Submarine-launched ballistic missile
From Citizendium, the Citizens' Compendium
An submarine-launched ballistic missile (ICBM) is a sea-based missile, in practice fired from an extremely quiet nuclear-powered ballistic missile submarine (SSBN). While the first SLBMs were of fairly short range, the current generation has range comparable to that of an intercontinental ballistic missile.. "Ballistic" describes its trajectory, with a powered boost phase into space, midcourse coasting along a suborbital phase, and unpowered reentry at one or more points determined by a precision navigational system. ICBMs were one of the main weapon systems of the Cold War, with a maximum of 400+ by the U.S. and Soviet Union/Russia, and in the tens by China. Modern U.S. and Russian SLBMs carried multiple independently targetable reentry vehicles (MIRV), with accuracy comparable to, or superior than, that of land-based ICBMs.
Of the strategic delivery systems of what been called the "Triad" of nuclear delivery systems, each presenting an adversary with a different defense problem:
SLBMs are considered the most survivable. Their numbers have been reduced both by bilateral arms control agreements between the U.S. and Russia, as well as advances in capability. China, France and the United Kingdom all deployed SLBMs; the British have used U.S. missiles with U.K. warheads.
Guidance and accuracy
First-generation SLBMs used inertial navigation, with inertial navigation systems both on the submarine, which determined the launch point coordinates, and aboard the missile. Later versions added s celestial navigation, primarily before reentry, in which they determine their location based on the bearings to a set of stars.
All known SLBMs use nuclear warheads. Operational SLBMs also use MIRVs, such that each missile delivers some mixture of nuclear weapons and penetration aids (e.g., radar decoys and jammers). SLBMs also are now of an accuracy comparable, or sometimes superior to, land-based ICBMs.
There is even more interest, however, in "kinetic kill" warheads for SLBMs than ICBMs. The kinetic energy of the reentry vehicle is so high that a conventional explosive warhead would not add as much energy as a dense inert mass.
While nuclear-armed ballistic missile submarines would be likely to fire from well-guarded waters near their homelands, if they were using conventional warheads, they could move significantly closer to the target, so their flight time would be much shorter than that of an ICBM. The scenarios most commonly proposed for conventional SLBM use would be against a rogue state or non-national terrorist group, which would be unlikely to have the missile launch detection and tracking systems available to Russia and the U.S. As a consequence, the target might have no warning before the warhead(s) strike.
The first SLBM, Polaris A1, with a single nuclear warhead and a range of 1000 nmi/1853 km, was accepted by the U.S. Navy in 1960, for use on the first George Washington class of SSBNs, made by modifying nuclear attack submarines under construction. UGM-27 Polaris missiles went through three generations, were replaced by the more advanced UGM-73 Poseidon, and then by Trident SLBMs: (UGM-96 Trident and UGM-133 Trident II.
There has been reduction, through arms control agreements, of SLBM rockets, and of their capabilities such as multiple independently targetable reentry vehicles (MIRV). In principle, verification of compliance with the treaties involves national technical means of verification and bilateral on-site inspections and overflights by monitoring aircraft. In practice, since deployed SSBNs are extremely difficult to detect, they present the most difficult verification problem of the systems of the Triad.