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need help designing a line launcher to help with my job

A place to ask general spud cannon related questions.
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need help designing a line launcher to help with my job

Unread postAuthor: Cableguy » Sun Oct 02, 2016 7:57 am

Hello good people!
My job requires running fiber optic lines over great distances in rough terrain. A device that could launch a string 500-600 ft would be a great help to me.
I built my first pneumatic cannon last week and although it wasn't as powerful as I had hoped, the results were encouraging.

It has a 4 by 8 inch 80 sch chamber and a 15 by 2.5inch sch 40 barrel. It also has a modified sprinkler valve with a push button trigger. To hold the line, there is a removable "shoot through" bow fishing reel mounted at the end of the barrel At 80 psi With a string attached the (unweighted) tennis ball only went about 100 ft. This launcher will be helpful when I have to get a line over smaller obstacles like trees and houses. But I am going to need a much more powerful launcher for the big spans.

Any suggestions on a design that can accomplish my goal of sending a line over 500 ft will be greatly appreciated.

I'm thinking water bottles may be a good projectile. They're heavy enough to carry the string but if I accidentally hit a transformer hopefully the bottle would explode instead of the unintended target.
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Re: need help designing a line launcher to help with my job

Unread postAuthor: Juggernaut12121 » Sun Oct 02, 2016 12:17 pm

What size is the sprinkler valve? If it's one of the smaller 1'' ones then I think the flow is going to be pretty constrained (I bet the projectile leaves the barrel before the chamber actually empties). I would try and make a piston valve if I were you, I'm not sure what fittings you have available but a 2'' piston valve should be sufficient.
Here are some tutorials and pages to show you how one operates:
spud_wiki/index.php?title=Piston_valve
http://www.instructables.com/id/Golfbal ... /?ALLSTEPS

Plus, a piston valve cannon isn't too far off material wise, as you could use your current chamber and sprinkler valve for the pilot, although you'd probably need a longer barrel.
Hope this helped!
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Re: need help designing a line launcher to help with my job

Unread postAuthor: jimmy101 » Sun Oct 02, 2016 2:30 pm

Download Dave Hall's Gas Gun Design Tool (GGDT) from
http://www.thehalls-in-bfe.com/GGDT/download.html
and play around with it. It has a ballistic trajectory calculator (under the "Tools" menu) that will estimate the range for a given gun and ammo. If you have any questions on how to use it ask on these forums. A quick calculation with your basic setup (had to guess at the valve parameters) 80 PSIG, tennis ball, can't model the affect of the string, ... max range should be almost 300 feet.

(Hint "Outer Diam" is the ID of the chamber, "Inner Diam" is normally set to zero)
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Re: need help designing a line launcher to help with my job

Unread postAuthor: Cableguy » Sun Oct 02, 2016 10:30 pm

Thanks for the suggestions fellas!
The piston valve is something I may have to work up to. It seems a bit complicated to me, with my current level of experience!
The valve on my launcher is indeed 1". Would the ideal valve diameter be the same as the barrel's?
My laptop crapped out on me so I've been reduced to using my phone for all of my online errands and such. I wasn't able to open the link to David hal'sl ggdt. From what I've read sounds like the pressure chamber should be 60-80% of the volume of the barrel.

I'm thinking for my next attempt I will try to beef up the original design. Go to a 24"by4" pressure chamber and a sdr21 barrel , to cut down on tennis ball drag, and maybe use a 2" ball valve instead of the 1" sprinkler valve. Are sprinkler valves really that much more efficient than ball?

Thanks again for the help!
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Re: need help designing a line launcher to help with my job

Unread postAuthor: jimmy101 » Mon Oct 03, 2016 2:56 pm

I think the 1" sprinkler valve is going to work better than the ball valve. Even if you don't mod the sprinkler valve.

You can estimate the performance (though GGDT would do a much better job) just based on your barrel length and target muzzle velocity. Say a 3' barrel with a target muzzle velocity of 300 fps. The time for the ammo to transit the barrel is the barrel length divided by the ammo's average velocity, which for simplicity we just assume to be half the muzzle velocity. So it takes 3'/(300fps/2)=20 milliseconds for the ammo to move through the barrel.

If you can open your valve in much less than 20 ms (say 2 ms) then the valve opening time wont affect the performance since the valve will be fully open before the ammo has even moved much. If you design a super-duper valve that'll open in 0.2 ms it wont perform noticeably better than the 2 ms valve. (GGDT is great for doing these kinds of calculations)

If it takes 20 ms to open the valve then the valve will reduce air flow during most of the actual launching of the ammo, which will decrease performance somewhat.

If it takes significantly more than 20 ms to open the valve then the valve is always limiting air flow and the pressure behind the ammo probably never reaches the maximum value, greatly reducing the gun's performance.

Opening a ball valve fast, say in less than 20 ms, is pretty hard to do. At that speed you may well shatter the valve or the valve's lever when you hit the open-stop. The force required to open the valve that fast may make it very hard to control which way the gun is pointing when it is fired. Certainly, if you use your hand to operate the valve, and try to do it fast, your ability to actually aim the gun is going to be limited.

If your target muzzle velocity doubles then the valve needs to open in half the time. So for a 600 FPS muzzle velocity and a 3' barrel you want the valve to open in less than 10 ms.

The bigger the opening in the valve the less the valve opening time will matter. Some valves (like sprinkler valves) have flows that are pretty much linear with how far open the valve is. Ball valves have very non-linear openings as a function of the handle position and they have pretty low flow early in their opening cycle.

For reference, a typically sized combustion gun with a 3' barrel and firing 2" diameter spuds at 300 FPS muzzle velocity takes a few tens of ms for combustion and that slow combustion rate limits the performance.
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Re: need help designing a line launcher to help with my job

Unread postAuthor: Cableguy » Tue Oct 04, 2016 7:29 am

Thanks for the detailed explanation jimmy!
I was really only thinking of using the ball valve because I couldn't find an alternative with a 2" diameter. I suppose that's why piston valves are popular. I watched a video where a guy made his piston out of bondo. I will go with another sprinkler valve for my next launcher. The third generation I will likely attempt to use a piston valve.
I tried opening the ggdt on my friends computer but the program won't run on a mac!

A launcher that works the way I need it to would help me make 500$+ per month more than I do now. I often spend a lot of time trying to throw my line over an obstacle or battling briars through a 600 ft span!
After I get the design figured out I will have my machinist friend build it out of metal. I'm thinking something like a fire extinguisher chamber with an exhaust pipe barrel would be durable and portable enough for daily use.
Thanks again for the advice!
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