The warhead of the photon torpedo comprises a maximum of 1.5 kilos of antimatter and 1.5 kilos of matter. These are divided into many thousands of small pellets suspended in a magnetic field - smaller yields can be achieved by reducing the number of such pellets in the torpedo.

Also included in the torpedo are target acquisition, guidance and detonation assemblies and a warp sustainer unit. The latter is charged by the launching vessels own drive field at launch, boosting the torpedo speed up to Vmax = Vl + (0.75 Vl / c), where Vl is the velocity of the launching vessel. If launched at low impulse flight the torpedo will accelerate to a 75% higher sublight velocity; launch at high impulse speed will not push the torpedo into warp. If launched during warp flight the torpedo will continue at warp until the sustainer is exhausted. Torpedo range can be extended by utilizing the matter / antimatter warhead to power the sustainer, although this causes a corresponding loss of warhead yield. For a mid-range yield the torpedo can achieve ranges of some 3,500,000 kilometres at sublight speeds.

The photon torpedo can be set to fly a ballistic trajectory, be steered by the launch vessel, can home in via its own guidance systems, or use a combination of these methods in a single flight.

The warhead of a photon torpedo can be removed and replaced by sensor packages or other equipment. Some advanced models are fitted with full warp drives for use as long range high speed probes - the Class VIII probe can cover 1.12 light years at Warp 9, while the Class IX probe can cover 2 light years at the same speed. On one occasion such a device was used to transport a Federation diplomat to an urgent rendezvous.

There are currently eight major photon torpedo launch systems in use with the Federation :

Torpedo tube (2nd class) : Dating from the late 2200's, this model is now only in service on the Excelsior, Miranda, Centaur, Oberth, and Constellation classes. This tube is capable of firing one photon torpedo approximately every four seconds.

Standard Torpedo Tube : A modern version of the 2nd class torpedo tube, this model can fire a torpedo every two seconds. It is more compact than the older model, has lower maintenance requirements, and is less prone to overheating with prolonged use.

Burst Fire, Type 1 : One of the most important advances in torpedo tube technology, the burst fire tube allows more than one torpedo to be launched simultaneously. The Type 1 model can load and fire a cluster of four photons every 2.85 seconds; although this makes the tube almost three times as bulky as a standard tube, it allows many targets to be engaged simultaneously. The Type 1 is fitted to the Ambassador class as well as various starbases and space stations.

Burst Fire, Type 2 : Designed for larger vessels, the type 2 burst fire torpedo tube can fire up to eight photons every five seconds, giving a greater overall rate of fire and increasing the number of targets which can be engaged simultaneously.

Burst 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 3 - or alternately can fire single rounds at a high rate of fire.

Burst Fire, 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 class battlecruiser.

Pulse Fire : Developed for the Akira class, the Pulse fire tube is a modification of the burst fire tube. The loading and pre-fire stages can hold up to four photons simultaneously, but the launch tube itself is only of sufficient size to fire one weapon at a time. The pulse fire tube therefore fires four rounds in one second, then pauses for three seconds to reload with the next four photons. Overall rate of fire is therefore one torpedo per second.

Micro tube : This system was developed to arm small vessels such as Runabouts and shuttlecraft. It fires a compact torpedo with a much smaller warhead than the standard models.

The Enterprise-A fires a photon torpedo in a battle in the Khitomer system. This shot was taken from the recovered flight data recorder of General Changs modified B'Rel class Bird of Prey, which the Enterprise and the USS Excelsior destroyed in this battle.
 

The Enterprise-D fires a photon from her aft torpedo tube; each of the Galaxies
three tubes could fire up to ten such weapons simultaneously. The ships main
impulse engine is at the top right of the image, with the port nacelle on the left.