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.Cook, James. "Final air miles for 'spy in the sky' crews." BBC, 26 March 2010. Retrieved 20 October 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).Haddon-Cave 2009, pp. 16–17. Such a replacement had been necessitated by the rapidly-approaching fatigue life limitations accumulated across the Shackleton fleet.Jefford et al. 2005, p. 87. 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.Chartres 1986, p. 12.Jefford et al. 2005, p. 131. 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.Fricker 1972, p. 593. 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.Neal 1970, p. 119.
R1
Three Nimrod aircraft were adapted for the signals intelligence role, replacing the Comet C2s and Canberras of No. 51 Squadron in May 1974.Lake Air International July 2001, p. 31.Fricker 1972, p. 594. The R1 was visually distinguished from the MR2 by the lack of a MAD boom.Haddon-Cave 2009, p. 17. 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.Lake Air International July 2001, pp. 30–31.
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.Lake Air International July 2001, p. 34.
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."Nimrod R1 makes final flight" Defence Management Journal, 28 June 2011. Retrieved 28 June 2011."Nimrod R1 aircraft in final flight for RAF." BBC, 28 June 2011. Retrieved 13 July 2011. 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."PICTURES: First RAF Rivet Joint aircraft arrives in UK."Flight Global. Retrieved: 18 December 2013
MR2
Starting in 1975, 35 aircraft were upgraded to MR2 standard, being re-delivered from August 1979.Donald 1996, p. 95. The upgrade included extensive modernisation of the aircraft's electronic suite. Changes included the replacement of the 1950s ASV Mk 21 radar used by the Shackleton and Nimrod MR1 with the new EMI Searchwater radar, a new acoustic processor (GEC-Marconi AQS-901) capable of handling more modern sonobuoys, a new mission data recorder (Hanbush) and a new Electronic Support Measures (Yellow Gate) which included new pods on the wingtips.Air International July 1981, pp. 9–10, 12–14.
Provision for in-flight refuelling was introduced during the Falklands War (as the MR2P), as well as hardpoints to allow the Nimrod to carry the AIM-9 Sidewinder missile to counter enemy Argentine Air Force maritime surveillance aircraft.Brown 1987, p. 110. In preparation for operations in the Gulf War theatre, several MR2s were fitted with new communications and ECM equipment to deal with anticipated threats; at the time these modified aircraft were given the designation MR2P(GM) (Gulf Mod).
The Nimrod MR2 carried out three main roles: Anti-Submarine Warfare (ASW), Anti-Surface Unit Warfare (ASUW) and Search and Rescue (SAR). Its extended range enabled the crew to monitor maritime areas far to the north of Iceland and up to out into the Western Atlantic. With Air-to-Air Refuelling (AAR), range and endurance was greatly extended. The crew consisted of two pilots and one flight engineer, two navigators (one tactical navigator and a routine navigator), one Air Electronics Officer (AEO), the sonobuoy sensor team of two Weapon System Operators (WSOp ACO) and four Weapon System Operators (WSOp EW) to manage passive and active electronic warfare systems.
Until 2010, the Nimrod MR2 was based at RAF Kinloss in Scotland (120, 201 and 206 Squadrons), and RAF St Mawgan in Cornwall (42 and 38(R) Squadrons). Following Options for Change, 42 Squadron was disbanded and its number reassigned to 38(R) Squadron. The Nimrod MR2 aircraft was withdrawn on 31 March 2010, a year earlier than planned, for financial reasons."Last flight of the Nimrod MR2." Ministry of Defence, 31 March 2010."Planning Round 10 is Going to be a Tough One." RAF Families Federation, 6 January 2010. The last official flight of a Nimrod MR2 took place on 26 May 2010, with XV229 flying from RAF Kinloss to Kent International Airport to be used as an evacuation training airframe at the nearby MOD Defence Fire Training and Development Centre.Wilson, Tom. "Historic plane ends its career at Manston." This is Kent, 8 June 2010. Retrieved 13 July 2011.
AEW3
In the mid-1970s, a modified Nimrod was proposed for the Airborne Early Warning (AEW) mission – again as a replacement for the Lancaster-derived, piston-engined Shackleton AEW.2. Eleven existing Nimrod airframes were to be converted by British Aerospace to house the GEC Marconi radars in a bulbous nose and tail. The Nimrod AEW3 project was plagued by cost over-runs and problems with the GEC 4080M computer used."BAe Nimrod AEW 3." Spyflight. Retrieved: 20 October 2010. Eventually, the MoD recognised that the cost of developing the radar system to achieve the required level of performance was prohibitive and the probability of success very uncertain, and in December 1986 the project was cancelled. The RAF eventually received seven Boeing E-3 Sentry aircraft instead.
MRA4
The Nimrod MRA4 was intended to replace the capability provided by the MR2. It was essentially a new aircraft, with current-generation Rolls-Royce BR710 turbofan engines, a new larger wing, and fully refurbished fuselage. The project was subject to delays, cost over-runs, and contract re-negotiations. The type had been originally intended to enter service in 2003 but was cancelled in 2010 as a result of the Strategic Defence and Security Review, at which point it was £789 million over-budget; the development airframes were also scrapped."Scrapping RAF Nimrods 'perverse' say military chiefs." BBC News, 27 January 2011. Some functions were allocated to other assets, with Hercules transport aircraft and E-3 Sentry Airborne Early Warning aircraft given some tasks, but the cancellation of the MRA4 resulted in a significant gap in long-range maritime patrol and search-and rescue capability.Hoyle, Craig. "IN FOCUS: UK left exposed by Nimrod cancellation, report says". Flightglobal, 27 September 2012. Retrieved 21 May 2016..
In July 2016, the Ministry of Defence announced the purchase of nine Boeing P-8A Poseidon aircraft for the RAF. The RAF declared the P-8 had reached initial operating capability (IOC) on 1 April 2020, by which time two of the planes had been delivered. The nine aircraft will be based at RAF Lossiemouth.
Design
Overview The Nimrod was the first jet-powered MPA to enter service, being powered by the Rolls-Royce Spey turbofan engine.Neal 1970, p. 120. Aircraft in this role had been commonly propelled by piston or turboprop powerplants instead to maximise fuel economy and enable maximum patrol time on station. Advantages of the Nimrod's turbofan engines included greater speed and altitude capabilities, and it was more capable of evading detection by submarines, as propeller-driven aircraft are more detectable underwater by standard acoustic sensors.Rininger 2006, p. 69. The Nimrods had a flight endurance of ten hours without aerial refuelling. The MR2s were later fitted to receive mid-air refuelling in response to demands of the Falklands War.
At the start of a patrol mission all four engines would normally be running; as the aircraft's weight was reduced by fuel consumption, up to two engines could be shut down, allowing the remaining engines to be operated more efficiently. Instead of relying on ram air to restart an inactive engine, compressor air could be crossfed from a live engine to a starter turbine. The crossfeed duct was later discovered to be a potential fire hazard.Haddon-Cave 2009, p. 20.Neal 1970, p. 121. Similarly, the two hydraulic systems on board were designed to be powered by the two inner engines that would always be running. Electrical generation was designed to far exceed the consumption of existing equipment to accommodate additional systems installed over the Nimrod's service life.