Trident is a submarine-launched inter-continental ballistic nuclear weapons system, which is currently carried by four Royal Navy Vanguard-class submarines.
The Missiles
The Trident II D5, currently in service, is supplied by the Lockheed Martin Corporation. It is a three stage, solid-fuel missile, 13 metres long, more than two metres in diameter and weighs more than 60 tonnes. Each missile can carry up to 12 nuclear warheads and each Vanguard-class submarine can carry up to 16 missiles.
The missiles are carried in vertical tubes in the mid section of the submarine and are fired from beneath the surface. Each missile is independently controlled by the submarine. Precise data concerning target location is fed to the missile by the submarine's strategic weapons system computers, together with information on the vessel's position, course and speed, determined by an inertial navigation system.
When fired, a missile is ejected from the submarine by high pressure gas and only when it breaks the surface does the first rocket stage actually fire. Then the missile's own inertial guidance system takes over, calculating actual flight behaviour and guiding it to a pre-determined point in space. After the third rocket motor has separated, the warhead carrier, known as the 'bus' takes a star sighting to confirm the missile's position and then manoeuvres to the point at which the warheads can be released to 'free-fall' on to their selected target trajectories.
The Submarines
From 1994 the Royal Navy's Vanguard-class submarines, equipped with the Trident weapons system, succeeded the Polaris missile boats which had maintained a continuous strategic nuclear deterrent for the United Kingdom for almost 30 years.
The current Trident submarines are giant vessels, 150 metres in length and displacing 16,000 tonnes when submerged. In spite of their huge size, they are among the quietest submarines in the world and are extremely difficult to detect.
The submarines’ nuclear reactors produce enough power to supply a small town and need to be refueled only after years of operation. The boats produce their own oxygen and fresh water and this enables them to remain at sea almost indefinitely. They can circumnavigate the world underwater if necessary, and patrols lasting several months are routine.