The Hawker Siddeley Nimrod is a retired maritime patrol aircraft developed and operated by the United Kingdom. It was an extensive modification of the de Havilland Comet, the world's first operational jet airliner. It was originally designed by de Havilland's successor firm, Hawker Siddeley; further development and maintenance work was undertaken by Hawker Siddeley's own successor companies, British Aerospace and, later, BAE Systems. Designed in response to a requirement issued by the Royal Air Force (RAF) to replace its fleet of ageing Avro Shackletons, the Nimrod MR1/MR2s were primarily fixed-wing aerial platforms for anti-submarine warfare (ASW) operations; secondary roles included maritime surveillance and anti-surface warfare. It served from the early 1970s until March 2010. The intended replacement was to be extensively rebuilt Nimrod MR2s, designated Nimrod MRA4. Due to considerable delays, repeated cost overruns, and financial cutbacks, the development of the MRA4 was abandoned in 2010. The RAF also operated a small number of the Nimrod R1, an electronic intelligence gathering (ELINT) variant. A dedicated airborne early warning platform, the Nimrod AEW3, was in development from late 1970s to the mid-1980s; however, much like the MRA4, considerable problems were encountered in development and thus the project was cancelled in 1986 in favour of an off-the-shelf solution in the Boeing E-3 Sentry. All Nimrod variants had been retired by mid-2011. Development MR1 On 4 June 1964, the British Government issued Air Staff Requirement 381, which sought a replacement for the aging Avro Shackleton maritime patrol aircraft of the Royal Air Force (RAF). Such a replacement had been necessitated by the rapidly-approaching fatigue life limitations accumulated across the Shackleton fleet. A great deal of interest in the requirement was received from both British and foreign manufacturers, who offered aircraft including the Lockheed P-3 Orion, the Breguet Atlantic and derivatives of the Hawker Siddeley Trident, BAC One-Eleven, Vickers VC10 and de Havilland Comet. On 2 February 1965, Prime Minister Harold Wilson announced the intention to order Hawker Siddeley's maritime patrol version of the Comet, the HS.801 as a replacement for Shackleton Mk 2. The Nimrod design was based on the Comet 4 civil airliner which had reached the end of its commercial life (the first two prototype Nimrods, XV148 and XV147, were built from two final unfinished Comet 4C airframes). The Comet's turbojet engines were replaced by Rolls-Royce Spey turbofans for better fuel efficiency, particularly at the low altitudes required for maritime patrol.