Rolls-Royce RB.80 Conway
In early jet engines the exhaust was much faster and hotter than it had to be (contrary to the ideal Froude efficiency) for efficient thrust; capturing some of that energy would improve the fuel economy of the engine. The turboprop is an obvious example, which uses a series of additional turbine stages to capture this energy to power a propeller. However there is a tradeoff in propeller efficiency compared to forward speed, so while the turboprops are efficient engines, they are only efficient at speeds of up to 500 mph (800 km/h; 430 kn). This meant there was a sweet spot between the high efficiencies of the turboprop at low speeds and the jet at high speeds that was not being directly addressed. This spot, between about 450 mph (720 km/h; 390 kn) and 700 mph (1,100 km/h; 610 kn), was precisely where the vast majority of commercial jet aircraft spent most of their time.
The basic concept of bypass had been studied from the earliest days of jet engine design. Alan Arnold Griffith had proposed a number of different bypass engine designs as early as the 1930s while he and Hayne Constant were trying to get their axial-flow jet engines working at the Royal Aircraft Establishment. Frank Whittle's Power Jets also studied a number of bypass configurations. However, the need to get jet engines into service during the war meant this work had to be put aside in favour of simpler designs with shorter introduction times. The ending of the war changed priorities dramatically, and by 1946 Rolls-Royce agreed that existing engines like the Rolls-Royce Avon were advanced enough that it was time to start work on new concepts like bypass.
Griffith suggested building a purely experimental design using parts of the Avon and another experimental jet engine, the AJ.25 Tweed. In April 1947 a 5,000-pound-force (22,000 N) design was proposed, but over the next few months it was modified to evolve into a larger 9,250-pound-force (41,100 N) design in response to a need for a new engine to power the Mk.2 low-level version of the Vickers Valiant bomber. A go-ahead to start construction of this larger design was given in October, under the RB.80 name. The Rolls-Royce RB.80 Conway was the first by-pass engine (or turbofan) in the world to enter service. The name "Conway" is an Anglo-Saxon permutation of River Conwy, in Wales, in keeping with Rolls' use of river names for jet engines.
During development it was decided to further improve the basic design by adding another feature then becoming common, a "two spool" compressor arrangement. Earlier engines generally consisted of a series of compressor stages connected via a shaft to one or more turbine stages, with the burners between them arranged around the shaft. Although this arrangement is mechanically simple, it has the disadvantage of lowering the efficiency of the compressor. Compressor stages run at their maximum efficiency when spinning at a specific speed for any given input air pressure - in a perfect compressor each stage would run at a separate speed. The multi-spool design, first used on the Bristol Olympus turbojet, is a compromise, the compressor is separated into "spools" designed to operate closer to most efficient speed, driven by separate turbines via concentric shafts. Two and three-spool designs are common, beyond that the mechanical complexity is too great.
The new version had a four stage low-pressure compressor driven by a two-stage turbine, and an eight stage high-pressure compressor driven by another two-stage turbine. Now known by the Ministry of Supply designation as the RCo.2, design work was completed in January 1950 and the first example ran for the first time in July 1952 at 10,000 pounds-force (44,000 N) thrust. By this time the low-level Valiant Pathfinder had been abandoned, and so the first example was also destined to be the last. Nevertheless it proved the basic concept sound, and "ran perfectly for the whole of its 133 hours life."
The new engine was similar to the RCo.2 in most ways, differing in details. The low-pressure compressor now had six stages, and the high-pressure nine, driven by two and one stage turbines respectively. The first RCo.5 ran in July 1953, and passed an official type rating in August 1955 at 13,000 pounds-force (58,000 N). Construction of the prototype V-1000 was well underway at Vickers Armstrongs' Wisley works in the summer of 1955 when the entire project was cancelled. Having second thoughts about the concept of basing the V-bombers away from the UK, the need for the V-1000 became questionable and it became an easy decision to drop the project.
The Conway was saved once again when it was selected to power the Handley Page Victor B.2 variant, replacing the Armstrong Siddeley Sapphire of earlier models. For this role Rolls designed an even larger model, the RCo.8 of 14,500 pounds-force (64,000 N), which ran for the first time in January 1956. However the RCo.8 was skipped over after receiving a request from Trans-Canada Airlines (TCA) to explore a Conway powered Boeing 707 or Douglas DC-8, having interested both companies in the idea. Rolls responded by designing an even larger model of the Conway, the 16,500 pounds-force (73,000 N) RCo.10, and offering the similar military-rated RCo.11 for the Victor. The new engine differed from the RCo.8 in having a new "zeroth stage" at the front of the low-pressure compressor, further increasing cold airflow around the engine. The RCo.10 first flew in an Avro Vulcan on 9 August 1957, followed by the RCo.11 in the Victor on 20 February 1959.
Boeing had calculated that the Conway, even though it had limited bypass in keeping with its original in-wing mounting, would increase the range of the 707-420 by 8% compared to the otherwise identical 707-320 powered by the non-bypass Pratt & Whitney JT4A (J75). In May 1956 TCA ordered Conway-powered DC-8s, followed by additional orders from Alitalia and Canadian Pacific Air Lines, while the Conway powered 707 was ordered by BOAC, Lufthansa, Varig and Air India. RCo.10's development was so smooth that after delivering a small number for testing, further deliveries switched to the 17,150-pound-force (76,300 N) RCo.12, which was designed, built and tested before the airframes had finished their testing. These models also featured a distinctive scalloped silencer, and a thrust reverser that could provide up to 50% reverse thrust.
The RCo.12 Conway was an axial-flow turbofan with a low bypass ratio of about 25%. It had a seven-stage low-pressure compressor, the first six stages made of aluminium and the last of titanium. Behind this was the nine-stage high-pressure compressor, the first seven stages of titanium and the last two of steel. The bypass housing duct was also made of titanium. The combustion area consisted of ten cannular flame cans. The high-pressure compressor was driven by a single-stage turbine using hollow air-cooled blades, which was followed by the two-stage turbine powering the low-pressure compressor. Accessories were arranged around the front of the engine, leading to a minimum of increased diameter.
Although successful in this role, only sixty-nine 707's and DC-8s were built with the Conway, due largely to the delivery of the first US-built bypass engines, particularly the Pratt & Whitney JT3D. Nevertheless the Conway was successful on these designs, and was the first commercial aero engine to be awarded clearance to operate for periods up to 10,000 hours between major overhaul.
The final development of the Conway series was the RCo.42, designed specifically for the Vickers VC10. As the need for wing-embedded engines was long abandoned by this point, Rolls dramatically increased the zeroth-stage diameter to increase the bypass from about 25% to 60%, and further increasing thrust to 20,250 pounds-force (90,100 N). First run in March 1961, it would be the most successful of all the Conways, powering all of the VC10 fleet, later models with the RCo.43.
By 1968 a Hyfil carbon-fibre fan assembly was in service on Conways of the VC10s operated by BOAC.