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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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Unread postAuthor: Lentamentalisk » Fri Jan 25, 2008 5:27 pm

I am somewhat confused on the whole spark plug deal. You guys seem worried that your steel pipe will burst, but seem to have no issue just epoxying a spark plug in. Is there something I'm not getting here? Why is that safe?
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Unread postAuthor: Novacastrian » Fri Jan 25, 2008 5:42 pm

Is has to do with the effective surface area on the sparkplug after installation.
It is not all that much (around 1/2 square inch) but at a pressure of say 500psi (around 4x mix) you would have 250 pounds of force trying to push the sparkplug out. This force only acts for a split second.
Having said that i would not be happy relying on epoxy to hold it in. I would be sure to thread the hole. That and not put any part of my body near the plug when firing. :binky:
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Unread postAuthor: Lentamentalisk » Sat Jan 26, 2008 1:42 am

so threading a gutted schrader valve into the pipe, and filling it with epoxy is also considered safe?
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Unread postAuthor: Novacastrian » Sat Jan 26, 2008 2:19 am

It has been done countless times without death or injury. So yes, i would consider it safe.
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Unread postAuthor: SpudBlaster15 » Sat Jan 26, 2008 2:38 am

I've used my hybrid launcher at 8x mixes (theoretical internal pressure of ~800PSIG) several times without spark plug failure. The small hole in the chamber should also not cause a noticeable decrease in it's strength.
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Unread postAuthor: Ragnarok » Sat Jan 26, 2008 3:34 am

SpudBlaster15 wrote:The small hole in the chamber should also not cause a noticeable decrease in it's strength.

A perfectly round hole in a uniformly loaded flat plate increases stress concentration around it's edge by three times. Threaded holes are worse, because a threaded/tapped hole in a chamber wall will lower the required load to yield the wall by around a factor of 4, regardless of the size of the hole. (Unless it's area is a significant fraction of the wall area - then it gets worse).

I'd call that a very significant decrease in strength. That's why pressure gauges and schraders have to be tapped through double layers of pipe.
Sorry to go all "engineer" on you, but there are some dangerous and common misconceptions being sprouted here.

But here's a interesting fact - did you know that the stress around the circumference of a pressurized cylinder is twice as high as the longitudinal stress? That's why a chamber will split along rather than around (unless it's PVC and smashes).
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Unread postAuthor: Hubb » Sat Jan 26, 2008 7:25 am

Another update:

The silicone didn't work. The reason: I put it on the wrong side :lol:

To clear up the spark question, the new spark plug I made (same design as the diagram, just longer leads to place the spark moreso in the chamber) works fine. It has fired at 4x with no problem. Also, I have it set up so that if it fails, (1) I will not be near it and (2) I can simply rebuild it.
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Unread postAuthor: Hubb » Tue Feb 19, 2008 11:50 pm

Bump for another question:

I'm going to be hooking up a stun gun to replace the piezo. The problem is, I do not have a soldering gun. Is there a way to secure the spark wires to the prongs without a soldering gun? The wiki didn't help and neither did the search.
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Unread postAuthor: Lentamentalisk » Tue Feb 19, 2008 11:58 pm

wireties and duct tape... maybe a few hose clamps...


Go get your self a soldering iron and some non-leaded solder!!!!
how can you call your self a person with out a soldering iron?

p.s. sry about the big spammy font, but the topic required it
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Unread postAuthor: Hubb » Wed Feb 20, 2008 12:03 am

Dude. Did you just ask me to kick you in the balls?

JK. I plan on getting a soldering iron, just not today and I'm ready to shoot the d*** thing. Maybe I'll just go with some good ol' fashioned JB Weld. I got a lot of that stuff.
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Unread postAuthor: Lentamentalisk » Wed Feb 20, 2008 12:06 am

make sure it doesn't get in between the two things you are trying to solder together, otherwise it will make a nice insulator, and you wont get any spark what so ever... that happened to me once with epoxy X(
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Unread postAuthor: Hubb » Wed Feb 20, 2008 12:24 am

Yeah, I know. When I built my spark plug, I used the JB Weld to hold the prongs in place. The first time, that's what I did. $2 later and the problem was fixed.

Is there any other type of suggestions that I can use to complete this task?
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Unread postAuthor: jimmy101 » Wed Feb 20, 2008 1:50 pm

hubb017 wrote:I'm going to be hooking up a stun gun to replace the piezo. The problem is, I do not have a soldering gun. Is there a way to secure the spark wires to the prongs without a soldering gun? The wiki didn't help and neither did the search.

Actually, a typical soldering gun won't work very well unless you crack open the stunguns case. Soldering to the bigass stainless (or chrome) steel probes is very difficult and most soldering irons won't do it. To get solder to work you'll have to get the probes so hot that the plastic case of the stungun will start to melt.

One solution is just to use wires with aligator clips on both ends. (You can get a set of 10 or so from radioshack for a ~6$). This has the advantage of making it easy to move the stungun from gun to gun.

Remember that you don't need a very good connection between the wires and the stungun electrodes. The spark is perfectly capable of jumping another small gap in the system. Simply wrapping the wires around the probes and then securing them with tape will work fine and have zero affect on the effectiveness of the ignition system. Also, you really don't care about the gauge of the wire. Bigass 12G wire is no better than 26G (fairly fine) wire. You would of course like the wire to have pretty good insulation to keep from shocking the crap out of yourself.
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Unread postAuthor: Hubb » Fri Feb 22, 2008 5:14 pm

Jimmy: I didn't think about the clips.

Right now, I JB Welded the wires to the electrodes (although I wished I'd have used the clips) and it works perfectly. I haven't been able to test the hybrid, because it has been raining in South GA for about two days now.
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Unread postAuthor: SpudFarm » Fri Feb 22, 2008 6:32 pm

oh i got the same problem over here :(
i just made my hybrid but cant test it bechause of the darn rain and snow :'(
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