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Hybrid cartridge material

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Hybrid cartridge material

Unread postAuthor: auxiliary » Sat Aug 29, 2009 12:21 am

I am dreaming up ideas of making hybrid cartridges that will hopefully be shot semi-automatically. My question is, can I get away with using 1/2" PVC as a cartridge with a 20X mix? I know that sounds crazy, but when the cartridge is fired, it will be housed in 3/4" type M copper receiver then ejected. The burst pressure for sch 40 1/2" PVC is 1,910 psi and for sch 80, 2,720 according to engineering toolbox. If 20x is not plausible then what is the highest safe mix that I could get away with. I would normally find the pressures generated by mixes using HGDT, but it doesn't work on this computer.

I have three main reasons for using PVC for a hybrid cartridges. The easiest and most compact method for attaching a schrader valve to the back of the cartridge (with insulation for a spark) would most likely be the way daccel did it BACH (epoxy with holes drilled in the cylinder for extra strength. He pressure tested it to 3,300 psi and it didn't fail. I believe the epoxied schrader would be the weakest link so there is no point to using metal if the back end is going to pop out first. Secondly, plastic is lightweight and hopefully that will aid the ejecting process. Lastly, PVC is cheap and easily mass producible.
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Unread postAuthor: inonickname » Sat Aug 29, 2009 12:28 am

I would never use PVC in a hybrid no matter what. Just a personal thing. It's up to you, but I wouldn't unless required.

If the diagram is to scale then the burst disk will not have enough force to clamp and seal it there.
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Unread postAuthor: PVC Arsenal 17 » Sat Aug 29, 2009 12:32 am

I know nothing about hybrids and I'm extremely unqualified to make any comments here, but I would at least suggest sleeving the inside of the PVC chamber with copper. PVC may be able to handle the pressure, but the sudden spike in pressure from hybrids is what has caused some to fail. There was a thread on this recently.
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Unread postAuthor: auxiliary » Sat Aug 29, 2009 1:07 am

Here is an updated design taking your suggestions into account:
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Unread postAuthor: Moonbogg » Sat Aug 29, 2009 1:36 am

I have some suggestions. Consider aluminum tubing. Very strong, especially when small diameter and also very cheap as well. Lose the epoxy in the back and use an aluminum disc with press fit retaining pins to hold it in place. Then tap an NPT hole in the plate and install the schrader.
In the front you could clamp the burst disc with an aluminum half coupling.
You may also simply use aluminum pipe nipples and tap a hole through aluminum plugs.
These are just the first thoughts that came to mind. I would start there and let it evolve to its final destiny. Thats just me though.
Also consider simply making a piston hybrid with easy fueling capabilities. The cartridges don't take any of the loading time or work out of using the cannon. They simply require you to do more work up front to shoot more times later.
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Unread postAuthor: john bunsenburner » Sat Aug 29, 2009 3:02 am

Well really if you had the time you could make an efficient fill station for your cartridges, also have you considered the following:

You have a metal tube that has internal threads on both ends. On the one side you put in a small piece of threaded metal with a hole in the middle where a threaded shrader was been screwed in. Now you have the other side, here you have a similar set up just that a brass nipple with ammo is screwed in. At the end of the nipple there is an O ring, that sits in grooves in the metal pipe that are at the right place.
To make this "live ammo" you do the following:

1.Unscrew the back with the shrader
2.Take a precut aluminium burst disk and drop it in.
3.Screw the back, back on
4.Put ammo into the nipple
5.Fill the entire set up with a hybrid mix

With practise this can be done in 20-50seconds per shot, not too bad
What you need to consider is the firing mechanism, you need a spark gap in there too, however that is not too hard to figure out by your self...
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Unread postAuthor: spudamine » Sat Aug 29, 2009 3:31 am

I thought about doing this a while ago, You're right abou the 20x mix, you will need at least this to get appreciable power with such a small chamber, or you could go for a smaller oxygen + propane mix number. Also consider having a larger diameter cartridge than projectile, like a real rifle cartridge.
I dont think PVC will be upto the job especially with holes drilled in it for the epoxy plug, plastic is plastic at the end of the day. I would suggest a combination of brass fittings and copper pipe, this way the whole assembly can be soldered/sweated together. The only difficult bit is retaining electrical isolation between the schrader and cartridge for the ignition circuit.
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Unread postAuthor: auxiliary » Sat Aug 29, 2009 12:41 pm

I decided that if I am going through all of the trouble to use PVC for a cartridge it is not worth it. My next design uses a 1/2" sch40 steel chamber and a 3/8" sch40 steel ammo tube. To seal the burst disk in, I would solder a washer down inside the threads to provide a sealing face and stopper. I can make a gasket out of some PVC bath liner I have to ensure a seal against the washer.

