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Help Building "V3" advanced combustion

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Help Building "V3" advanced combustion

Unread postAuthor: metallover » Sat Feb 07, 2009 11:30 am

I am starting work on my third advanced combustion cannon. It will have stun gun iginition, chamber fan w/ custom controller, propane fueling system attached, and all the other stuff...

I need to know what wire to get to use for the stungun ignition. I picked up the cheapest stun gun I could find on ebay. It's 200k volts and I'm not sure if that will be too much when it comes to battery life? I just need to know what wire to use. It's my first stun gun.

My other question is about the fan circut. I'm a total noob to that kind of stuff. I have read latke's page about a fan circut, but I want something a little different.
-Use a 9.6v nimh rc car battery
-One button you press down and the fan runs for a timed period (555 chip?)
-Another button runs the fan manually
-LED tells you when the fan is on

Any help is appreciated.
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Unread postAuthor: starman » Sat Feb 07, 2009 11:44 am

You can just use standard 22 gauge "hookup" wire and then 2 layers of heat shrink tubing over it. Real HV wire is a little expensive and hard to find...and not really necessary here. Keep the leads short and separated as much as reasonably possible.

Since you typically fire the stun gun only about 1 second to fire your cannon, you'll find the battery will last longer than you think. A 9.6v RC battery would work great to power both the fan and your stun gun if you want to go that route.

While the 555 timer circuit is easy to build, I've found that I tend to leave the fan running almost all of the time while shooting. You're either airing out or mixing/firing. I would try running without the circuit first and see how you like it. The RC battery will last a long time just driving a computer fan. Then if you want, you could have a spare charged battery waiting just in case.
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Unread postAuthor: Combustion Monkey » Sat Feb 07, 2009 12:04 pm

The wires I use on my 200k volt stun gun are from a junk car neon underbody light kit. I know its a bit of a rare find but i've tried to zap myself through the insulation and have had no problems.

The stun gun I have runs on two 9v batteries. The battery pack also runs the fan. Even though the fan is rated for 12v the higher voltage hasn't hurt it yet and it gets run quite a bit. To control the fan I use momentary switches. One is on the ignition control box, and I just rewired my deadman switch to also run the fan during firing.

Battery life has been good. My first two batteries lasted through the R&D process, and several hundred shots between showing off the spark strip and actual firing. I hope this helped some.
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Unread postAuthor: metallover » Sat Feb 07, 2009 12:05 pm

Thats a good point. The battery is 3800mah and unassembled at the moment. I'll run without any circuts for now and see how it goes.

With the wire that sounds like a plan with the wire with added shrink. I just ordered 12m of 5mm heatshrink along with 4-6m of other sized from hobby city. That's the place to get heatshrink.. I'll also look around and see if I find any beefy wire with a lot of insulation.

$0.16 a meter! :o http://www.hobbycity.com/hobbycity/stor ... oduct=3833

All the other sized are about a quarter a meter, but that's still an amazing price. :wink:
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Unread postAuthor: jimmy101 » Sat Feb 07, 2009 2:35 pm

The wire really isn't all that critical, pretty much anything will work as long as you keep the wire well separated and the spark gap(s) is/are reasonable.

The "200KV" should jump a gap of nearly 8 inches with sharp gap points. In reality the stungun manufacturers tend to grossly over estimate the voltage of the guns. Even in a multi gap setup your total gap length is going to be less than 1/2 inch (13mm). So the wiring etc. only has to withstand about 13KV for sharp gap points (1KV/mm) and perhaps 40KV for very blunt points (3KV/mm). Your stungun will ony reach whatever voltage is required to jump the gap(s).

So, if the wires never approach each other to a distance less than the total spark gap distance then you won't have arcing problems. Just remember to route the wires such that you aren't touching them.

On a metal gun the insulation and wire routing is much trickier than it is on a PVC gun. For a metal gun the heat shrink is probably a good idea. On a PVC gun the tubing really isn't necisary.

One more thing about stunguns. It is absolutely imperative that there is always a spark gap that is short enough to actually spark. Nothing will kill a stungun faster than being fired without a spark gap.

The other thing that'll kill a stungun is just firing it to watch the cool spark. Cheap stunguns have a pretty limited life. If you only fire it when your are actually trying to fire a spudgun the stungun will last a very long time. If you fire the stungun just to watch the cool spark the total lifetime of the gun is probably measured in tens to hundreds of seconds. Furthermore, the gun should never be triggered for more than a second or so. You'll overheat the internals and burn it out.

On the fan controller circuit, I'm with Starman, the timer circuit for the fan really doesn't accomplish much. The fan will be running pretty much all the time anyway so you might as well just use a switch.
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Unread postAuthor: metallover » Sat Feb 07, 2009 5:03 pm

OK. So far I know that I can run the fan the whole time and I don't need a whole controller.

