Lightweight air tank ideas

[btrettel]

I'm in the early planning stages of a new gun that I want to have a lightweight air reservoir on. The pressure won't be particularly high because I want the gun to be pressurized by an on-board pump. I'd need about 75 to 125 cubic inches of volume and it has to take a pressure of up to 150 psi.

My first thoughts were to use a pressure vessel made from aluminum tubing with two plates welded to the end. This would end up weighing less than a pound and a half. However, I've learned that the weld would reduce the 6061 T6 aluminum to 6061-0 aluminum near the weld because of the heat treating and unless I retreated it, the weld area would be significantly weaker. That would have reduced my safety factor from 13.5 to 2, and to me a safety factor of 2 is unacceptable. After examining the heat treating process I decided not to do this. I figure someone here might actually do this on a regular basis so I thought it might be worth mentioning.

Next I looked at SDR 21 and 26 PVC pipe, but I had a lot of difficulty finding any. I emailed McMaster-Carr first thing--they don't carry any SDR pipe at all. Eventually I located Ryan Herco Products. They carry the pipe but not the fittings. Without SDR fittings the air tank would only be about half a pound lighter, which helps but isn't worth the hassle of getting the pipe.

So now I'm considering either scrapping the idea of a lightweight low pressure reservoir and continuing use of my HPA tank or trying something else.

Does anyone know a good place to get cheap and lightweight aluminum pressure vessels? I started researching it a bit and found this place, but no prices are visible. I emailed them to find out. I looked at eBay too but all my keywords seemed to be wrong, so if anyone could suggest a keyword I'd appreciate it.

On a whim I started looking up aluminum water bottles. If they have similar dimensions to my original plan they'd work. Sigg bottles look adequate, though I'd prefer something larger. They seem to have a 1.5 liter version listed in a chart on their website but I can't find it anywhere. Has anyone tried these? I could find one person who considered them when searching, but no reports of how well it worked.

If anyone has any other ideas or suggestions I'd appreciate it.

[POLAND_SPUD]

I don't see any point in making a light weight tank if you have HPA...
if something works well don't change it.. you can always put hte tank in the backpack or something

If you really want to reduce weight you can always use 12g CO2 cartridges or 88g ones... but of course you would have to buy a reg

[jackssmirkingrevenge]

inspiration?

[john bunsenburner]

You could buy a carbon fiber hpa which is very light. When making a pressure vessel it is very important to have rounded ends and not flat ones which decrease safety by alot. The thought of casting the pressure vessel jsut crossed my mind but it would not really work...I enjoy using an iron or aluminium pipe with two end caps, which are either self made(cast) or commercial. I would just buy an alli pressure vessel, or an iron one which i would hook up to a hose and have it in a back pack...

[btrettel]

HPA and 20 ounce CO2 tanks aren't as light as what I want, and they cost too much. I already own an aluminum HPA tank and a regulator, but the weight really turns me off, at least for what I intend to do with it. If I were just shooting things in my backyard, HPA would be great, heck, I'd probably look into getting a much larger cylinder. But I want to design Nerf and paintball guns, so weight is a concern.

12 gram CO2 cartridges are inconvenient and too expensive in the long run.

Carbon fiber would be nice if I were rich. Though it might be cheaper than I anticipate so I'll look into it.

I have no intention to use a backpack. I want to use the air tank as a stock.

I'll make what I'd like a little clearer: a light tank that can hold 150 psi, is about 12 inches long, is reasonably cheap, and has a volume of about 75 to 125 cubic inches. The theory is that this will be cheaper and more convenient in the long run if I pressurize with a hand pump and get 30+ shots per air reservoir fill.

Edit: My HPA tank weighs 4 pounds, which isn't obscene, but definitely the majority of the setup's weight. I'd like something that's as light as 1 pound with a lower air capacity and ability to be filled whenever.

Aside from the company producing low pressure aluminum air tanks, there doesn't seem to be anything like what I want available, so I'll probably stick with my HPA tank unless those little aluminum tanks are pretty cheap.

Lighter HPA tanks (like carbon fiber ones) are worth looking in to. A small 48ci aluminum HPA tank sounds reasonably cheap... I'll look into them.

[john bunsenburner]

would a bulk tank with a very long hose be an alternative, remeber if you fill your tank with only 150psi then it will run out very quickly!!!

[btrettel]

Long hoses aren't practical with Nerf, paintball, airsoft, or any game for that matter.

Also, when shooting small projectiles and designing with air efficiency in mind, I should be able to get about 20 shots out of the chamber I've described and still have very good power. I think if I design it right I can get 30, but that's just a guesstimate.

Edit: After some thought, I think I'll keep using the HPA tank unless these small aluminum tanks I've been looking at are really cheap. I don't even know why I posted this really--there isn't that much out there like what I'm thinking of.

[maggotman]

ive been filling my empty c02 fire extinglisher with 600psi hpa and it dosent last long at all

[john bunsenburner]

what size is your barrel, and what valve do you plan to use, and yes stick to hpa its probably best.

[btrettel]

ive been filling my empty c02 fire extinglisher with 600psi hpa and it dosent last long at all

Did you read my post above yours? I'm shooting small projectiles like Nerf darts and I'm eventually thinking about shooting paintballs. I'm intentionally limiting performance to an acceptable minimum. When the energy requirements are lower that translates into less gas needed. And I'm confident in my ability to optimize the system to use less gas for the same performance. See BAGS in my signature--I've written a simulation like GGDT to help optimize my guns. My planned gun should be significantly more efficient than the average gun that's thrown together without any concern for efficiency.

what size is your barrel, and what valve do you plan to use, and yes stick to hpa its probably best.

You've seen my FANG gun. My plans are for something similar. The barrel has a diameter of 0.53 inches and I will use either a 1/4 inch or 3/8 inch QEV piloted by a directional control valve. But, the volume of the air chamber is very likely to be less than 2 cubic inches

And no, HPA isn't "probably best", at least for me. As I said, it lends itself to tanks that are heavier than what I would like. And I find refilling the tanks inconvenient. HPA is a compromise, one I'm likely to take, but still a compromise.

[john bunsenburner]

so wait, you rather refill some tank you cant get eery 10shots than a tank you have already every 10 000 shots?!?!

[btrettel]

Something to that effect is what I've been saying all along. It's lighter and I don't mind the work. In fact, I bet I would use less energy pumping every 20 or so shots than I would lugging around something that weighed twice as much. Aiming would be easier with a lighter gun too.

The idea is to reduce weight, not make the gun impractical in some other way. 20+ shots is significant.

Edit: Getting tired of the lack of an email response, I called Catalina Cylinders. Their cylinder #1686 costs only $17.50. I'll have to thread it myself, but that shouldn't be a problem. I'm definitely going to order one then because that's a great price and it should do precisely what I want to. I just need to look at what threads I can fit on it.

[TurboSuper]

Standard SCH 40 pressure-rated PVC sounds well up to the task you're describing, am I missing something?

[btrettel]

Yes, you are. Sch. 40 PVC pipe is too heavy.

I mentioned SDR pipe as too heavy so I thought it would logically follow that sch. 40 is also too heavy, though that wasn't completely obvious.

[john bunsenburner]

Yes that also confuses me: with only 150psi, you can use nearly anything from SCH-40 PVC to any alumium pipe, safety should not really be an issue at pressures so low!