For my whole life, I've been interested in projectiles. As a kid, I liked to throw rocks on our farm. When my dad taught me how to fire a rifle, I enjoyed target practice. Ballistics in general fascinate me, whether it's a well thrown basketball or a Hohmann transfer orbit.
Living in a fairly densely populated area in Los Angeles, I couldn't exactly take my .22 out and plink cans, so I shelved the practical side of ballistics and went back to playing video games. In 2000, I stumbled across the Backyard Ballistics page and became interested in the idea of potato guns.
I built my first few using PVC and fired them using Piezo-electric BBQ igniters that would create a spark and set off the mixture of air and hairspray, starter fluid, or propane. When it worked, it was great. The problem was, it didn't work all the time.
Getting air mixtures correct is tricky, especially when you have to rely on something as small as a spark to ignite your mixture.
It has great range, is easy to play around with, and you don't need to keep a fire extinguisher handy.
I used 2" PVC for the pressure chamber and barrel. In the photo to the right, you can see the pressure vessel on the left. The nub at the top is a Schroeder (bicycle) valve I got at Pep Boys for a couple bucks. A couple feet of tubing are below it, then at the bottom I use some reducer bushings to drop it down to 1" going into the green solenoid valve. You can find the valve at Home Depot in the sprinkler system, they run about $10-15 US.
From the valve, it goes through another bushing to feed back up into the barrel. I wired a simple switch in a project box to the barrel. The switch interrupts a serial circuit for two 9V batteries. The solenoid valve says it's designed for 24V DC, if I recall correctly, but 18 volts seems to set it off just fine.
The procedure for discharging it is as follows:
At step 5, the air discharges immediately and pushes the projectile out at high speed. For my purposes, the small air chamber provides enough volume, but for long distance or higher muzzle velocity, more volume is probably better.
Because I still live in a high population density area, I usually just pour a cup or two of water down the barrel and fire it. It punches the water out as an aerosol cloud (more on this in the next section) and can completely soak someone. I like firing water because it kicks, but I don't have to worry about breaking someone's car window with an errant projectile.
After firing it like this for a while, my imagination began to wander. What else could it do? I've designed and built a few things. A little time, and I might have a good attachment that fires a grappling hook for climbing. I've also considered the utility of using it to launch lit fireworks up to altitude so they look and sound more spectacular, I'll have to find an isolated area to safely try that.
The 'Flame Machine'
It became clear to me that the high pressure air was blowing through the water and blasting it apart into a mist. I began to wonder at the possible uses of this aerosol and in a flash of inspiration, came up with an device christened by my police department (more on that in a moment) as a 'flame machine'.
I purchased a MAP gas (it could work just as good with Propane, MAP was just more convenient) tank with a remote torch nozzle (safety first) and mounted the torch head at the end of the air chamber at a 45 degree angle that would put the flame directly in front of the mouth of the barrel.
I attached the tank and tubes to the assembly with wire ties and a big hose clamp, then fired it up to make sure the torch worked. After shutting it down and building a simply frame to hold the contraption facing upwards, I was ready to try it out.
Modified firing sequence:
At step 9, the compressed air forces its way through a liquid like usual, but this time, it's flammable alcohol. It blasts an aerosol of this fuel past the piece of tape and through the flame from the torch. Much of it ignites, and as it travels upwards, it turns into a fireball approximately 30-40 feet up that expands to a mushroom cloud at its apex.
I discharged this for my neighbors and received a number of compliments. The next day, I heard that the police showed up five minutes later and were asking around about someone who has been reported as having a 'fire machine'. I'm sure that what I did was legal, and I consider it a compliment that my display drew their attention on the 4th of July when boxes of firecrackers and other illicit fireworks didn't.
I'll post some photos soon.