A pneumatic cannon is a cannon powered by pressurized gas, usually air or CO2. They generally require a little more work to build than combustion cannons, but using typical compressor pressures and a decent valve, they will outperform most combustion launchers of the same size.
Pneumatic launchers consist of five basic elements:
- Compressed gas storage reservoir (chamber for short)
- Filling valve (bicycle Schrader valves, quick disconnects, etc.)
- Firing valve (ball valves, sprinkler valves, piston valves, bolt-style valves(see piston valves) quick-exhaust valves, and burst disks are the most common)
- Pressure gauge (for safety & repeatable launches)
Operation & Theory
In order to fire, the operator loads a projectile into the barrel, fills the chamber with a compressed gas (see Pneumatic air sources for common filling methods), and activates the main valve thus dumping the air in the chamber to the barrel.
The expansion of the compressed gas propels the projectile down the barrel. The barrel length determines the amount of time the projectile has to accelerate and ultimately the velocity of the projectile.
Higher performance launchers use techniques such as:
- Faster valve opening times - see sprinkler valve modification
- Higher flow valves to maximize the airflow through the valve (Especially true for cannons with very long barrels)
- Using higher gas pressures.
Less common, but also effective in increasing power, some people will also use a gas with a higher speed of sound and particle speed (e.g. helium).
Efficiency of a cannon can be roughly determined by observing how much noise it makes when fired. A relatively quiet gun is often efficient, as there is little energy left over to create sound.
Common types of launchers
Ball Valve Launchers
Ball Valve Launchers release pressurized gas by way of a ball valve. These valves can be made of metal or plastic, and are available in many sizes and pressure ratings. Ball valves are simple to operate, and have very high flow. However, their opening times are comparitively long compared to other valves such as piston valves. Ball valves are typically used in simple designs, or on launchers with long barrels where large airflow is crucial. They can also be used as breech loaders, or venting ports on combustion cannons.
Sprinkler Valve Launchers
Sprinkler Valve Launchers release air using a commercially manufactured sprinkler valve. These valves are simple to use and install, but are designed to open slowly. This is due to their main application in water systems, to prevent water hammer from destroying the valve. Since air is being used, one can modify the valve to actuate faster, using a larger pilot valve, such as a blowgun, without fear of hammer effects. Sprinkler valves are the most cost-effective option if a design calls for moderate performance. However, their relatively low flow rates and number of internal bends cause the performance to reduce to roughly 60% of that of a piston valve of the same diameter. Sprinkler valves are typically rated to ~120 psig, but it is unadvisable to use them at this pressure, as they are designed for use with water.
Piston Valve/Diaphragm Launchers
Piston valve launchers use a piston in a valve that either seals against the barrel or chamber. Releasing air through the pilot valve will cause the remaining air to push the piston away from its sealing position and air will go out the barrel.
This is generally an efficient type of launcher, as the opening time is extremely fast and there is very little constriction of airflow in the actual valve. The only real loss of flow can be attributed to the bends inherent in the design that cause gasses to lose energy as they change direction.
A diaphragm launcher operates on the same principles as a piston valve. The only difference is that the piston cylinder is replaced by a flexible diaphragm that flexes towards the pilot valve to release air into the barrel. These valves open faster than piston valves, but have slightly lower flow rates. While piston valves require a bumper to protect the piston housing, diaphragm valves do not.
QEV launchers use a similar principle as the Piston valve only these can be bought from the store. These valves open even faster than piston valves because their pilot volume is much lower, and the valve is commercially manufactured and optimized.
"Valveless" launchers do not have conventional valve, rather the projectile itself is held in the barrel sealing off the pressure chamber, and is fired by physically releasing the projectile. Another variation of the theme is to have the projectile held in a seal that will only release the projectile once a certain pressure is reached (commonly referred to as a "detent"). This allows the use of a relatively inefficient slow/low flow valve to fill to the required, while still having maximum performance for the given parameters (zero "opening time" with instantaneous full flow) exceeding any other form of mechanical valve.
Downloading GGDT (Gas Gun Design Tool) is highly recommended to those interested in pneumatic launchers.
A page on sprinkler valves and a PDF book can be found here; The Sprinkler Valve