ignition for my combustion

[ramses]

In a hybrid, you need a tiny spark, because the voltage required to jump a given gap increases roughly linearly with the absolute pressure.

so that system that would give you 1 inch sparks at atmospheric pressure would give a maximum of 1/16" at 16x. That's why Larda used a glow plug at 200x; a LOT of voltage would be required otherwise.

Is it possible for you to put the spark at the middle of the chamber? Yours is really rather long and skinny, so it would certainly be beneficial, as would multiple gaps. It would be as simple as sawing your chamber in half, adding a coupling, and tapping in your sparkplug there. you wouldn't even need a tap!

depending on where your gas inlet is, you may be having extra trouble. You should really use a fan for a chamber that long.

I hate to bring this up again, but you may be getting abnormally close to DDT. (ddt=bad, unless you plan for and exploit it). Multiple gaps would work to prevent this possibility while simultaneously improving performance. I should add that a fan increases the chance of DDT.
Depending on your spark size, multiple gaps should not be an electrical problem.

[cfb_rolley]

I would say it would be a mixture issue. What about fan positioning? If it's blowing towards the barrel, the spark should be between the fan and the barrel or does it not really matter? So for all the questions but hair spray, being a vapor rather than a gas like propane would probably settle wouldn't it? Causing a mixture too rich at the bottom?

[cfb_rolley]

By DDT do you mean detonation or something? Or is that where the flame front is too quick? I think I've read about it but can't remember. Worse come to worse I can just do what you suggested and cut/shorten/couple it back up then tap the spark in there. Though multiple gaps would be my preferred option.

Edit: got it to fire today with a fan just behind a peizo ignitor, but it was incredibly unreliable

[spudtyrrant]

I hate to bring this up again, but you may be getting abnormally close to DDT.

i highly doubt it, to my knowledge DDT has never happened in a hybrid( possibly high mix hybrids, such as Larda's, but it is impossible to tell), let alone a simple combustion, their are plenty hybrids with narrower and longer chamber's than his that have never experienced it.

[Crna Legija]

this guys did..

[spudtyrrant]

this guys did..

a copper barrel exploding at 20x doesn't automatically mean DDT, 20x creates a huge amount of pressure, and the ammo he had basically acted like a plug, try igniting a 20x mix in a closed copper pipe and see what happens, there is a reason you don't see copper hybrids all over spudfiles

[SpudBlaster15]

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[Hotwired]

That copper pipe does not have a working pressure of 2000psi

If you're using copper for major components you should be going nowhere near the burst pressure of the pipe.

[cfb_rolley]

I read a little more into DDT, it's a bit worrying. In my opinion I think it would be highly unlikely in an advanced combustion, but I wouldn't rule it out. Though in hybrids I would say it's a higher possibility and in a 200x mix it wouldn't surprise me that DDT could occur. But it seems as though your regular ghetto gun builder would find it difficult to replicate, let alone exploit. Though has anyone tried using a hybrid setup to achieve it and use it to their advantage?

[ramses]

I believe Larda got DDT at 100x with oxy/propane, but only because he embedded brass ~.5mm INTO hardened steel. that doesn't happen too often.

I believe most copper pipe DOES have a working pressure of around 2ksi. I believe burst is around 9ksi.

I'm just trying to be cautious in this case. turbulence+long chamber + end ignition

Multiple gaps, fan, and central gaps can only improve performance and make this whole thing safer.

This thread is about ignition trouble, not DDT.

[cfb_rolley]

Haha yeah kind of went off on a tangent... I will be moving my spark gap when I can, and I think with a decent propane metering setup my problems should be solved. How about a simple capacitor discharge for ignition? All it would need is batteries, a capacitor and something that collapses the circuit. Am I on the right track or am I still lost?

[Hotwired]

I believe most copper pipe DOES have a working pressure of around 2ksi. I believe burst is around 9ksi.

You do not have a source for that.

http://www.scribd.com/doc/7824801/Copper-Tube-Handbook

Very small bore tubing does have the kind of ratings you think of but not the size of pipe used on the cannon.

Also, soldered/other joins can be much weaker than the rating of the pipe and the act of soldering can lower the pressure rating of the pipe.

Standard copper pipe is not a suitable material for high pressure combustions.

How about a simple capacitor discharge for ignition? All it would need is batteries, a capacitor and something that collapses the circuit. Am I on the right track or am I still lost?

Make it a capacitor charged by a disposable flash camera board then have a switch to dump the capacitor through an ignition coil.

[ramses]

Replace the stock capacitor on the flash board with one around 1-5 uF for much faster recycling, little to no loss of power, increased battery life, and increased circuit life.

Also, Don't get shocked. it hurts. a lot.

I believe most copper pipe DOES have a working pressure of around 2ksi. I believe burst is around 9ksi.

You do not have a source for that.

http://www.scribd.com/doc/7824801/Copper-Tube-Handbook

Very small bore tubing does have the kind of ratings you think of but not the size of pipe used on the cannon.

Also, soldered/other joins can be much weaker than the rating of the pipe and the act of soldering can lower the pressure rating of the pipe.

my bad

but 1" pipe seems to lurk around 500psi rated for type M. I don't think 2 KSI would rupture it. In fact, type "M" (the thin stuff) bursts at ~3800 PSI, as long as it is not annealed. 50/50 solder gives a working pressure of 200PSI. The copper shouldn't be annealed unless you overheat it, which is easy if you have a big torch. I don't see why it is a poor choice for a high pressure combustion.