I just picked up a all the connecters, a 10' peice of 1.5" for the barrell, and a foot of 4" chamber. This is way too frickin huge. :D About what size should I cut it down to to where the barrell won't bend and to where it's still portable. I want it as big as possible.
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Unread postAuthor: Combustion Monkey » Sat Feb 07, 2009 5:06 pm

You may want to consider making it a coaxial or semi coaxial. That way you have a good bit of barrel living in your combustion chamber. I have found that this allows the use of a longer barrel in a shorter overall length. It also does nice things for the balance of the gun.

Check out the gun in my signature.
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Unread postAuthor: metallover » Sat Feb 07, 2009 5:12 pm

Co-axial looks like a great idea. Do you just take the volume of the barrel that is inside the chamber and subract it from the volume of the chamber when finding your C:B ratio, or is there more to it?
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Unread postAuthor: Combustion Monkey » Sat Feb 07, 2009 5:16 pm

Thats a good way to get an idea of your ratio. Due to all the odd bits in my chamber I like to do a water volume so I know exactly what the displacement is. Something else to consider is fuel mixing. I built a prototype to test my theories and found that i needed to figure a way to get a fan in the chamber somehow. With the coax barrel it took the fuel and air forever to mix on there own.
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Unread postAuthor: starman » Sat Feb 07, 2009 7:45 pm

metallover wrote:OK. So far I know that I can run the fan the whole time and I don't need a whole controller.

I just picked up a all the connecters, a 10' peice of 1.5" for the barrell, and a foot of 4" chamber. This is way too frickin huge. :D About what size should I cut it down to to where the barrell won't bend and to where it's still portable. I want it as big as possible.


If you're just shooting spuds, I would cut it around 5'. You won't realize much more performance with a longer barrel simply because of the drag and irregularities of the potato. If you are shooting a hardened projectile, that is a different story. However, at 5' the barrel stays straight and is still portable.
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Unread postAuthor: Fnord » Sat Feb 07, 2009 7:52 pm

To avoid the downfalls of the coax design, I would suggest making a 1" thick pvc plug with a hole offset from the center. This will plug the front of the chamber and allow you to put a barrel in near the top of instead of right through the middle. You'll have more room for fans and stuff.

I'd also stick an endcap on the front just for extra strength.
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Unread postAuthor: Combustion Monkey » Sat Feb 07, 2009 8:05 pm

_Fnord wrote:To avoid the downfalls of the coax design, I would suggest making a 1" thick pvc plug with a hole offset from the center. This will plug the front of the chamber and allow you to put a barrel in near the top of instead of right through the middle. You'll have more room for fans and stuff.

I'd also stick an endcap on the front just for extra strength.


Not to hijack the topic, but have you built a coaxial combustion?

Back on topic. I think you should build whatever you think is fun. I did a coax just because I wanted to figure out how to do it. I also believe that greater performance can be achieved through a more traditional advanced combustion. In fact my next project is going to be much more traditional.

Don't let us loons influence you too much. Sure get some great ideas, ask a ton of questions, but build your own gun in the end. You'll love it that much more!

Sorry if that was a bit of a rant. I think the best ideas are the ones that come from the individual builders imaginations.
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Unread postAuthor: inonickname » Sat Feb 07, 2009 8:42 pm

Cable from a flyback (eg. an old tv) is awesomely insulated. Try running extra cells in parallel for my runtime. If the timer you're using can't take that voltage, use a relay.
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Unread postAuthor: metallover » Sat Feb 07, 2009 11:10 pm

No, no. Keep the ideas coming. I think a Coaxial is a great way to get more performance out of a smaller overall design. I'll do some searching, but what are the main pros/cons of a coaxial and what problems will need to be addressed.

-I'm using a cam-lock with the barrel and it would be perfect to make into a coaxial.
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Unread postAuthor: Combustion Monkey » Sun Feb 08, 2009 10:44 am

Ok, this is what I learned from building my coaxial combustion.

The first thing you should decide is if you want to breach load or muzzle load. The only downfall to breach loading is that it makes it interesting to get an 80mm fan into the chamber, thats why I built my fan into a wye. If muzzle loading is the way you want to go its much easier. You can mount your fan in the traditional location and have the coaxial barrel stop just short of it. The last part will also work with breach loading but you will have to remove the coaxial barrel to load the gun (pain in the arse). If you do chose to breach load I recommend getting another set of cam locks to mount to the breach. My screw in end cap had a nasty habit of just about fuzing itself to the chamber to the point that it required 2-3 people to remove it!

For some reason having the barrel in the chamber does odd things to fuel mixing. I would call some sort of fan mandatory unless you want to wait 5-10 min between shots. I have also noticed that the coaxial barrel acts as a heatsink. The first few shots will be less powerful than those that are fired after the chamber is up to temperature. You have to warm it up like an old car before it runs right :lol:.

For your ignition I would recommend a spark strip. Standard electrodes will interfere with the internal barrel. I like the spark ring I use on The Bettie. It is easily built out of odd bits of wire and pvc that are most likely kicking around anyhow.

Thats all I can think of at the moment. Everything else is the same as a standard combustion. If you think of something I missed don't hesitate to ask.

Good luck on the build!
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