The U.S. Navy is upgrading and refurbishing its Trident II SLBM (Sea Launched Ballistic Missile) so that these weapons will still be effective for at least another 25 years. There have already been upgrades to the electronics and mechanical components in the guidance system. Upgrades are underway to the reentry body (heat shield and such that gets individual warheads to the ground intact). Some of the upgrades are classified and details on all of them are kept secret for obvious reasons.
The Trident II is one of those rare complex systems that consistently perform flawlessly. They do exist. For example, test firings of production models of the Trident II have never failed. There have been 148 of these missile launches each involving an SSBN (ballistic missile carrying nuclear sub) firing one of their Trident IIs, with the nuclear warhead replaced by one of similar weight but containing sensors and communications equipment.
The test results for the Trident while in development were equally impressive, with 87 percent successful (in 23 development tests) for the Trident I and 98 percent (49 tests) of the Trident II. The Trident I served from 1979-2005, while the Trident II entered service in 1990 and may end up serving for half a century.
Trident II is a 59 ton missile with a max range of 7,200-11,000 kilometers (depending on the number of warheads carried). Up to eight W76 nuclear warheads can be carried, each with the explosive power equal to 100,000 tons of high explosives. The navy recently bought another 108 Trident IIs at a cost of $31 million each.
The success of the Trident is in sharp contrast to the problems Russia and China have had developing SLBMs. The latest Russian SLBM, the Bulava (also known as R-30 3M30 and SS-NX-30), was almost cancelled because test flights kept failing. The Bulava finally successfully completed its test program on December 23rd, 2011. That made 11 successful Bulava test firings out of 18 attempts. The last two missiles make five in a row that were successfully fired. As a result of this, the Bulava has been accepted into service, with a development test firing success rate of 61 percent, but some last minute glitches led to more tests being scheduled and Bulava has yet to enter service.
Then there is the Chinese JL (Julang) 2 SLBM, which was supposed to enter service in 2008 and still hasn’t. This missile has had a lot of problems, as have the SSBNs that carried them. The 42 ton JL-2 has a range of 8,000 kilometers and would enable China to aim missiles at any target in the United States from a 094 class SSBN cruising off Hawaii or Alaska. Each 094 boat can carry twelve of these missiles, which are naval versions of the existing land based 42 ton DF-31 ICBM. No Chinese SSBN has ever gone on a combat cruise because these boats have been very unreliable and they have no dependable SLBMs to carry.