Rolls-Royce’s Trent XWB

By mid-summer this year, the world’s most efficient, sophisticated and state-of-the-art aero engine will be fulfilling its destiny and powering the prototype of tomorrow’s airliner on its first-ever flight, according to Geoff Thomas.

Rolls-Royce’s Trent XWB engine — the latest member of the ultra-successful Trent family — will take to the skies over Toulouse as it thrusts the A350-900 into public consciousness and on to a packed series of tests before it carries its first paying passengers in 2014.

With around 630 firm airframe orders from 35 customers, the Trent XWB has become the fastest-selling engine for Rolls-Royce and the company is already gearing up to produce more than 1,200 engines over the coming years. And as the Trent 1000 engine is also offered as a popular option on the A350’s major competitor — Boeing’s 787 Dreamliner – the UK engine original equipment manufacturer (OEM) is gearing up for a major expansion in manufacturing in future years and decades.

While the new airliner (MSN#001) may have its maiden flight just before this year’s Paris Air Show, Airbus says that it won’t be appearing at Le Bourget as the flight test schedule could be adversely affected. All being well, though, the Trent XWB-powered A350 should appear at the Farnborough International Airshow in 2014.

Like any new engine project — albeit one that follows in the footsteps of the Rolls-Royce three-shaft Trent family that has accumulated more than 50 million hours’ service over the past 18 years — certification is a major milestone. And this was achieved on February 7, 2013, when the European Aviation Safety Agency (EASA) issued the certificate, clearing the path for the engines to fly on the prototype airliner.

Two engines — ESN21002 and ESN21003 — were subsequently delivered to Airbus’ Toulouse plant and installed on the XWB airframe ready for flight after the relevant ground running and vibration calibration tests were completed.

It’s a measure of how the success of the Trent family of engines has transformed the fortunes of Rolls-Royce that in the 18 years since the first Trent was produced, 2,000 were delivered. Current plans mean that this total will be easily surpassed with around the same number being delivered over the coming five years.

The Trent XWB ran for the first time on a test bed in 2010 and since then, 12 full engines have been taken to extremes of performance, endlessly measured and examined and, in some cases, deliberately tested to destruction. The test engines have so far run for more than 4,000 hours to examine all aspects of the new powerplant including robustness, integration, cyclic tests, endurance, the low-pressure system, performance, mechanical integrity and other tests on the flying test bed.

After running initially on a test bed, the next major milestone for any engine is the first time it takes to the air and in this case the Trent XWB made its airborne debut, mounted in-board (No.2) on the port wing of Airbus’ A380 flying test bed. This happened early in 2012 with the new engine replacing one of the super jumbo’s four Trent 900 engines.

Weight, noise, fuel and flight

As with any of today’s major airliner engine projects, from the outset (and well before construction, initial testing and first flight) Rolls-Royce had to guarantee, within one per cent: how much the engine would weigh; how much noise it would make; how much fuel it would use; and the date of the first flight.

Delivering on those promises was a massive challenge and more than 2,000 scientists and engineers worked together on hundreds of test rigs to ensure that the more than 18,000 individual components were capable of running in perfect harmony.

By early 2013, the XWB engine had operated for more than 3,000 hours and this included an exhaustive flying test bed programme that featured: -29C icing tests in Canada; altitude and crosswind tests in the US; high temperature tests (40°C) in the Gulf; endurance tests in Spain; additional flight tests in France; and further test bed performance trials in the UK.

Before certification, EASA also had to be confident that the entire engine was exhaustively tested. As always for engine OEMs, the most demanding and significant tests concerned the fan module, including full engine blade-off tests. These were successfully completed in Germany and the UK by the end of 2012, clearing the path for February’s certification ceremony in Paris.

Head of marketing for the Trent XWB is Tim Boddy, and he explains that the latest member of the Trent family of engines is now in what he describes as the “realisation phase”, having achieved all its major milestones so far, on time and with the powerplant performing up to or in excess of specification.

Rolls-Royce designed and built today’s Trent XWB engine, utilising 3-D computational fluid dynamics modelling with new supercomputers offering the equivalent of 700,000 additional manhours of research and technology (R&T).

Rolls-Royce’s annual R&T budget of more than $1bn has also provided its scientists and engineers with an X-ray machine so powerful it could take a medical X-ray of a human body at a range of one kilometre, as well as discovering tiny misalignments of components that in the cruise phase of flight could amount to a 0.2 per cent improvement in specific fuel consumption (SFC) simply by adding a 50-cent shim.

As Boddy says, it is anticipated that Rolls-Royce will be producing the XWB engine at Derby in the UK well into the next decade — and probably into the 2030s or even longer — so ensuring that production is optimised is vitally important.

In the past, engine OEMs tended to design and build their new engines without going through what is known in the motor industry as "manufacturing pre-production testing". Engineers from Toyota’s nearby car plant at Burnaston in Derbyshire, UK, have been working with the Rolls-Royce engine builders to ensure the assembly sequence of this modular engine is practised, time and time again, so that everyone in production understands how to support the Airbus XWB’s demanding production schedule.

Rolls-Royce believes it is the first aero engine OEM to build a fully operational engine that production engineers may use to assemble and disassemble in a dedicated pre-production facility, before ramping-up production as entry-into-service (EIS) approaches in a year’s time.

For the first time, a fully assembled Rolls-Royce aircraft engine will not fit into the cargo hold of either of the world’s major air freighters (Boeing’s 747F or 777F) when it is completely assembled. So each Trent XWB has to be partially disassembled into its modular components before being flown around the world. Engineers at Derby accomplished the first active split in February this year and have now honed the process down to 40 manhours in a single eight-hour shift.

