|UGM-133A Trident II|
A Trident II launch from a submerged Royal Navy submarine.
|Place of origin||United States|
United States Navy|
|Manufacturer||Lockheed Martin Space Systems|
|Unit cost||$37,320,070 (2014) |
|Mass||130,000 lb (59,000 kg) |
|Length||44 ft 6.6 in (13.579 m)|
|Diameter||6 ft 11 in (2.11 m) (1st stage) |
|Warhead||8–14 MIRV (Multiple Independently-targetable Re-entry Vehicle) W88 or W76 thermonuclear weapons  |
|Engine||Three solid-fuel rocket motors; first & second stage – Thiokol/ Hercules solid-fueled rocket; third stage – United Technologies Corp. solid-fueled rocket |
|Propellant||Nitrate ester, Plasticized Polyethylene Glycol |
|More than 7,500 mi (12,000 km)   (exact is classified) |
|Speed||Approximately 18,030 mph (29,020 km/h) (Mach 24; 8,060 m/s)  (terminal phase)|
|MK 6 astro-inertial guidance which is able to receive GPS (Global Positioning System) updates  |
|Single movable nozzle actuated by a gas generator |
|Accuracy||90 m with Mk5 RV |
|Ballistic missile submarine|
The UGM-133A Trident II, or Trident D5 is a submarine-launched ballistic missile (SLBM), built by Lockheed Martin Space Systems in Sunnyvale, California, and deployed with the American and British navies. It was first deployed in March 1990,  and remains in service. The Trident II Strategic Weapons System is an improved SLBM with greater accuracy, payload, and range than the earlier Trident C-4. It is a key element of the U.S. strategic nuclear triad and strengthens U.S. strategic deterrence. The Trident II is considered to be a durable sea-based system capable of engaging many targets. It enhances the U.S. position in strategic arms negotiation with performance and payload flexibility that can accommodate active treaty initiatives (see New START). The Trident II's increased payload allows nuclear deterrence to be accomplished with fewer submarines,  and its high accuracy – approaching that of land-based missiles – enables it to be used as a first strike weapon.   
Trident II missiles are carried by 14 US Ohio and four British Vanguard-class submarines, with 24 missiles on each Ohio class and 16 missiles on each Vanguard class (however, the number of missiles on Ohio-class submarines will be reduced to 20 each in coming years, in compliance with the New Strategic Arms Limitation Treaty). There have been over 161 successful test flights of the D5 missile since design completion in 1989,  the most recent being from USS Nebraska in March 2018.  There have been fewer than 10 test flights that were failures,  the most recent being from HMS Vengeance off the coast of Florida in June 2016.  The D5 is the sixth in a series of missile generations deployed since the sea-based deterrent program began 60 years ago. The Trident D5LE (life-extension) version will remain in service until 2042. 
The Trident II was designated to be the latest longer-range missile, performing greater than its predecessor ( Trident C-4) in terms of range and payload capacity. In 1972, the US Navy projected an initial operating capability (IOC) date for the Trident II in 1984. The US Navy continued to advance the IOC date to 1982. On 18 October 1973, a Trident program review was administered. On 14 March 1974, the US Deputy Secretary of Defense disseminated two requirements for the Trident program. The first was an accuracy improvement for the Trident C-4. The second requirement asked for an alternative to the C-4, or a new Trident II missile with a larger first stage motor than the C-4.
Studies were conducted to determine whether the more-expensive Trident II could be constructed similar to the US Air Force's MX ICBM. This was done primarily to decrease budget costs. It was established that the Trident II would be 83 inches in diameter and 44 ft in length in order to maintain performance with the existing MX ICBM. Modifications to the guidance system, electronics hardening, and external protective coatings were incorporated into the design. While this satisfied the US Naval study requirements, it did not accommodate the US Air Force payload requirements.
Propulsion stages were proposed to be used between the first stage and second stage motors, effectively making the Trident II a longer three-stage missile than the C-4. Studies were delayed in 1978 when Congress only approved $5 million of the suggested $15 million for the Naval/Air Force program studies. By December 1978, the US Navy and Air Force own studies agreed with each other that the savings made by a similar missile structure would not be effective. It was determined that the US Navy and Air Force maintain and be responsible for their own unique weapon systems. The US Navy continued with their own weapon design of the Trident II.
