The Trident missile is a submarine-launched ballistic missile (SLBM) equipped with multiple independently targetable reentry vehicles (MIRV). Originally developed by Lockheed Missiles and Space Corporation, the missile is armed with thermonuclear warheads and is launched from nuclear-powered ballistic missile submarines (SSBNs). Trident missiles are carried by fourteen US Navy Ohio-class submarines, with US warheads, and four Royal Navy Vanguard-class submarines, with British warheads. The missile is named after the mythological trident of Neptune.The launch from the submarine occurs below the sea surface. The missiles are ejected from their tubes by igniting an explosive charge in a separate container which is separated by seventeen titanium alloy pinnacles activated by a double alloy steam system. The energy from the blast is directed to a water tank, where the water is flash-vaporized to steam. The subsequent pressure spike is strong enough to eject the missile out of the tube and give it enough momentum to reach and clear the surface of the water. The missile is pressurized with nitrogen to prevent the intrusion of water into any internal spaces, which could damage the missile or add weight, destabilizing the missile. Should the missile fail to breach the surface of the water, there are several safety mechanisms that can either deactivate the missile before launch or guide the missile through an additional phase of launch. Inertial motion sensors are activated upon launch, and when the sensors detect downward acceleration after being blown out of the water, the first-stage motor ignites. The aerospike, a telescoping outward extension that halves aerodynamic drag, is then deployed, and the boost phase begins. When the third-stage motor fires, within two minutes of launch, the missile is traveling faster than 20,000 ft/s (6,000 m/s), or 13,600 mph (21,600 km/h).
Minutes after launch, the missile is exo-atmospheric and on a sub-orbital trajectory. The Guidance System for the missile was developed by the Charles Stark Draper Laboratory and is maintained by a joint Draper/General Dynamics Mission Systems facility. It is an Inertial Guidance System with an additional Star-Sighting system (this combination is known as astro-inertial guidance), which is used to correct small position and velocity errors that result from launch condition uncertainties due to errors in the submarine navigation system and errors that may have accumulated in the guidance system during the flight due to imperfect instrument calibration. GPS has been used on some test flights but is assumed not to be available for a real mission. The fire control system was designed and continues to be maintained by General Dynamics Mission Systems.
Once the star-sighting has been completed, the "bus" section of the missile maneuvers to achieve the various velocity vectors that will send the deployed multiple independent reentry vehicles to their individual targets. The downrange and crossrange dispersion of the targets remains classified.
The Trident was built in two variants: the I (C4) UGM-96A and II (D5) UGM-133A; however, these two missiles have little in common. While the C4, formerly known as EXPO (Extended Range Poseidon), is just an improved version of the Poseidon C-3 missile, the Trident II D-5 has a completely new design (although with some technologies adopted from the C-4). The C4 and D5 designations put the missiles within the "family" that started in 1960 with Polaris (A1, A2 and A3) and continued with the 1971 Poseidon (C3). Both Trident versions are three-stage, solid-propellant, inertially guided missiles, and both guidance systems use a star sighting to improve overall weapons system accuracy.
https://en.wikipedia.org/wiki/Trident_(missile)
