Semi auto design

[ramses]

good job so far. I would like to point out that unless you close off fueling very well after ignition, and it stays closed, you will have a mini blow torch where the air/fuel enters the chamber, assuming you have a flash arrestor. If you left out the flash arrestor, you will have the whole reservoir go up.

Make sure the reservoir can take the pressure of combustion should it decide to go up all at once (100psi per "X" that you store in it)

maybe you could post a diagram of your regulator so its operation is more clear.

[daccel]

Thanks. I've been scratching my head all day trying to address the problem you brought up about closing off fueling until the temp has dropped back below ignition point. You could in theory have some way to delay the piston return as I originally was thinking, but any method of doing that seems complicated.

In the end I settled on having an on/off valve attached to the trigger. May not work if you're firing too fast, but it's the easiest way I could think of doing it.

I've changed the chamber/breech design once again. It needed to be more compact to line up the breech with the magazine.

The flame arrestor is based off one I saw on Youtube. It's basically just fine bronze wool in a tube. I put it after the regulator, because I figured that only a small volume of burning gases might pass through before the regulator sealed, which would be easier to cool than the full volume off the chamber.

Let me know what you think.

[ramses]

that looks very good; holding the trigger will seal off the reservoir. I would add a check valve right by the chamber, then the flame arrestor, then the reg. If it works out, I REALLY don't want to be shot with it.

[SPG]

WOW, excellent my dreams are coming true, finally a small calbre semi-auto hybrid actually being built, instead of like mine where it exists on here and in my head only.

A quick thought on venting. You're obviously going for semi-auto only with your current set up. If it were me, I'd have the chamber piston lock back after each shot, this is how it'd work:

You currently have a two-stage trigger
#1 stage: chamber fill valve open
#2 stage: piezo ignition

If your "chamber piston" is locked back after each shot you'll need a simple linkage so that at #1 stage of your trigger pull, a sear is released allowing the "chamber piston" forward movement, sealing the chamber. Combining this with the chamber fill valve opening, will allow a tiny bit of your fuel/air pre-mix into the chamber before it is sealed flushing the spent combustion gases out through the now closing breach. With the "chamber picton" now fully forward and the breach fully closed the fuel/air mix pressurises the chamber. Then you simply squeeze off the trigger for #2 stage.

I'd also be very tempted to use an "air spring" behind the "chamber piston" as you'd first thought. The obvious advantage to this is that you can adjust the pressure behind the "chamber piston" so that it opens when you want it to. It seems very hard to find springs that give just the right pressure unless you wind your won, and also an air-spring can be more compact.

Finally with a fuel/air tank as small as you're proposing I wouldn't worry too much about explosions. If you make the seal at the bottom end your weak point, then it'll act like a burst disc and assuming your firing with a standard pistol shooting stance any buring gasses will be directed down away from you.

I do totally love this thing, and ca't wait to see it built. I'm hugely jealous you're actually doing it, not just dreaming it like me.

[daccel]

ramses - I don't plan on shooting anyone with it. Just wanted the shell for looks. Even at 1x it's more than double the stock fps.

SPG - Glad I'm not the only nut who is excited about this .

That's not a bad idea about locking the piston back. Except, how I've envisioned it, the fill valve is always open until the trigger is pulled, so then fuel/air would always be venting unless you continued to pull the trigger. I suppose you could have it only fill at the midpoint of trigger pull, but then you might not be able to squeeze that fast. Also, what about throwing off aim with the piston snapping forward?

For now, I think I'm going to simplify things by only have an on/off slide valve between the chamber and reservoir to speed up the build. Then modify from there if it fails.

I know I'm getting ahead of myself here, but since you mentioned dreaming... I've been toying with the idea of using a pressure switch to make it fully auto. So that when the chamber filled to the desired pressure, the switch would activate the spark.

And building off your idea of using a sear to lock back the piston, you could have a pneumatic ram to release the sear only when the pressure dropped back to atmospheric. You could also have a slide valve for filling (though it might require a dual surface area component like a pop off valve) that only opened when the pressure dropped back down.

Edit: Life keeps getting in the way of this build, but I hope to have something for the showcase section soon! Thanks for putting up with my overabundance of ideas and lack of finished product .

