Burst disk

Cross-section of burst disk in a union; A is chamber, B is barrel, C is the fragile disk, and D is the union.

A burst disk consists of a thin disc of airtight material that will rupture at a certain pressure, held in place inside a union or cam lever coupling. (See the burst disk material page.)

Since the burst disk ruptures almost instantly when the pressure gets to a certain point, they have the fastest opening time of any valve in existence, short of a "valveless" launcher. Since they are pressure-triggered they are nearly always used in hybrids, they are also occasionally used as the main valve in a pneumatic cannon because of their superior performance. Sometimes they are used as a secondary valve in a pneumatic cannon to compensate for the slower opening time of the main valve.

Burst disks are a very close rival to a ball valve in cost, at roughly $13 for a 2" union in sch 40 PVC. More expensive (but convenient) devices exist, such as camlock couplings.

To use, solvent weld your chamber to one half of your union or cam coupling. One then 'loads' the valve by putting one or more layers of burst disk material (such as aluminum foil, or a piece of a beverage container) in between the two halves of the union or coupling. The union or cam coupling is then tightened down, locking the burst disk in place.

In a hybrid cannon, burst disks fail when the pressure builds due to the combusting gases. Prior to this, the chamber was containing the flammable gases at a relatively low pressure. They are also sometimes used on combustion cannons - for their noisemaking capabilities, and/or their (debated) performance improvement.

In a pneumatic cannon, they can be used in the same way, simply pointing the cannon in the correct direction and filling it until it fires. However, many opt to use another system because of the inherent safety issues with this. Such systems include puncturing it with a pointy object (such as a nail) attached to the end of the projectile, melting it with an electrically heated wire, or having a pneumatically triggered burst disk system using dual burst disks.

Since Burst disks only last for a single shot and need to be replaced before each round, Burst Disk Cartridges (BDC) have been created.