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 fixed-wing aerial platforms tasked primarily with 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 three Nimrod R1s, 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 the Boeing E-3 Sentry, which provided an off-the-shelf solution. 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 Rolls-Royce Avon turbojet engines were replaced by Rolls-Royce Spey turbofans for better fuel efficiency, particularly at the low altitudes required for maritime patrol. Major fuselage changes were made, including an internal weapons bay, an extended nose for radar, a new tail with electronic warfare (ESM) sensors mounted in a bulky fairing, and a MAD (magnetic anomaly detector) boom. After the first flight in May 1967, the RAF ordered 46 Nimrod MR1s. The first example delivered (XV230) entered service in October 1969. A total of five squadrons using the type were established; four were permanently based in the UK and a fifth was initially based in Malta.
R1
Three Nimrod aircraft were adapted for the signals intelligence role, replacing the Comet C2s and Canberras of No. 51 Squadron in May 1974. The R1 was visually distinguished from the MR2 by the lack of a MAD boom. It was fitted with an array of rotating dish aerials in the aircraft's bomb bay, with further dish aerials in the tailcone and at the front of the wing-mounted fuel tanks. It had a flight crew of four (two pilots, a flight engineer and one navigator) and up to 25 crew operating the SIGINT equipment. Only since the end of the Cold War has the role of the aircraft been officially acknowledged; they were once described as "radar calibration aircraft". The R1s have not suffered the same rate of fatigue and corrosion as the MR2s. One R1 was lost in a flying accident since the type's introduction; this occurred in May 1995 during a flight test after major servicing, at RAF Kinloss. To replace this aircraft an MR2 was selected for conversion to R1 standard, and entered service in December 1996. The Nimrod R1 was based initially at RAF Wyton, Cambridgeshire, and later at RAF Waddington, Lincolnshire, and flown by 51 Sqn. The two remaining Nimrod R1s were originally planned to be retired at the end of March 2011, but operational requirements forced the RAF to deploy one to RAF Akrotiri, Cyprus on 16 March in support of Operation Ellamy. The last flight of the type was on 28 June 2011 from RAF Waddington, in the presence of the Chief of the Air Staff, ACM Sir Stephen Dalton. XV 249, the former MR2, is now on display at the RAF Museum Cosford, West Midlands. The R1 was replaced by three Boeing RC-135W Rivet Joint aircraft, acquired under the Airseeker project; the first aircraft was delivered in late 2013.
MR2
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