Chamber to barrel ratio

The chamber to barrel ratio, or C:B ratio, is the volume ratio between the chamber and barrel. The ratio determines how much of the available energy is transferred to the projectile, and a good C:B ratio is a major factor in combustion cannon performance. The goal of an optimal ratio is to have the barrel end at the exact point where the projectile stops accelerating.

Higher ratio cannons are louder, since more energy is wasted as noise when the projectile exits the barrel. Too low a ratio will hurt performance, since a vacuum is formed behind the projectile when the combustion product cool. In extreme cases, it may even be sucked back into the chamber. [1]

Experiments have shown that a C:B ratio of about 0.6-0.8:1 is the most efficient for a given chamber using potatoes as projectiles. The ratio will vary somewhat depending on the weight, friction and blowby of a certain projectile, but it's in that area for most common projectiles.

Two main theories exist when determining the optimal chamber for a particular barrel. A brief summary of both is found below.

1. A chamber volume that exceeds the optimal value for efficiency will increase performance due to more energy being available within the chamber. This theory is based on the fast combustion model, in which the fuel burns quickly, and the pressure rises to it's peak long before the projectile has moved a significant distance down the barrel, even if the chamber is largely oversized. When designing a launcher, a larger chamber will produce higher levels of performance.

2. The most efficient ratio will usually also produce the highest level of performance. This theory is based on the slow combustion model. A chamber that is too large takes longer to burn the fuel, meaning the pressure rises more slowly and to lesser levels, resulting in lower projectile acceleration. This phenomenon is similar to a slow opening pneumatic valve. If the chamber is grossly oversized, combustion may not complete until after the projectile leaves the barrel, further reducing performance. When designing a launcher, the chamber and barrel should be matched to each other for optimal performance. A ratio anywhere between 0.5:1 and 1:1 will be ideal under most circumstances.

No raw data has been presented to support either theory.

In short, C:B ratios are good for determining which barrel will yield the highest performance given a fixed chamber volume, and also for maximizing fuel efficiency and controlling noise output. When a definite answer is found, they will also be effective at determining which chamber volume will deliver optimal performance given a fixed barrel. A single value should not be interpreted as the optimal ratio for all cannons, however.

Burst disks can be used to increase the performance of high ratio cannons, since they let higher pressure build up before the projectile starts to move. They will not increase the performance in near optimal cannons. Source

The C:B ratio of pneumatic cannons is much less fixed, since factors like air pressure and valve performance play a greater part in determining overall performance. It is advisable to use GGDT to determine the best design for pneumatic cannons.