BAE Systems Nimrod MRA.4 ZJ518 (Ex Nimrod MR.2 XV234), Ca. 2009

---

Introduction

Named after the biblical king of Old Testament renowned for his description as the “Mighty hunter”, Hawker Siddeley Nimrod was developed upon the airframe of De Havilland Comet to be the mainstay RAF maritime patrol and anti-submarine warfare asset throughout the cold war era.

Development started in the mid 1960s, with the need to replace the aging Avro Shackleton in the maritime patrol role. The Shackleton, itself based upon WWII-era Lancaster bomber, was rapidly approaching its fatigue life limitation accumulated through its long service life. In 1964, the British government issued Air Staff Requirement 381, which attracted a wide range of proposals including the French Breguet Atlantic and US Lockheed P-3 Orion being a few of them

Nimrod MR.1

In 1965, the British government awarded Hawker Siddeley a contract to develop a brand-new maritime patrol aircraft based on the De Havilland Comet civil jetliner (De Havilland being one of the predecessors to the Hawker Siddeley) to replace the aging Shackleton. The new aircrafts, converted from two unfinished Comet 4C airframe and those which had reached the end of their commercial life, were to be designated HS.801 by Hawker Siddeley and, upon acceptance to service by the RAF in 1969, the "Nimrod Maritime Reconnaissance Mark 1 (MR.1)"

Part of Nimrod's modification from baseline Comet was the replacement of Rolls Royce Avon turbojets with more fuel-efficient Rolls Royce Spey turbofans, which was critical for long endurance required by the maritime patrol role. The most striking alteration, however, was the addition of an unpressurized avionics and weapon bay underneath the pressurized fuselage section, establishing Nimrod's iconic "double bubble" fuselage configuration. Moreover, the extended nose cone housed a 1950s era ASV Mark 21 search radar, a football-shaped fairing on the top of the vertical fin housed the ESM suites, and a Magnetic Anomaly Detector (MAD) was fitted to the extended tail boom.

A total of 46 airframes were converted from Comet 4C, which started entering RAF strength in October 1969. The Nimrod MR.1 carried out three primary missions: Anti-Submarine Warfare (ASW), Anti-Surface Unit Warfare (ASUW), and Search and Rescue (SAR). One of the top-priority missions was to protect Royal Navy ballistic missile submarines, cooperating with Royal Navy frigates and attack submarines for the task.

Nimrod R.1

During the early 1960s, Comet R.2 and Canberra B.6(RC) aircrafts bore the responsibility of signal intelligence gathering (SIGINT) role for the RAF. in 1964, the London Signals Intelligence Committee make the decision to procure a new type of aircraft to replace the aforementioned aircrafts in SIGINT role. Several potential options were considered including a variant of Boeing 707, but it was determined that a variant of Hawker Siddeley Nimrod, then in development for the RAF, was to be the cheapest option.

In 1969, a total of three Nimrods were ordered for conversion to SIGINT aircraft, which were designated as R.1 to differentiate them from the MR.1 maritime patrol version. Three airframes were constructed as part of the overall Nimrod production line before being delivered to the RAF with no equipment fitted. This was owing to the highly secret nature of the equipment intended for use on the aircraft - instead, they were fitted out at RAF Wyton, the home base of 51 Squadron.

The R.1 airframes could be easily recognizable by the absence of the MAD tail boom of the MR.1 variant, replaced by a much smaller antenna dome. They also had a litter of blade antennas, as well as antenna domes on the front of the leading-edge fuel tanks. The bulk of the aircraft's detection equipment was installed in the weapons bay, including an array of internal rotating dish aerials.

The first aircraft was delivered in July 1971, and was eventually completed more than two years later, being formally accepted into service in May 1974. Owing to the secretive nature of its SIGINT role, the Nimrod R.1 was officially only known as "radar calibration aircraft" by the RAF.

Nimrod MR.2

The Nimrod MR.1 was originally outfitted 1950s era avionics and detection suites inherited from the preceding Avro Shackleton. Starting in 1975, 35 MR.1 airframes were upgraded to MR.2 standard, being re-delivered from August 1979. Among the extensive modifications of the aircraft electronic suites were new EMI Searchwater radar, new GEC-Marconi AQS-901 acoustic processor which enabled the MR.2 airframes to utilize more modern sonobuoys, a new mission data recorder and new Yellow Gate ESM equipment marked by pods on the wingtips.

During the 1982 Falklands war, provision for in-flight refueling was introduced as MR.2P modification, as well as hardpoints to allow the Nimrod to carry two AIM-9 Sidewinder missiles for self-defense and, possibly, to counter Argentine Air Force maritime patrol aircraft.

