Guy Norris/LOS
A little over a year from now the world's longest jet airliner is due to fly from Everett, Washington. Powering the first flight of the Boeing 777-300 will be Rolls-Royce Trent 800s - a testament not only to the growing market penetration of the UK engine maker, but also to the success of the Trent family itself. Until the first -300 flight, it will have been 27 years since an R-R engine last powered a widebody airliner on its first flight, and 15years since a non-US engine powered the maiden flight of any Boeing airliner.
For R-R, the growing success of the Trent 800, building on that of the 700, is viewed by the company as something of a watershed in its perpetual struggle with the two US giants, Pratt & Whitney and General Electric. In order terms, R-R is running second in the 777 race with 32% of the firm orders, behind P&W and ahead of GE. Although only narrowly ahead of the GE offering the GE90, R-R feels confident that its growing power base in the Asia-Pacific market is the start of a comeback. The smaller Trent 700 has also clinched 32% of the Airbus A330 market, again beating GE back into third place.
Part of the excitement stems from the coup of winning the huge Singapore Airlines/ Singapore Aircraft Leasing order in 1995. Not only was this a significant victory over P&W, the incumbent engine supplier, but it more than made up for the body blow of losing the British Airways 777 order to GE with the GE90 in August 1991.
At the core of the revival, in more ways than one, is the same triple-shaft design philosophy which began with the first RB211 in the 1970s (see box overleaf). "This engine comes of age with higher thrust. We're cashing in on the design heritage," says Trent chief engineer, Mike Terrett. "At this stage, the triple-shaft design allows you to give more thrust with only a little increase in weight."
The feeling among R-R and customer airlines is that "the hour" of the three-shaft design is finally at hand. "The original design was over-engineered," admits Terrett. "The RB211-22B, for example, had a huge amount of weight that could be taken out. But, because of that, we didn't get huge bending deflections, and that's one of the main features which gave us confidence in the long term."
The extra weight also meant that the RB211 option was always heavier than the competition, a significant drawback on some longer ranged missions where every kilo counted. Now, for the first time on any Boeing widebody, the R-R engine is lighter. Terrett says: "With the -524 we were heavier, with the -700 we achieved parity and with the -800 we are lighter. Roughly 30% lighter in the case of the GE90 and maybe 136kg per shipset over the PW4084 we think."
BIGGER FAN
"The fan is probably a third of our weight advantage," explains Terrett. Fan diameter was increased from the Trent 700's 2.5m unit to the 800's 2.8m "for straightforward thrust reasons", he says. "The 700 fan would probably have gone to 356kN (80,000lb) if we had chosen to, but we'd have had to make some radical changes to strengthen the blades for bird strikes and so on."
The fan consists of 26 super plastic diffusion bonded (SPDB) wide chord fan blades. In the SPDB process, three layers of titanium are laid upon each other and heated to the point where they can be formed, then inflated with inert gas into the required shape, with the centre layer becoming the internal reinforcement. The diffusion bonding of the internal layer is based on existing RB211 honeycomb wide-chord blades and the super plastic property allows high stretch forming without "necking." The resulting reinforcement is known as a Warren Girder structure.
R-R plays its fan technology as a trump card in the big-engine battle. It introduced the 535E4 some 12 years ago as the world's first high bypass ratio engine with a wide chord (or snubberless/clapperless) fan, and has built on the tradition ever since. "The fan is one of our product differentiators as far as competition is concerned," says Terrett. "Our fan technology gave us a head start and now we're into the second generation."
At the centre of the fan, is a 780mm-diameter glass fibre reinforced-plastic nose cone, which rotates at 3,500RPM. The "Nicholas Spinner", as the cone is known, has the familiar collar extension, which was added originally to improve the airflow through the fan root into the core of the -524G. The collar extends the full depth of the cone to 94cm.
The spinner is bolted to the front of the low pressure (LP) disc. Each blade is engaged into the disc by an axial dovetail slot and held in place by a single "key". A beefy slider assembly prevents any radial movement of the blades.
