The Nimrod is a maritime patrol aircraft developed in the United Kingdom. It is 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 can carry and drop torpedoes, mines, bombs and other stores. Sonobuoys for tracking submarines are dropped from special launchers in the rear of the fuselage.
It has been the Royal Air Force's primary Maritime Patrol Aircraft (MPA) since the early 1970s, when it replaced the piston-engined Avro Shackleton. The RAF uses 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.
Other MPA designs have been jet powered, including the US Navy's S-3 Viking and future P-8 Poseidon.

Development
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.

MR1
Three Nimrod aircraft were adapted to the SIGINT 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. New Bombardier Sentinel R1 (ASTOR) aircraft due for delivery from mid 2004 may take on some duties performed by the R1. 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.
The Nimrod R1 is based at RAF Waddington in Lincolnshire, England and flown by 51 Sqn.

MR2
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 (see picture). 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.

AEW3
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 has taken longer than anticipated and the first of 12 MRA4s have not yet entered service. 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
On July 30th, 2007, the Nimrod MRA4 successfully released the Sting Ray torpedo for the first time. The safe separation trial to demonstrate the ability to deploy this store from the MRA4 bomb bay took place at Aberporth range off the coast of West Wales during the 75th flight of development aircraft PA02. Three MRA4 development aircraft have been built and are undergoing an intensive flight-test programme. PA02 achieved its first flight in December 2004 and is being used to test elements of the mission system and the air vehicle.[1] [2]

MRA4
As of late 2006, 15 Nimrod MR2 and 3 Nimrod R1 remain in operation .

Active Operators
Five Nimrods have been lost in accidents :

Accidents and incidents

Crew: 12
Capacity: 24 POB (Persons On Board)
Length: 38.63 m (126 ft 9 in)
Wingspan: 35.00 m (114 ft 10 in)
Height: 9.45 m (31 ft)
Wing area: 197.05 m² (2,121 sq ft)
Empty weight: 39, 009 kg (86,000lb)
Max takeoff weight: 87,090 kg (192,000 lb)
Powerplant: 4× Rolls-Royce Spey turbofans, 5,515 kg (12,160 lb) each
Maximum speed: 923 km/h (575 mph)
Cruise speed: 787 km/h (490 mph)
Range: 8,340-9,265 km (5,180-5,755 miles)
Service ceiling: 13411 metres (44,000 ft)
R1: none
MR2: AIM-9 Sidewinder, AGM-84 Harpoon, Sting Ray torpedo, depth charges, nuclear depth bombs (until 1992) Performance