Repeater BB Hybrid - I think I've cracked it.

[SPG]

Finally I think I've cracked the main part of my Holy Grail - the repeater hybrid. I've pulled together a load of ideas from Jack's auto-piston theories to give what I think would be a true burst-disc free repeating hybrid. Personally I'm happy for it to be "semi-auto", but I could see how with a little extra circuitry it could be selective fire.

So here's a schematic of the combustion chamber:

Step one - Fueling The gun has a two stage trigger. the first part of the pull would open a valve from the regulator from a pressurised fuel/air mixture tank. As the mix enters the chamber it pushes the piston forwards, sliding along the barrel, against a spring (not shown).

Step two - Ignition Once fully pressurised the trigger is squeezed further, closing the valve, against a switch, firing the ignition circuit. The combusting gas pushes against the piston which moves further forward, against the barrel sleeve, pushing the barrel forwards. This brings the BB-loading port into the chamber. The combusting gases travel into this port and the BB is pushed along the barrel.

Step three - Exhaust The pressure drops in the chamber, the pistons sweeps forwards, sliding along the barrel, pushing the remaising combusted gases out of the BB-loading port. When it reaches the sleeve it pushes the barrel back, loading another BB.

And then we start again.

Body I found some chomolloy steel tubing which I see as being suitable for the chamber, and some very tight fitting nylon rod which I will use for both ends of the chamber and the piston (machined down a fraction), the barrel is hydraulic brake line. The plan is to run the steel tube the full length of the gun, with the sections of the rod forming the various parts inside. This would be drilled and tapped so that it is held in place with small allen bolts.

Fuel/air tank A seperate piece of steel tube slung under the barrel like an over-under shot gun, the muzzle side end cap of which would be deliberately weakened, so that in the unlikely event of combustion inside this fuel/air tank, it would act as a burst disc.

Ignition I've been looking at the electronic ignition systems for small RC engines. They use a coil-less system and of course are capable of firing at engine speeds (up to 10-12k rpm) so that's plenty past my rate of fire.

Fully auto The ignition system (in the RC install) uses a small Hall effect magnet fixed to the crankshaft, to trigger the ignition as it passes the system's sensor. I could install this in the piston so that when it reaches fully pressurised postion it fires the ignition. I can see I'd need to install a small check valve in the fueling system to prevent "flashback" into the fuel/air tank, as this set up would mean that the fuel valve at the trigger would remain open. I'm sure a small bit of circuitry and a switch would enable me to fire fully or semi.

But like I said, for me semi is the real goal.

[bigbob12345]

Maybe it could work Im not sure.
It would not be an easy feat it would definatly require a lathe and the timeing would have to absolutly perfect otherwise it wont work.
I dont think it is worth the effort I would just suggest a high pressure pnuematic semi auto which would get the same power if you use great enough pressure and it would be much easier to set up and you wouldnt have to deal with the perfect timeing thing.
From me youve got it might work.

[mopherman]

So you wold be keeping a bulk tank with fuel air at high X? sounds safe. How are you going to keep the barrel from getting pushed all the way forward at the filling stage? springs would be tough to calibrate. not to mention tolerance issues.

[th3p0p0]

i like it. it sounds like it will work if you have to timing down just right (like he said^^). if you have the shop to do this then by all means do so.

[jackssmirkingrevenge]

Interesting, very interesting. Agreed that there's going to be some careful tuning needed, and that a premixed fuel-air supply might be a tad dangerous...

[SPG]

I don't get the timing issue here, like I say squeeze to first trigger pressure and fuel the chamber, squeeze further and fire the gun, no timing issues unless you count firing before the chamber is fully pressurised - and if you think about that it'll still fire, because the fuel/air ratio is always right no matter what pressure you're at.

If I went for fully auto then again, there's no timing issues, as it's the piston that triggers ignition, so unless I'm missing something (which is why I posted at this stage)...

How do I set up the spring, well it's relatively easy. I could do the math using Hooke's Law, or I could do it trial and error (more likely to do that actually). I can simply test how much force is needed to compress a certain spring a certain distance, just by loading it with weights. Then I can calculate the force on any given piston by looking at the pressure behind it in relation to the area of the piston's face.

What I invisage doing is tailoring the piston face to the spring (this is easier than winding my own spring), after all if you think about it a truly tiny piston 1/10 of an inch squared in area at say 100psi would have a force of 10lbs against it. I don't see the face being that small, but you get the idea. That said I've actually got 4 springs out of a weighing scale (human not kitchen) and they're capable of taking a shared weight of 300+ lbs.

About the precharged fuel air mix. It's only the same as DR did back on Spudtech with his Gen II water pressurised system, and as I'm thinking small calibre, small chamber, I only need a small fuel/air tank, I was only thinking 200cc or so. The plan here isn't a strafer capable of firing thousands of BBs in one go, it's a 20-30 shot rifle.

[jimmy101]

I don't get the timing issue here, like I say squeeze to first trigger pressure and fuel the chamber, squeeze further and fire the gun, no timing issues unless you count firing before the chamber is fully pressurised - and if you think about that it'll still fire, because the fuel/air ratio is always right no matter what pressure you're at.

Well, the big timing issue is keeping the fuel resevoir from grenading. The fuel inlet valve must be 100% closed before ignition. You might want to consider some kind of interlock between the inlet valve and the ignition system.

Other than that, it looks very interesting.

[SPG]

Well the big thing to stop that is the check valve, which would of course prevent the combusting gas traevlling back down into the fuel/air tank.

And of course the fact it's one single two stage trigger operating both... so it's not a timing issue it's a mechanical issue.

[Sticky_Tape]

You should just make it a pneumatic

[SPG]

You should just make it a pneumatic

Well that's no fun is it, part of this is the challenge. And anyway this forum is littered with people's attempts at pneumatic auto-BB's.

The other reason for hybrids on this scale of course is that it's very portable. No compressors to worry about and still good full power multiple shots. The only thing that would come near is vHPA or CO2.

[PinHead]

I think it looks very good, in fact I was expecting something much more complicated. My main worry would be the massive impact from the piston inside the gun; figure that the piston is going to experience equal the pressure that your projectile is going to, but in actuality it could be even more, since it has a larger surface area for the pressure to act on. You better have a VERY strong bumper in there - maybe one on both the piston AND the chamber wall. I can only imagine the amount of recoil produced by the piston too. But then again, you have the force of the projectile and the piston acting in opposite directions, so would they cancel each other out, even somewhat?