The regular potato gun that every serious DIYer has built utilizes just a regular combustion of gases at atmospheric pressure. This is the most basic of potato guns and usually the least powerful. The next most powerful type of gun is a pneumatic cannon that uses pressurized air to launch a projectile. After that comes the hybrid potato cannon which is a combination of the other two (hence "hybrid").
A hybrid is powered by the combustion of pressurized gases. This allows for much more fuel and oxidizer to react inside the combustion chamber and it will produce much higher pressures than the other two types of potato guns. More information can be found at http://www.spudfiles.com/spud_wiki/index.php?title=Hybrid_launcher
There are 4 major components to a hybrid potato cannon:
- Combustion Chamber
- Metering System
- Ignition System
The potato gun is first fueled via the metering system which injects just the right mixtures of fuel and air into the combustion chamber. The fuel mixture is then ignited by two spark plugs connected to a stun gun. The resulting combustion produces enough pressure to break the burst disk holding the gases inside of the combustion chamber and to shoot the projectile out at high speeds
- One of the most important things to consider when building any potato gun is the chamber to barrel volume ratio. Experiments have shown that a chamber to barrel ratio of 1.5 : 1 will efficiently utilize all of the guns power.
- The next thing to consider is the barrel length. The longer the barrel is the more time a projectile has to accelerate. However, the larger the barrel, the larger the chamber. So you have to strike a compromise and just decide on a practical and economical barrel length.
- For the meter pipe I got the largest pipe I could at the longest length that would fit. A meter pipe with a larger volume means that you don't have to pressure it up as high when measuring fuel. This is what I would suggest.
- When you incorporate multiple ignition sources you increase the power of the cannon. Igniting the fuel mixture at multiple points will create multiple pressure waves which will increase the velocity of the gases. I have built two ignition sources for now, but I have left room for two more.
My main resources when researching this project were:
- The next step will demonstrate how to build the barrel.