{|The development of the Nimrod patrol aircraft began in 1964 as a project to replace the Avro Shackleton

Avro Shackleton

The Avro Shackleton was a British long-range maritime patrol aircraft for use by the Royal Air Force. It was developed by Avro from the Avro Lincoln bomber with a new fuselage...

. The Nimrod design was based on that of the Comet 4

De Havilland Comet

The de Havilland DH 106 Comet was the world's first commercial jet airliner to reach production. Developed and manufactured by de Havilland at the Hatfield, Hertfordshire, United Kingdom headquarters, it first flew in 1949 and was a landmark in aeronautical design...

 civil airliner which had reached the end of its commercial life (the first two RAF aircraft were unfinished Comets). The Comet's turbojet engines were replaced by Rolls-Royce Spey turbofan

Turbofan

The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...

s for better fuel efficiency

Fuel efficiency

Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...

, particularly at the low altitudes required for maritime patrol.