UGM-73 Poseidon SLBM

The first US SLBM to have MIRV warheads, Poseidon originated from a series of studies for a Polaris A3 successor; the initial Polaris B3 designation was dropped by early 1963, as the resulting missile would clearly be a new design. Just as each Polaris model had been longer-ranged than the previous one, so Poseidon would be even more capable, with a planned range of more than three thousand miles. Although no “magic bullet” method of detecting SSBNs had been developed, the Soviet Navy was building large numbers of ASW ships as part of its expansion into a “blue water” force, and it was desirable to provide American missile submarines with an expanded operating area to minimize the chances of detection.
Poseidon would be around the same length as the A3, but the new requirements meant that its diameter would have to be increased to 74 inches. This was incompatible with the width of existing Polaris launch tubes, but if the tube linings were stripped out, the new missile could be fitted. Electronic changes would also be necessary, such as installing the Mk.88 fire-control system.

Poseidon would be the second US missile (after the Minuteman III) to be equipped with MIRV warheads, vastly increasing the number of targets an individual SSBN could engage, even when two or three warheads were assigned to a single targets. The maximum warload was 14 Mk.3 RVs, each with a 50-kiloton yield W68 device; in this configuration, Poseidon’s range was about that of Polaris A3. This was raised to over 3,200 miles if only ten RVs were carried, which was probably the standard fit. Accuracy is believed to have been improved by a factor of two when compared to Polaris, but Poseidon was still not a counterforce weapon.

In 1965, funding for Poseidon was approved, and in November 1966 Secretary of Defense MacNamara publicly announced that he had recommended proceeding with deployment of the new missile as a means of countering Soviet ABM development.

In May 1967, British Prime Minister Wilson confirmed that the UK would not be buying the Poseidon system to arm its still-building Polaris boats, partly due to the desire not to become overly dependent on American weapons. Although the possibility of a British Poseidon buy would be raised again, no UGM-73s would be exported, and the Royal Navy would end up operating Polaris until the mid-1990s.

Flight testing began with a pad firing from Canaveral on August 16 1968, and the first underwater Poseidon launch from an SSBN took place off Cape Canaveral on August 3, 1970, from the USS James Madison. The system was in operational service the following year when the Madison went to sea with Poseidons loaded. Poseidon backfits were applied to boomers of the Madison, Benjamin Franklin, and Lafayette classes throughout the 1970s. The earlier boats of the Ethan Allen and George Washington classes retained Polaris until retired as SSBNs.

Poseidon submarine launched ballistic missile

Throughout the 1980s, Poseidon boats were gradually retired from the strategic role as more Trident subs became available, thus allowing the US to stay within SALT-II limits. Several became moored training ships, while others were reclassified as SSNs before being mothballed. As part of President Bush’s major nuclear draw-down of September 1991, submarines still equipped with Poseidon were taken off alert. The Trident refits lasted in the strategic role a little longer, but these too were gone by 1993.


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