As for the spark, I will solder a steel washer to the end of the schrader and epoxy it in so it is not quite touching the inside wall of the chamber.

Here's the new design:
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Unread postAuthor: ramses » Sat Aug 29, 2009 4:00 pm

I would suggest against a sch 40 chamber: do you really trust .06" thread depth to 20x?

If you use 1/2" sch 80, you actually have to drill out the end to get the recommended hole size. Depending on length, you can drill out the whole inside to get a larger ID, maybe even drill out most of the diameter to the sch 40 specifications.
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Unread postAuthor: Moonbogg » Sat Aug 29, 2009 5:16 pm

I am not fond of tiny little high mix hybrids. I suggest buying a rifle.
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Unread postAuthor: theBOOM » Sat Aug 29, 2009 5:49 pm

Moonbogg dont this the minis :cry:
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Unread postAuthor: Moonbogg » Sat Aug 29, 2009 6:44 pm

theBOOM wrote:Moonbogg dont this the minis :cry:


Sorry. I have respect for them and those who build them, I just felt the urge to share my personal opinion which should have probably been withheld, especially in someone elses thread.
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Unread postAuthor: GalFisk » Mon Aug 31, 2009 9:52 am

This last design looks promising to me, I like the washer spark gap. I second using oxygen if possible, to get more power out of the limited volume.
I think it will be sufficient, maybe even better, to thread the inside of the chamber where the epoxy sits rather than drilling holes in it. And since you're already threading the other end, fabrication requires less tools.
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Unread postAuthor: THUNDERLORD » Mon Aug 31, 2009 11:05 am

When the shell is pressurized it will probably flex (bulge).
Maybe visibly undetectable, but enough to not fit in the chamber.

To avoid that a thicker more rigid casing would be used.
Then the problem is since it's rigid, when it fires it will kick back out of the chamber almost instantaneously.
In blow-back action firearms (ex. walther ppk, mac-10/11 etc.) it's not a problem because the cartridge flexes, temporarily "glueing" itself in the chamber (until pressure drops after proj. exits).

But in this hybrid shell case(pre-pressurized) when it ejects, it will be too rapid, probably before the projectile exits, causing poor function and venting pressure from the ejection port, and maybe bursting the shell case...

To prevent this a thin tube extended from front of shell could be used that will flex on firing causing it to temporarily be glued in place (front of "bullet" and burst-disc).
The advantage is that if the shell case can withstand firing pressure, only a short chamber is needed to enclose the thin tube at cartridge front. (pro=shorter loading and ejection movement, con=larger diameter shells)

Unless a complex locking mechanism like a gas assisted bolt is used.
Then it will need a magazine feed, etc....Get's more complicated than a firearm rifle while even more complicated because it's a protoype.

Besides the whole shell concept defeats the advantage of liquid/ gas powered weaponry to begin with.

IMO a blow forward bolt at chamber front with spark (ignition switch) set just at end of spring travel can't be beat, or a combination cylinder ,like a springer airgun except with hybrid chamber compressing air (other side piston) at a blow forward bolt. :roll:

Oh, since your cart.s have shraders or prestas, you could try N2O carts in a bike tyre filler for oxydizer...all together looks like expensive shells and time consuming....but cool 8)

BTW, got me wondering if a SMG will cycle faster with thicker shell casings. 8)
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Unread postAuthor: john bunsenburner » Mon Aug 31, 2009 11:28 am

Really the shells can be as complex as they are mode seeing as how they, once built, last along time.
To solve the problem you could have a non self ejecting shells, and stickign an end cap of some sort behind them.
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