In 1957-58, British firms had taken an interest in a NATO competition for a maritime reconnaissance aircraft to replace the Lockheed Neptune. Avro had discussions with Bregeut, who had won the NATO contract with the twin-Tyne Atlantic. The firms jointly proposed a modified Atlantic, the Avro-Breguet 2A, with two additional RB.153-61 jet engines fitted on under-wing pylons and with wing tip fuel tanks. This came to nothing when Avro couldn't agree with Breguet's use of honeycomb for the fuselage shell.

In November 1961 Avro published a brochure for the aircraft they felt would best meet the RAF's requirements. The Avro 776 was based on the Trident fuselage with a redesigned and extended wing, and thee RB.179 turbofans giving 16300lb thrust each. Maximum takeoff weight was 179600lb with a weapons load of 16920lb. Transit speed to and from the search area was 460 knots at an altitude of 35000ft.

In 1963 the RAF issued OR.357 for a maritime reconnaissance aircraft to replace the Shackleton. This was required to have three or more engines (jet or turboprop), transit to the operational area at a speed of 400 knots or better, and around 8 hours search thereafter. The Avro 776 appeared to be the likely winner of OR.357, but on June 4th 1964 the RAF issued a new requirement, OR.381 (apparently without informing Hawker Siddeley, Avro's parent company).

Through a mixture of political expediency, a wish to cut costs, and a belief that the Shackleton couldn't last until 1970, this OR for an "interim" MR aircraft was almost exactly that for the Breguet Atlantic, which the RAF had originally rejected because it had insufficient speed, endurance and payload.

The Avro team was stunned, but in seven days they had produced a design for an aircraft largely using existing tooling, a production engine, and yet retaining high speed and long endurance. This aircraft, based on the Comet 4 wings and fuselage married to the RB.168 engine, was known as the HS.801.

The intention to order a version of the Comet 4C to replace the Shackleton was announced in Parliament on February 2nd 1965. The configuration of the HS.801 was agreed on September 30th 1965.

On January 19th 1966 a production contract for 38 aircraft was signed. The first HS.801 prototype flew on 23rd May 1967; in the same month the new type was named the Nimrod. The first production aircraft flew on 28th June 1968, and the RAF took delivery of its first aircraft on 2nd October 1969.

A further eleven airframes were ordered in the mid-1970s, but there was no requirement for these; it was a purely political decision to maintain employment.

Since 1960 the carrier-based Fairey Gannet, equipped with the AN/APS-20 radar, had been providing airborne early warning for the Royal Navy. The decision to phase out conventional fixed-wing aircraft carriers had the effect of depriving the Fleet of AEW cover. Accordingly, the RAF undertook to provide an interim AEW capability, utilising surplus Shackleton airframes (which wouldn't last until 1970!) and APS-20 radars. Number 8 Squadron was re-formed on January 1st 1972 equipped with twelve Shackleton AEW.2s. Their envisaged role of Fleet air defence was quickly pushed into second place by the need to provide AEW for the whole UK Air Defence Region. The lack of an organic AEW capability was to cost the Navy dear in the Falklands conflict on 1982.

Meanwhile the search was on for a proper AEW aircraft, both in Britain and at NATO. The inability of the NATO countries to reach agreement on the Boeing E-3 (although agreement was eventually reached) caused the British Government to decide go it alone with the Nimrod AEW project, which by 1977 had reached an advanced design stage and was costing about £1 million per month.

The British AEW project utilized the eleven surplus Nimrod airframes ordered in the mid-1970s. The airframes were modified with large fore and aft radomes, but the mission system continued to have serious problems. Finally, in September 1986 the Government decided that the Nimrod AEW wasn't going to be viable, and re-opened the whole competition for an AEW aircraft. In December 1986 the Boeing E-3 Sentry was announced as the winner. Approximately £1 billion was wasted on the Nimrod AEW project.

Three airframes were completed as Nimrod R.1s, a specialised signals intelligence aircraft built to replace Comets which had been operated in a similar role. Number 51 Squadron received the aircraft between 1974 and 1976.

Starting in 1975, Nimrod MR.1s began to be converted to MR.2 standard. Externally the aircraft looked much the same, but internally the mission systems were completely upgraded. Upgrades continued thru 1984.

The RAF's Staff Requirement (Air) 420 for a new Maritime, Reconnaisance and Attack aircraft to replace the Nimrod MR.2 was endorsed in November 1992, when an initial data gathering phase was authorised. A Request for Information was issued to the seventeen contractors who had registered an interest, and a competitive tendering phase was initiated in January 1995. Four companies submitted proposals. Dassault offered the Atlantique 3, Lockheed the Orion 2000, Loral/Marshall the Valkyrie (rebuilt Orions) and British Aerospace the Nimrod 2000. Also considered were whether to refurbish the existing MR.2, and whether to do nothing and continue with the MR.2 as it was.

On July 24th 1996 the Nimrod 2000 was announced as the winner of SR(A) 420. British Aerospace was contracted to supply twenty-one Nimrod 2000 aircraft, together with a training system and initial logistic support. The aircraft was required to be able to deploy anywhere in the world for 10 days without re-supply. A £2.4 billion contract was awarded in December 1996. Thirteen years later, the Nimrod MRA.4 had still not entered service.

On October 18th 2010 the UK government announced as part of its Strategic Defence and Security Review that the Nimrod MRA.4 was not going to enter service. This is despite the fact that development had largely been paid for.

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