Kinglet primary

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Kinglet was a boosted fission primary used in two designs of American thermonuclear weapons, the W55 and the W58; and was also adapted by the British as a super-hardened primary known as Harriet used in the Chevaline improvements to the British Polaris A-3TK missile.

Primary is the technical term for the boosted fission trigger of a thermonuclear weapon, and is used to compress, heat and ignite the fusion fuel in the thermonuclear secondary.

The Kinglet primary was used in the US W55 warhead of an NDB ("nuclear depth bomb") carried by the SUBROC anti-submarine missile launched horizontally from a submarine to attack at longer ranges and at shorter times than torpedoes can manage, and with a much higher kill probability. The two yields of the W55 are a common feature of NDBs. A high yield will be used in deep oceanic water. A low yield will be required for use in shallow or coastal waters, and in deep oceanic water to minimise risk to nearby shipping. The lower yield is likely to be obtained without the boosting, which in turn reduces the fusion element almost completely.

Kinglet was also used as a primary in the three W58 thermonuclear warheads of the US Polaris A-3 missile. The British had safety concerns about Kinglet and the W58 and initially rejected it for their Polaris A-3 missiles, and substituted a different all-British primary. The most recently declassified documents suggest that the British later adapted the Kinglet primary as a basis for Harriet used in Chevaline because it could be hardened against the effects of exo-atmospheric ABM warhead bursts before re-entry, whereas their original primary would be vulnerable, and unable to defeat the Soviet Galosh ABMs defending Moscow.

Characteristics of these weapons are:

Kinglet primary based nuclear weapons
Model Max Yield (kt) Diameter (in) Length (in) Weight (lb)
W55 4.5 or 250 13.5 39.4 470
W58 200 15.6 40 257

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