The Poseidon missile was the second US Navy ballistic missile system, powered by a two-stage solid fuel rocket. It succeeded the Polaris missile beginning in 1972, bringing major advances in warheads and accuracy. It was followed by Trident I in 1979, and Trident II in 1990.
A development study for a longer range version of the Polaris missile achieved by enlarging it to the maximum possible size allowed by existing launch tubes started in 1963. Tests had already shown that Polaris missiles could be operated without problems in launch tubes that had their fiberglass liners and locating rings removed.
The project was given the title Polaris B3 in November, but the missile was eventually named Poseidon C3 to emphasize the technical advances over its predecessor. The C3 was the only version of the missile produced, and it was also given the designation UGM-73A.
Slightly longer and considerably wider and heavier than Polaris A3, Poseidon had the same 4,600 kilometres (2,500 nmi) range, greater payload capacity, improved accuracy, and MIRV capability. Poseidon could deliver up to fourteen W68 thermonuclear warheads contained in Mark 3 reentry vehicles to multiple targets. The high-re-entry-speed design was intended to counter sprint-type terminal ABM defenses. See Atmospheric re-entry for blunt body theory.
The low-yield warheads were apparently selected to make the weapon unsuitable as a first-strike weapon against hardened targets in the Soviet Union, but could be used in a retaliatory strike against soft targets, or in a pre-emptive strike against unhardened surface targets such as airfields, SAM sites, radar sites and other similar targets, opening a pathway for heavy bombers. In later years this targeting technique has been demonstrated by the use of cruise missiles to neutralize airfields, SAM sites etc. in the opening phases of the Gulf War.
As with Polaris, starting a rocket motor when the missile was still in the submarine was considered very dangerous. Therefore, the missile was ejected from its launch tube using high pressure steam produced by a solid-fueled boiler. The main rocket motor ignited automatically when the missile had risen approximately 10 metres above the submarine.
The first test launch took place on 16 August 1968 and the first test launch from a submarine took place on the USS James Madison on 3 August 1970. The weapon officially entered service on 31 March 1971. It eventually equipped Lafayette-, James Madison- and Benjamin Franklin-class submarines.
About 620 UGM-73A missiles were built between 1970 and 1978.
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