3/4" barrel on it, that would GREATLY increase the range/power of the gun.

[Technician1002]

Thanks for all the help everyone. tech so what you're saying is that my barrel to restriction ratio should be 2:1? I'll probably just downsize the barrel for golf balls. Now i know what to improve for my next project, thanks.

Actually a bigger valve is better. 2 inch on a 2 inch barrel if you can do it.

If the valve turbulence limits flow speed to about supersonic, then a 4X area increase (2X diameter) from the valve to the barrel limits the speed in barrel to 1/4 supersonic not counting gas expansion. On my Quick dump valve cannons for fruit and the 2 inch valve I get fantastic performance on the 2 inch and 2.5 inch barrels. On the 1 inch valve, attaching a 1 inch or 1.25 inch barrel provides good results.

On the 1 inch valve I am not considering any larger diameter barrels except for a antenna launcher shooting tennis balls. Extreme power is not needed to toss a tennis ball over a tree.

Here is a view into the valve on the 1 inch launcher with the valve open. The ports have a larger cross sectional area than the pipe diameter. The valve ports are directly inside the pressure chamber.

[D_Hall]

Agree with Tech. A 3/4" valve just isn't up to pushing 2" diameter projectiles of significant mass.

Shouldn't that be of "low mass"? As the mass of the ammo increases (or the friction) doesn't the efficiency (flow, opening time etc.) of the valve become less important?

I believe we're both right. On the heavy end, yeah, valve efficiency doesn't matter as much. But on the (very) light end, negligable pressures can push your projectile just fine (so you don't need much air at all). Inbetween the two extremes.... You have spuds.

[covey12]

what ammo are you shooting out of that barrel, if your only using potatoes make sure there is a perfect fit around them, i suggest using a 1.5" for potatoes and sharpen the edges a little, youll be able to shoot a good distance, and use golf balls aswell

[mark.f]

3/4" sprinkler valves don't have as much flow as 1" sprinkler valves, yes, but you should also know that 1" sprinkler valves don't have that much flow either. A commonly quoted figure is that they have the flow of about a 9/16" hole.

But, still, a 1" sprinkler valve will not only be cheaper and generally easier to modify, but it will also give a lot more flow than the 3/4" valve, even if it is a little restricted.

[Technician1002]

Agree with Tech. A 3/4" valve just isn't up to pushing 2" diameter projectiles of significant mass.

Shouldn't that be of "low mass"? As the mass of the ammo increases (or the friction) doesn't the efficiency (flow, opening time etc.) of the valve become less important?

I believe we're both right. On the heavy end, yeah, valve efficiency doesn't matter as much. But on the (very) light end, negligable pressures can push your projectile just fine (so you don't need much air at all). Inbetween the two extremes.... You have spuds.

I found for very light projectiles the valve efficiency is extremely important. The projectile when approaching the speed of sound is plowing a lot of air, so having lots of pressure behind it at high speed is important. Energy = Mass Times Velocity Squared. If possible, doubling velocity is more important than doubling mass. Valve efficiency matters.

Did you know you can dent car doors with marshmallows?

***EDIT***
Went out and took the dent photo. Dented the door from 15 feet. Full size marshmallow, 1 inch barrel, 20 inches long, 54 cu in chamber.