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Not impressed with my "pneu" pneumatic

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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Not impressed with my "pneu" pneumatic

Unread postAuthor: xpitxbullx » Sun May 18, 2008 9:23 pm

I've made a few spud guns long ago and have recently decided to try again. I was always making combustion ones and decided to try pneumatic to see how much better it is. I think I need to experiment some more with chamber sizes.

I used the same dimensions of my combustion model to build my pneumatic gun with a modded 1" rainbird model 100 sprinkler valve. I'm only seeing about a 25% increase in distance.

Here are the dimensions of my pneumatic that are launching a potato 180yrds at 100psi (only 40 yards further than my combustion gun)

Chamber = 4" diameter x 12"
1" Rainbird model 100 sprinkler valve with blowgun trigger
Four feet of 1.5" barrel

4" x 12" makes a fine combustion chamber. Is it different for air powered?

Jeff
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Unread postAuthor: bigbob12345 » Sun May 18, 2008 9:27 pm

Well your sprinkler is constricting flow a bit there.
I would reccomend a 2in piston valve with a chamber that size.
Then I bet you will get atleast 250yrds.
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Unread postAuthor: DYI » Sun May 18, 2008 9:31 pm

You must have a very poor combustion gun there if you were seeing 25% improvement from a 100psi pneumatic with a terrible valve like that.

Propane/air at atmospheric pressure can generate up to ~125psig in 2000+ degree fahrenheit combustion gases, so you should be impressed that a 100psi pneumatic using cold gases and a poor valve could outperform it at all.

You're going to have to use pressures much higher than 100psig if you want to see any real significant power gains over an atmospheric fuel/air combustion cannon.
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Unread postAuthor: xpitxbullx » Sun May 18, 2008 10:17 pm

I'll need a different chamber if I'm going to trust going over 100psi. I read a lot of experienced people using the 1" rainbird. I wasn't under the impression it was a terrible valve but for my needs I guess I'll have to look at piston valves (is this something I can just buy?)

I use Gumout Carb Cleaner (ether) for my combustion gun and BBQ igniter. I'll have to read more on what you guys are doing for combustion guns and make some modifications.

Jeff
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Unread postAuthor: bigbob12345 » Sun May 18, 2008 11:11 pm

A sprinkler is not a bad valve
Your chamber is just a little too big for one

And about the piston valves
Here is a useful link
And no it is not something you can buy, you make it yourself.
It is not very hard at all, I have made several. Just takes some trail and error.
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Unread postAuthor: n-strike » Sun May 18, 2008 11:22 pm

I don't see what's so bad about 180 yards...it's not bad. Unless you're like me who's crazy and wants powerful guns. Then I think you should make a mega launcher. I have always preferred pneumatis over combustions. I think you need to make your chamber two times bigger. It is a little more different for air powered guns, as you actually need pressure rated PVC and fittings. And whoever is reading this, no, you still need pressure rated PVC and fittings for your combustions, but it's just not as crucial. You do need it more with pneumatics though because you are putting stress on them so they need to last instead of exploding.
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Unread postAuthor: no-limit » Sun May 18, 2008 11:55 pm

Piston are hard to make for some of us, I have made 4 and can only hope this last one works, will try it tomorrow.

Yes you can buy them and I just might have to do that myself.
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Unread postAuthor: MaxuS the 2nd » Mon May 19, 2008 2:06 am

If your piston isn't working, you're doing something wrong.

My first was a [female doggy] to get working, then the second one was easy, then I did the third in a day (the whole gun that is).
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Unread postAuthor: psycix » Mon May 19, 2008 5:36 am

MaxuS the 2nd wrote:If your piston isn't working, you're doing something wrong.


Cant agree more with that.
Design 5 times, rethink 2 times, measure 6 times, cut once, assemble once.
Fire many times. :D

If you just take the time and really make sure your doing it the right way, it will work without any tweaking right on the first shot!

pitbull, just take your time, it might be overwhelming at first, but once your fammiliar with it, you will notice its not that hard. Do alot of research and build one.
Construct a barrel sealer, and use that sprinkler as pilot valve.
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Re: Not impressed with my "pneu" pneumatic

Unread postAuthor: jimmy101 » Mon May 19, 2008 10:19 am

xpitxbullx wrote:Chamber = 4" diameter x 12"
1" Rainbird model 100 sprinkler valve with blowgun trigger
Four feet of 1.5" barrel

4" x 12" makes a fine combustion chamber. Is it different for air powered?

Welcome to the forum.

Lets see, your chamber volume is 151in<sup>3</sup> and your barrel is 84<sup>3</sup> so your CB ratio (chamber/barrel) is 1.8. Which is reasonable. A bigger chamber would be better but the increase in performance probably wouldn't be all that much.

According to GGDT, using the parameters for a 1" mod'd Orbit WaterMaster valve, and a 100g spud with 10psi friction, I get a muzzle velocity of 260 FPS. Maximum range at a 37degree launch angle and a Cd of 0.75 is 230 yards.

Modeling the same basic geometry in a combustion gun using HGDT gives a muzzle velocity 300 FPS and a maximum range of 270 yards.

Assuming I didn't screw somthing up in GGDT or HGDT your performance are a bit lower than expected but still reasonable.

You might want to download a copy of the simulators GGDT (for pneumatics) and HGDT (for hybrids but can be used with standard combustions as well) from D_Hall's site.
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