SpudFiles

Hey guys, have you all ever had problems with steel chambers and high voltage ignition sources? What sort of insulation do you use?

epoxy works well, but standard spark plugs can also be found in standard NPT threads

or, you can usually get spark plug thread taps as well if you need to

Epoxy!

I used a spark strip for my metal chamber. I supportd the spark strip with screws, and then fed the wires through another set of holes. The rubber coating of the wires is enough to prevent shorting. Epoxy was used to seal the holes and not for shielding.

On my all metal launcher, I drilled out steel 1/4m x 1/8"f reducer bushings and fed two electrodes through with .015 nomex around the outside and .045 fiberboard inbetween the two electrodes. I filled the remaining cavity wit PC7 epoxy and ran the whole assy. through a 30" of mercury vacuum chamber to remove voids while curing. Voids in high voltage insulation materials cause internal shorting and carbon tracking. Extensive research was done to get to this point with regard for pressure testing. The Spark bushings would easily withstand 150 psi air pressure and thread into the chamber via 1/4" npt half couplings welded to the sides of my chamber. Most folks have never ran their combustion launchers through any sort of pressure testing. If you have ever had a ball jam in your barrel you would know why this is so important. Check out my build pics if you are intersted. Happy Building, hope this helps.

So epoxy can, Moon, can withstand hybrid-level pressures, and an additional set of holes wouldn't weaken the chamber too much? (The cobras seem fine )

I may consider a similar, albeit less equipment requiring method, motor...

Grignacke wrote:So epoxy can, Moon, can withstand hybrid-level pressures, and an additional set of holes wouldn't weaken the chamber too much? (The cobras seem fine )

I may consider a similar, albeit less equipment requiring method, motor...

The epoxy I use is only to surround the wire as it feeds through the hole. The epoxy has very little surface area. It just fills a tiny gap around the wire to make it air tight. The epoxy is not relied on in any significant way. Rubber cement would also work for this purpose. I would not fill a large hole or use the epoxy as a major part of the chamber and expect it to withstand the punishment. Some people do that, but I wouldn't trust it. I use it as a sealing compound only.

I'm using an NPT spark plug for my hybrid ATM, you just have to look around for some standard threaded sparkplugs if you don't want to use epoxy