Tennis Ball Combustion Cannon

[SpudBlaster15]

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[jimmy101]

Yeah, it's getting sticky and hard to unscrew now. I'll just get a new plug today.

You might spend some time and clean up the threads on the plug and the fitting. Sometimes there is a pretty severe "flash" ridge in the threads. This keeps the cap from screwing on properly since it seizes up way to early.

Latke talks about cleaning up the threads on his site.

[joannaardway]

The pressure rises enough to get the ball moving before the flame front reaches the back of the tennis ball.

That's unlikely to be the reason. The more likely factor is that after the propane has completed combustion, there is no oxygen left for the tennis ball to burn in.