Julian Moxon/PARIS
With its Trent family, Rolls-Royce has laid to rest any doubts about its relationship with Airbus Industrie, dating from the difficult early days when the consortium's aircraft were powered only by US engines. As the sole engine supplier for the Airbus A340-500/600 with the Trent 500, holder of a claimed 38% share of the A330 powerplant market with the Trent 700 and the first company to sign an agreement to power the A3XX, with the Trent 900, R-R has become thoroughly integrated into the Airbus family.
While the Trent 500 is a derivative of the Trent 700 and 800 and therefore presents a relatively straightforward technical development, the engine is setting the Derby company one of its toughest production challenges.
Such is the demand for early deliveries of the A340-500/600, of which 129 have been ordered (representing sales of more than 550 engines), that R-R has been forced to meet an unprecedented schedule for initial engine delivery. It builds from 18 engines this year, for the flight test programme, to 50 in 2001 and 120 in 2002, when the first A340-600 enters service with Virgin Atlantic Airways, and then increases to 160 the following year.
"It is by far the fastest ramp-up of a big-fan engine production programme we have ever seen," says Trent 500/Trent 900 director Ian Kinnear. "We have increased annual production from a few hundred engines 10 years ago to more than 1,000, so we're ready for it." During that time, R-R has carried out major modernisation of its production facilities, creating an ultra-modern, clean and colourful environment that leaves an impression of studied calm rather than being the place where one-third of the world's big-fan engines are made.
The Trent 500 is the latest in the range of Trent engines to be fielded by R-R. It follows the Trent 700 for the Airbus A330-200/300, which entered service with Cathay Pacific in 1995, and the Trent 800 for the Boeing 777 (Thai International in 1996). Its three-shaft design philosophy mirrors that of all the company's big-fan engines, going back to the first RB211 developed for the Lockheed TriStar in 1970.
The fact that this design has survived from its initial application on a three-engined aircraft to a quad (the Boeing 747), then to the A330 and Boeing 777 twins and, with the A340, back to a quad, reflects the ease with which a three-shaft layout can be scaled to suit the power requirements of each type. In the Trent range, turbines and compressors have been scaled independently to create a family with thrusts ranging from 50,000lb (222kN) to over 100,000lb.
The Trent 700 high-pressure core has also been retrofitted to the RB211 to create the RB211-524G/H-T with improved performance and on-wing life. Two new derivatives - the Trent 900, for the Airbus A3XX, and the Trent 600, for the Boeing 747X, are under development, both with swept fans. The Trent 500 and 700 fans are 2.47m (97.5in)in diameter, increasing to 2.6m for the Trent 600 and 2.8m for the Trent 800 and 900.
Scaled Trent 800
Inits layout, the Trent 500 is essentially a scaled version of the Trent 800 (see technical description, Flight International, 11-17 September, 1996), but it uses the smaller Trent 700 fan. The derivative approach means that only proven components are used. "It was one of our precepts that there should be no unproven technology in this engine," says Robert Nuttall, Trent 500 head of marketing. This is intended to help the engine achieve a zero in-flight shutdown (IFSD) rate from entry into service.
"We're being much more aggressive on reliability," says Nuttall. "We sat down with Airbus at the beginning of the programme and decided that to target any other IFSD would be unacceptable." This demand stemmed partly from the sole engine supplier position held by R-R on the A340-500/600, which Nuttall says "makes it even more important - everyone is watching". He also says that as sole supplier, "it is important that we deliver because Airbus is relying on us. They have nowhere else to go".
The engine is now just over half-way through its development programme, with about six months to run to certification in December. The programme involves seven engines, which between them have amassed about 1,750h running time to date, the lead engine having built up 2,000 cycles. This is now being borescoped to look for potential failures before going back on to the test stand for a programme that is aiming for a total of 15,000h running time - or 4,000-5,000 cycles - before entering service. This is equivalent to a complete service lifetime.
"We're running it for even more hours than we did the Trent 800," says Kinnear. "We've got the fleet leader on pure cyclic work to build up the hours of maturity. One of the lessons you learn is that however well you design an engine, nothing beats running it to get experience."
Problems during testing have been limited to minor failures in the turbine area, for which fixes have been designed. "It has done exceptionally well on test," says Kinnear. "The engine handles and performs flawlessly. It has a good reputation internally, coming in under the weight target and showing plenty of potential for growth. We think it could grow easily to 62,000lb thrust."
In its internal architecture, the Trent 500 is a reproduction of the Trent 800, but with a few important differences leading from advances in technology and from the scaling factor. The engine will be produced in two thrust variants - 53,000lb for the A340-500 and 56,000lb for the -600. The higher power is needed because the A340-600 is longer and takes off at a lower angle of incidence. The two aircraft have virtually the same climb performance.
