FLUG REVUE December 2004: Rolls-Royce has high-tech history

ROLLS-ROYCE REMAINS AT THE FOREFRONT OF TECHNOLOGY

By Patrick Hoeveler

The Singapore Airlines Airbus A340-500 glides majestically overhead in the direction of the airport. After over 14 hours in the air, the airliner with its four Trent 500 engines comes in to land on the runway in Los Angeles. It was 96 years earlier that a visibly enthusiastic Charles Rolls landed in a field near Le Mans in his Wright Flyer. His flight had lasted only four-and-a-half minutes. Despite his enthusiasm, he could surely not have even dreamed that a company bearing his name would achieve so many feats. The name Rolls-Royce has been one of the big names of the propulsion industry for many decades. But what were the most important engines it built?

Not an easy question, even for Rolls-Royce's Director Engineering and Technology, Dr. Mike Howse. “Rolls-Royce's 100-year history is peppered with many important aero engines, and choosing would be difficult,” he says. Two of his favourites are the Merlin and the Welland. Whereas over 168,000 of the former were built, only one-thousandth of that number of the latter were produced, yet they were both of enormous importance to a company that abounded in contrasts. Thus, the car enthusiast Charles Rolls, born in 1877, came from the upper crust of society, whereas Henry Royce, born in 1863, had to laboriously work his way up from a humble background.

In 1904 the two met for the first time and agreed to found their own company. Rolls soon discovered his passion for aviation: “There is nothing so fascinating or exhilarating as flying,” he said in 1908 after meeting the Wright brothers near Le Mans. However, he was unable to persuade his partner to develop an aero engine or to follow his suggestion that they build the Wright Flyer under licence. Royce considered the market too small. Rolls on the other hand possessed two Wright Flyers and was the first person to fly from England and France and back again in one of them, on 2 June 1910. Tragically he could not enjoy his fame for long, being the first Englishman to be killed in an aircrash, during a display at a flying show on 12 July 1910.

This appeared to seal the fate of aviation as far as the motor car company was concerned, until, following the outbreak of the First World War, the Board unexpectedly decided to move into aviation with the manufacture under licence of French Renault engines for the British government. Royce now began work on the development of his first aero engine, and the water-cooled Eagle with 12-cylinders in a 60 degree “V” arrangement was already running on the test rig in Derby by the end of February 1915. With a 20.32 litre engine capacity, it generated 184kW (250hp) at 1600 revolutions per minute. Apart from the planetary gearbox, which reduced the loading on the engine crankcase, it featured a conventional design. Its first flight took place on 18 December 1915 on the Handley Page 0/100. From virtually nothing, Royce had created an aero engine that was to have a production run of over 4,600 and which enabled the Vickers Vimy to achieve several records, such as the first direct flight across the Atlantic and the first flights to South Africa and Australia.

Despite this success, the development of aero engines virtually ceased after the war. In 1926, the Kestrel V12, an important precursor to the Merlin, marked the return to aero engine development. It adopted as its trademark two rows of six cylinders cast as a single piece from aluminium to save weight. It even flew in the prototypes of two fighters that were later to be used by England's enemy, the Messerschmitt Bf 109 and the Junkers Ju 87. Further glory was achieved by Rolls-Royce with the R Engine for the S.6 racing seaplane, with which Supermarine was hoping to win the Schneider Trophy in 1929. Due to lack of time, Royce remained true to his principle of simplicity and used a Buzzard 12-cylinder as the base, on which he reinforced the components and aligned the crankcase, the valve gear and accessories into a better streamline shape. The secret of his success lay in the double-sided supercharger with ram air intake manifold, which drove more air through the engine. Here, a rotor accelerated the air through the centrifugal force and forced it into a diffuser. In this way, the speed of the air was converted to pressure so that a greater air mass would be squeezed into the cylinders for combustion purposes, thus producing more power. These experiences with compressors were later to prove helpful when it came to the gas turbines. At any rate, after Supermarine won the prestigious race, Reginald Mitchell, the designer of the S.6 and later of the Spitfire, was rewarded with a Rolls-Royce car.

