Introduction: Sci-fi Propane Pop Gun
Right from the start let me thank fellow Instructables author Eric Rocher for inspiring this project through his own article that can be seen here.
I would not have guessed that this project would work until I saw it myself. It can be really tricky to get just the right amount of fuel into a small volume of air to create a flammable mixture, but a torch seems to do the job excellently.
The way this project works is that once the torch has fed a mixture of propane and fresh air into the vinyl tube, being ignited on one end causes the advancing flame front to push the not yet ignited fuel forward until it reaches the larger volume inside the soda bottle. Having a larger surface area inside the bottle allows the flame front to propagate much more rapidly, creating a loud pop.
An interesting effect I noticed was that the bottle occasionally created an invisible vortex ring which would slam into objects on the far side of the room. In a future video I may experiment more to create a way for those vortex rings to be made more consistently.
The torch I used is a Bernzomatic ts8000, and the vinyl tubing is 5/8" ID by 7/8" OD. I used a 24oz soda bottle with a standard cap size. Some of my viewers on YouTube have suggested trying this project with a micro torch or jet lighter and smaller tubing, which is a very interesting idea. I have doubts about how it might work, but it could be a fun experiment!
I hope you enjoy!
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