Not sure where I got this from but...
Air Staff target 357 laid down in June 1963 requested feasibility studies for aircraft capable of transiting at 450kts and searching at 180kts for eight hours at 1,000 nautical miles form base while carrying 18,620lb of stores.
BAC offered a VC-10 with RB177-22 Medway engines, the Maritime Reconnaissance Vanguard, a variant of the Canadair CL-44 and a Mach 2 variable geometry aircraft with reheated RB177-22 engines. Hawker Siddeley, who submitted their studies in October 1963 proposed the Avro 776 with Medways, Trident MR1 and MR2, the AW681 MR1 and MR2, Avro 784 and a variable geometry aircraft based on APG/1011F. Their preferred solution was still the Avro 776, which it felt, could meet the specification completely. The Trident was nearly as good, but had restricted scanner size capability, as did the AW681, the latter also suffered comparatively in having a maximum transit speed of only 420kts allied to high search speed (220 reducing to 185kts). The Avro 784, which was rather similar in appearance to the rival Vanguard, had a wing span of 145ft and a length of 106ft 9”. Powered by four RR Tynes it could carry the stores and operational equipment demanded and meet the overall performance requirements, except transit speed. The same general performance and capabilities applied to the Vanguard and CL-44 derivatives, which virtually eliminated them, while both VG designs were soon out of the running as too complicated and definitely too expensive.
At this stage it was rather belatedly realised that the full AST 357 requirements could not be met by 1968, the latest deadline for the start of the Shackleton replacement. Reluctantly the Ministry accepted that the new aircraft would have to go into service in an ‘interim’ condition if the very tight delivery dates were to be attainable, but insisted on it having as a minimum the operational capability of the current Shackleton with plenty of potential ‘stretch’. The time scale eliminated all proposals except those based on existing designs (shades of the Orion – and the Shackleton) and in the autumn of 1963 the BAC VC-10 and Trident were assessed on their low level capabilities by a mixed team of test pilots and experienced maritime aircrew. When it was suggested that the RAF would only accept a four-engines aircraft, Hawker Siddeley worked overtime to re-vamp their tentative MR Comet proposal of 1961, and the Mk4C variant joined the VC-10 and Trident as an option.
The VC-10 was judged unnecessarily large and expensive and it became a matter of choice between the two Hawker Siddeley projects. The company re-examined their Trident MR1 and MR2 proposals and between March and June 1964 put forward a series of HS 800 variants based on the aircraft, in parallel with the HS 801 MR Comet. Meanwhile the Ministry was formulating ASR 381 and the die was cast, for when issued by OR23 (RAF) it was seen to detail an interim maritime reconnaissance aircraft to replace the Shackleton MR2 based firmly on the Comet. A series of HS 801 brochures was produced by Hawker Siddeley as the project design team refined their ideas, culminating in a Type Specification on December 1964. This formed the basis of the Ministry’s Specification MR254D&P issued in April 1965 for the aircraft, which received the formal ‘instruction to proceed’ in June and subsequently became the Nimrod.
A replacement for the MR Shackleton had been found at last, albeit supposedly only an interim solution. In practice, of course, it has become the solution – and a very successful one at that.
