A giant underwater missile launcher becomes the main arm of America's nuclear deterrence. The Trident missile is a submarine-launched ballistic missile (SLBM) equipped with multiple independently targetable reentry vehicles (MIRV). The Fleet Ballistic Missile (FBM) is armed with nuclear warheads and is launched from nuclear-powered ballistic missile submarines (SSBNs). Trident missiles are carried by fourteen active US Navy Ohio-class submarines, with US warheads, and four Royal Navy Vanguard-class submarines, with British warheads. The original prime contractor and developer of the missile was Lockheed Martin Space Systems. Trident I (designated C4) was deployed in 1979 and retired in 2005. Its objective was to achieve performance similar to Poseidon (C3) but at extended range which improved survivability of the submarine. Trident II (designated D5) had the objective of improved accuracy, and was first deployed in 1990, and was planned to be in service for the thirty-year life of the submarines, until 2027. Trident missiles are provided to the United Kingdom under the terms of the 1963 Polaris Sales Agreement which was modified in 1982 for Trident. British Prime Minister Margaret Thatcher wrote to President Carter on 10 July 1980, to request that he approve supply of Trident I missiles. However, in 1982 Thatcher wrote to President Reagan to request the United Kingdom be allowed to procure the Trident II system, the procurement of which had been accelerated by the US Navy. This was agreed in March 1982. Under the agreement, the United Kingdom paid an additional 5% of their total procurement cost of 2.5 billion dollars to the US government as a research and development contribution. The launch from the submarine occurs below the ocean 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, which 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 engine 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). The missile attains a temporary low altitude orbit only a few minutes after launch. The Guidance System for the missile was developed by the Charles Stark Draper Laboratory and is maintained by a joint Draper/General Dynamics Advanced Information Systems facility. It is an Inertial Guidance System with an additional Star-Sighting system, which is used to correct small position and velocity errors that result from launch condition uncertainties due to the sub navigation system errors and some errors that have accrued by 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 Advanced Information 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 there is no direct relation between these two missiles. 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.

• published: 15 Mar 2014
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