The entire disc attaches to the LP shaft with a curvic coupling, behind which is a roller bearing. To the rear of the bearing is an electrical pick-up, which is used in conjunction with a phonic wheel to measure LP compressor speed. At the rear end of the LP shaft are internal splines, which engage the LP turbine shaft and a ball bearing to maintain the shaft in the correct position. In addition to the LP ball bearing there are eight main line bearings in all. The LP shaft has a roller bearing forward and rear, plus a central location bearing. The intermediate pressure (IP) shaft has a similar arrangement, while the high pressure (HP) shaft has a forward location bearing and a rear roller bearing.
The zero-loaded bearing, which forms the inter-connect between the LP and IP shafts, is a vital part of the triple-shaft strategy, and one which R-R fought hard to get right. "It had a number of problems, the lubricating oil supply was not good. We finally fixed it in the 1970s and, once we did that, had an advantage," says Terrett.
Another area with past problems was the "hot strut" supporting the rear roller bearing between the HPT and IPT. "The arrangement benefits performance in terms of better surge margin and tip clearance, if you can get it right," says Terrett. It was essential to get this right as it provided the key to using a much shorter shaft. R-R argues that "the shorter the HP shaft, the better", a message that was amplified for the 777 competition. The Trent 800 HP shaft is 1.04m long compared with 1.32 for the PW4084 and 2.13m for the GE90. The company believes the shorter shaft is less prone to "shaft whip or whirl" and therefore retains better tip clearance control for longer.
At one stage early in flight testing R-R feared that its past troubles with bearings had come back to haunt it when the test team picked up indications of distress in the LP shaft tail bearing after the first flight in May 1995. The trouble turned out to be caused by vibrations of the aerodynamic fairing at the aft of the strut for which Boeing is responsible. Loads were generated by a high frequency tone and transferred through the strut support structure to the rear bearing. Relieved that the engine was not at fault, R-R later salvaged solace from the incident by proving it could quickly change out the troubled part on-wing.
The fan casing module is directly derived from previous RB211s with an aluminium rear case, a fan containment ring made up of an aluminium isogrid and Kevlar. The job of the isogrid is to keep the casing circular after a blade fails, while the Kevlar's job is to keep the blade from penetrating the containment ring. A series of "bolt-in" ice impact panels and rear linings coat the inner surface of the casing.
A series of structural and non-structural outlet guide vanes (OGVs) are mounted in the casing directly behind the fan. Some 25 are non-structural carbon composite while 33 are made of structural hollow titanium vanes. The use of structural OGVs, and the necessary weight penalty, was driven by the "hybrid" engine mount of the -800.
All RB.211s before the Trent series have a mount on the fan case and on the LP turbine case. In this way, loads from the core engine are transmitted up to the fan case mount via structural fan OGVs. In the case of the A330, however, there was insufficient ground clearance for this approach, so the engine is core mounted. The fan OGVs are non-structural and carry the fan case only while the lower part of the nacelle is flattened slightly . la CFM56.
For the heavier Trent 800, Boeing and R-R opted for a hybrid solution with a mix of fan case and core mounting. The front mount links to the fan case OGV ring and takes vertical and side loads. The rear mount, linked to the turbine rear frame, absorbs vertical, side and torque loads. These supports are augmented by "A" frames which horizontally span the bypass duct from the core to the rear fancase stiffening ring, and by twin thrust struts which sit between the rear mount and the intercase ring.
The heart of both the GE90 and PW4084, like all two-shaft engines, is considered to be the HP compressor. In the Trent 800, however, the IP compressor is the key to the engine performance. "Its main function is to supercharge the core," says Terrett. "The IP shaft can run at its own optimum speed (in this case 7,700rpm), so it gives you the ability to grow the engine without having to do weird and wonderful things to it like pushing the entire radius out."
Compared to the Trent 700, the -800's eight-stage IPC is changed in materials and aerodynamics to match the higher power and larger size of the -800 operating requirements. The most obvious change is a steeper inner annulus line which, when viewed in section, drops down more sharply into the core of the IPC. The larger opening and bigger dimensions increase flow by 5% and area, in the first half of the IPC, by 3%.