[SPG]

Oh right, I hadn't realised in yours that the fill valve was only shut when you squeezed the trigger, I thought in your diagram you were probably on the midpoint of your trigger pull.

As for locking back the piston and releasing it on first pull of the trigger, I don't think it'd be a big problem, you'd have time to aim as the chamber fills, I know your rate of fire might not be quite as high, but I reckon you'd probably still be squeezing off rounds as quick as you could aim. After all if you're firing too quick you won't have time to adjust your aim between shots.

Mind you, I've been pondering this too, and can see a way to perform the same lock-back/release function. I'm just not certain they'd fit in something as small as your pistol.

You'd have a sear built into the piston itself. Imagine a tiny piston/sear built into the top of your piston pushing upwards with pressure into the chamber wall. As you fill with fuel/air mix it'd be pushed up into the chamber wall, but not engage as there'd be no notch to engage with. Then when you ignite, your piston moves back, as it does so it passes the notch, and your sear engages holding the piston back till the pressure drops sufficiently for the sear/piston to drop down allowing the chamber to vent and the piston to move forwards. If your sear was wedge shaped I can imagine that this sear/piston could work with gravity (no need for a spring) which would mean your chamber would vent well.

As for fully auto, well that's a completely different and more complicated thing.

I'll do a sketch, but first, lunch.

[ALIHISGREAT]

I'd still be worried about reservoir ignition... i think that the resevoir needs to be combustion proof so if it does occur, there won't be any harm done to the gun or user.

[john bunsenburner]

How about splitting the resevoir up so that the two gasses are at specific pressures liek thatt some kind of valve can be opened to chaieve good mixing while there ir no danger of cumbustion in the resevoir.

[SPG]

I don't think there's any worry there, for a start it's a very small reservoir, and it's very easy to build in a burst disc style safety. Logically this'd be at the bottom so that any "blast" is directed out of harms way.

If it were a huge tank, then maybe there'd be more concern, but here, I can't see the problem.

[daccel]

I think I understand your sear idea. Definitely hard to integrate on something this small. Wouldn't the spring pressure behind the main piston create enough friction on the sear that it wouldn't drop down with gravity? Unless you mean wedge with the angle on the front side so that it could be depressed by forward motion?

Given the low mix I plan on using this at, I'm not overly concerned about reservoir ignition. At any rate, if the valve is air tight and closed, the only way that could happen would be if the piezo got out of sync with the valve, so I'll just make sure it's fixed in place well enough. However, I will keep in mind where the weak point is in the system and where it's pointed.

I have thought about splitting the fuel/air and using a single slide valve with two pathways, and a check valve on the propane reservoir so that air couldn't enter it. But that would require more space than I have. Something to note for future cannons though.

I was really hoping for a successful test today, but I think I need more precise gauges to meter something this small at such low mixes (~.1 cu in), because I couldn't get ignition.

See below for the final chamber/piston design.

I did, however, manage to test it using just air. It fires at 50 psi, so I need to put a stronger spring in there. But that's good, because it will help sealing. I was worried the spring would be too strong and have to switch to a weaker one that wouldn't seal it. At this pressure, it only dents a pop can, no penetration. But I'll try to install a spring that releases at close to the 200 psi I have on tap, and see what that can do.

So, after all that, I've basically just made a pop-off valve . At any rate, enjoy:

50 psi, 6mm .12gm bb
[youtube][/youtube]

Multiple shots, 50 psi, no bb
[youtube][/youtube]

[SPG]

So you made a pop-off valve work by accident and JSR's been trying to make one for years?

Quick fetch the Epoxy-meister.

[daccel]

Yeah, I've got skills 8)

No, as much as I'd like to one up him on that, I'm afraid it's not auto cycling. I had it hooked up to my bike pump and it fired and and reset with each stroke.

I tried putting in a stronger spring, but it didn't seem to really change the opening pressure, because the limited space meant that I couldn't precompress the larger spring as much and still have the valve open.

I think there is also a problem with the spark gap, because I couldn't even get fuel/air from a torch flowing through with the valve out to ignite. And of course, it's now thoroughly epoxied in place.

Someone slap me next time I don't make something completely serviceable.