SpudFiles

Well I'm building a true micro hybrid (not out of a pill bottle with a syringe to pump pressure..). The main chamber is a 2 layer (maybe 3 layer soon) pressure PVC pipe, with a schraeder valve to pump pressure. The valve is bolted, o ringed and epoxied in. My plan is to use a screw in reducer (one end of the pvc is screw type) to reduce down to a barrel. A heavy duty piezo sparker will be used (from a hot water system) to provide ignition.

I have several problems with this.

1. I do not have propane readily available, is it ok to use butane? If so, what are good ratios and pressures to use for 2x, 3x and so on?

2. So far (barrel not complete) the weakest part is rated to 15 bar (safe manufacturer recco.). Is this safe or do I need to beef it up?

3. I haven't found anything useful for barrels or burst disks so far. Any reccomendations? I was thinking of aluminium foil.

(endcap for it is 15mm class 18 pvc.)

Any help greatly appreciated, as this is my first hybrid.

I'll post pictures of what I have soon.

Butane will be fine. Why do you think propane isn't available?

Yes, aluminum foil usually works well. Have you considered your burst disk holding mechanism? If your burst transition is very small, you may have a challenge finding the right layer count (it will be lower than you think).

The field is really wide open to you for barrels. Decide what you want to shoot for ammo and plan for the proper barrel size to accomodate it. Brake line, commercial pellet and BB barrels, copper...there are several options.

Have you built other basic combustion or pneumatics cannons before?

Propane is available, but it's a fair drive and very expensive. The burst disk is held in place by a washer between two threaded parts, kind of hard to explain.

Yes, I've built several other cannons before.

I think we have some copper pipe around which I'll use.

btw for butane, what ratios would it be used in/pressures?

instead of a PVC chamber, i would strongly recommend using copper, or malleable iron fittings.

inonickname wrote:1. I do not have propane readily available, is it ok to use butane? If so, what are good ratios and pressures to use for 2x, 3x and so on?

This is unlikely to be a problem, but be aware that butane's pressure is much lower than propane's, and the pressure will fall very drastically not much above freezing.

It's not a gas for cold days, but you shouldn't be too worried by it's lower than normal pressure, as it can, at a typical room temperature (although I don't suggest firing inside) still be capable of mix numbers up to around 100x. You shouldn't be going that high however.

btw for butane, what ratios would it be used in/pressures?

Butane's ratio is about 3.23% to air, for a total of 3.13% of the mix.

What exact numbers you will need will depend on your fueling method (captive volume or manometric), but if you take pressures for propane, then multiply by 0.776 (or just 3/4 would be roughly right) to get the butane pressure - remembering the final pressure will need to be the same, topped up with extra air. Just bear in mind, if you're doing captive volume metering, the meter pressure cannot exceed approximately 30 psi, as that is the typical pressure of butane at 20 degrees C.

Ali, I don't have any copper appropriate sizes available. Also conductive containers make ignition difficult. The pvc combo should withstand a few hundred psi easily. I have a burst disk setup ready but I'm hoping that a spud pellet will have enough friction to hold the (air) pressure before firing.

Aussie summer here, so theres no way that it's going to be too cold.

Thanks

inonickname wrote:I have a burst disk setup ready but I'm hoping that a spud pellet will have enough friction to hold the (air) pressure before firing.

Afraid not. Spuds will start moving at just a couple of psi at the outside, so you wouldn't even make it to 2x before the friction was exceeded.

Put another way, I have fired spuds with pneumatics using 8 psi of pressure (I was testing the valve's ability to work on low pressures) - spuds will not stay in place at 8 psi. 2x happens at 15 psi, and you couldn't even make 1.5x.

You'll be needing that burst disk set-up.

Ragnarok wrote:

inonickname wrote:I have a burst disk setup ready but I'm hoping that a spud pellet will have enough friction to hold the (air) pressure before firing.

Afraid not. Spuds will start moving at just a couple of psi at the outside, so you wouldn't even make it to 2x before the friction was exceeded.

Put another way, I have fired spuds with pneumatics using 8 psi of pressure (I was testing the valve's ability to work on low pressures) - spuds will not stay in place at 8 psi. 2x happens at 15 psi, and you couldn't even make 1.5x.

You'll be needing that burst disk set-up.

Bah, alright. Thanks for the formula though, got filling worked out now. (i wrote 1x, 2x and so on pressures on my bike pump (it has an inbuilt guage) and marked locations for 1x, 2x and so on for the syringe (butane).

Waiting for the epoxy to dry, then going to solvent weld it all together.

Thanks for the help