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Background
— Aurora Alternatives — BAE Systems MRA4 Nimrod |
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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".
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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.
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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.
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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) |
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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.
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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.
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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.
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[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.
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