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Hybrid Questions

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Hybrid Questions

Unread postAuthor: DonTheLegend » Mon Mar 23, 2009 6:09 pm

Looking at everyone else's hybrids has got me wanting to make one too. I've been kicking ideas around for a little while and I have some questions. My budget is flexible, but I can't afford to keep dumping money into this so I want to try to plan something that will work as much as possible the first time (I know nothing works as expected the first time but you get my point).

First, plans on what I want to build:

Chamber: 2" steel pipe 6 - 12" long (approx chamber volume 30 ci)
Ignition: Stun gun through spark plug
Burst disk holder: ???
Barrel: Many barrels, but 3/4" to start (will fire custom molded ammunition)
Operating Mix: 4x - 10x minimum. Would like to move up to 20x + once a high pressure air source is found.
Fuel: Air/Propane
Metering system: TBD

I am trying to stay away from having anything machined. I have done this in the past and if absolutely necessary I'll do it again, but I'm trying to avoid it.

Commercial burst disks and holders look too expensive for my budget.

Questions:
1. What to use as the burst disk holder. It seems like people are recommending flanges for use with higher mixes because it is easier to hold the disk but I can't seem to find any that with a high enough pressure rating. A union is pretty self explanatory, but it seems like these are difficult to seal at higher pressures and thicker burst disks complicate this.

2. I am planning to build this out of seamless SCH 80 steel pipe and 3000 psi fittings to allow me to test higher mixes once I get a high pressure air source, but if I stay away from really high mixes(15x+) is this really necessary? I've read all about DYI's 20x gun, but does anyone else have experience using the cheap malleable iron fittings?

3. What to use for ignition. I was planning on getting a 100K volt stun gun and making a custom spark plug inside a bushing with epoxy that would thread into a tee on the chamber and the spark would just jump to the chamber wall, but would a spark plug from a car be easier? What kind of threads are these? Also, I realize that depending on what mix I use, a spark may no longer be feasible (not sure I really want to go to that high of a mix number either).

At this stage pretty much everything is up in the air so if anyone has suggestions about something else, please let me know.

Here is a pic of the basic planned layout:
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Unread postAuthor: jonnyboy » Mon Mar 23, 2009 6:19 pm

Burst disk holder: ???

A union or a piston but a piston may be a little ambitious if you've never made a piston valve for a pneumatic gun before.

As far as those fittings..

I wouldn't want to be near it when you fire it as in remote firing. But I would feel safe with it at anything under 10x.

Your metering options are manometric or the regular combustion 1x mix way.

A stun gun would be good but make sure to keep your gap as small as possible.
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mobile chernobyl wrote:I can shoot a Canuter Valve off my '82 Chevy Ram F150 AT LEAST 3/4 Mile with 'ma cannon made of soup cans duct taped together, then I just squirt some bacardi 151 in the chamber and hold up my cigarrete lighta and WHOOSH! That thing flies at least 3/4 mile
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Unread postAuthor: DonTheLegend » Mon Mar 23, 2009 6:25 pm

Yea I'm not worried about metering, that part is pretty straight forward, I just have to decide which method to use. And I've built piston valves before, I'm just not wild about them in a hybrid, especially since I want to look at higher mixes. Stopping the mass of a piston without seriously damaging something at higher pressures gets difficult.
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