R-R details Trent XWB development strategy

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The stakes could not be higher as Rolls-Royce begins transitioning its newest large turbofan into production. The Trent XWB is the sole offerable powerplant to equip the European airframer's flagship 250- to 350-seat A350 XWB long-range twinjet family, designed to single-handedly see off the formidable two-pronged competitive threat posed by Boeing's General Electric GE90-powered 777-200LR/300ER models and the long-delayed mid-market 787.

Airbus can perhaps sleep a little easier in the knowledge that the Trent XWB is the latest evolution in a long line of three-shaft large turbofans that began with the introduction of the Trent 700 on the A330 in the 1990s. The family subsequently grew to include the Trent 800 for early 777 models, followed by the Trent 500 for the A340-500/600, Trent 900 for the ultra-large A380 and, most recently, the 787's bleedless Trent 1000.

airbus
 © Airbus

Despite the Trent XWB's heritage, R-R has not missed the opportunity to incorporate cutting-edge technology into the new powerplant and has made some significant advances even compared with the Trent 1000, which is yet to fly on the 787.

UNPRECEDENTED CHALLENGE

An unprecedented challenge for the engine maker has been the requirement to design from the outset a common engine for all three major A350 variants: the baseline -900, -800 shrink and -1000 stretch, due to enter service in 2013, 2014 and 2015 respectively.

"There is a clear mix of heritage: a great platform to build on, but great technologies being built into the engine," says Trent XWB chief engineer Chris Young.

Airbus's decision to abandon its initial plans for the A350 (which would have essentially involved bolting 787-technology engines on to a revamped A330 airframe) and switch to the all-new XWB concept, left R-R with the task of addressing the 74-92,000lb-thrust (329-409kN) band with one engine family.

Young describes this as "a reasonably large thrust range, but nothing outside the capability of what we've done in the past. It's pretty much the same as the Trent 800". He adds, however, that a key difference is that a "very planned approach right up front" is being taken to that thrust range.

Historically, engines at the higher end of such a family have tended to be "throttle-pushed" and run at hotter temperatures, while those at the lower end have been oversized for the airframe and therefore too heavy.

"The challenge is catering for a very large thrust range while making sure that you don't kill the maintenance costs for the engine at the top of the thrust class, and you don't kill the fuel burn of the one at the bottom of the thrust class," says Young.

THRUST RATINGS

Following recent airframe weight increases announced by Airbus, the final thrust ratings for the three A350 variants are due to be revealed to airline customers at a programme progress review in Hamburg on 2 April. The current ratings, nominally 74,000lb (for the -800), 83,000lb (-900) and 92,000lb (-1000), are expected to be revised slightly upwards to cater for the new airframe operating weights.

"Our basic strategy on this engine is we have a single engine type," says Young. "We design it originally for the -900, nominally an 83,000lb-rated engine at the moment, and that engine we absolutely optimise for a 2013 entry-into-service level of technology.

"The -800 engine is identical, we just derate down to that [74,000lb] level, but that allows us to deliver outstanding hot-and-high performance," he adds.

However, the 350-seat A350-1000, which will have a maximum take-off weight of 298t, will require a modified engine.

"What we have said is that rather than build a slightly bigger core or slightly bigger fan [to cater for the -1000], we'll optimise it as well as we can for [the -900 in] 2013, then we'll insert new technology for 2015 that will allow us to build the thrust rating up," says Young. The message from existing and prospective A350 customers was "make sure that you don't compromise the smaller aircraft through total commonality", he adds.

The engine that will power the A350-1000 will therefore incorporate two significant modifications. The first of these is a new fan system, although the basic 3m (118in) fan diameter and blade aerodynamics will be retained. "If you looked at it from 10 paces you wouldn't notice the difference," says Young.

For the -1000, the fan will be run at a higher speed to achieve the required flow rates, and will therefore be equipped with thicker titanium fan blades and a stronger fan casing.

Young points out that using the strengthened fan system across the whole engine family would have added excessive weight to the variant for the A350-800, while the fan "doesn't need much maintenance and can stay with aircraft", therefore mitigating the impact of the loss of commonality.

The second change to the A350-1000's Trent XWB will be an increase in the operating temperature capability of the core, taking advantage of new turbine technologies developed through the European Environmentally Friendly Engine (EFE) research programme.

"We've a number of options as to what we need to do [for the A350-1000], but we don't need to finalise our decision until around about the middle of next year," says Young.

The changes will mainly concern turbine blade design, materials, coatings and cooling technology, and capitalise on concepts due to be tested on the EFE demonstrator.

"That suite of technologies, if they all come good, is more than we need [to achieve 92,000lb for the -1000]," says Young. "We can have a drop-out rate of them not quite being ready on time, and we'll still be perfectly capable of achieving the required thrust rating."

Due to the common fan size, which enables the use of an identical titanium forging for the casing, "the level of commonality actually ends up being huge", says Young.

Although the baseline Trent XWB - optimised for 83,000lb of take-off thrust for the A350-900 - would be able to run at up to 92,000lb for the -1000, the fan and core changes are being introduced to maintain durability.

"The technology insertion is really more around recovering your time-on-wing back to the same sort of levels as the 84,000lb engine, by giving you a greater turbine gas temperature margin," says Young. "It's a maintenance cost resetting via use of technology."

The engine core improvements developed for the 2015 entry-into-service of the A350-1000 could eventually be incorporated into earlier versions of the Trent XWB that will by then be in service on the -800/900 models.

