The Poseidon missile was the second SLBM, 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.
Poseidon, although slightly longer and considerably wider and heavier than Polaris A3, had the same 4600 km range, although with increased payload weight. It also had improved accuracy and up to fourteen W68 thermonuclear warheads contained in Mk.3 RVs that were of a new MIRVed high-re-entry-speed design intended to counter sprint-type terminal ABM defences.
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 neutralise airfields, SAM sites etc in the opening phases of the Gulf War. Similarly a new more accurate celestial/intertial guidance system was not developed because it was felt a highly accurate invulnerable weapon might destabilize the nuclear deterrence balance with the Soviet Union.
As with Polaris, starting a rocket motor when the missile was still in the submarine was considered very dangerous. The missile was ejected from its launch tube using high pressure steam produced by a solid-fueled boiler.
The first test launch took place on 16 August 1968 whilst 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 all 31 Lafayette-class submarines, the submarines' original Polaris outfit being replaced with Poseidon during refits.
