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update 11/07: TOW tube launcher is done, success!

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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Unread postAuthor: Fnord » Mon Sep 24, 2007 1:37 pm

Maybe if you put your team logo on it?
Any type of logo or school colors tends to make people feel a bit more comfortable about some type of unidentified weapon.
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Unread postAuthor: dongfang » Mon Sep 24, 2007 1:37 pm

Hi,

Welcome to spudding GTLandser!

I agree with Pete Zaria; don´t make your first gun out of that tube. Most people mess it up more or less badly building their first gun. There is always a lot to learn about which construction techniques work and which don´t, and about building stuff in general. And when learning by doing, scrap piles are milestones on the way to progress....

So build a regular PVC gun first, I´d say.

I also agree that pointing a TOW tube at people is not a nice or safe thing to do. Some hero could potentially save the crowd from a terrorist maniac by immediately gunning you down, and not with potatoes. At least paint it pink with ROTC on it (or whatever non-weapon colors), and shoot into the air.

Regards
Soren
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Unread postAuthor: singularity » Mon Sep 24, 2007 2:26 pm

something very similar to what you plan on doing has been done <a href="http://arniesairsoft.co.uk/?filnavn=/projects/at4_custom/at4_custom.htm"> project details can be found here</a> i don't know how useful it will be because it doesn't have any detailed technical instructions but i though it may come in useful

also i have built an at-4 mock up from only PVC its a coaxial with a sprinkler valve for a pilot a link to it can be found in my sig, i bet if you chopped it down a bit you could fit it into the TOW rocket tube
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be sure to check out my <a href="http://www.spudfiles.com/forums/ak-style-airsoft-vortex-gun-t10959.html">AK Styled Vortex Gun</a> and my <a href="http://www.spudfiles.com/forums/at-4-t9627.html">AT-4 Rocket</a>

upcoming projects... finalized clip fed BBMG and ball point pen sniper
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Unread postAuthor: GTLandser » Mon Sep 24, 2007 5:20 pm

Thanks again for the replies.

@Pete Zaria
I elected not to go coaxial upon further examination; the TOW tube does have a couple holes in it, I guess where the firing system interfaced with the mount or something. Also, I am short on time and did not want the hassle of getting 6" PVC for a chamber when a much smaller one would seem to work. I elected to try a 3" PVC chamber and barrel, each 18" long, with a blowgun actuated 1" sprinkler valve in between. All the components should fit in the TOW tube, and I checked to ensure the pipe sections were rated to 250psi.

I am in the process of building, but before I settle on a permanent barrel and chamber ratio, I had some questions:

I dl'd the GGDT and....

Oddly, using:
temp 80F
psi 100
dia 3in
length 18in

default settings for a generic valve

barrel bore 3in
barrel length 18in

projectile:
friction 5psi
mass 250g
dia 3in

I get a 'barrel unnecessarily long' warning, and muzzle velocity around 120 ft/s...and still get the long barrel warning as short as 8 inches...what gives? Are my assumptions for this setup even correct?



@William
I am rolling my shirts in such a way (and including a brochure rolled in the shirt) that is ensures a pretty snug fit of the shirt in the barrel. I definitely have to nudge it with my hand to move it down the barrel...it is not simply falling through from one end to another.

Where would I get a safety valve? I was going to use my psi gauge to keep the chamber under dangerous pressure...that doesn't prevent overpressure if the t-shirt is jammed though, I guess. Am I going past a good safety margin here?

@Jimmy, et al
We have a pretty cozy relationship with the GTPD, and it is SOP to call ahead when we walk around campus with our dummy M16's (which we do on occasion). I'm asking questions to make sure I build it right, and I'm going to test it before it's even pointed at the stands (and when it is, we'll be lobbing the shirts at a high angle). Your concern is noted and appreciated.

Our presence with the TOW tube will be called in before hand, as we are expected at the games in any case. We are allowed to launch shirts at the pleasure of the GTPD and Athletic Association, so whatever they are comfortable with is as far as we go.
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Unread postAuthor: veginator » Mon Sep 24, 2007 6:28 pm

Can you give me the exact specs on your spent TOW because I might do this with my 105 mm howitser round. :D
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Unread postAuthor: DYI » Mon Sep 24, 2007 7:05 pm

Are you sure that the friction is as high as 5 psi? In a 3" barrel, that would mean that it takes ~35 pounds of force to push it down the barrel. If it does take this much pressure, you may want to make the fit a little less snug. Also, for a 3" barrel it such low pressures, you should have a valve that has at least 2" of flow. A 1" sprinkler valve has less than 3/4" of flow, so it's really killing your performance.
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Unread postAuthor: GTLandser » Mon Sep 24, 2007 7:38 pm

@veginator
Sorry, nothing to it but being 6" id and 48" oal....either aluminum or a very nice fiberglass job, it has such an odd tone when struck I honestly can't tell.

@DYI
Nope, I put down 5psi as a wild ass guess. And sadly, I am too far along now wih my 1" valve to change it; that and it was the biggest one they had at Home Depot.

Anyone have anything to say about my assumptions about barrel and chamber length above? I want to get that right before making anything permanent...
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Unread postAuthor: GTLandser » Mon Sep 24, 2007 7:40 pm

BTW, we have plenty of ROTC stickers, we'll likely be adding those to the tube....
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Unread postAuthor: thespeedycicada » Mon Sep 24, 2007 10:19 pm

you should be fine you will have to use more than 5 psi to get any distance but make sure you will be able to take it out of the missle tube to service it or you will be sorry!
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current projects: co-axial piston valve.Status DONE!
S.P.E.C.S update mk 1 construction begining in febuary all the maths for it are done plans are drawn up and parts are listed.
NEXT project:auto piston valve.
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Unread postAuthor: GTLandser » Mon Sep 24, 2007 11:08 pm

Ok, so I ended up with an 18" chamber and 15" barrel, basically as discussed. I did not glue the sprinkler valve in, in case there was a failure of some kind.

