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A Few 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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A Few Hybrid Questions

Unread postAuthor: Hubb » Mon Jan 07, 2008 9:01 am

Apart from my pneumatic project, I want to construct me a hybrid. Now, I've never built a hybrid but I do know the principles of such. I'm looking at using a steel pipe, 2" X 12" for the chamber.

I just have a few questions (searching just didn't give me the answers I wanted):

1) Will a chamber fan increase the performance of a hybrid like it does for a combustion?

2) Other than drilling a hole and filling it with epoxy of some sort, what is the best way to run wires into the chamber?

3) I would like to operate it at 4x, but, on occasion, 6x. Would a seemed pipe be safe enough to use for this?
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Unread postAuthor: spanerman » Mon Jan 07, 2008 9:13 am

1. no, the air injection causes turbulance that mixes them..
2.seems best way to me....
3. yup fine. i use mine at 8x...
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Unread postAuthor: Hubb » Mon Jan 07, 2008 9:23 am

Thanks for clearing that up for me. Now, I'll build a hybrid. :D
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Unread postAuthor: Fnord » Mon Jan 07, 2008 11:01 am

The air injection mixes them, but doesn't cause near the turbulence of a fan, and the air slows down as soon as you stop filling.

But, I can't see a chamber fan surviving for too long in a hybrid chamber. I had a piece of pvc separating the wires in my spark gap, and it was completely burnt black when I took it out, to the point where the spark would arc across it.

2.If you have a schrader valve somewhere on your chamber you can run one wire through the brass part, and hook the other to the chamber walls.
Reducers seem to be thicker than most other fittings. You should probably drill through one of those

3.As spanerman has already said, you'll be fine with 6x. It's the normal mix I run at now.
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Unread postAuthor: Hubb » Mon Jan 07, 2008 11:33 am

About the fan, that's what I was thinking it would do, but wasn't sure.

As far as the spark, I know it will have to be a tiny one, but there should be no reason I would need more than one, right? Also, will a piezo work or will I have to use a hotter spark method?
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Unread postAuthor: Fnord » Mon Jan 07, 2008 11:38 am

A piezo will work so long as you keep the spark gap down to a mm or so.

You shouldn't need multiple sparks. There are rare cases when it would give an advantage, but none of them apply to you right now.
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Unread postAuthor: Hubb » Mon Jan 07, 2008 12:14 pm

As of now, all my questions have been answered. Thanks guys.
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Unread postAuthor: spanerman » Mon Jan 07, 2008 12:17 pm

for you ignition id recomend a spark plug...nice and simple :)
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Unread postAuthor: Hubb » Mon Jan 07, 2008 12:36 pm

spanerman wrote:for you ignition id recomend a spark plug...nice and simple :)


For placement of the spark, yes. But, in order for it to operate, I would need an ignition coil and a large battery, correct?
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Unread postAuthor: spanerman » Mon Jan 07, 2008 12:38 pm

nope, just conect one end of peizo to tip of plug and the other to the chamber....the spark plug is just a method of getting the current into the chamber..
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Unread postAuthor: Hubb » Mon Jan 07, 2008 12:47 pm

That makes sense. I'll consider it.
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Unread postAuthor: MrCrowley » Mon Jan 07, 2008 4:54 pm

Sparks plugs are great, they already have a small spark gap, which can be changed. Though you have to drill a bigger hole for them.
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Unread postAuthor: DYI » Mon Jan 07, 2008 4:59 pm

I'd really only use a fan in a hybrid if I wanted to cause a detonation, and a conventional computer case fan wouldn't survive too long in a high mix hybrid chamber, as the heat is more intense and lasts longer than in a normal combustion (apparently the pipe in _Fnord's 11x test was too hot to hold for a while after the shot).

All the rest of your questions seem to have been answered quite well. The burst pressure of 2" SCH 40 welded steel pipe has been calculated at over 2kpsi, so even a 10x mix would leave you with a reasonable safety margin.
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Unread postAuthor: Hubb » Tue Jan 08, 2008 6:42 am

Again, thanks guys. I've considered the spark plug idea and it appears to be a better one that what I had in mind. As far as drilling a bigger hole, here is what I thought:

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From the diagram, the spark plug will be placed into the back of the launcher in a tee. This will keep me from having to drill into the chamber or a fitting. Of course, I'll probably have to epoxy the spark plug into the tee, but that's okay.

Any additional thoughts are welcome.
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Unread postAuthor: spanerman » Tue Jan 08, 2008 6:48 am

move the scrader to the fuel input...this way the air injection will force the propane into the chamber
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