Upstream of the first stage is a new variable inlet guide-vane (VIGV) which is angled to turn at right angles to the annulus. The unison ring holding all the VIGVs is made of composite for low cost and weight. Throughout the remainder of the IPC is a series of VIGVs and variable stator vanes, which control compressor stability at low rpm. These are moveable through about 40¡ via unison rings, which are turned by fuel-operated actuators acting on bellcranks and drag links.
In material terms, the biggest change is in the rear stages, which are made of higher-temperature-resistant IMI834 titanium. All rotors and discs in the -700 IPC are made from titanium 64. The IPC drive-arm assembly has also been redesigned slightly from the 700 configuration to match the higher stiffness requirements of the 800.
The HP compressor is a largely unchanged six-stage design based on the familiar RB.211 arrangement. The discs for stages one to four are made from high-temperature titanium and are bolted to the stage five and six discs which are made from Waspalloy. The first stage blades are set in axial dovetail slots, whereas all the other stage blades are located in circumferential dovetail slots. This gives R-R the option of changing blade numbers later in the life of the engine, and reduces the leak of "windage" at certain guide-vane positions. The root sealing of the first stage HPC has also been improved over the Trent 700 and the modifications will be retrofitted to the Airbus engine.
SIXTH-STAGE DISC
The sixth-stage disc is welded to a Waspalloy "cone", which connects directly to the HP turbine mini disc. The compressor case surrounding the blades and discs is made up of six bolted cylindrical casings. The stiff casing also supports mounting points, which are more rigid and help prevent carcase distortions. The flanges between the case segments are used for the vanes and abradable rotor path linings. These are deeper than previous linings to avoid rubs. The complex flange design borrows directly from the International Aero Engines V2500 and provides improved rotor tip clearances.
Holes are bored through the casing to allow bleed air to be drawn from the third-stage HP compressor. Three valves on this stage, and four on the fourth IP compressor stage, are used to bleed off compressor air at low power or during acceleration or deceleration to prevent a stall or surge. The valves are controlled by the electronic engine control (EEC), and scheduled as a function of shaftspeed, ambient pressure and IP compressor inlet temperature. Taking a leaf out of earlier RB.211 configurations, the valves are designed to be fail safe so that they will always fail in the closed position. They could not be designed to fail open because bleed air at full power is too hot.
The same "Phase 5" combustor lies at the heart of both the Trent 700 and 800. R-R says that the combustor "...has been very successful at reducing NOx [nitrous oxides], beating its 30% NOx reduction target without impacting on other emissions." A Phase 6 double-staged combustor is in development for future generations with the promise of a further 20% reduction in NOx.
A set of 24 airspray injectors is used in the combustor, each with increased air swirling for better combustion. Air is swirled by a set of inner swirl vanes and by two sets of outer swirl vanes. To avoid "thermal distress", the combustor is also built with a mechanically isolated burner heatshield and external heat transfer ribs on the flame tube itself.
TURBINE TECHNOLOGY
The single-crystal HP turbine blades are made from CMSX4 material and fitted to a single-stage disc, which is connected to a mini disc to the rear of the HP compressor drum. The flow of cooling air to the blades is controlled by two sets of seal fins, which are attached to the front face of the disc.
Like all RB.211s, the turbine blades are shrouded - a tradition which differentiates the R-R engine from the unshrouded turbine blades of the GE and P&W products almost as much as the three- versus two-shaft arrangement. R-R has stuck to the shrouded blades because it believes that they offer better performance retention, although they are heavier and cannot spin as fast.
Unlike previous engines and Trent 700, however, the shroud interlocks have been deleted. This was possible because the inclined root dampers at the base of each blade produce a much lower vibration level. Another contributor to the reduced vibration level is the redesigned nozzle guide vanes. "As on the Trent 700 there are 40 NGVs compared with 36 on the other engines. We've changed to a 'wake-off' 3-D design which cuts the impact of the vane wake on the blades and moves the vane/blade resonance out of the operating range," says Terrett.
