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Repeater BB Hybrid - v2.0

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Repeater BB Hybrid - v2.0

Unread postAuthor: SPG » Mon Feb 25, 2008 12:05 pm

This is version 2 of the 4.5mm Repeater Hybrid HERE which I think answers a lot of the crits. The main one was about the spring rate, and the pressure that it would have to hold back. To this end, I plan two things.

First of all, more room for the spring. More room, means a bigger spring, which means I can get a stronger spring, it means too, that I'll only use a small fraction of its travel which means the spring rate is more even. I'm looking at either a) the spring from an MTB front fork or b) an air-rifle spring.

Secondly, the rear "piston". This will be only a little smaller than the sliding front piston. The point being that the force pushing the front piston against the spring at combustion is (if my physics is right) lessened. As an example, if the front "slide" piston is 2 square inches and the rear is 1 3/4 square inch, then the force on it is 2-1 3/4 x pressure. So if the chamber pressure before combustion is 100psi, then the difference is 25psi - so I don't need such a strong spring.

Here's the revised diagram.


*If JSR can start a new thread for every version then I figure I can
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Unread postAuthor: jackssmirkingrevenge » Mon Feb 25, 2008 12:37 pm

So, have you recruited the services of a good, reasonably priced machinist yet? :D
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Unread postAuthor: SPG » Wed Feb 27, 2008 6:42 am

Quick addition to this, relating to the one bit which isn't on the diagram, the spring.

In actual fact there'll be a twin spring system, the first is a weak spring which just pushes the piston back and forth during it's sliding stroke at fuelling and exhaust. This is mainly just to clear the chamber of the combustion gases, so strength isn't an issue as they can vent out of the barrel at the BB port.

The second spring however needs to hold the lot closed against the pressure of the fuel/air mix and only compress when combustion takes place. This will be the shorter in length so that when fully extended it just meets the piston as it slides forwads on fuelling.

Initially I'd wondered about a spring from a BB gun I have but I'd like to be able to adjust the spring rates and hardness. Then I remembered the elastomers used in mountain bike suspension. This is basically a synthetic rubber compound. In an MTB suspension it's provided using elastomer stacks, multiple elastomers lined up one after the other. By changing out some or all for different hardness or rigid spacers you can adjust the spring rate etc. And in MTB use they all come with a convenient hole through the centre, and they're also quite cheap.

So elastomer stack it will be, giving me greater adjustment, and because of their nature a slower rebound rate which will allow greater venting. Diagram to follow as soon as I've got off the phone with this client.
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Unread postAuthor: Jared Haehnel » Wed Feb 27, 2008 7:30 am

Nice design. It would be interested to if you could get it to work... looks like a lot of machining though....
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Unread postAuthor: Skywalker » Wed Feb 27, 2008 9:42 am

SPG: very nice! It looks relatively simple for what it's doing; I especially like that you combined the ammo-loading with the rest of the operation, and that your piston sweeps the exhausted air/fuel out on the return stroke. I take it you will have electronic controls for ignition and fuel/air injection, activated by the movement of the barrel assembly.

The only trouble I can see, and this would apply to any valved hybrid, is that it's going to be h-e-double-hockey-sticks for the orings around your pistons. In fact, since your front piston is going to be holding back the whole pressure difference between atm and firing pressure, and having to move some distance under that condition, that sounds a bit harsh. I think Jack's right, it's probably beyond the level of home-building with epoxy etc.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 27, 2008 9:52 am

Skywalker wrote:I think Jack's right, it's probably beyond the level of home-building with epoxy etc.


The problem is that epoxy tends to soften when heated, I would worry that the repeated heating from auto fire would cause it to fall apart pretty quickly, at least around the chamber area.

I would go for proper machining, it wouldn't be *that* expensive, and a reasonable well machined fit would eliminate the need for o-rings in practice.
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Unread postAuthor: SPG » Wed Feb 27, 2008 12:19 pm

Yeah I was avoiding o-rings completely in that assembly, and firing and fueling would be on the trigger, not with any complex electronics, but then to start with I'm only aiming for semi- not fully- auto.

Also because it's only going to be fuelled for a few fractions of a second, I'm not even bothered if there's a few tiny leaks.
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