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Hybrid Semi-Auto 0.177 Mk.3

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Hybrid Semi-Auto 0.177 Mk.3

Unread postAuthor: SPG » Thu Mar 06, 2008 3:49 pm

This is the Mark 3 version of my repeating BB hybrid. I think it resolves a lot of the issues found HERE and HERE. To me it's also innately more satisfying as it has a fixed not reciprocating barrel.

It's probably clear enough, but a quick run through:

Barrel: Fixed, 0.177 calibre.
Magazine: Tubular, Under Barrel.
Valve/Bolt: Sliding vertically against spring with "pop-off" action.
Ignition: Electronic Ignition from RC Engine.
Fuelling: Pre-mixed Propane/Air mix.
Trigger: Double Pull, 1st stop opens fuel valve, 2nd stop closes fuel valve and fires ignition.

And that's it. The obvious thing is that the sliding "valve/bolt" has a small piston face, likely to be little more than 7.5mm² overall. Then the port to this is even smaller at only 0.75mm². So if we imagine an opening pressure is 100psi, then the spring only has to hold a force of 1lbs (port area * pressure). A 1lb spring is easy to find, in fact I have some clothes pegs with similar springs. Because it has the "pop-off" action, it needs a high initial pressure to open, but shouldn't close till the pressure has dropped to as little as 10psi, plenty of time for the chamber to exhaust.

I'm only aiming for a working semi-automatic, but I can see with the additio of a siimple check valve in the fuel line, and a timing circuit in the ignition there's no reason why adjustable rate of fire fully-auto wouldn't be possible, or even three round burst.
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Unread postAuthor: Sticky_Tape » Thu Mar 06, 2008 3:53 pm

Looks pretty cool but it seems a little dangerous
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Unread postAuthor: Jared Haehnel » Thu Mar 06, 2008 3:54 pm

Nice design.
The only issue you might have is clearing the chamber of the exhaust gases after each shot. Even with the barrel open you will still have residual gases left that were no able to escape before the valve closed and may cause misfires in the future shots.
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Unread postAuthor: SPG » Thu Mar 06, 2008 3:58 pm

Yeah, that was my worry too, but someone on here, and I forget who has a coaxial piston hybrid, which of course has much the same problem and he has no problem with that as of course there's no oxygen in the exhaust gases to throw the mix out.

And as I said before, the pressure at which the valve will fully close is very very low.
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Unread postAuthor: Jared Haehnel » Thu Mar 06, 2008 4:34 pm

There was an Idea here but it was useless so I removed it... see my next post...
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Last edited by Jared Haehnel on Fri Mar 07, 2008 7:40 am, edited 1 time in total.
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Unread postAuthor: STHORNE » Thu Mar 06, 2008 6:25 pm

Jared Haehnel wrote:Nice design.
The only issue you might have is clearing the chamber of the exhaust gases after each shot. Even with the barrel open you will still have residual gases left that were no able to escape before the valve closed and may cause misfires in the future shots.


Maybe a break away modification towards the center of the chamber would fix the problem? (female adapter with male threaded coupling)
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Unread postAuthor: Jared Haehnel » Thu Mar 06, 2008 9:00 pm

Or you can hook up the compressor or a shop vac to suck out the remianing exhuast gases as it introduces the new fuel.... you'll lose a bit of fuel but it will clear the chamber....
It probably a bit more sensible idea then my previous solution. 8)
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Unread postAuthor: STHORNE » Thu Mar 06, 2008 9:05 pm

you could do that but i say my solution is easier (requires less equipment and expense). still like your's though :)
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Unread postAuthor: jackssmirkingrevenge » Thu Mar 06, 2008 11:00 pm

Very pretty :) my one gripe is that the pressure pushing the sliding thingy through the barrel port might cause enough friction for it to lock up. Oh, and you *really* need a friendly machinist for this one ;)
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Unread postAuthor: Solar » Fri Mar 07, 2008 4:22 am

Hell yes jack. That is a thing of beauty. I am not into hybrids, but damn that is nice. I would suggest o-ring grooves and the slidey thingy though. I actually might be able to help you with this one. I have threaded aluminum tubes with pressure caps that can be bored up from the 1/4" npt to accept a spark plug, and you could use the 1" threaded end to install the barrels. I will make some detailed pics for you. I might have a few extras after this run that is VERY near complete. Can't wait to show you guys ALL the design details.
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Unread postAuthor: SPG » Fri Mar 07, 2008 12:11 pm

Ahh, no no compressors or shop vacs for this one, it's a BB and so has to be portable, and I don't think it needs to break open in the middle either, like I said the pressure at which the barrel port "slidey thing" closes should be very small indeed.

I'm thinking of a reed (sliding) valve for the fuelling, so this will mean that at rest it's closed, at half pull on the trigger it's open and at full pull it's closed again. of course when I release the trigger it means a tiny bit of fuel/air mix will be released into the chamber, which should act to flush it out.

The second option is to have it so that the slidey thing "catches open" at the top of it's stroke, and isn't released until the trigger is also released. This means that the whole venting flushing thing can take place with no bother, of course it'd mean it really would be only a semi, but hey a semi-auto hybrid is still pretty cool.

And Jack, this one's much easier to fab than the last, the only bits that need machining are the two end caps, and one of those only needs a tapped hole in it for the spark plug (and conveniently RC engine spark plugs have metric threads of standard bolt sizes).

So in fact it's just the slidey thing end... shouldn't cost a lot.
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Unread postAuthor: Jared Haehnel » Fri Mar 07, 2008 12:24 pm

A reed valve would be fun. The fuel and air would flow into the chamber until you pulled the trigger the higher pressure caused by the expanding mixture closes the valve and when the pressure drops low enough the valve opens again...
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Unread postAuthor: Skywalker » Sat Mar 08, 2008 12:02 pm

Oh yeah, the "Blow-up" loader! I thought of something like that one, but not as a valve, just to load ammo. I would be a little concerned about the possibility of it jamming due to sideways force and friction, but the theory is sound.
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