first piston valve

[Gun Freak]

You can get SDR 26 at Home Depot/ Lowes but I don't think they carry SDR 21. You would probably have to get that at a plumbing supply.

[kotabuuki]

alright, I think i need to take a golfball with me next time i go shopping. ugh, i can't wait to get it assembled.

[Technician1002]

Is there an optimal C/B ratio for piston valves?

There are many optimal C/B ratios, not just one. The optimal will vary with pressure, valve CV, and projectile mass. Barrel geometry plays a role too. A 100 foot 1/2 inch barrel is not going to have an optimum chamber size at all because the length to diameter of the barrel is too restrictive to properly flow. The barrel length would need optimized first, then a chamber engineered to match.

Some of the variable relationships are;
lower pressure use larger chamber

smaller diameter barrel, use higher pressure and smaller chamber.

Short barrel use high pressure and smaller chamber.

Small valve (poor flow) use smaller chamber and higher pressure to compensate. High flow valve, use larger chamber.

Optimum is a fuzzy size. A point of diminishing returns is reached. Double the volume and the performance increases, but by a small amount. Optimum is smaller if you have a limited air supply such as a hand pump instead of a compressor.

[kotabuuki]

Optimum is smaller if you have a limited air supply such as a hand pump instead of a compressor.

could you define your idea of a small chamber? i figure that since you have been in the community longer than i have you most likely have a different idea of what small is. i was planning on making my chamber out of 3" pipe and 25"-30" long. is that a small medium or large?

I have a 3 gallon air compressor, with an additional 5 gallon tank that i can hook up to it, h=giving me about 8 gallons of air. additionally my compressor only goes up to about 110. some clarification would be appreciated. thanks

[Technician1002]

I would recommend picking up a copy of Gas Gun Design Tool (GGDT) which is an excellent simulation. An example of a small chamber reasonably matched to a barrel is my Marshmallow Cannon with a 2.5 inch t shirt barrel. It uses very little air per shot but is still able to launch shirts ~200 feet.

Here is an example of what I consider a small chamber and large barrel. The valve opening is 1 inch.


The chamber volume is much smaller than the barrel volume. Due to the expansion of air, it is very efficient. Higher power at lower efficiency is the result of making the chamber larger.

This on the other hand will launch stuff, but will fail to launch at low pressure as the air does not expand enough. This combination launches much slower than the short barrel at ~100 PSI. If used at ~300 PSI, GGDT predicts this would do much better than the short barrel. I have not tested this at 300 PSI.


This takes much more air to operate as the chamber is 700 Cubic Inches instead of 54. At 100 PSI it is very powerful. When playing with this launcher much of the time is spent waiting for the compressor to catch up. With the larger 2 inch valve and chamber it has very respectable power at low pressure. This design was able to launch t shirts into the 300 section in a basketball stadium. It was built for an engineering competition. It won the competition.


You can hear the one the school made for the competition here. This shot is at about 35 PSI. At the very end of the video you can see me with the prototype I built.
[youtube][/youtube]

[kotabuuki]

every time i watch that video i want one :p

I have GGDT, but it doesn't work on my computer for some reason. which is odd, because my computer is less than a year old, It's an asus with windows 7 home, and i5, 4gigs of ram, ect. but the program says

Run-time error '339'"

Component "MSCHRT29.OCX' or one of it's dependancies is not correctly registered: a file is missing or invalid

which is weird, because no matter how many times i delete and try to un-install it it says the same thing. I tried to install it on my brothers 4 year old computer and it worked first time, unfortunately he doesn't like me on his computer. so i don't know what's wrong with it.

[Technician1002]

A lot of people have had problems with Windows 7 and GGDT. Somewhere there is a discussion about it and a fix. See if you can search it or maybe one of the other members will link to it.

[Labtecpower]

I can run it without problems. Just set the (compatibility mode?) on windows XP

[kotabuuki]

-removed due to faulty information-

[seeyouinhell2mf]

what is the tank you used for the school cannon?

[kotabuuki]

what is the tank you used for the school cannon?

I haven't built it yet. I'm trying to figure out what the best tank size is for my build.

[Crna Legija]

idk about windows 7 but on vista just just ave to run as admin.


also you ggdt is set at chamber seal, you probably want barrel seal.

[kotabuuki]

-removed-

[kotabuuki]

so my brother let me use his computer and i started using ggdt. here are my results. I believe that my values are right, but if you see anything out of the ordinary please inform me.

here are is my design. i think i may make the chamber 25" though.


here is what ggdt said the optimum chamber would be for my 48 in barrel, chamber seems rather small. and i don't think that it would be able to propel heavy projectiles.


here is the "optimum" barrel length if i kept my chamber at 30". it is excessively long, and i definitely won't be making this one.





Thanks, and as always input is welcome.

[Technician1002]

The chamber size is about right for a 2 inch barrel and a 1.5 inch valve orifice. A larger tank would still be hissing air out the valve after the projectile is gone. If you increase the valve seat to 2 inch and use a 2.5 inch piston, the better flow would allow higher pressure on the projectile and could utilize a larger chamber.

If you plan on doing a lot of shooting at lower pressure, there is advantage again to a little larger chamber.

After building and then playing with GGDT, I found the 700 cu in chamber on mine is oversize for the job except at very low pressures. This is fine as much of the time it is used at under 50 PSI where it performs very well.

Part of the reason for the large chamber I used is the contest originally included launching a 4 inch foam ball. The chamber was sized for a 4 inch max barrel. For a 2 inch barrel the chamber is oversize. It works well for the 3 inch shirt barrel and 4 inch foam ball barrel.

4 inch foam ball barrel set up for some chronograph shots


When modeling your launcher, model it for a range of operating pressures. This will give you a good feel for a decent chamber size.

Edit;
One common error in using the barrel optimization is it adjusts length for the point the projectile stops accelerating.

The fault with this is due to flow resistance along the length, the muzzle velocity on the "Optimum" length will be much lower than from a shorter barrel that has it still accelerating as it leaves the muzzle.

I directly saw this when checking the best length for a t shirt barrel. A barrel cut to where the acceleration is dropping off will result in higher acceleration and a higher muzzle velocity. Take the optimized parameters and then manually start trying shorter barrels changing nothing else. You will see there is a better optimum length for maximum muzzle speed.

My results are here;
https://inteltrailblazerschallenge.wiki ... rim+method

For our school cannon, we used an old disposable Freon tank kindly donated from the trash at an auto AC and Radiator service shop.


R134a Freon tank

It was slightly modified for higher flow.