In March 1980, US Secretary of Defense Harold Brown proposed an increased level of funding for the submarine-launched ballistic missile modernization. Emphasis was strained for the need of increased accuracy. The House Armed Services Committee (HASC) recommended no funding, while the Senate Armed Services Committee (SASC) recommended full funding of $97 million. The SASC asked for a plan which incorporates "the fullest possible competition... (and) should consider competing among contractors for each major component, including the integrated missile." $65 million was awarded for the submarine-launched ballistic missile modernization.
On 2 October 1981, President Reagan called for the modernization of the strategic forces.  The Defense Department directed the Navy to fund all development of the Trident II D5 missile with a December 1989 IOC. All research and development effort would be directed toward "a new development, advanced technology, high accuracy Trident II D5 system." In December 1982, Deputy SECDEF Frank Carlucci advised Secretary of the Navy Caspar Weinberger to include funding for a new RV/warhead combination for Trident II. The reentry vehicle was to be designated as the Mk 5, which was to have an increased yield than the Mk 4. The development contract for Trident II was issued in October 1983. On 28 December 1983, the Deputy SECDEF authorized the Navy to proceed with Full Scale Engineering Development of the Trident II D5. The first Trident II launch occurred on 15 January 1987,  and the first submarine launch was attempted by USS Tennessee,  the first D-5 ship of the Ohio class, on 21 March 1989 off the coast Cape Canaveral, Florida. The launch attempt failed four seconds into the flight because the plume of water following the missile rose to greater height than expected, resulting in water being in the nozzle when the motor ignited. Once the problem was understood, relatively simple changes were quickly made, but the problem delayed the IOC of Trident II until March 1990.   IOC for SWFPAC completed on schedule in 2001 which allowed Trident II SSBN to be supported in the Pacific theater.
The Trident II is a three-stage rocket, each stage containing a solid-fuel rocket motor. The first motor is made by Thiokol and Hercules Inc. This first stage incorporates a solid propellant motor, parts to ensure first-stage ignition, and a thrust vector control (TVC) system. The first-stage section, compared to the Trident C-4, is slightly larger, allowing increased range and a larger payload. In addition to a larger motor, the D-5 uses an advanced and lighter fuel binder ( polyethylene glycol) than the C-4.  This fuel is more commonly known as NEPE-75. (NEPE stands for nitrate ester plasticized polyether.)  
Both the first- and second-stage motors are connected by an interstage casing, which contains electronic equipment and ordnance for separation during flight. The second stage also contains a motor made by Thiokol and Hercules Inc., parts to ensure the second-stage ignition, and a TVC system. The first and second stages are both important to the structural integrity of the missile. To ensure that the stages maintain a maximal strength-to-weight ratio, both stages are reinforced by a carbon-fiber-reinforced polymer hull. 
The second- and third-stage sections are connected by an integrated equipment/adapter section (ES). The equipment/adapter section is modified to be shorter and more compact than the C-4's adapter section.  The D-5's equipment section contains critical guidance and flight control avionics, such as the Mk 6 navigation system. The equipment section also contains the third-stage TVC system, ordnance for ejecting from the second-stage motor, and the MIRV platform. The nose fairing shields the payload and third-stage motor. Mounted within the nose cap (above the nose fairing) is an extendable aerospike.  This aerospike effectively decreases drag by 50%. The third-stage hull is also reinforced by carbon fiber and kevlar. 
Before the launch sequence is initiated, the on-board MARK 6 navigation system is activated. The specified mission trajectory is loaded onto the flight computer. 
Once the launch command is given, a steam generator system is activated, igniting a fixed solid-grain small rocket motor.  The exhaust is fed into cooling water, causing expanding gas within the launch tube to force the missile upward, and out of the submarine.  Within seconds, the missile breaches the surface of the water and the first-stage Thrust Vectoring Control (TVC) subsystem ignites. This enables hydraulic actuators attached to the first-stage nozzle. Soon after, the first-stage motor ignites and burns for approximately 65 seconds until the fuel is expended; in addition, an aerospike atop the missile deploys shortly after first-stage ignition to shape airflow. When the first-stage motor ceases operation, the second-stage TVC subsystem ignites. The first-stage motor is then ejected by ordnance within the interstage casing.  
Once the first stage is cleared, the second-stage motor ignites and burns for approximately 65 seconds. The nose fairing is then jettisoned, separating from the missile. When the nose fairing is cleared of the missile, the third-stage TVC subsystem ignites, and ordnance separates the second-stage motor. The third-stage motor then ignites, pushing the equipment section the remaining distance (approx. 40 seconds) of the flight. When the third-stage motor reaches the targeted area, the Post Boost Control System (PBCS) ignites, and the third-stage motor is ejected.
