The Type 42 destroyers were designed principally to provide air defence for a task force, replacing the County Class destroyers, which were becoming increasingly expensive to operate, but they are also utilised to engage surface and sub-surface targets. All the ships were named after large British cities, commemorating many of the Town Class cruisers that fought with such distinction in the Second World War. The ships are armed with the British Aerospace Sea Dart surface-to-air missile system, for defence against both aircraft and missiles, as well as being equipped with a Vickers 4.5 inch gun and anti-submarine torpedo tubes. In addition, as a result of experience in the Falklands War, the ships are also fitted with 20 mm Phalanx CIWS and chaff decoys for close range defence. Each ship carries a Lynx helicopter which extends the range of the ship's radar systems and, armed with Sea Skua missiles or torpedoes, provides a powerful antii-ship or anti-submarine capability. The Type 42 destroyers are fitted with controllable pitch propellers and powered by two Rolls-Royce Tyne Engines that provide a cruising speed of 18 knots. In addition they are equipped with two Rolls-Royce Olympus engines that allow the ships to "sprint" at up to 29 knots for short periods.