This is a crucial facet of the entire building process because, when the production phase is in full swing, an average of one Trent XWB will be delivered to Airbus at Toulouse each working day to meet customer demand. Each of Boeing’s large freighters will be able to carry three Trent XWBs at one time and Rolls-Royce anticipates that, in time, there will be around 1,200 shop visits a year, which works out to three or four transportations each working day.

“We are now moving on to testing engine capability and robustness prior to the A350’s EIS and, as a result, we will see our test hours accelerate quickly,” explains Chris Young, Rolls-Royce Trent XWB programme director. “Importantly, we are also moving into the initial engine production phase that will eventually see deliveries reach the ‘one each working day’ level.”

Continual development

Part of the Trent family’s continual development programme has concentrated on delivering a series of small but vital improvements to each Trent engine design: enhancing the aerodynamics; enabling the engine to operate at greater temperatures and pressures; and reducing both the parts count and overall weight.

This ongoing programme has resulted in highly efficient engines that are more cost-effective to own and maintain, as well as giving greater time on-wing before maintenance is required. Underpinning this performance, as with all Rolls-Royce engines, is technology. For example, each of the Trent XWB’s high-pressure turbine blades works in an environment where the surrounding gas temperature is at least 200°C above the melting point of the blade’s alloy.

Each blade also forms part of a disc that rotates at 12,500rpm, with the tips reaching 1,200mph — or twice the speed of sound at sea level. Each time the aircraft takes off, this single blade develops the same horsepower as a Formula 1 racing engine, yet it can travel ten million miles before it needs replacement.

This commitment to engineering excellence extends throughout the engine to more humble components. Even the bolts that hold each engine’s modules together have to deal with immense loads that will be generated from powering an A350 on a daily basis. Coming in sets of 30, these bolts are so strong that two fully laden A380 super jumbos could be hung from them.

A combination of engineering excellence, continued component improvements and motivated working teams, has continued to drive forward engine performance, resulting in the Trent XWB being around 16 per cent more efficient (per passenger kilometre) than the first-generation Trent engines that entered service in 1995.

The current EASA certification applies to the 84,000lbf thrust Trent XWB engine that will power the A350-800 and -900 aircraft types. A more powerful 97,000lbf thrust Trent XWB is also being developed to power the stretched A350-1000 version of the aircraft that will feature both enhanced range and passenger capacity.

The increased thrust needed by the larger -1000 from the 97,000lbf Trent XWB-97 engine will be achieved through a combination of new high-temperature turbine technology, a larger engine core and advanced fan aerodynamics — although the external dimensions of the fan and its associated casing remain the same. Designed specifically to deliver additional thrust without impacting upon SFC or on-wing life, Rolls-Royce has now completed the preliminary design review and the new engine variant is on track for a first engine run in 2014.

As the Rolls-Royce statistics demonstrate, SFC is increasingly important for airlines. As the entire industry worldwide uses around 65 billion US gallons of aviation kerosene each year — and it is not long since this fuel cost around 80 US cents a gallon, compared to today’s level of around US$3 — it is hardly surprising that airlines and their passengers have a vested interest in engines being as fuel-efficient as possible. As most industry pundits believe that the cost of aviation fuel is unlikely to fall far below today’s levels, the importance of extracting the maximum calorific value from each gallon burned is increasingly important.

The Trent XWB-97 will feature a new high-flow fan system (although still utilising Rolls-Royce’s favoured titanium blades of the same diameter as its slightly smaller siblings), along with an inflected annulus to give greater through-put of air. The XWB-97’s nacelle will also retain the same aerodynamic profile as the 84,000lbf engine, despite the slightly larger core that will give increased flow rates through the high capability turbines that feature tip clearance control, upgraded materials and advanced coatings and cooling technology. The XWB-97 will also be flight-tested on Airbus' A380.

Additionally, the Trent XWB family will feature the lowest carbon emissions of any widebody engine in service today and this is important not only for environmental reasons, but to demonstrate that the engine is both efficient and cost-effective for its users and customers.

Around 80 per cent of the line replaceable units are common with the XWB-84, while technological advances — such as adaptive cooling, shroudless HP turbine blades, advanced tip cooling and newly developed high-temperature materials and coatings — also assist with both performance and economy.

With the first engine run scheduled for around a year’s time, first flight on the A380 in mid-2016 and EIS a year later, the XWB-97 is an exciting addition to the Trent family of engines. It also has identical pick-up points, common tooling, externals and consumables to assist with airline familiarity and transportation. The compressor blisks will utilise linear friction welding which helps to avoid materials wastage, and manufacturing trials are already underway for the IP compressor and the shroudless HP turbine module. It will even be possible to repair minor damage to blisks on-wing, utilising newly developed technology called laser blending that will re-profile as necessary and then deposit new material.

Trent XWB specifications:

  • Three-shaft high bypass ratio: 9.3
  • Take-off thrust: 75,000–97,000lbf (330–430 nautical miles per hour)
  • Fan diameter: three metres (118in)
  • Fan: single stage, swept
  • Airflow: approx. 1,440kg (3,200lbf) per second
  • IP compressor: eight-stage axial blisks
  • HP compressor: six-stage axial
  • HP turbine: single-stage, air-cooled
  • IP turbine: two-stage, air-cooled (extra stage relative to previous marks of Trent)
  • LP turbine: six-stage, uncooled
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