Design
The Nimrod was the first jet-powered Maritime Patrol Aircraft (MPA). Earlier MPA designs used piston engines or turboprop engines to improve fuel economy and to allow for lengthy patrols at low altitudes, as with the Lockheed P-3 Orion. Jet engines are most economical at high altitudes and less economical at low altitudes; the aircraft can travel to the operational area at high altitude which is economical on fuel and fast compared to earlier piston aircraft. On reaching the patrol area the Nimrod descends to its working altitude.
On patrol at high weight all four engines are used, but as fuel is consumed and weight is reduced first one and then a second engine is shut down, allowing the remaining engines to be run at an efficient RPM rather than running all engines at less efficient RPM. A "rapid start" system is fitted should the closed-down engines need to be restarted quickly; instead of relying only on ram air for restarting an engine, compressor air from a live engine is used in a starter turbine which rapidly accelerates the engine being started. All engines are used for travel back to base at high altitude.
Read more about this topic: Hawker Siddeley Nimrod
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