Despite the success, Royce never lost sight of reality and predicted the need for a new engine for modern monoplane fighters. As a result, development of the twelve-cylinder P.V. 12 began in 1932 as a private venture. With the benefit of hindsight, this was indeed a courageous decision, as Rolls-Royce's rivals in the UK had a hefty lead at the time, at least in numbers. For example, only 35 out of a total of 721 engines produced in the United Kingdom in 1929 were built by Rolls-Royce. The later Merlin seemed at first to be ill-fated. The death of Royce after a long illness overshadowed the first full engine test of the P.V. 12 on 15 October 1933. Furthermore, cracks in the single-part aluminium casing made separate cylinder blocks necessary.

Once the engineers had got to grips with the problems, things took a sharp turn for the better. As the engine which powered the Hurricane, Spitfire and Lancaster, the Merlin achieved fame throughout the world. On 28 May 1940 came the order from the Air Ministry to send a complete set of drawings to the USA. Packard and Continental ended up manufacturing the engine, which also helped the American P-51 Mustang achieve its breakthrough. The successor to the Merlin, the Griffon, proved even longer lived. It was to remain in service with the Avro Shackleton of the Royal Air Force until 1991. One of its distinguishing features was lubrication of the main bearing and connecting rods through the hollow crankshaft.

The next milestone was not long in coming. As Mike Howse explains, Rolls-Royce soon heard of the activities of Frank Whittle. “Our experience with superchargers led to involvement in early Whittle PowerJets/Rover gas turbines. It was the W.2B/23 that marked the company's direct involvement in gas turbine development.” After starting with the production of turbine blades, Whittle suggested to the company that it should take over production of his W.2 engine, which was then built under the name of the Welland.

Unlike the piston engines, which bore the names of birds of prey, the jet engines were named after British rivers to symbolise the continuous flow of air and the combustion process in gas turbines. According to Howse in his Whittle lecture, the challenge in the W2 lay in the combustion chamber. Ten combustion chamber cases were distributed around the engine. The air flowed from the compressor into the chambers. There it was ignited upon the introduction of fuel, following which it was conveyed forwards again, emerging eventually from the rear out of a single nozzle.

Not long afterwards the designers increased the air flow from 14.6 to 17.5kg/s for the Derwent. Today, the Trent 900 for the Airbus A380 devours 1179kg/s. It was from the Derwent that the first turboprop engine in the world was developed: on 20 September 1945 a Gloster Meteor flew for the first time with two RB.50 Trents, which powered two five-blade propellers. This was followed by the RB.39 Clyde, which had a nine-stage axial compressor plus a radial compressor. It was the first two-shaft engine to have separate turbines, which drove the high-pressure compressor and, via a gearbox, the axial low-pressure shaft with propeller. However, the Clyde never got beyond the prototype stage. Large-scale success came with the Dart, as Howse emphasises. “Vickers' selection of the Dart to power the Viscount ushered Rolls-Royce into the US airline market. And almost 60 years after its design, there are still around 2,000 Dart engines in service today.“

Further developments on the jets came in quick succession. In only nine months, the specialists from Rolls-Royce had designed what was then the most powerful engine in the world, with a thrust of 22.22kN – the Nene. The distinguishing feature of this engine proved to be the significantly more efficient compressor, which pumped the air into a total of nine combustion chambers. Compressor and turbine ran on two separate shafts that were linked via a quick disconnect plug. The famous RB designations made their debut with the Nene, as Whittle engines like the W.2B/23 were shortened, for example, to the B.23. To avoid confusion with bomber version designations, the letters “RB” for Rolls and the Barnoldswick research site were chosen.

If combustion chambers were still in the foreground in the early gas turbines, in axial engines like the Avon and Conway the compressors assumed an ever more important role, as Howse explains. Whittle's first product still had a single-stage centrifugal compressor and a pressure ratio of 3:1. On the Conway, there were 17 stages and a pressure ratio of 15:1. The company's current top performer, the Trent 900, has 15 stages and an overall pressure ratio of 42:1.

Rolls-Royce had little experience of axial compressors initially. Its first engine to have an axial compressor was the Avon, designation AJ.65 (standing for axial jet with 6500 lb/28,89kN output), which had a twelve-stage compressor, eight combustion chambers and a single-stage turbine. Compared with other models, it had an inauspicious beginning: the test engine was difficult to start and the blades in the first stage broke. It was an arduous task just to tease 22.22kN, the output of the Nene, out of it. But also in this case the difficulties were surmounted and the Avon became a success. It was later upgraded to include a two-stage turbine and an annular combustor.

