Structure and Systems

Airframe

The Nimrod MRA.4 upgrade involves dismantling 80% of each MR.2's airframe and replacing 60% of it with new stuctures, including the complete wing. Wing span is being increased by 3.71m, and area by 23%. Modern turbofan engines are being installed. The whole flight deck is being stripped and replaced with a 2-man cockpit, and an all-new tactical operators' area is being installed. Other new parts include the entire landing gear (in steel to allow for higher weights), all four weapons bay doors, the fin fillet, rudder, auxiliary fins and fin-top pod, which will house a towed decoy. MR.2 parts retained are being "zero-lifed". All this will keep the aircraft in service for a further 25 years, based on a utilization of 650 hours per year.


Crew

The Nimrod MR.2 has a minimum of 13 crew members: two pilots, flight engineer, route navigator and nine tactical operators. In the MRA.4, this is reduced to a total of 10, with the elimination of the flight engineer, route navigator, and one tactical operator.

Dimensions

 R.1MR.2MRA.4
Wing span35.0m35.0m38.71m
Length35.97m38.63m38.63m
Height9.08m9.08m9.29m
Wing area191.2m²191.2m²235.8m²
Empty weightn/k33483 kg51150 kg
Maximum weight87098 kg87098 kg106217 kg

Powerplant

RR/BMW BR710 cutaway
BR710 (Rolls-Royce)

The Nimrod MR.2 has four Rolls-Royce Spey RB168-20/250 turbofans, each of which generate 55.06 kN (12140lb) of thrust. In the Nimrod MRA.4 the Speys are replaced by the Rolls-Royce/BMW BR710 turbofan, which generates 66.3 kN (14900lb), a 23% increase.

The first BR710 engine for the Nimrod MRA.4 completed its first run in early September 1998 at the BMW Rolls-Royce Development and Assembly Centre in Dahlewitz, near Berlin. The engine's certification is planned for the summer of 2000, with series production beginning in early 2001.

The BR710 is also used in the Gulfstream V, but it required considerable modification for the Nimrod. Numerous components have had to be made corrosion-resistant. A new mixer is required as a result of the aerodynamic conditions caused by the engines' location, and the engine's electronics must be interfaced with the Nimrod's systems.

The BR710 completed a 4-week altitude testing programme at the DERA test site at Pyestock, Hampshire in early December 1998.

The engine successfully completed a 150-hour endurance test at the Rolls-Royce BMW test facility at Dahlewitz in May 1999.

The Nimrod MR.2 has a total fuel capacity of 85840lb, distributed between tanks in the fuselage, wings and pods. All marks are equipped for air-to-air refuelling.

Avionics

Nimrod MR.2

Six aircraft were equipped with the L-3 Wescam MX-15 electro-optical turret in 2003. Project Broadsword, which was implemented in early 2006, introduced the capability to transmit real-time video imagery from the MX-15 to ground stations and commanders.

Nimrod MRA.4

At the heart of the MRA.4 is a Tactical Command System (TCS) jointly developed by BAE Systems and Boeing. The system will be able to gather, process and display up to 20 times more data than the MR.2. This data will be acquired from several types of sensors. These include an advanced surveillance radar electronic support measures, sonobuoys, electro-optics (FLIR) and a magnetic anomaly detector.

The MRA.4 will reportedly contain 5.4 billion lines of computer code, which is three times as much as the B-2 Spirit.

The aircraft's primary sensor is the Racal Searchwater 2000MR radar. This is a state-of-the-art high-performance set designed for all-weather Anti-Submarine and Anti-Surface Warfare. It is optimised to have a high probability of detecting small targets in poor weather conditions, and uses an extensive range of proven techniques to reduce sea clutter. It can automatically track over 100 surface targets, and can also detect and localise aircraft using its pulse-Doppler mode. It also has SAR and weather modes.

The aircraft is also equipped with an ELTA EL/L-8300UK ESM system to detect and locate hostile electronic transmissions, and a CDC UYS503/AQS970) acoustics system. Both of these were originally developed for the AP-3C Orion upgrade.

A comprehensive Defensive Aids Sub-System (DASS) is fitted, which includes a Radar Warning Receiver, Missile Approach Warning System, Towed Radar decoys, and flares and chaff.

Nimrod MRA.4's two pilots will have an entirely new instrument panel derived from that of the Airbus A340. Most information is presented on seven full-colour glass screens. Flight aids include a flight management system, collision warning and surface proximity alerting.

