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Modified QDV for hybrid use

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: saefroch » Sat Jan 08, 2011 4:56 pm

Fnord wrote:as well as the limit of my collar-bone's impact strength.
:lol:

UHMWPE does stretch enough. When I was making my check-valved piston, I put the piston into a vise to tap for the valve housing, but when I screwed in the bolt, I found it had increased significantly in diameter and had to be put back into the drill press and made smaller.

Do we have any numbers on heat resistance? The source of all data on plastics has data on UHMWPE, but I can't find anything substantial on nitrile rubber.
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Unread postAuthor: Fnord » Sun May 20, 2012 4:34 pm

As per request I'm putting the 2.2 version of this valve up here (thought it was already here but apparently I'm getting over-the-hill lately).


Operation is the same as the original design, except the trigger piston is being forced against the port from below rather than internally.
This valve will re-set itself and prevent purging, but at the mixes it was intended to use, the small amount of spent gases which are left over should have a negligible impact on the next shot.

An issue which came up in recent discussion was the significant o-ring friction present at mult-kpsi chamber pressures. It may be necessary to modify the inner piston/port dimensions for a little more opening kick, as well as using telfon-coated o-rings.

Otherwise this design appears to be a more simple and durable alternative which does not require extreme tolerances.
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Unread postAuthor: saefroch » Sun May 20, 2012 7:58 pm

Any chance of the other version (shall we call it 2.3)?
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Unread postAuthor: Fnord » Mon May 21, 2012 4:45 am

I'll draw up a better diagram later, but I have other stuff to attend to tonight. Watch this space.
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Unread postAuthor: MrCrowley » Mon May 21, 2012 5:01 am

Still not a fan of the piloted piston hybrid? :D
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Unread postAuthor: Mimikool » Mon May 21, 2012 8:01 am

I'm not sur if i understand or not.

There is only a spring that prevent the translation of the piston, by keeping the sealing of the neopren part ?

The spring should be very strong in order to support the initial pressure, but not too much, the piston should be able to recoil easily.


Right ? ^^
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Unread postAuthor: saefroch » Mon May 21, 2012 8:37 am

You are mostly correct, Mimikool. There has however been some recent discussion on how strong the spring needs to be, since there are significant performance gains if the spring prevent translation of the piston until combustion is complete. Of course, that requires a spring about 10 times as strong.
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Unread postAuthor: Mimikool » Mon May 21, 2012 12:47 pm

I designed a system wich need a weaker spring and keep a good acceleration.

The inbalance can be more or less large, depending on weither the spring strenght.

It can be optimised easily.

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Unread postAuthor: saefroch » Mon May 21, 2012 3:38 pm

That's what I'm working on, Fnord came up with a very similar sketch. Check the force required to hold that piston in the closed position until combustion is finished with a 20X mix. It's pretty crazy.
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Unread postAuthor: MrCrowley » Mon May 21, 2012 3:42 pm

Do you think you guys can even find a suitable spring? I believe both Fnord and I have experience with springs being crushed in hybrids.
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Unread postAuthor: al-xg » Mon May 21, 2012 4:52 pm

Personally I'd go for a completely balanced piston with a spring pulling it open but held back by a latch. Then a much smaller adjustable relief piston would open the latch at the set pressure.
The piston could then just be a solid block, flow passages would be simplified and the external relief valve would be easier to adjust service for best performance.
It avoids having to use a hefty spring.

The piston opening time would still be at least as good as a normal QDV, but one could have the relief valve open the trigger at a lower pressure to synchronise opening with peak pressure (whcih would of course be determined through experimenting with the adjustable relief valve).

Fnord and I have experience with springs being crushed in hybrids.

Am I right in thinking that this problem could be linked with the spring being compressed too far (ie spires edit: wrong word in english... the coils? touching at some point) ?
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Unread postAuthor: saefroch » Mon May 21, 2012 6:00 pm

Suitable springs appear to be available for my requirements, but that's only because I'm working with a .5" valve.

I'm going to need a diagram of your design, al-xg, I can't visualize what you're saying.
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Unread postAuthor: SpudFarm » Tue Jun 26, 2012 6:39 am

Bump

I do like your design Fnord, looks like it will work and perform as a dream..
The design is also pretty simple and could easily be made by most in this forum.

Now get crackin!
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