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Expert design ideas needed

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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Expert design ideas needed

Unread postAuthor: c9d » Wed Dec 24, 2008 12:25 am

After conversing with Mr Crowley about this subject, I figure I would bring a design drawing to the table for your guys' help/opinion. There are only a few walls I am running into. Here is my write up and drawings:

Image


1. My first idea was to use a small electric solenoid to act as the release valve. Running thin wires from a switch system to a solenoid would allow me to retain the retracting/extending properties of the launcher. However with my limited space to begin with, any electrical system I fit in here would be a low voltage DC system, and none of the solenoids I can find are rated very high in their pressure differential and operate at a low voltage.

2. I could try a pneumatic release system, however without knowing how small the tubing CAN be to work, I can never be sure if I can keep the retracting/extending properties of the launcher. I would not however have to worry about an electrical system, but I would have to find a lever activated release valve that could fit where the original trigger already sits.

3 The second obstacle would be what materials to use. Steel is way to thick and heavy to use. I already know that 2.5in SCH 40 PVC's outer diameter will not fit in the launcher tube unless I shave/sand the whole OD of the PVC down. I figured I would go with aluminum, but how do I achieve a good seal, while still being able to disassemble the back end of the coax for maitenance?

Any opinions and ideas are greatly appreciated. I did a mock up of this coax with all the PVC I had bought before I knew it wouldnt work. I just wish they made PVC in a SCH 30-35... lol. I havent advanced this build any farther until I have a better direction to take. Thank you all in advance.

Garrett
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Last edited by c9d on Wed Dec 24, 2008 10:10 am, edited 1 time in total.

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Unread postAuthor: jrrdw » Wed Dec 24, 2008 9:41 am

My thoughts, use a PVC main tube with a aluminum extender (so you can custom fit with out thinning the PVC) use o-rings on the extender to seal the chamber. Use a pneumatic activation. Build the pilot chamber small enough to be activated with a schrader valve size pilot, 1/8" with a gate type valve for activation at the trigger (that size has been done before for piloting).

All the above can be customized to fit your needs.
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Unread postAuthor: c9d » Wed Dec 24, 2008 4:18 pm

Could I use Extruded Aluminum Tube 6061 T6 for my cylinder? I can't seem to find a lot of info on the web about this stuff, i.e. bursting pressure, etc. I can get this stuff welded together by a friend who TIGs, but do not want to order the wrong stuff. I doubt my pressures would exceed 400 psi as I would only be pushing a sabot mini nerf vortex football.

Jrrdw, I am having a difficult time visualizing what you are saying with the PVC to alum extender.
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Unread postAuthor: ramses » Wed Dec 24, 2008 6:04 pm

I think he means find a size of aluminum tube that fits well in the pvc that you use, cutting an o-ring groove into the pvc on the inside, and sliding the aluminum pipe into the pvc. unfortunately, when you pressurize, a large amount of force will pull the aluminum out of the pvc, and release all your pressure.

I don't know how important performance is to you, but I would suggest having the whole launcher be only the length of the collapsed version, and having the whole thing attached to the breech end. The tank wouldn't have to change lengths, neither would the barrel. Then the muzzle could appear to have the real-steel ID.

For piloting, I would suggest 2 metallic strips on the outside of the chamber, connected to a momentary switch. these could have a small amount of current supplied by a button cell, that would trigger a transistor, putting a 9volt across a sprinkler valve.

It should be mentioned that sch40 and sch80 have the same OD, it is the inside diameter that changes.
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Unread postAuthor: grumpy » Wed Dec 24, 2008 7:42 pm

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Unread postAuthor: jrrdw » Wed Dec 24, 2008 8:16 pm

c9d wrote:Could I use Extruded Aluminum Tube 6061 T6 for my cylinder? I can't seem to find a lot of info on the web about this stuff, i.e. bursting pressure, etc. I can get this stuff welded together by a friend who TIGs, but do not want to order the wrong stuff. I doubt my pressures would exceed 400 psi as I would only be pushing a sabot mini nerf vortex football.

Jrrdw, I am having a difficult time visualizing what you are saying with the PVC to alum extender.


PVC as the outer tube and aluminum as the inner tube. If you hollow the PVC to accommodate the aluminum, it would lower the rating of the PVC, make it weak. You can turn down the aluminum to fit the PVC and it will still be strong.

6061 should hold the pressure fine. The extender is what I'm calling the part that slides out.

Ramses makes a good point, once you pressurize the chamber it will be hard to hold the pieces together. You would have to build a internal catch that will hold/be air tight.

The o-rings would be better placed on the outside of the extender, rather then the inside of the main tube.
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Unread postAuthor: c9d » Wed Dec 24, 2008 9:16 pm

This whole coaxial assembly must fit in the outer tube of a "real" M72 LAW launcher. I am not making a whole new launcher. I am sorry if I left that part out. Ramses is on the same track as I was. The total length of the coaxial would be 24", the collapsed length. The inner diameter of the launcher tube is 2 3/4in. and the ID of the extension tube is 2 1/2 in.

I don't think a two piece system would be very practical.

Ramses, I have looked at many sprinkler valves, and all of them are waaay to big to fit in the extender tube. Like I said before, none of the electrical options seem feasible, as the voltages I could use would be way less than an acceptable level for the pressure differentials I would be using. What would be a good pressure to have in the cylinder to push that little sabot football out to about 150-200 ft with minimal arc(less than 45 degree angle of trajectory)? I did try to use that GGDT thing, and still do not understand how to use correctly.
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Unread postAuthor: jrrdw » Thu Dec 25, 2008 8:29 am

This whole coaxial assembly must fit in the outer tube of a "real" M72 LAW launcher. I am not making a whole new launcher. I am sorry if I left that part out. Ramses is on the same track as I was. The total length of the coaxial would be 24", the collapsed length. The inner diameter of the launcher tube is 2 3/4in. and the ID of the extension tube is 2 1/2 in.


Yes, you left all of that out, or most of it. Let us know how it turns out.
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Unread postAuthor: c9d » Thu Dec 25, 2008 9:21 am

in my defense, it was kinda implied by the picture, but I apologize for not stating it.
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Unread postAuthor: jrrdw » Thu Dec 25, 2008 10:19 am

c9d wrote:in my defense, it was kinda implied by the picture, but I apologize for not stating it.


Haaahaha, you don't have to defend yourself nobody is mad at you. :lol:

Do you have any of it put together yet?
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Unread postAuthor: c9d » Thu Dec 25, 2008 11:14 am

I did do a mock up in PVC, after finding that the PVC wouldnt work, but havent started construction in the Aluminum yet. I am placing my order for the aluminum the day after christmas ( 3x what I need, in case of mistakes).
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