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Piston cartridge

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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Piston cartridge

Unread postAuthor: daccel » Mon Jan 05, 2009 2:07 am

Thinking about making a piston cartridge, but at smaller sizes the coaxial layout doesn't end up leaving much chamber volume, so I came up with this. Actually very similar to JSR's pneumatic cartridge now that I think about it.

I'm concerned the sealing face might end up melting. I couldn't find anything on that in other threads, so I'm assuming it's not an issue?

Also, would the gap around the sealing face restrict flow too much?
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Unread postAuthor: starman » Mon Jan 05, 2009 2:22 am

You're going to need one hellacious spring back there to hold your prefire mix. I don't think you would have to worry about melting sealing faces, at least at sane hybrid mixes. I believe it has been recommended by _fnord to use air pressure back there instead of trying to calibrate a spring to do the job.

You show igition wires but no gap detail...are you still dreaming that up?
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Unread postAuthor: daccel » Mon Jan 05, 2009 2:39 am

How I imagined it was the piston being essentially a thick epoxy washer, isolating the center part from the chamber. The thin vertical line in the middle is a piece of wire which makes the spark gap(s) between itself and the chamber wall.

The part at the back is a check valve for filling, the spring is just there to hold it in place and allow the current to flow through the center.

You wouldn't need a spring to hold the prefire mix, the pressure seals it. Unless I'm missing something..

And because the back is sealed, it should act as an air spring. Though I'm not sure how the timing would work out if that part ignited as well. It may seal the barrel back up before the projectile has cleared the barrel.
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Unread postAuthor: jon_89 » Mon Jan 05, 2009 2:43 am

Would this have more power than a regular hybrid with the same mix and dimensions.
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Unread postAuthor: JDP12 » Mon Jan 05, 2009 10:01 am

no most likely less power actually.
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Unread postAuthor: ALIHISGREAT » Mon Jan 05, 2009 10:27 am

you could potentially get ignition in the spring chamber at the back if you fill through it, you'll need another method of fuelling for this to work, maybe a check valve/schrader either through the chamber wall or on the piston front or something.
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Unread postAuthor: Fnord » Mon Jan 05, 2009 5:47 pm

A cartridge design is one of the few I'd suggest using burst disks with.
Imo getting ONE piston hybrid working right was hard enough. Mass producing them will be ridiculously annoying.

Simpler:

Union(B-disk) -> reducer -> chamber pipe -> endcap with hole drilled -> schrader.


Put the schrader through the endcap with the rubber still on it, and use it as one half of the spark gap circuit.
The other half will be the chamber wall.


*****
Your sealing face won't melt.

You don't want to cut flow down more than about 2/3 your barrel bore area; it'll reduce performance if you do.
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Unread postAuthor: daccel » Tue Jan 06, 2009 12:30 am

Well the piston front could act as a check valve to fuel through, then orings on the piston to seal it from air behind.

Hate to let go of an idea, but fnord makes a good point. The effort might be better spent finding a way to simplify and speed up burst disk production/installation, like a punch or something.
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