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Background  —  Aurora Alternatives  —  BAE Systems MRA4 Nimrod

Update  Oct 2010: The UK government has now cut the £3.6B MRA4 Nimrod saying that the project had run "more than eight years late".

The BAE Nimrod maritime patrol aircraft has been in RAF service since 1969.  The current service version is the Nimrod MR2 ( Maritime Reconnaissance Mark 2, left) which will  be replaced by the highly-modified Nimrod  MRA4 (right). The MRA4 is a major rebuild of the MR2 incorporating a new, extended wing  and modern engines. [1] This rebuild program turned out to be much more involved than expected resulting in essentially new-build airframes and an opportunity.

Rolls-Royce BR710s power the MRA4. This is a high-bypass turbofan  which  increases total power greatly (an almost 30% increase over MR2 engines) and  fuel efficiency also improved.[2]  However, MRA4  re-engining was very complicated  compared with other jet aircraft designs. The Nimrod engines are "buried" in the wing structure, requiring an entirely new wing centre section for MRA4.

  BAE Systems  [3]  MRA4  Nimrod   —   Specifications
  Dimensions:   span: 38.7m, length: 38.6m, height: 9.45m
  Powerplant:   4 x 68.8kN (15500 lbst) RR  BR710 turbofans
  Performance:   max speed: 960 km/h,  max cruise: 860 km/h
  Weights:   empty weight: 46,500,  MTOW: 105,376 kg
  Maximum range:   11,110 km  (6000 nm, 6900 statute miles) [4]
  Crew:   3 x flight crew, 7-to-9 x systems operators
  Armament:   Torpedoes, missiles (AS, AA, and cruise)

The difficulties in upgrading '70s-vintage airframes resulted in virtually new-build aircraft which in turn opened the possibility of new-production Nimrods. This idea has suggested before [5] but has become more economical due to the MRA4. More important from a Canadian perspective is timing. Were MRA4 sensors adopted,[6] such new production Nimrods could be built in time for an Aurora phase-out dictated by airframe life. This is seen by many as a major advantage over newer designs.

Many also see the Nimrod as a real ASW aircraft while the P-8A and  its ilk are merely warmed-over airliners. Ironic then, that the Nimrod sprang from the original jetliner, the DeHavilland Comet of the 1950s. [7] Despite much reengineering, the Nimod inherits Comet design disadvantages – eg: engines built into wing structures [8] –  but, on the other hand, an outward-retracting main gear allows an uninterrupted  bomb bay of great length.

If anything makes the Nimrods real ASW aircraft, it is that immense bomb bay (left). This raises a critical policy question: does Canada wish to emphasize ASW ?  If not,
a smaller sovereignty patrol aircraft might suit. If yes, Canada must buy larger, more capable aircraft. Timing  is critical since a CP-140 life-extension has been ruled out. If the answer to ASW is 'yes', Nimrod  has the timing.

[1] Very little of the original MR2 Nimrod airframe remains after the rebuilds  –  basically just some tail components and the fuselage shell.
[2] Of course, there are other measures. The basic weight for the BR710 is 2.1 tonnes, the MR2's Spey 250 (RB168-20) weighs just over 1.2 tonnes. A BR710 is 1.22m in diameter, a Spey 250 is 0.82m. As mentioned, the BR710 is a high-bypass turbofan  (with a bypass ratio of  4.2 compared with the Spey 250's 0.64) meaning that BR710s demand much great air flow which further complicated re-engining of the MRA4.
[3] By the time the Nimrod was developed, DeHavilland had merged with Hawker Siddeley. The Nimrod emerged as two Comet 4C-based HS 801 prototypes.  Hawker Siddeley was then merged with  BAC to become BAe  (British Aerospace)  which later became BAE Systems.
[4] Regardless of engine type, Nimrod fuel burn is also reduced by technique. In transit, RAF crews fly on all four engines.  But, for lower altitude patrols, one or two engines are shut down to improve fuel economy and extend range. 'Live' engine bleed air allow quick restarts.
[5] In 2002,  BAE Systems offered the US Navy a "new-production version of  the MRA4 Nimrod " to replace EP-3E Orion  ELINT aircraft.
[6] There is some 'Can Con' in the MRA4 sensors but the point is the inevitable delays and expense of integrating a different sensor suite.
[7] The RCAF operated two D.H.106 Comets (5301 and 5302) beginning in 1953. A year later, the aircraft had to be taken out of service for structural reinforcement (the early Comets had famously exposed the dangers of explosive decompression) but  then served on until 1963.
[8] The engines and their extended tailpipes pass through the wing spars. Since the MRA4's engine is half again the diameter of the MR2 engine, a complete redesign of the centre section was necessary. This style of engine installation also has structural disadvantages. Most modern jetliners have wing pylon-mounted engines for two reasons: suspended engines contribute to aeroelastic stability –  reducing the required structural strength/weight  –  and reducing the danger from an engine fire. An internally-mounted engine installation such as the Nimrods may reduce drag slightly but with an attendant weight penalty and greater structural risk in the event of (albeit rare) engine fires.