"With the interlock in place, we'd need to keep cool, but by not having to rely on the interlock we can operate at higher temperatures. As a result, the HP turbine will never be a temperature limiting factor on the Trent," he adds.
One major benefit of the three-shaft design, says R-R, is the simplicity of an un-cooled IP turbine. This single-stage unit rotates at 70% of the speed of the HPT, "...so it's running at only half the stress and at cooler temperatures," says Terrett. The blade is made of single crystal RR3000 and a new short chord IP vane is protected with an advanced plasma vapour deposited aerofoil thermal barrier coating. At an early stage R-R also changed the material used for the HP/IP bearing structure between the two shafts from steel to nimonic. In a similar way to the -524 and -535, the annulus running between the HP turbine and the large diameter LP turbine also rises steeply.
An IP turbine case cooling system, controlled by the Lucas-made full -authority digital engine-control (FADEC) system, maintains optimum tip clearance for the IP and LP blades. At take-off, flow from the cooling manifold is reduced to prevent excessive tip rubs. At cruise, flow is increased to achieve the smallest clearances.
Reflecting the higher thrust levels of the Trent 800, the LP turbine has a fifth stage, one more than the Trent 700 and two more than the RB.211. The first stage of the LPT is made from higher-temperature-resistant Mar M 002 material, which used to make up the IPT and HPT blades in earlier RB.211s. "It's like a tide of materials technology moving back through the engine," comments Terrett.
The solid cast blades are welded in pairs to control vibration and, reflecting their 3-D computational fluid dynamics (CFD) origins, are highly curved, thin and almost boomerang-looking. "We are looking at improving this even further using CFD, possibly by taking a few aerofoils out," Terrett says.
The LPT vanes are also solid cast and R-R claims that this method makes both blades and vanes cheaper to produce than the previous hollow RB.211 designs.
SYSTEMS CHANGES
R-R has been able to benefit from the combined Trent 700/800 effort particularly in systems design. The benefits have been twofold. Firstly, the majority of systems are common, with significant changes to only four line-replaceable units out of around 70. Secondly, the Trent 700 programme ran 12 months ahead of the -800, so problems, which cropped up on the first were dealt with on the second. "We had a wish-list for both, but, because the -800 had those extra 12 months, it got more of the list," says Terrett.
The air system, for example, is common to both Trents and similar to the RB.211. In the -800, however, carbon seals are used to control leakage of oil into the bearing chambers, as opposed to labyrinth seals on the -700. Changes were also made to the IP turbine cooling-air overheat detection system. In the -700 no overheat warning is sent to the cockpit unless both thermocouples in the same assembly read the same. In the -800 a failed thermocouple is disregarded if it indicates outside a set temperature range, leaving the other device still active.
The Hispano-Suiza high-speed external gearbox has new features on the -800 including a single hydraulic pump supported by aircraft-mounted hydraulic pumps, which are powered both electrically and pneumatically. It also supports a variable speed constant frequency generator, which is a back-up engine electrical generator.
Another system, the fuel-oil heat exchanger , which heats the cold fuel from the wing tanks before delivering it to the engine, is basically common with the -700 but is mounted higher on the fan case for easier transportation. The oil system will also be fitted with carbon seals with a design life of 30,000h in place of the -700's labyrinth arrangement. The vibration monitoring system has an additional pick-up and allows engine balancing using weights attached to the LP turbine as well as the fan disc.
The thrust reverser is a different design to the one which gave many headaches on the Trent 700 because of problems with the tertiary lock system introduced late onto the engine after the Lauda accident. The Boeing-designed -800 system uses translating cowls instead of the four tilting buckets on the Trent 700. The cowls are moved by mechanically linked hydraulic actuators, which block bypass air and deflect it forward through turning cascades. "We were able to do much more cyclic testing with this one, so we had a chance to get ahead of the game," Terrett says.
Getting ahead of the game has become something of a mantra at the UK engine maker's Derby headquarters, where work on the next stage of the Trent programme, the 895, is now well advanced.
Source: Flight International
