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The Nimrod was born from an extensive modification of the de Havilland Comet, the world's first jet airliner. It was originally designed by de Havilland's successor, Hawker Siddeley, now part of BAE Systems. A major modification was the fit of a large weapon bay under the fuselage that could carry and drop torpedoes, mines, bombs and other stores. Sonobuoys for tracking submarines could be dropped from special launchers in the rear of the fuselage.
The Nimrod has been RAF's primary Maritime Patrol Aircraft (MPA) since the early 70s, when it replaced the piston-engined Avro Shackleton. The RAF used two Nimrod variants: the MR2 variant in the Maritime and for the Reconnaissance role the R1 variant in a reconnaissance and electronic intelligence gathering capacity (ELINT).
The Nimrod was the first jet-powered MPA of any significance. Earlier MPA designs used piston or turboprop engines to improve fuel economy and allow for lengthy patrols. Jet engines are most economical at high altitudes and less economical at low altitudes. However, the transit to the operational area can be made at high altitude and in a jet aircraft this is not only economical on fuel but fast as well, compared to earlier piston-powered aircraft. After transit, the Nimrod descends to its patrol area.
On patrol, at high weight all four engines are used, but as fuel is used and weight falls, first one engine is closed down and then a second is closed down when weight is lower. This allows the remaining engines to be run at an efficient RPM rather than running all engines at less efficient RPM. A special "rapid start" system is fitted should the closed-down engines have to be started quickly again. Instead of relying only on airspeed for re-starting an engine, compressor air from a live engine is used in a starter turbine which rapidly accelerates the engine being started. For transit back to base, the closed-down engines are re-started and the aircraft climbed to altitude.
MR1
Nimrod development began in 1964 as a project to replace the elderly Avro Shackleton. Like many other successful maritime patrol aircraft, it was based on a civil airliner which had reached the end of its market life - in this case, the Comet 4. The first two RAF aircraft were unfinished Comet 4 airliners. The Comet's turbojet engines were replaced with 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 a first flight in May 1967, the RAF ordered 46 Nimrod MR1s. The first example entered service in October 1969. Five squadrons were eventually equipped with the MR1.
R1
Three Nimrod aircraft were adapted to the SIGINT(for "SIGnal INTelligence") role, replacing the Comet C2s and Canberras of No. 51 Squadron in May 1974. The R1 is distinguished from the MR2 by the lack of a MAD boom. Only since the end of the Cold War has the role of the aircraft been officially acknowledged. Officially these were once described as "radar calibration aircraft". The R1s have not suffered the same rate of fatigue and corrosion of the MR2s and will continue in service long after the MR2 is replaced by the MRA4. One R1 has been lost in a flying accident since the type's introduction; this occurred in May 1995. To replace this aircraft an MR2 was selected for conversion to R1 standard, and entered service in December 1996.
MR2
Starting in 1975, 32 aircraft were upgraded to MR2 standard, including modernisation of the electronic suite and (as the MR2P) provision for in-flight refuelling and additional ESM pods on the wingtips. The in-flight refuelling capability was introduced during the Falklands War, as well as hardpoints to allow the Nimrod to carry the AIM-9 Sidewinder missile (giving rise to the aircraft being called "the largest fighter in the world"). Eventually all MR2s gained refuelling probes and the "P" designation was dropped.
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 4,000 km out into the Western Atlantic. With Air-to-Air Refuelling (AAR), range and endurance was greatly extended. The MR2 was a submarine killer carrying sensors and data processing equipment linked to the weapon systems. In addition to weapons and sonobuoys, a searchlight could be mounted in the starboard wing pod for Search and rescue (SAR) operations.
The crew was comprised 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. Two of the WSOps would be used as observers positioned at the port and starboard beam lookout windows when flying in dense air traffic. The MR2 had the longest bomb bay of any NATO aircraft.
AEW3
In the mid-1970s the Nimrod's duties were expanded to include AEW - again as a replacement for the Lancaster-derived, piston-engined Shackleton which was still in service in that role. The aircraft were modified by BAe at the former Avro plant at Woodford, Greater Manchester to house the GEC Marconi radars in a bulbous nose and tail. From the start of the first flight trials in 1982 the Nimrod AEW3 project was plagued by cost over-runs and electronic difficulties. Eventually, the MoD realised 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 Sentries (AWACS) instead, with proven radar performance, and electronic enhancements to the original USAF systems to address UK-specific requirements. Of the 11 RAF Nimrods that were selected for conversion to AEW3 standard, none returned to the maritime reconnaissance role: all were eventually reduced for spares to support the maritime Nimrod fleet.
MRA4
In 1992 the RAF started a Replacement Maritime Patrol Aircraft (RMPA) procurement programme to replace the Nimrod MR2 aircraft. To meet the requirement BAe proposed rebuilding each Nimrod MR2 with new engines and electronics which it called Nimrod 2000. The RAF considered bids from Lockheed with its P-3 Orion, Loral Corp. with rebuilt ex-US Navy Orions, and Dassault with the Atlantique 3, but in December 1996 awarded the contract to BAe for the Nimrod 2000 as the Nimrod MRA4.
The MRA4 is essentially a new aircraft, with current-generation Rolls-Royce BR710 turbofan engines, a new larger wing, and fully refurbished fuselage. Much larger air intakes are required because the airflow of the BR710 engine is significantly higher than that of the original Spey 250. The rebuilt aircraft borrows heavily from Airbus technology; the wings are designed and manufactured by BAE Systems (a former Airbus partner) and the glass cockpit is derived from that of the Airbus A340.
Development took longer than anticipated. The contract was initially for the supply of 21 rebuilt Nimrods, but due to technical problems the project was halted. Early in the contract BAE discovered that none of the Nimrod airframes supplied by the RAF for refurbishing were to a common standard. This considerably complicated the refurbishment process.
The British House of Commons Defence Committee, in July 2004, reported a forecast cost of £3.5 billion compared to £2.8 billion approved at Main Gate. They noted that the in-service date had slipped to 2009, compared to the date of 2003 approved at "Main Gate".
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