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The Trident missiles are submarine-launched ballistic missiles (SLBM). The Trident I is the successor to the Poseidon (C-3) SLBM. The first Fleet Ballistic Missile (FBM) was the Polaris (A-1). Since then, the SLBM has been evolved through Polaris (A-2), Polaris (A-3), Poseidon (C-3), and today's Trident I (C-4) and Trident II (D-5).

The Trident I (C-4) was developed to replace the Poseidon (C-3) SLBM. It has almost the same dimensions as the Poseidon, but a doubled range. While Poseidon had a range of approx. 2000nm, Trident has a range of approx. 4000nm. To achieve this increased range, it was necessary to improve the propulsion system. Trident I got a three-stage, solid-propellant power plant. Each of the three stages has a boost rocket motor with advanced propellants, improved case materials, and a single lightweight movable nozzle with a TVC system of lightweight gas-hydraulic design. A reduction of the missile's electronic components also contributed to reduced package sizes, weights, and calibration, thereby allowing more volume for propulsion.

The first Trident I missile was launched on January 18, 1977 from a flat pad at Cape Canaveral, FL. USS Francis Scott Key (SSBN 657) became the first ship to launch a Trident missile. This took place on October 20, 1979. USS Florida successfully launched 6 Trident missiles in rapid succession in February, 1995.
Each Trident submarine (OHIO-Class) can carry up to 24 of these missiles. Each missile can be targeted independently.
The only SSBNs still carrying Trident I are the OHIO-class submarines operating in the Pacific. Trident I is scheduled to be phased out in the early 2000s.

Trident II (D-5) was a development that became possible because of the new, larger OHIO-class (also known as Trident-class). The D-5 looks like the C-4 but is bigger. Many improvements in all fields were made: missile (guidance and reentry system), fire control, navigation, launcher and test instrumentation (non-tactical) subsystems, resulting in a missile with additional range, improved accuracy, and heavier payload.

Trident II is fired by the pressure of expanding gas in the launch tube. When the missile attains sufficient distance from the submarine, the first stage motor ignites, the aerospace extends and the boost stage begins. Within about two minutes, after the third stage motor kicks in, the missile is traveling in excess of 20,000 feet (6,096 meters) per second.

With the launches of the D-5 missiles the Navy has longest string of successes in the history of United States' ballistic missiles. The first launch took place in 1989 and the last one on December 18, 1997 aboard USS Louisiana. 76 successful launches had been completed before.

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Relating Books (in association with amazon.com):

From Polaris to Trident: The Development of US Fleet Ballistic Missille Technology (Cambridge Studies in International Relations, Vol 30) by Graham Spinardi

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