The burst fire tube allows more than one torpedo to be launched simultaneously.
Type 1: The Type 1 model can load and fire a cluster of four photons every 2.85 seconds. The Type 1 was fitted to the original Ambassador Class as well as various starbases and space stations.
Type 2: Designed for larger vessels, including refitted Ambassador Class vessels, the Type 2 burst-fire torpedo tube can fire up to eight photons every five seconds, giving a greater overall rate of fire.
Type 3: Developed to arm the Galaxy and Nebula classes, this tube can fire up to ten torpedoes every five seconds, a 25% increase over the Type 2.
Type 4: The type 4 is the most powerful photon torpedo tube currently in service with the Federation. It can fire a burst of twelve torpedoes every five seconds and currently is only fitted to the Sovereign & Excalibur Class explorers.