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hybrid cartridge protoype without burst disk

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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Unread postAuthor: jackssmirkingrevenge » Fri Jul 08, 2011 12:35 am

If it's not sealing with a spherical projectile, I doubt a cylinder is going to help.

One of the issues is that you need to depressurise the fill rig rapidly in order to shock the projectile into sealing (almost like an exhaust valve). I'm using a schrader valve to do this, and the fill rig is 26.5mm inner diameter, more than double the cartridge, so there's a lot of dead space.

I could scale up to 3/8" calibre and probably work better but the Sherline is a miniature lathe ;)

One thing I discovered making this is (as LeMaudit had already pointed out, but you have to feel it to understand) is that it was significantly easier to cut the M10 thread for this one than the M12 for the pen gun, so making a small threaded burst disk holder shouldn't be a problem. It adds to the loading process but it should be more reliable.

Still, the "valveless" principle is too good to let go of without a fight. One thing I thought about was threading the outside of the cartridge in order to connect it directly to a schrader adapter, but it would be easier to just make a smaller fill rig directly.

Another option could be to have the projectile already placed in the seal, and with a separate fill valve in the back (that would also allow you to poke out any projectiles which failed for some reason...)

Yes, I have a lot of brass to go through :)
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Unread postAuthor: POLAND_SPUD » Fri Jul 08, 2011 1:03 am

Another option could be to have the projectile already placed in the seal, and with a separate fill valve in the back (that would also allow you to poke out any projectiles which failed for some reason...)
One one hand that sounds nice, but on the other... ohh damn that's going to increase mass and complexity

Have you considered creating a screw in plug for the cartridge ? either the pellet holder or the back of the cartridge could be screwed in in place after filling the cartridge
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Unread postAuthor: jackssmirkingrevenge » Fri Jul 08, 2011 1:58 am

I see what you mean, but now you've added another seal between the two cartridge halves.

I like the idea of simplifying the structure though:
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Unread postAuthor: POLAND_SPUD » Fri Jul 08, 2011 2:02 am

but now you've added another seal between the two cartridge halves.
o-rings are quite cheap you know

and that solves all the problems that adding a shrader valve won't solve
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Unread postAuthor: jackssmirkingrevenge » Fri Jul 08, 2011 2:40 am

POLAND_SPUD wrote:...and that solves all the problems that adding a shrader valve won't solve


For the extra complication though you could still fill with the rig, but add a simple "drawing pin" valve at the back. I wasn't proposing adding a schrader, trying to avoid those for cartridges ;)
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 10, 2011 4:42 am

I drilled out the back of the cartridge and added a simple check valve in order to try it out as a "conventional" burst disk cartridge. Epoxy curing, testing this evening hopefully.
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Unread postAuthor: POLAND_SPUD » Sun Jul 10, 2011 4:45 am

a "conventional" burst disk cartridge
A pneumatic cartridge ?? you diagram suggests that there is a metal insert in the epoxy plug so...
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 10, 2011 4:59 am

POLAND_SPUD wrote:A pneumatic cartridge ?? you diagram suggests that there is a metal insert in the epoxy plug so...


Still a hybrid, the intention is to use the fill rig and have the igniter contacts with the valve and cartridge body.

Edit: spark gap works well, now to see if it seals. Watch this space :)
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Unread postAuthor: warhead052 » Sun Jul 10, 2011 10:53 am

Jsr, why not use a piston? It may work a little bit better, and if you get spring strength right, then you wont even need a pilot valve for the piston, the main combustion of the hybrid could smash the piston back wards opening up the barrel allowing the exhaust to flow out. I would try it if I had some syringe heads.
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 10, 2011 12:13 pm

warhead052 wrote:Jsr, why not use a piston? It may work a little bit better, and if you get spring strength right, then you wont even need a pilot valve for the piston, the main combustion of the hybrid could smash the piston back wards opening up the barrel allowing the exhaust to flow out.


Simpsons did it.

While it's more practical than a burst disk, there are some disadvantages, namely if it's a coaxial the cartridge is much more bulky, and it complicates ignition.

In other news, tried out the modded cartridge with air at 75psi, held pressure beautifully :) The epoxy bulged out a little though, indicating it's not cured yet so I put it aside. Still, looks promising.
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Unread postAuthor: jackssmirkingrevenge » Sun Jul 10, 2011 10:27 pm

Update after some frustrating testing, tried it at 6x - it held the pressure well enough, but the spark was arcing externally, clearly the spark gap inside was not small enough. It needs to be put together with more precision.
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Unread postAuthor: LeMaudit » Mon Jul 11, 2011 10:26 am

How is it going? :shock:

Did you consider machining the plug in some gluable plastic (Delrin?) or simply use a plastic sleeve and a rubber washer to insulate the central part?
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Unread postAuthor: jackssmirkingrevenge » Mon Jul 11, 2011 10:53 am

LeMaudit wrote:How is it going? :shock:


Right now, it isn't... well, not very fast at least.

Did you consider machining the plug in some gluable plastic (Delrin?)


I had started to make an acrylic plug but it shattered when I was trying to thread it... got some teflon rod cutoffs from work today, but they have a 10mm hole through the centre... but yes, when I source some suitable material, it will be a perfect idea.

or simply use a plastic sleeve and a rubber washer to insulate the central part?


Since we like making graphic tutorials, my turn ;) the valve stem could be shortened, but then you would also have to suitably insulate the end of the electrode which goes in to make contact with it.
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Unread postAuthor: Gun Freak » Mon Jul 11, 2011 11:05 am

So what's to stop you from insulating the valve stem? Or using a plastic one?
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Unread postAuthor: LeMaudit » Mon Jul 11, 2011 11:15 am

Oh! I see... I didn't catch the pressure had an influence... of course it makes sense...

Alright, here's a better insulated-no thread-no glued-self sealing one :D
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