"The intent is that the new core [for the -1000] would have the capability of being read across to all of the previous [-800/900] engines" says Young, who adds that decisions to upgrade engines already in service will be looked at on a case-by-case basis.

Ultimately, the cruise thrust required across the three A350 derivatives is broadly similar, because they share a highly common wing.

"When people ask how can you effectively and efficiently do one [engine] that covers the whole [A350] family - well, it's not the headline take-off thrust number that you're really trying to optimise around," says Young.

The basic architecture of the Trent XWB was finalised in mid-2007, including fan diameter, the number of compressor and turbine stages and core size. Subsystem configuration was frozen in September 2008. Towards the end of last year contracts were agreed with the 12 risk- and revenue-sharing industrial partners.

"We're on the point of freezing the design, launching all of the parts into manufacture and some of the long lead time parts are there already," says Young. Bench testing of the first engine is to begin in the second quarter of 2010, with certification targeted for the end of 2011.

The flight-test programme for the A350 itself is scheduled to get under way in early 2012, but the Trent XWB is due to get airborne aboard A380 MSN001 during the first half of 2011.

COMPUTER MODELLING

Young says that to make maximum use of the latest computer modelling techniques and rig-testing to reduce the risk of development problems arising downstream, "far more resource" has been put into this programme in the early stages than before, "when it can have the biggest effect and we have time to react".

On previous programmes maturity testing began after certification, but these two "streams" of work are being done in parallel for the Trent XWB. "It's a restructuring of the way we run these programmes," says Young.

The Trent XWB's 3m, 22-blade titanium fan is 51mm (2in) wider than the Trent 900's and therefore the biggest yet produced by R-R.

"We've ended up with a core a bit smaller than the Trent 900, even though the thrust is a bit higher," says Young. The 9.3:1 bypass ratio XWB is also the first Trent to feature a two-stage intermediate-pressure turbine, which Young says "puts growth capability into the engine", and is the first to incorporate blisks, which have seen several years of successful service in the Boeing 717's BR715 turbofan.

Another milestone for the Trent XWB is the ability to modulate the amount of air bled to cool the turbine blades, further optimising cruise efficiency.

Relative to the engines powering first-generation widebodies such as the 747, R-R predicts the Trent XWB will deliver 28% better fuel burn, and expects a 15% improvement relative to the first iteration of the Trent 700.

Having worked so closely with Airbus to tailor the three-shaft Trent XWB to meet the requirements of the A350, R-R sees challenges for rival engine manufacturers GE or Pratt & Whitney, should they choose to offer an alternative powerplant.

"We've had the benefit of being able to set the [A350's] engine cycle around the three-shaft engine," says Richard Keen, R-R head of marketing for Airbus programmes. "There'd be compromises for a two-shaft engine that tried to adapt to the same cycle and fan size that we've set," he adds.

The 12 partners on the Trent XWB are responsible for just over 40% of the workshare (see graphic), of which three are described by R-R director of risk and revenue sharing partners Bill Boyd as "cornerstone". ITP, in which R-R owns a large minority stake, is responsible for the design and manufacture of the low-pressure turbine (LPT) module, while Kawasaki Heavy Industries has the intermediate-pressure (IP) compressor module, and Mitsubishi the LPT blades, IP turbine discs and parts of the combustor.


LONG-RANGE DESIGNS FOR A CLEANER FUTURE

by Aimee Turner

The design of future engines for long-range operations could benefit from a unique piece of UK research aimed at optimising control strategies for a cleaner exhaust.

Professor Haydn Thompson at the Rolls-Royce Control & Systems University Technology Centre within the Department of Automatic Control and Systems Engineering at the University of Sheffield explains: "Emissions from the engine depend on temperatures in the combustion chamber, so the trick is to control the temperature cleverly, optimising the engine for reduced fuel burn while increasing component life and improving engine performance."

The trouble is that modern turbofan engines are already highly optimised with even the most concerted effort to advance the status of materials technology only standing to pay back around a 1% improvement in terms of specific fuel burn.

"Certainly, we have to do something innovative as we are reaching the limit of current engine technology," says Thompson.

Studies as part of the Omega aviation research initiative were undertaken to investigate the impact of minor changes to the inlet guide vane (IGV) gain schedule of a typical high-bypass gas turbine engine. The results revealed not only a potential for modest savings in terms of fuel consumption and therefore a proportionate reduction in CO2 emissions but also the potential to reduce temperatures in the turbine section.

A review of the current life enhancing control technologies was then undertaken to plot the possible future roadmap in this field and some new approaches using adaptive and predictive control-based technologies were proposed.

This led ultimately to the development of a novel generalised predictive control (GPC)-based system to optimise the IGV schedule on a high-fidelity GTE model, with the results analysed during a typical flight profile of an aircraft at take-off, cruise and descent.

"It was observed that changing the IGV gain schedule during cruise by a small amount has some potential benefits in terms of reducing the CO2 emission rate, enhancing engine life and generating greater thrust," says Thompson.

Because the engine control system is already highly optimised, the percentage changes are small, but they are significant because in a typical flight profile, most time is spent in cruise. For short-haul flights, this varies between one to three hours and for long-haul flights from 8-10h.

While savings are multiplied by time spent in cruise mode, it is useful to remember that an aircraft's size is based upon the fuel capacity that it needs to meet range and load requirements. A reduction in engine fuel flow therefore has the potential added benefit of reducing required mission fuel, which in turn would reduce aircraft weight.

This could allow the aircraft to be redesigned with smaller fuel tanks, potentially leading to a reduction in aircraft drag as fuel tank size has an impact on the aircraft's profile.