In preparation for operations in the 1991 Gulf War, further modernization in communications and ECM equipment were carried out to deal with anticipated threats. These modified aircraft were given the designation MR.2P(GM) (Gulf Mod)

Nimrod AEW.3

The development of Grumman E-2 Hawkeye and its associated equipment in the 1960s prompted the British government to consider developing a dedicated Airborne Early Warning (AEW) aircraft of their own. At the time, the only aircraft designated for AEW role in British possession was the propeller-driven Fairey Gannet operated by the Royal Navy Fleet Air Arms out of their aircraft carriers. The Gannet was outfitted with the AN/APS-20 search radar, which had been developed during World War II and was rapidly becoming obsolete.

Avro Shackleton returned to the picture as an interim solution while a brand-new aircraft was being developed. The AN/APS-20 search radars were stripped from surplus Royal Navy Gannets to be outfitted to elderly Shackleton airframes on the verge of retirement from the RAF. The Shackleton AEW.2, as they were officially known in the RAF, entered service with 8 Squadron in April 1972.

At the time, Britain's aircraft industries were undergoing major rearrangements, with Hawker Siddeley being amalgamated with several other companies to form the British Aerospace (BAe). The new company inherited the task of creating an AEW platform based upon the Nimrod airframe, and a total of eleven existing Nimrod airframes were to be converted by British Aerospace at Woodford.

The Nimrod AEW.3 was easily distinguishable by two bulbous radomes on the nose and tail that housed the GEC Marconi pulse-doppler radar scanners. GEC also supplied GEC 4080M computer to integrate data collected by the pulse-doppler radar, ESM, new Cossor IFF system, and the aircraft’s own inertial navigation system.

The complexity of the AEW requirement proved too much for British industry to overcome by itself. Even with the addition of oversized radomes, Nimrod airframe turned out not to be big enough to do the job and the computer, having only 2.4 Megabytes of memory, was woefully inadequate to integrate all of the aforementioned systems into a single mission package. GEC Marconi and BAe, having been appointed joint project leader, ended up blaming each other for Nimrod AEW.3 shortcomings and the program went practically nowhere.

In the end, the British government decided to put a plug on the program and, in October 1987, procured seven Boeing E-3D Sentries from the US instead. Throughout the whole fiasco, Shackleton AEW.2 aircraft soldiered on as RAF only AEW platform until it was completely replaced by E-3 Sentry in 1991.

Nimrod MRA.4

Nimrods flew in support of the invasion of Afghanistan in 2001 and the invasion of Iraq in 2003. By that time the MR.2 was showing its age and the British government sought the Replacement Maritime Patrol Aircraft (RMPA) procurement program to replace the aging Nimrods. Several prospective options were considered, including French Dassault (formerly Breguet) Atlantic 3, the US Lockheed P-3 Orion 2000 and a potential conversion of civilian Airbus A310.

BAE systems, successor to the British Aerospace, ultimately settled on a proposal that involved an extensive rebuild of existing Nimrod MR.2 marketed under the name Nimrod 2000. The new airframe would have its Rolls Royce Spey turbofans replaced with even more fuel-efficient Rolls Royce RB710 turbofans, requiring the wings to be enlarged to fit the wider diameter engines. The reworked wings coupled with more efficient engines was projected to result in a twofold increase of the aircraft’s operational range.

In addition to the engine upgrade, the airframe would receive a fully refurbished fuselage, new hydraulic and electrical system, and updated avionics, including a modern glass cockpit, improved datalinks and satcom links, new Thales Searchwater 2000M radar and an electro-optic / infrared (EO-IR) imager turret behind the nose gear. New ESM, sonobuoy receiver system, and an upgraded MAD gear was to be a minor update of the MR.2

In June 1996, the Equipment Approvals Committee recommended that the Nimrod 2000 bid be selected to meet the RAF's requirement. In July 1996, it was announced that the £2 billion contract had been awarded to British Aerospace to produce the Nimrod 2000. By the time of formal contract award in December the aircraft had received the designation, Nimrod MRA.4.

The Nimrod MRA.4 was originally scheduled to reenter service in April 2003. However, BAE systems discovered that the MR.2 airframes supplied by the RAF were not built to a common standard, themselves being rebuilt from MR.1 airframes and, in turn, Comet 4C jetliners. A huge number of variations were found between airframes, considerably complicating the refurbishment process. To make matters worse, the new wing was flawed, which resulted in the project being put on hold while another wing design was developed.

Due to the delays and cost overruns, only five Nimrod MRA.4 were ever converted from the planned 21. The first development airframe eventually made its maiden flight in August 2004, followed by several years of trials and evaluations by BAE systems and RAF.

The end came in October 2010, with the publication of the Strategic Defense and Security Review, which heralded sweeping cuts in British military capability and equipment. The British government announced that the Nimrod MRA.4 would not enter RAF service. All converted MRA.4 airframes were scrapped in March 2011

To fill the gap left on Britain’s maritime patrol capability, the British government instead procured nine Boeing P-8 Poseidons, which will undertake the range of tasks that were undertaken by the Nimrod MR.2 and intended for the MRA.4. The Nimrod R.1 was replaced in SIGINT role by three Boeing RC-135W Rivet Joints, procured in October 2014. These, incidentally, were the only RC-135s ever operated by a foreign power.