Hybrid cannon
A hybrid is a type of spudgun that combines principles of the combustion and pneumatic spudgun. It uses a pressurized mixture of fuel and air to get more power out of a given chamber volume. The hybrid is capable of higher velocities than a combustion or pneumatic spudgun; projectiles fired by a hybrid have broken the sound barrier. This is possible because of the higher pressure and temperature gases generated by a hybrid combustion.
A hybrid launcher consists of seven basic elements:
- High pressure combustion chamber
- Pressure-triggered main valve (usually a burst disk, see below)
- Barrel
- Fuel system (usually metered propane)
- Air injection valve (usually schrader or quick connect)
- High-voltage Ignition source
- One or more pressure gauges
Operation theory
A hybrid is first injected with a measured amount of fuel - much more than would be used in a regular combustion. However, without adequate oxygen, the mixture would be far too rich to ignite, so the chamber is then with pressurized air. The amount of air depends on how much fuel was added, and is calculated to achieve a stoichiometric fuel/air ratio in the chamber.
A hybrid using twice as much fuel and air as a comparable combustion gun is said to be using a 2X mix. Higher mixtures can be used and will produce even higher pressures. The fuel and air needs to be measured and matched carefully to ensure reliable operation, pressure gauges and fuel meters are used for this.
The ignition of a hybrid gun is more difficult than a normal combustion, as the length of a spark is roughly inversely proportional to the density of the gas. This necessitates the use of high-powered ignition source such as a stungun, flyback, or other high-voltage source; grill igniters frequently won't jump a spark gap in pressurized fuel/air environments.
The gases resulting from the combustion of the fuel/air mixture are released to the barrel via a self-operating valve of some sort, most commonly a burst disk because of it's simplicity and reliability, although self-venting piston/diaphragm valves have been used. The valve is required to contain the pressure of the mixture pre-ignition, in other words, to prevent the pressurized fuel/air mixture from exiting out the barrel before ignition. Upon ignition of the fuel-air mix, the pressure rises and the valve opens.
Safety
A hybrid is the most dangerous type of launcher that can be built. It is advised to only attempt to build one with pipe that can handle the high pressure environment (300+ PSI) such as Sch. 80 PVC, or even better, metal pipe and fittings. As with any launcher, the entire chamber is only rated to the spec of the lowest-rated part, so care must be taken when constructing the chamber of a hybrid. Remote ignition is a must, and the limit of a 4x propane/ 4 atmosphere mix should never be breached when using any type of plastic pipe. If a higher mix is attempted, DDT (deflagration to detonation transition, where the flame front exceeds the sound barrier in the combustion gases) can occur, causing an extreme pressure spike. No plastic pipe can handle this pressure spike, and will probably shatter, launching plastic shards everywhere. A chamber made of steel, iron, or aluminum can survive higher mixes than 4x propane/ 4 atmospheres, and have been used at up to 8x propane/ 8 atmosphere mixtures. However, utmost care must be taken with these launchers, in construction and materials used.
