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Piston Separate from Sealing Face

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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Piston Separate from Sealing Face

Unread postAuthor: Secret Squirrel » Mon Nov 29, 2010 1:30 pm

I know I've seen this somewhere before, but for my next pneumatic I'm planning on having the piston at the back of the chamber coax style, but then instead of running the barrel through the chamber, I was going to connect the piston to a sealing face at the other end of the chamber via threaded rod. This way I can have the compactness and clean looks of a coaxial with the ability to change barrels easily.

Just an idea I had, not taking credit for it though as I'm sure I've seen it elsewhere, I just can't remember where.
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Re: Piston Separate from Sealing Face

Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 1:44 pm

Secret Squirrel wrote:Just an idea I had, not taking credit for it though as I'm sure I've seen it elsewhere, I just can't remember where.


You mean the legendaryToolies valve?

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Unread postAuthor: Secret Squirrel » Mon Nov 29, 2010 1:57 pm

That's it! But I'm going to build it as an inline instead of an over under. I just have a thing for shoulder fired rocket launchers and recoilless rifles.

Edit: JSR, how is it you find these links so fast?
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 4:06 pm

Secret Squirrel wrote:JSR, how is it you find these links so fast?


Easy when you know how ;)

I just have a thing for shoulder fired rocket launchers and recoilless rifles.


Something like this then?

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Unread postAuthor: Gaderelguitarist » Mon Nov 29, 2010 10:53 pm

With a setup like you're planning, Performance might not be too spectacular. A standard coaxial setup might fit your want for a shoulder fired rocket launcher style.
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 29, 2010 11:44 pm

Gaderelguitarist wrote:With a setup like you're planning, Performance might not be too spectacular


No reason why it shouldn't be as good as a coaxial, or even better give the "straight through" flow layout.
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Unread postAuthor: Gaderelguitarist » Mon Nov 29, 2010 11:57 pm

My reasoning was based on the scale of the launcher in the picture you provided, JSR. For this to be "powerful", I would think it would have to be much too long to be comfortably fired from the shoulder, looking like some type of jousting lance of PVC.
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Unread postAuthor: Secret Squirrel » Tue Nov 30, 2010 12:11 am

I've been working on this project for a while now, and it's gone through many iterations.

The first version was a coaxial, but there were a few reasons I didn't like that layout.

First of all the coaxial made for a rather bulky launcher in the barrel sizes I need. Ideally I want a 2" barrel (for Nerf pocket vortexes or cup fulls of airsoft BB's) but on the first prototype I went with a 1.5" barrel because otherwise I would be stuck with using a 4" chamber. Even with a 1.5" barrel and 3" chamber the coaxial weighs much more than I'd like, far too heavy to lug around all day. PVC adds up quickly in weight. This inline style uses a single layer of tube as both the barrel and chamber instead of the double layer in a coax.

Since the entire point of this project has been to build a launcher suitable for airsoft use, performance isn't particularly my main goal. In fact the coaxial was a bit more powerful than I'd like, I know I'd never want to be hit with anything coming out of it. Maximum performance isn't really the goal.

The launcher will probably end up being about three feet in length, made almost entirely from 2" PVC, a bit undersized, but 2.5" is impossible to find. All I'm looking to do it toss a Nerf a few hundred feet, and given that they are made to be projectiles, that shouldn't require too much power.

JSR, that's pretty much exactly what I mean. One version of this launcher was going to use large combustion cartridges to provide a more realistic launcher with breech loading, though after building one cartridge I realized it was a bit cost prohibitive to build several advanced combustions, and after burning some ammo during tests I decided airsoft and combustibles don't go well together, and so abandoned the idea.
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 30, 2010 12:51 am

My reasoning was based on the scale of the launcher in the picture you provided, JSR. For this to be "powerful", I would think it would have to be much too long to be comfortably fired from the shoulder, looking like some type of jousting lance of PVC.


I see your point, some calculations:

let' say 60 inch overall length, of which 5 inches is piston and pilot.

With a 2" barrel and a 3" chamber, a coaxial will give you a 55" barrel with 216 cubic inches of chamber volume.

