Low emissions and noise loom large in Rolls-Royce's vision for future engines as environmental factors begin to outweigh the drive for out-and-out performance
In a few weeks, Rolls-Royce will deliver for testing in Spain a demonstrator engine containing a range of technologies that for the first time will place emissions reduction at a higher premium than out-and-out performance. It is a sign of the times and reflects the increasing emphasis placed on reducing aviation's contribution to global warming.
"Emissions and noise have come to the fore in the last 10 years and are now the two strongest design drivers," says Stephen Morgan, R-R's chief engineer, research and technology, civil aerospace. "They condition the design of the engine and, therefore, my spend on technology."
As it celebrates its 100th anniversary, R-R is looking to the century ahead from a position of strength, having expanded its line-up with the takeover of Allison in the USA, created its Rolls-Royce Deutschland subsidiary and achieved significant success with its Trent line-up of civil aeroengines. Now, it is preparing for the next 20 years and beyond with a structured research programme split into three main components: Vision 5, Vision 10 and Vision 20.
Vision 5 takes off-the-shelf technologies that can be applied immediately. Several feature in the Trent 900 powering the Airbus A380, including the swept, 2.9m (116in) diameter titanium fan, a new low-emissions combustor, and contra-rotating high- and intermediate-pressure shafts to minimise noise and reduce fuel burn.
Vision 10 covers technologies likely to become available within the next 10 years, while Vision 20 looks at developments on the way over the next two decades, and takes into account future aircraft design and its effect on engine requirements.
The Affordable Near Term Low Emissions (Antle) engine comes under the Vision 10 strategy. Antle incorporates a European Commission funding element, which targets a 12% improvement in specific fuel consumption compared with today's levels, along with a 60% reduction of oxides of nitrogen (NOx) and a one-third cut in engine life-cycle costs. Another goal is to cut the development time for new engines by 30%.
Partners
The Antle demonstrator is based around a Trent 500 (the engine powering the Airbus A340-500/600), modified progressively with upgraded components supplied by R-R's partners in the programme - Avio, Hispano-Suiza, Rolls-Royce Deutschland, Techspace Aero, TRW and Volvo. Tests will take place at the INTA test facility near Madrid. "Compared with what we have today, we can expect to see core size going down and fan diameter increasing, helping to increase bypass ratio. We also want more torque to drive the considerably increased electrical requirement of 'all-electric' aircraft; and a hotter, more efficient core," says Morgan.
Lower noise is an increasing priority, as was shown recently in Singapore Airlines' requirement that its newly ordered A380s should meet the London Heathrow airport QC4 noise limits due to be introduced in 2006. This meant that the Trent 900 fan diameter had to be increased, to provide a higher bypass ratio - leaving it 101mm wider than the optimum. "It's the first time environment has been a factor in engine design," says Morgan.
He says the contra-rotating high-pressure (HP) and intermediate-pressure (IP) spools introduced on the Trent 900 were made possible through improvements in bearing technology, which can handle much higher rotational speeds between the two shafts. "The spools used to spin the same way to keep bearing speeds down," he says. "Contra-rotation means you get a higher net swirl out of the HP compressor, which puts it more or less in the right orientation for the IP stage. We save about 1% in fuel."
According to Morgan, capability and cost have increased about equally in the past 30 years. "We believe we need to be more imaginative on increasing capability while keeping cost at the same, or lower, levels," he says. This is particularly true of the fastest-growing segment of aviation - unmanned air vehicles. R-R, through its North American operation, powers Northrop Grumman Global Hawk and Fire Scout UAVs. These use existing engines, but the emphasis in future will be to develop "clusters" of powerplants suitable for different applications.
R-R is involved in the US Versatile Affordable Advanced Turbine Engine (VAATE) programme, one focus of which is UAV power. "The emphasis on UAVs has shifted since the Iraq campaign started," says Morgan. "We have moved away from short- to medium-range subsonic UAVs towards providing a long-range supersonic strike capability. We're looking at combined-cycle gas turbines to meet that requirement, and with our Concorde heritage we're uniquely placed to do so."
R-R's latest civil engine, the Trent 1000, was chosen along with General Electric's GENX to power the Boeing 7E7, a relief for the company, which had to wait a long time to power the 7E7's predecessor, the 767.
Robert Nuttal, vice-president, marketing, for R-R's airlines division, says the 7E7 requirement was met with an engine that is aerodynamically based on the Trent 900. The major difference in the 7E7 design is the "dramatically" higher electrical power needed for the aircraft's pressurisation system, which is now supplied by electrically powered compressors instead of using air bled directly from the HP compressor.
For R-R, this provided an unexpected benefit. Analysis showed that taking the generator drive from the intermediate instead of the HP shaft improved compressor aerodynamics and resulted in significantly lower idle speeds, reducing noise while taxiing, as well as delivering a 50% reduction in fuel burn - and therefore pollution - during descent.
Higher ratio
The Trent 1000 also brings a significantly higher bypass ratio compared with the Trent 900 - 11:1 against 9:1. This is done by increasing the hub-to-tip ratio of the 2.8m-diameter fan with a smaller-diameter core and improved three-dimensional computer-designed aerodynamics at the fan root. All modern aeroengines now feature 3D designs, which has resulted in improved prediction of airflow and compressor and turbine performance. "The inhibitor is computer power," says Morgan. "Today we can do stresses on single blades and the fan, but we need an order of magnitude improvement in computer power to develop a complete physical understanding of the engine."
Under the Vision programme, technologies are spun into new engine designs as they become available. The Trent three-shaft family has benefited most to date because of the common basis for its respective models, but R-R wants to repeat the process in its two-shaft line-up, which includes engines from R-R North America, International Aero Engines and R-R Deutschland. "We're trying to bring to the small end what we've done with the Trent and develop a single, scaleable engine covering the 7,000lb to 30,000lb-thrust [30-135KN] range," says Morgan. Elements on the drawing board include a new nine-stage boosterless HP compressor, under development at R-R Deutschland, and a new fan, which R-R NA is studying. "The next two years will see rig testing of major components," he adds.
The demonstrator programme covers two- and three-shaft designs, with the goal of improving efficiencies, reducing the size of the engine and pushing up pressure ratios. "We want to do more with less," says Morgan. "We're currently achieving pressure ratios of around 4.5:1 in six stages and we're aiming at 5.5:1 in five stages. That's a 20% increase in loading per stage."
The same applies to turbine development, where "excellent" results have been obtained from a new soluble ceramic manufacturing process, which means that cooling air can be directed more accurately at turbine-blade hot spots, improving cooling and enabling the blades to run at higher temperatures without affecting life. Optimising the process in this way halves the amount of cooling air needed and results in lower NOx production. Stage numbers will be reduced, the goal being to halve the number of nozzle guide vanes, while reducing the number of turbine blades by 30%.
Nuttal says the approach to engine design has changed to reflect the particular operational specifications of modern commercial aircraft. "Every engine in the last 15 years has been a bespoke design. No one engine is suitable for another aircraft - we have to take into account subtleties of design to give Airbus and Boeing the best life-cycle costs with respect to the way the aircraft is operated."
JULIAN MOXON / LONDON
Source: Flight International