Well, I couldn't resist testing it, even if the glue was not fully cured. I took it up to 60psi, and everything worked as I hoped; the shirt shot across the garage with enough force to bounce all the way back! :D

Tomorrow, I will add some plywood centering rings so the pvc parts are supported in the TOW tube, and run it at various pressures to see how far it will go. I definitely plan to make ease of access and maintenance a priority.

The one negative so far, is that my n2 tank won't fit inside the TOW tube--the PVC assembly is simply too long. I have several options to deal with that, so not a big deal.

Thanks all for the info, and thank God for the GGDT!
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Unread postAuthor: jimmy101 » Tue Sep 25, 2007 12:17 pm

GTLandser, sorry if it sounded like I was being an ahole. We get a lot of posters on the board that really have no concept of how dangerous a spud gun can be. Add onto that the idea of making one look like a real gun, and given the current environment concerning guns on campuses, and it sounds like a recipe for disaster.

Anyway, assuming everything is kosher in terms of the police etc.

One possibility for a safety relief valve is the valve used in hot water heaters. A decent hardware store should have'm for a couple bucks. IIRC, they are usually rated at about 70~100 PSI which is about what you want. Unfortunately, they can't be used as fill regulators since once they open they tend to stay open until the pressure drops way down. (Too bad, if they closed quicker you could omit the regulator and pressure gauge and just use the tank control valve to fill the chamber till the vent opened.) Of course, if you have the time and $$$ you can get both preset and variable "pop-off" valves from McMaster and many other suppliers.

My favorite t-shirter-
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(not mine)

Regarding you GGDT results, the "barrel too long" message and the prediction of needing a very short barrel, I suspect the problem is the default "dwell time" value. This value is how long the valve stays open before it closes itself. The default value gives a valve that immediately closes itself. The tshirt is still in the barrel after 2.5mS (the default opening time), so make the "dwell time" bigger, perhaps 100 mSec. Now the warning is gone and if you graph velocity versus position you see that the tshirt is still accelerating significantly when it leaves the barrel. This is what you would expect since the CB is 1 and the chamber is charged to 100 PSIG, so there is still ~50 PSI pushing the round when you run out of barrel. The 50 PSI in the chamber should be enough to keep a generic valve open.

Please post pictures when done.
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Unread postAuthor: williamfeldmann » Tue Sep 25, 2007 1:04 pm

I'm excited to see the finished product. It's a shame you couldn't get the six inch. That would have been a monster :twisted: Give us some range estimates at various psi when you get a chance. Be interesting to know how much range you get out of it.

Jimmy was right about your opening time. Check that, it's something I always forget to change the first time I run a new design.

Anyway good luck fitting it together and please post some pics of it. We are all waiting anxiously...
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Unread postAuthor: GTLandser » Tue Sep 25, 2007 4:36 pm

Thanks for the info folks; I am getting there. I need to assemble everything in the tube, then mark the locations of my centering rings so I can drill holes for some screws to hold everything in place. And then the paint has to dry...

The two forward rings on the barrel are epoxied in place, while the rearmost ring is only a loose fit to keep things from shifting too much. I figure the barrel is less likely to fail than the chamber, but absent the screws, the entire assembly should be removable.

I couldn't use a jig saw, so making those centering rings with a coping saw was a PAIN.


@jimmy
I like that t-shirt cannon! What are those valves on top of the soda bottles?!
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Unread postAuthor: jimmy101 » Wed Sep 26, 2007 1:34 pm

The "valves" on the soda bottle tshirt guns aren't actually "valves".

Well, I guess one is, the greenish thing is a standard sprinkler valve that has been mod'd to be operated by a blowgun trigger. The black cover on the sprinkler valve, that the blowgun is screwed into, is a custom replacement part manufactured by DR (?) on the SpudCom site.

The black tube between the bottle and the valve is a custom adaptor (DR again) that has a "soda bottle thread" on one end and the suitable thread for the sprinkler valve on the other end (there may be a threaded nipple between the adaptor and the valve). This adaptor also has a tapped hole for the pressure gauge, and another for a Schrader valve (both 1/4 NPT?) to fill the chamber.

The bottle adaptor is a really cool piece. They are supposed to be for sale on the SpudCom site but last I checked they weren't available yet.

The custom plate for the valve is pretty cool to but I believe there is a limited selection available. Maybe the same as the valve your local hardware store has, maybe not. In any case, it is pretty straight forward to do the blowgun mod of a sprinkler valve.

The clear PVC barrel and the clear soda bottle chamber really make this gun look cool. To bad the clear PVC is soooo $$$$$$. Heck, you can even get clear PVC fittings if you pockets are deep enough.
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Unread postAuthor: GTLandser » Tue Nov 13, 2007 6:53 pm

Well, I have been very busy with work, so an update was not possible. I have photos of the construction and can shortly take photos of the final product. How do I post pics?!

The t-shirt cannon has been a hit at the football games, and we have been able to reach out to about 50 meters with it. We even got the attention of some interesting people (read into that) at the Army vs GT game.

Sadly, there is no money in the budget to make improvements, or to build any additional cannons, or even tubes to make them with (my boss wanted two more made!). Apparently one is not supposed to have a spent TOW tube lying around. :P
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