Nimrod MRA.4 cockpit
MRA.4 cockpit (BAE Systems)

Navigation duties are handled by twin laser-gyro inertial navigation systems with embedded GPS.

Communications are handled by the Communications Management System, which allows voice communications from low frequency to UHF, as well as providing datalinks (Link-11, Link-16), and SATCOM capabilities in SHF and UHF modes.

Flight testing of the Northrop Grumman Electro-Optical Surveillance and Detection System (EOSDS) began in May 2006. The purpose of the initial flights was to assess the aerodynamic effects of the deployment and retraction of EOSDS as well as to determine the impact of opening the bomb bay doors of the Nimrod MRA.4. During these test flights to date, the system was deployed at 5,000 feet and remained deployed up to 10,000 feet while flight operators tested all three EOSDS sensors and examined the aerodynamic effect on the aircraft.

EOSDS is a gimbal-mounted electro-optical system designed to give the Royal Air Force the capability to observe and track targets of interest in the visible and infra-red spectrums by means of both a television camera and two thermal-imaging cameras. EOSDS has three detection modes and can automatically track targets through nadir to allow continuous surveillance even when over-flying a target.

Nimrod R.1

Equipment installed on the R.1 is more difficult to be sure about. The aircraft carry an extensive array of antennae that can intercept communications. Three conical helix antennae are mounted on the tailcone and wings. The large weapons bay has probably been filled with a great deal of electronic equipment, possibly including a SLAR for radar imagery, although there is no evidence of additional ram scoops for cooling air. ESM pods are mounted on the wingtips, although these are not the same as the "Yellowgate" pods on the MR.2. The fintip "banana" may contain a satellite communications dish.

A new piece of equipment called SRIM 6113 Starwindow is believed to have been installed on all three R.1s during 1996/97. Key aspects of Starwindow, which was built by E-Systems, are thought to include two new, high-speed search receivers; a network of 22 digital intercept receivers; a wideband digital direction-finding system; an intercept capability against frequency-agile transmitters; and in-flight analysis equipment. Other features are believed to include a digital recording and playback suite, a multi-channel digital data demodulator and an enhanced pulsed signal processing capability.

In 2000 a £100m contract for a new automated electronic warfare collection system for the Nimrod R.1, known as "Project Extract", was awarded to Raytheon Strategic Systems. A Nimrod R.1 arrived at Patuxent River on April 17th 2003 to test the new system over the US Navy's Atlantic Test Ranges. After flying six missions, the aircraft returned to the UK on May 4th. In September 2003 the RAF announced that the program had been completed (presumably just the installation of the equipment - see below).

In October 2002 Thales won a contract to study a future antenna suite for the aircraft, and it was also announced that the aircraft will be fitted with the JTIDS datalink.

On April 26th 2004 the UK's Defence Logistics Organisation announced the selection of three US contractors to undertake first-stage assessment studies for Project Helix - a 400 million program intended to progressively upgrade the mission suite of the Royal Air Force's three Nimrod R.1 signals intelligence aircraft. Two of the three companies will be selected to proceed into a definition phase in 2005. A single contractor will then be chosen to undertake a risk-reduction phase in 2006.

Implementation of Project Helix will be undertaken in two major increments, the first of which - the acquisition element - is valued at 200 million. A contract award for this phase of the project is scheduled for 2007.

On September 21st 2005 Raytheon announced that it had successfully completed flight trials and acceptance testing for the "Extract" system installed on the R.1 fleet.

On April 3rd 2007 L-3 Communications announced that it had been chosen as the preferred contractor to execute the Project HELIX Demonstration and Manufacture contract, which has a projected value of up to £400 million, over a 7-year period, with first aircraft delivery scheduled in early 2013.

Performance

The Nimrod has the ability, unique in the ASW/MPA world, of being able to transit rapidly to an operational area on four engines, then shut down one or two for economy whilst on patrol. Despite being based on an airliner the Nimrod has the low wing loading necessary for manoeuvability 70m over the sea. This agility is vitally necessary for the tight "hunting circles" flown when attempting to trap a submarine, and also in the rapid weaving needed to check the identities of all the vessels in a loosely grouped fishing fleet.

The Nimrod MRA.4 will have an endurance of over 15 hours, equivalent to a range in excess of 11112 km (6000 nm)

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© David Hastings