The astro-inertial guidance uses star positioning to fine-tune the accuracy of the inertial guidance system after launch. As the accuracy of a missile is dependent upon the guidance system knowing the exact position of the missile at any given moment during its flight, the fact that stars are a fixed reference point from which to calculate that position makes this a potentially very effective means of improving accuracy. In the Trident system this was achieved by a single camera that was trained to spot just one star in its expected position. If it was not quite aligned to where it should be, it would indicate that the inertial system was not precisely on target and a correction would be made. 
The equipment section, with the MIRV, then aims the reentry vehicles (RV) towards earth. The payload is then released from the MIRV platform. To prevent the PBCS correctional thrust from interfering with the RV when released, the equipment section initiates the Plume Avoidance Maneuver (PAM). If the RV will be disrupted by the PBCS nozzle's thrust, the nearest nozzle will shut off until the RV is away from the MIRV. The PAM is used only when a nozzle's plume will disrupt the area near an RV. The PAM is a specialized design feature added to the Trident II to increase accuracy. 
- Range: With full load 4,230 nautical miles (7,840 km);
with reduced load approx. 7,500 mi (12,000 km) (exact is classified) 
- Guidance system: The MK 6 Astro-inertial guidance navigation system which is able to receive GPS (Global Positioning System) updates.
- CEP: Requirement: 90 metres (300 ft). (Information from flight tests is classified.)[ citation needed]
- Warhead (in US usage only): The Mark 5 MIRV can carry up to 8 W88 (475 kt) warheads, while the Mark 4 MIRV can also carry 14 W76 (100 kt) warheads.   START I reduced this to eight. New START provides for further reductions in deployed launch vehicles, limiting the number of submarine-launched ballistic missiles (SLBM) to 288, and the number of deployed SLBM warheads to a total of 1,152, therefore on average a missile will carry only 4 warheads.
- Warhead (in UK usage): Under a 1958 agreement, the U.S. supplies the UK with blueprints of its own warhead designs but the design, manufacture and maintenance of UK warheads are purely a UK responsibility. The Atomic Weapons Establishment (AWE) at Aldermaston constantly manufactures (and along with the maintenance and remanufacturing plant at Burghfield) a range of warheads of varying yield for fitting to Trident II missiles while pursuing ongoing research into new and improved warheads. Between 2005 and 2008 a £1.1bn program was undertaken to simulate and validate the safety and operation of the entire stockpile in to the mid 2020s without underground testing. The AWE has contributed scientists and £85m to the development of the Mk4A arming, fusing, and firing system at Sandia with the intention UK production at Aldermaston and fitting to existing warheads would commence in the latter half of the 2010s. The AWE is currently researching a fundamentally new warhead design to replace the existing design from the mid 2020s.[ citation needed]
The Royal Navy operates its missiles from a shared pool, together with the Atlantic squadron of the U.S. Navy Ohio-class SSBNs at King's Bay, Georgia. The pool is co-mingled and missiles are selected at random for loading on to either nation's submarines.  
- K Missile family
- M45 (missile)
- M51 (missile)
- R-29RMU Sineva
- R-29RMU2 Layner
- R-39 Rif
- RSM-56 Bulava
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During the past 15 years, the United States has done so much to upgrade its first strike capabilities – most notably by deploying Trident II D-5 missiles throughout the entire ballistic missile submarine (SSBN) fleet, placing high-yield W88 warheads on many of those missiles, and deploying stealthy B-2 bombers – that today a first strike could succeed even if the performance of key U.S. weapon systems fell far short of their expected accuracy, reliability, or both.
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By the end of the 1980s, however, the submarine-launched ballistic missile had turned another page. The accuracy of the Trident II (D-5) SLBM, planned as the replacement for the Trident I with Trident II deployments beginning in 1989, was comparable to that of the MX/Peacekeeper ICBM, the most accurate land-based missile in the U.S. strategic arsenal. Owing to its improved accuracy and larger payload compared to its SLBM predecessors, Trident II would be able to attack hardened targets in the Soviet Union that were not previously vulnerable to sea-launched ballistic missiles. Although U.S. planners might assume that these strikes against hardened targets in the Soviet Union would be retaliatory attacks, a Soviet net-assessment of U.S. first-strike capabilities would have to include the improved sea-based missiles.
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On 15 July 1980 my predecessor announced the Government's choice of the Trident submarine-launched ballistic missile system to replace the United Kingdom's current Polaris-equipped strategic deterrent force.
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