By contrast with the Avon, on the Conway, the first bypass engine, not all the air flowed into the compressor but, to reduce the fuel consumption, some of it flowed around the core. The bypass ratio was only 0.3:1 To put this in perspective, today's Trent 900 has a gigantic 8.1:1. Even so, the Conway was the first engine to have air-cooled engine blades. The ratio of the air in the primary and secondary airflows rose on the RB.178 demonstrator of 1966 to 2.3:1; despite having only two shafts, this laid the groundwork for the new concept of an engine with three shafts instead of two.

“Our edge in the civil market in recent times can be traced to the development of the three-shaft engine – the first of which, the RB.211-22B, entered service in 1972 powering the Lockheed L-1011 TriStar. The three-shaft design has many inherent advantages, not least the ability of each shaft to run at its own optimum speed and the use of fewer components leading to stiffer, stronger engines”, explains Howse.

The ambitious RB.211 programme for the Lockheed L-1011 TriStar came about as a result of the requirement for a high bypass ratio, and this engine was to consume 25 per cent less fuel than the Conway. For the first time the development department used three shafts and composite materials (Hyfil) in the fan. Rolls-Royce gave up its plans to build the RB.207 for the later Airbus A300 in favour of the RB.211, as it felt that the European project was too uncertain. The result was a tough price war against the General Electric CF6, which the company did win, but only at a high price: at the end of 1970 the estimated cost of manufacturing each engine actually exceeded the selling price by around £5,000.

The light Hyfil fan blades proved vulnerable to bird strike, and so the composite fan with its 25 Hyfil blades was abandoned in favour of 33 titanium blades. This was not the only disaster to hit the company: at the time of the maiden flight of the TriStar on 16 November 1970, the service life of the combustors was no more than 35 hours. These problems finished off the company, which by now was heavily dependent on loans. On 4 February 1971 Rolls-Royce went bankrupt. But once again there was another comeback, this time with the RB.211-524 for the 757. This was the first Boeing airliner not to be powered by a Pratt & Whitney type as launch engine. With the Trent family which followed, the company finally recovered its leading position.

So what challenges faces the company in the future? First and foremost, in Mike Howse's view, is the need to cut emissions. Reducing carbon dioxide as the biggest challenge would lead to new milestones. The stated objectives of the Vision 20 research programme include a 50% reduction in emissions of carbon dioxide and as much as 80 percent in nitrogen oxide emissions. The next major step could be recuperative engines like the Rolls-Royce WR-21 marine gas turbine, but these are proving too heavy for use in the air. Preliminary tests are currently under way in Stuttgart for such a powerplant, with intermediate cooling and energy recovery using heat exchangers.

Another approach, according to Howse, would be to separate propulsion and energy generarting systems, as has been done on the NASA research air vehicle Helios, using solar cells and electric motors. Fuel cells are a further possibility. Rolls-Royce is currently engaged in intensive research on components for more electric engines, for example, magnetic bearings. However, issues such as the temperature stability of the electric and magnetic materials, weight and insulation still need to be addressed.

Further research on possible pulse detonation engines is also needed. Here, a mixture of air and gas is ignited in a pipe. The shockwave compresses the air-fuel mixture at five times the speed of sound and ignites it. But to generate a useful amount of thrust, dozens of such explosions are needed every second. The biggest challenges here are probably injection systems and highly heat-resistant materials.

Notwithstanding all this research for the future, a phase of consolidation stands ahead of Rolls-Royce in the immediate future. After all, the company is involved on all the major aerospace programmes: the A380, 7E7, JSF, Eurofighter and A400M. But there will be no standstill, as Dr. Mike Howse pointed out in the closing words of his Whittle lecture: “Due to its inherent power density, efficiency and adaptability, the gas turbine and its technologies will continue to play a dominant role in aerospace, bringing with it massive new challenges and exciting opportunities for the engineers of the future.”

From FLUG REVUE 12/2004

Last updated 11 November 2004

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