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Project Chevaline

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Penetration Aid Carrier (PAC) of the Chevaline Front End Improvement for the British Polaris A3T SLBM. Copyright holder allows anyone to use image for any purpose, provided Chris Gibson and the Skomer website is attributed as source.
Chevaline Penetration Aid Carrier
© Chris Gibson

Project Chevaline was a secret project which was carried out in part at the Royal Naval Torpedo Factory (RNTF) Greenock. The project was intended to improve the survivability of Britain's Polaris missile against Anti Ballistic Missile (ABM) systems.

Pictured is the Chevaline Penetration Aid Carrier (PAC) of the Chevaline Front End Improvement for the British Polaris A3T SLBM, on display in the Bristol Aero Collection at RAF Kemble. The warheads lay in recesses to the left and right of the bus, which are not obvious in the image. The bus was a multiple warhead platform which allowed the penaids (penetration aids or decoys) to be deployed further apart in space. The silver tubes near the top and bottom (pointing fore and aft), some wrapped in cork sheet, contained the decoys. The decoy tubes were capped, and opened by the solenoid operated latches visible, before the decoys, 27 in total, were rocket-ejected. The silver tank held hydrazine fuel for the positioning thrusters used to adjust the PAC attitude in space. At the bottom centre and left are two hot-gas jet exhausts spaced at 90° (positioning thrusters) fuelled from the hydrazine tank. Two thrusters used to rotate the PAC about its fore and aft axis can be seen centre left, the bronze-coloured objects. The orange object at the extreme top is one of the manoeuvring engines, its liquid fuel being stored in the gold coloured cylinder in front of it, one of two such items. Underneath, hidden from view are the twin nozzles of the solid-fuel rocket motors used to separate the PAC from the second-stage booster. Not visible is the internal construction of the PAC main structure, formed from a lightweight composite material made up of balsa wood sandwiched between thin sheets of aluminium.

Chevaline employed a variety of techniques to achieve this aim, and involved the replacement of one of the three nuclear packages normally carried by Polaris with a decoy package, designed to deploy numerous false targets indistinguishable from the real warheads. These were intended to overwhelm any opposing ABM system by providing so many potential targets that it would be unable to correctly determine the genuine threats. In addition, the two remaining nuclear warheads received additional hardening, to increase their survivability in the event that they were successfully targeted by the ABM system. These methods were intended to maintain the deterrent effect of Polaris by ensuring that enough warheads would get through an aggressor's defences to be a reasonable deterrent to a first strike.

In the 1970s the British government revealed they were working on a secret Polaris upgrade programme known as Chevaline, which resulted in the Polaris A3TK, which entered service in 1982, and was eventually replaced by the Trident missile in 1996. The project was highly secret, and survived in secrecy through four different governments before finally being revealed.

Completion of work on this final project in 1969 also marked the closure of the Greenock factory, and the transfer of its remaining work to Weymouth.

Polaris update

Two mid-life update programs were instituted for the Polaris missile.

The first and best known was the Chevaline (aka "Super Antelope" and "KH793") program. It began in secret (as is true of all British nuclear programs) in the late sixties when the Soviet Union began deploying an ABM system around Moscow. Although this system eventually turned out to be very limited in scope, concern about the continuing potency of the British deterrent developed and proposals were made to develop a countermeasure system to improve the ability for Polaris to penetrate these defenses. The program was not an original British undertaking, but was based on a classified U.S. program called Antelope which had made available to the UK in 1967. In June 1967, the Labour Government announced in Parliament its decision not to upgrade the Polaris system by purchasing Poseidon missiles from the United States. Instead of deploying Poseidon, it was decided to re-direct work at Aldermaston to investigate the possibilities of designing a new warhead capable of penetrating Soviet defences using decoys, hardening techniques and penetration aids. Studies of the concept were made in 1967 and the decision to proceed was made by the first Wilson government that same year. By 1969 the Chevaline concept was defined and by 1972 the system had been worked out in detail. It was approved for deployment by the Heath government (1970-74), a decision finally ratified by the second Wilson government in February 1974. At the time of the Wilson decision to proceed the cost was estimated at £250 million. By 1975 this cost had increased to £400 million, and a review was held to determine whether the program should be cancelled in September. This was an important moment in British nuclear policy making because the key issue on review was more than just Chevaline - it was whether the British could afford to maintain its deterrent and competitive in the arena of nuclear arms.

The existence of Chevaline was first disclosed on 24 January 1980 during a debate in Parliament by Conservative Defence Secretary Francis Pym. The total cost of the project was given as £1,000 million making Chevaline the most expensive defence project not to be made public. The high cost resulted in a highly critical report by the Public Accounts Committee published in 1982.

Chevaline was a complex system was based on the coordination of the 16 missiles on a single submarine, maneuver by the RVs to elude interceptors, along with multiple decoy re-entry vehicles, and hardening of the warhead against ABM weapon effects. Each missile would fly a different trajectory so that all missiles would arrive simultaneously over the target (Moscow) and release two real warheads (reduced from the three of the AT3) plus four decoy RVs, and a large number of decoy balloons. The defense would be presented with 96 simultaneous maneuvering targets to intercept (even after the balloon decoys burned up). The system proved far more difficult to develop and deploy than expected.

The first Chevaline warhead was tested 23 May 1974 (possibly designated the TK-100). Sea trials of Chevaline were conducted in November 1980. Production of the Chevaline warhead ran from 1979-1982 with 100 warheads being produced. Chevaline went on patrol for the first time in mid-1982 aboard HMS Renown, with deployment completed in 1987. The estimated yield of the Chevaline is 225 kt.

In 1999 the decommissioning of the Chevaline system (presumably meaning warhead disasembly) was expected to be completed at the AWE during 2000.

The second update program for Polaris involved remanufacture of the solid fuel motors. This program began in 1981, and led to the installation of new motors in all missiles during 1986-87.

- History of the British Nuclear Arsenal[1]


1 Britain's Nuclear Weapons. History of the British Nuclear Arsenal.

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