Origins At the end of World War II, the Bristol Engine Company's major effort was the development of the
Hercules and
Centaurus radial piston engines. By the end of 1946, the company had only 10 hours of
turbojet experience with a small experimental engine called the
Phoebus which was the
gas generator or core of the
Proteus turboprop then in development. In early 1947, the parent
Bristol Aeroplane Company submitted a proposal for a medium-range bomber to the same
specification B.35/46 which led to the
Avro Vulcan and
Handley Page Victor. The Bristol design was the
Type 172 and was to be powered by four or six Bristol engines of thrust to the Ministry engine specification TE.1/46. The thrust required of the new engine, then designated B.E.10 (later Olympus), would initially be with growth potential to . The
pressure ratio would be an unheard of 9:1. To achieve this, the initial design used a low-pressure (LP)
axial compressor and a high-pressure (HP)
centrifugal compressor, each being driven by its own single-stage
turbine. This two-spool design eliminated the need for features such as variable inlet guide vanes (Avon, J79), inlet ramps (J65), variable stators (J79) or compressor bleed (Avon) which were required on single spool compressors with pressure ratios above about 6:1. Without these features an engine could not be started nor run at low speeds without destructive blade vibrations. Nor could they accelerate to high speeds with fast acceleration times ("
spool up") without
surge. The design was progressively modified and the centrifugal HP compressor was replaced by an axial HP compressor. This reduced the diameter of the new engine to the design specification of . The Bristol Type 172 was not selected for development in favour of the Vulcan and Victor, but the engine design was promising enough that development continued to be funded for use on the Vulcan and other projects.
Initial development The first engine, its development designation being BOl.1 (Bristol Olympus 1), had six LP compressor stages and eight HP stages, each driven by a single-stage turbine. The combustion system was novel in that ten connected flame tubes were housed within a
cannular system: a hybrid of separate flame
cans and a true
annular system. Separate combustion cans would have exceeded the diameter beyond the design limit, and a true annular system was considered too advanced. In 1950, Dr (later Sir)
Stanley Hooker was appointed as Chief Engineer of Bristol Aero Engines. The thrustmeter showed . The next development was the BOl.1/2 which produced thrust in December 1950. Examples of the similar BOl.1/2A were constructed for US manufacturer Curtiss-Wright which had bought a licence for developing the engine as the TJ-32 or J67 for the projected
F-102. The somewhat revised BOl.1/2B, ran in December 1951 producing thrust. The engine was by now ready for air testing and the first flight engines, designated Olympus Mk 99, were fitted into a
Canberra WD952 which first flew with these engines derated to thrust in August 1952. In May 1953, this aircraft reached a
world record altitude of . Fitted with more powerful Mk 102 engines, the Canberra increased the record to in August 1955. The first production Olympus, the Mk 101, entered service in late 1952 at a rated thrust of 11,000 lb, a weight of 3,650 lb, and with a
time between overhauls (TBO) of 250 hours.
200 series Rolls-Royce had introduced its advanced
Conway design in the early 1950s, as the world's first engine to feature bypass, today known as a
turbofan. They had a string of bad luck as one aircraft design after another selected the engine as its power plant and was subsequently cancelled. The bad luck finally ended when their 16,500 pounds-force RCo.11 version was selected as the power plant of the later versions of the
Handley Page Victor, replacing the earlier model's
Armstrong Siddeley Sapphires. The Conway would go on to win a number of civilian orders for airliners as well. Rolls-Royce began to try to convince the Air Ministry that the same engine should be used in later versions of the Vulcan, as it would lower the per-unit cost of the engines as well as simplifying logistics and maintenance. Faced with the possibility that the only design win for the Olympus might be taken away, the team at Bristol began a major upgrade to the Olympus layout in 1952. The new model, the BOl.6, did not use bypass, instead they changed the arrangement of the compressor stages, reducing the high-pressure and low-pressure sections by one stage each, but increasing airflow. The result was a design that was almost identical in size and weight as the original versions, but increasing the power to 16,000 pounds-force to complete with the Conway. Their efforts were successful, and the slightly modified BOl.7, known in the RAF as the Olympus Mk. 202, became the definitive engine on the Vulcan.
300 series The BOl.7 was originally designed for the
thin-wing Javelin interceptor aircraft project, in which case it would also feature reheat. As the Javelin design process continued, Bristol offered a series of further updates to this design to provide more thrust as the weight of the aircraft increased. The ultimate end of this series of developments was the BOl.21, which added a "zero stage" to the low-pressure compressor and further increased airflow to offer 21,000 dry and 28,000 lbf in full reheat. This development process ended in 1956 when the Javelin project was cancelled. Although the aircraft was cancelled, Bristol convinced the RAF to use the new engine as the basis for a further upgrade for the Vulcan fleet. Lacking reheat, the resulting Olympus Mk. 301s were fitted to all of the later production Vulcans as well as a small number of earlier models that were upgraded. These were later de-rated in service to 18,000 lbf, although this was increased to the original power for the
Black Buck missions. In 1962, Bristol won the contest to power the
TSR.2, and responded with BOl.22, or as the RAF referred to it, the Mk. 320. This was a further modification to the 301, slightly reducing rated dry thrust to 19,610 lbf, but offering a new reheat design that increased wet power to 30,610 lbf. Even the initial versions managed to beat these ratings, although development was not entirely smooth and the engine was de-rated when fit to the prototype aircraft. All of this work came to nothing when the TSR.2 project was cancelled in 1965. ==Variants==