3d throughout
For the first time in an R-R big-fan engine, three-dimensional aerodynamics are used throughout, reducing fuel consumption by about 1%. "We believe it is a world first for a complete engine," says Kinnear. In the six-stage high-pressure and eight-stage intermediate-pressure compressors - which are scaled down by 20% from those of the Trent 892 - full viscous flow has been achieved. In other words, the airflow through the compressors follows the solutions to the Navier-Stokes equations for three-dimensional flow. This has only recently become possible with the arrival of enough computing power and results in blades and stators having highly curved profiles from tip to root, increasing efficiency and reducing losses.
Further efficiency gains have been achieved in the turbines. Again for the first time, the high, intermediate, and low-pressure turbines and stators - 90% scale versions of those used on the Trent 892 - are fully three-dimensional and are made from single crystal (CMSX-4) alloy to give longer life with less cooling. Thermal barrier coatings on the aerofoils extend life.
A new development for an R-R engine is the installation of a dedicated active clearance control system on the high-pressure turbine casings, in addition to those already installed on the intermediate- and low-pressure turbines. This followed the decision to scale the engine core from that of the Trent 800 - which, if applied to the relationship between blade and casing, would have resulted in excessive tip clearance, and hence leakage.
Active control is not used on the high-pressure compressor casing, R-R preferring to match casing and disc materials to cater for expansion. Considerable effort has been made to transmit thrust loads from engine to airframe axially, to minimise case bending.
The blades of the five-stage low-pressure turbine have high-lift aerodynamics and are thinner and broader than previous designs, allowing fewer aerofoils, and reducing weight and centrifugal forces. The blades and stators are highly curved at the forward end but, as the airflow is straightened out, become much less curved at the turbine outlet.
The combustor is designed for increased lifetime and to ensure the Trent 500 meets forthcoming emissions criteria. A new single-annular design has been used, with four and five rows of tiles on the inner and outer walls, respectively. This takes advantage of the simplicity and reliability of a single-annular combustor compared with the double-annular designs used by General Electric and CFM International, which burn fuel in two chambers, one optimised for idle power, the other for high power.
R-R has advanced double-annular designs in the pipeline, but says it has taken the single-annular concept further than other manufacturers. "We have a two-track approach," says Hamish Low, engineering director of combustion systems. "We are staying with single-annular as long as we can for cost and reliability reasons, but are ready to go with staged combustion as soon as it is fully proven." Maintenance costs of staged combustors are "around double" those of the single annular type, he adds.
The overlapping tiles, made from high-temperature nickel, allow the combustor casing to move as it heats up, eliminating stress and allowing a much longer lifetime - about 15,000-20,000h, the planned life of the engine. The burning volume is also larger, since less cooling air is required, which Low says results in reduced emissions - equivalent to 20% of the International Civil Aviation Organisation's CAEP/2 standard for carbon monoxide,10% of the limit for unburned hydrocarbons and 40% of the smoke limit. For nitrous oxides (NOx) emissions, the combustor achieves 66% of CAEP/2, or about 75% of the still unratified CAEP/4 limit.
The major change to the engine front is in the fan casing, where the containment material has been changed from Kevlar to ribbed Armco steel. This design, following that used for the BR715 and Trent 8104 (a development engine designed for the Boeing 777X), came after changed certification requirements for containment. These set new standards for ensuring that, in the event of a fan failure, the fan case-mounted accessories would not be affected.
Kevlar, while providing a light, extremely tough containment band, develops a wave-like action when it is struck by a failed fan blade, which can disrupt accessories, potentially resulting in leakage of inflammable materials. "This meant you couldn't use 100%of the casing for mounting accessories," says Nuttall. "With Armco, we can distribute them much more logically from the maintenance point of view."
The 26 titanium, wide-chord, superplastically formed, diffusion-bonded fan blades are little changed from those used on the Trent 700, although incorporating the aerodynamics from the more recent Trent 800. R-R is developing a new swept fan for the Trent 900, but chose not to use it for the Trent 500, a decision that Kinnear says was an "issue of timing".
Flight-testing of the Trent 500 began in late June on a modified A340 testbed aircraft with the engine installed on the number two pylon, replacing one of the 34,000lb-thrust CFM International CFM56-5C4s. The 2.5h test went without incident and testing will build to about 50h during the summer, leading to certification at the end of the year. Testing so far has covered starting at sea-level and altitude, windmill relighting, water ingestion, bird strike, fan blade off and cross-wind performance.
The first production engines for flight testing will be shipped to Airbus this month and will be mounted on three A340-600s and a single A340-500 for a programme lasting 1,600h for the A340-600s and 300h for the A340-500.
Initial ground testing took the Trent 500 to 68,000lb thrust to establish running limits, but the engine will be certified at 60,000lb, which leaves plenty of temperature margin at the 53,000/56,000lb operational thrusts. This is intended to ensure it begins its working life with the exceptional reliability that has become expected of all modern aircraft powerplants from the day they enter service.
Source: Flight International
