The Nimrod is a maritime patrol aircraft developed in the United Kingdom. It is an extensive modification of the de Havilland Comet, the world's first jet airliner. It was originally designed by de Havilland's successor, Hawker Siddeley, now part of BAE Systems. A major modification was the fit of a large weapon bay under the fuselage that can carry and drop torpedoes, mines, bombs and other stores. Sonobuoys for tracking submarines are dropped from special launchers in the rear of the fuselage.
It has been the Royal Air Force's primary Maritime Patrol Aircraft (MPA) since the early 1970s, when it replaced the piston-engined Avro Shackleton. The RAF uses two Nimrod variants: the MR2 variant in the Maritime and for the Reconnaissance role the R1 variant in a reconnaissance and electronic intelligence gathering capacity (ELINT).
The Nimrod was the first jet-powered MPA of any significance. Earlier MPA designs used piston or turboprop engines to improve fuel economy and allow for lengthy patrols. Jet engines are most economical at high altitudes and less economical at low altitudes. However, the transit to the operational area can be made at high altitude and in a jet aircraft this is not only economical on fuel but fast as well, compared to earlier piston-powered aircraft. After transit, the Nimrod descends to its patrol area.
On patrol, at high weight all four engines are used, but as fuel is used and weight falls, first one engine is closed down and then a second is closed down when weight is lower. This allows the remaining engines to be run at an efficient RPM rather than running all engines at less efficient RPM. A special "rapid start" system is fitted should the closed-down engines have to be started quickly again. Instead of relying only on airspeed for re-starting an engine, compressor air from a live engine is used in a starter turbine which rapidly accelerates the engine being started. For transit back to base, the closed-down engines are re-started and the aircraft climbed to altitude.
Other MPA designs have been jet powered, including the US Navy's S-3 Viking and future P-8 Poseidon. Wikipedia