With an inline piston in a 4" chamber of 10", you'll get a 45" barrel and chamber volume of 125 cubic inches.

The coaxial will give you more power (assuming the same pressure, pilot etc.) for probably not much extra weight, something Secret Squirrel might want to keep in mind ;)
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Unread postAuthor: Technician1002 » Tue Nov 30, 2010 4:56 am

performance isn't particularly my main goal.


With air cannons, the power is adjustable on high performance cannons.
Precision golf ball launches becomes possible. Note the distance can be dialed in with a good pressure regulator. Other than hitting the side of the target, the distance was dead center.
[youtube]http://www.youtube.com/watch?v=g496mBQox7Y[/youtube]
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Unread postAuthor: Secret Squirrel » Tue Nov 30, 2010 9:58 am

I guess performance wasn't really the best word to use there. I intend on making this a fairly high quality build (for a PVC gun) but I don't need particularly high power.

When it comes to weight, the coax will be heavier for the same length of launcher because it consists of two layers of pipe with the chamber in between rather than one layer with the chamber at one end. I realize that a coax would provide more chamber volume and a longer barrel, but for this project I want to build the lightest smallest launcher possible with enough power to be effective for airsoft. The inline will let me use what amounts to a single piece of 2" pipe as the launcher. To separate the chamber and barrel there will be a pair of bushings reducing down to 1.5" or 1.25" pipe and then back to 2".

Besides, I already built a coaxial this size, and want to try something different.
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Unread postAuthor: c9d » Fri Dec 17, 2010 2:46 am

I know you said you tried a coax before and it was too heavy, but honestly... if you want compactness, its gonna have a little weight to it. That being said, how bout something like this then? I based my build off of the M2 Carl Gustav found in Battlefield Bad Company 2. The entire launcher is 3 feet long and is currently powered by a single CO2 cartridge. It has some weight to it, has a 2" barrel, and uses 3" PVC for the chamber instead of the 4" PVC you mentioned before.

Image

This particular launcher got turned down at an airsoft event for being "too realistic"... as in if the pocket vortex, going 220 fps out of the barrel, hit someone in the head... it could break their neck. I already have a system designed to tune this down to make it less than lethal, and give me more than a single shot on one charge of gas... lol. Just need to get the funds to finish her up.
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Unread postAuthor: Secret Squirrel » Fri Dec 17, 2010 7:51 pm

That looks fairly similar to what I was planning on building. I had previously underestimated the volume between 2in. and 3in. pipe. I'm leaning towards the coax layout now for a few reasons.

If I'm going to have to hand pump this launcher, efficiency should be a priority. There's no reason to spend more energy forcing air into the chamber and getting nothing in return. The coaxial layout is better for this purpose because it has less of a flow restriction, a lighter piston, and a longer barrel.

After actually weighing my old coax instead of just guessing at it's weight, I've found it weighs in around 7lbs., which is less than my AUG, so it's not really a big a deal as I once thought, and with the addition of a sling a weight in that range shouldn't be such a big deal.

Now for a question. With the onset of winter here, it's consistently cold out now, which is getting me kind of worried about PVC and it's brittleness. Does ABS have similar problems in the cold? I know that it is not pressure rated, but from GGDT the only "safe" pressures for this launcher are below 70psi, any more and the football is launched dangerously fast. ABS seems like it would be lighter, and safer, I'm just wondering how it handles cold weather.
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Unread postAuthor: c9d » Fri Dec 17, 2010 8:36 pm

Since my standard operating pressures will be in the 50-60 psi range... i should be good. Perhaps a wrapping of duct tape would be in order. However I would still be very worried taking her out in this type of weather... personally winter airsoft sux for me anyways... so its not one of my major concerns.
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Unread postAuthor: Lockednloaded » Fri Dec 17, 2010 8:41 pm

wrapping in duct tape will not help. If you drop the launcher while it is pressurized it WILL FAIL. ABS is a little more impact resistant, but you have to treat it like it WILL FAIL if you drop it or knock it too hard on anything. For safety in the field, I recommend a sch10 aluminum shell covering your ABS construction. It will not add much weight, but it will add a lot of protection
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