Rolls-Royce Conway

The RB.80 "Conway" was the first by-pass engine (or turbofan) to go into service in the world. Development started at Rolls-Royce in the 1940s, but it was used only briefly in the late 1950s and early 1960s before other turbofan designs were introduced that replaced it. The Conway powered versions of the Handley Page Victor, Vickers VC10, Boeing 707-420 and Douglas DC-8-40. The name "Conway" is an Anglo-Saxon permutation of River Conwy, in Wales, in keeping with Rolls' use of river names for jet engines.

History

In early jet engines the exhaust was much faster and hotter than it had to be 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 convert|500|mph|km/h kn|abbr=on|lk=on or slower. 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 convert|450|mph|km/h kn|abbr=on and convert|700|mph|km/h kn|abbr=on, was precisely where the vast majority of commercial aircraft spend 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 Haine 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 favor 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 Tweed. In April 1947 a convert|5000|lbf design was proposed, but over the next few months it was modified to evolve into a larger convert|9250|lbf design in response to a need for a new engine to power a low-level version of the Vickers Valiant bomber, the Mark 2 known as the Pathfinder. A go-ahead to start construction of this larger design was given in October, under the RB.80 name.

Early models

During development it was decided to further improve the basic design by adding another feature then becoming common, a "two spool" compressor arrangement. In earlier designs the engine generally consisted of a series of compressor stages connected via a shaft to one or more turbine stages. Although this arrangement is mechanically simple, it has the disadvantage of lowering the efficiency of the compressor, where a higher rotational speed is useful at higher compressions. In the multi-spool design the compressor is separated into "spools" designed to operate at their most efficient speed, driven by separate turbines via concentric shafts.

For the new version a two-spool layout was selected, with 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 as the RCo.2, design work was completed in January 1950 and the first example ran for the first time in July 1952 at convert|10000|lbf 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." [Kay, pp.113]

The work on the RCo.2 was soon put to good use. In October 1952 the Royal Air Force awarded a contract for the Vickers V-1000, a large jet-powered strategic transport that was intended to allow the V bomber force to be supported in the field through air supply only. Vickers also planned on developing a passenger version of the same basic design as the VC-7. The V-1000 design looked like an enlarged de Havilland Comet, but from the Valiant it took the wing layout and added a compound sweep (a passing vogue in UK design). It also featured the Comet's wing-embedded engines, demanding an engine with a small cross-section, which limited the amount of bypass the engine could use. It nevertheless required higher power to support a convert|230000|lb|kg gross weight, so Rolls responded with the larger RCo.5.

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 convert|13000|lbf. Construction of the prototype V-1000 was well underway at Vicker's Wisely 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.

Production versions

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 the an even larger model, the RCo.8 of convert|14500|lbf, 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 convert|16500|lbf 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. [Kay, pp.114] RCo.10's development was so smooth that after delivering a small number for testing, further deliveries switched to the convert|17150|lbf 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.

Although successful in this role, only 69 707'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. [ [http://100.rolls-royce.com/products/view.jsp?id=393 Rolls-Royce, a century of innovation] ]

Rolls continued working on the Conway, delivering the RCo.15. This was similar to the earlier RCo.12, but had a larger zeroth-stage along with a larger engine housing to fit it. This improved cruise fuel consumption by 3% while at the same time increasing take-off thrust to convert|18500|lbf. The designs were otherwise similar, and RCo.12's could be re-built into RCo.15's during overhauls.

Final versions

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 convert|20250|lbf. First run in March 1961, it would be the most successful of all the Conway's, powering all of the VC10 fleet, later models with the RCo.43.

Description

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 aluminum 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 two-stage turbine using hollow air-cooled blades, followed by the single-stage turbine powering the low-pressure compressor. Accessories were arranged around the front of the engine, leading to a minimum of increased diameter.

The engine produced convert|17150|lbf for takeoff, weighted convert|4500|lb|kg and had a specific fuel consumption of 0.87.

Engine variants

References

*Kay, Antony, "Turbojet, History and Development 1930-1960", Vol 1, Great Britain and Germany, Crowood Press, 2007. ISBN 978 1 86126 912 6