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 anti-submarine 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 ships radar systems, and, armed with the Sea Skua missile, is also the ship's principle anti-submarine weapon. The Type 42 destroyers are fitted with controllable pitch propellers and are powered by two Rolls-Royce Tyne Engines, which provide a cruising speed of 18 knots. In addition they are equipped with two Rolls-Royce Olympus engines, which allow the ship to "sprint" at up to 29 knots for short periods.