Generation II Hybrid
First, you'll have to excuse the name. Back in the day this was considered the next great thing in hybrids, but they have fallen out of use for one reason or another.
A hybrid launcher works, as you recall, but creating a pressurized fuel:air mixture in a chamber which is then ignited, breaking either a burst disk or opening a special valve to launch the projectile.
Usually, this mixture is created by injecting the proper amount of propane through a meter, and then pressurizing the chamber to the correct pre-ignition pressure with air.
The generation II hybrid works on a simpler principle. First, a fuel:air mixture is created in a large tank like you would fuel a standard combustion launcher. Then, water is injected into the tank at pressure (from a garden hose) to compress this mixture into a smaller tank at the desired pre-ignition pressure. They are closed off with a ball valve and the smaller tank is ignited, breaking a burst disk or opening a special valve like a normal hybrid.
This design uses this concept somewhat akwardly. First off, my tank was not as big as it should be for my chamber. so, to get any sort of respectable number of shots from it, I charged it with a 2 atm fuel mixture instead of 1 atm. This tank is connected to the ignition chamber by way of a coiled hose to visually ensure I didn't get water into the ignition tank.
The tank itself requires several features which allow it to function as intended. Due to the system of injecting water via a garden hose, there needed to be a way to purge air from the hose so that it didn't throw off the fuel mixture. This was solved with a tee and two ball valves. The top valve was opened and the bottom one shut, and then the garden hose was connected and allowed to run for a minute or two to purge all of the air from the hose and connections.
Also, there obviously needed to be a way to inject fuel and air, because this tank is designed to hold a 2 atm mixture instead of the 1 atm mixture in a pure generation II hybrid. This is the scrader valve and propane meter mounted on the top.
Finally, there needed to be a way to drain water when the fueling tank was depleted. I simply added a draincock to the bottom of the fueling tank, which I would open, and then pump in air through the scrader valve at the top to re-introduce fresh air and expel all of the water.
The fuel mixture of course went through the curly hose and into the ignition chamber. The coiled and clear hose was used so that I didn't get a shot of water into a plain steel ignition chamber.
The ignition was accomplished by drilling two small holes through a piece of 1/4" aluminum rod and running insulated wires through them with some epoxy. This was in turn fed through a 1/4" compression adapter which was then threaded into the launcher at the back of the chamber. The voltage source was a simple piezeelectronic BBQ grill igniter.
Finally, it was all slapped onto a wooden base, with a union for loading burst disks and ammo, and I had the launcher I wanted.
However, like most of my favorite launchers, I had to scrap this one when I moved. I was able to get consistent performance out of it after fine tuning my initial (compression tank) fueling by taking exact volumetric measurements of both the propane meter and fueling tank. However, during the first few weeks of playing around with this thing, I had plenty of misfires. If I ever rebuilt one of these, I would definitely use a 1 atm compression tank...