ANALYSIS: General Electric bets big on exotic composites for 777X engine

Washington DC
This story is sourced from Flight International
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After the costly, 40-month delay ­ushering the "game-changing" 787 into service, the last thing Boeing and its shareholders want is another leap into the unknown. That makes the 777X programme an unusual contradiction. It promises a 787-like 20% improvement in fuel efficiency, but for the most part relies on ­technologies already in service.

Thus, Boeing bills the all-composite wing that accounts for roughly half of the ­fuel-­saving efficiency of the 777X as a fourth iteration of the aerofoil pioneered with the 787-8 test fleet - refined with the first of the type ­delivered to customers, and improved again with the pending release of the 787-9 to flight test.

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General Electric

GE intends to use ceramic matrix composites for many engine components

The other half of the fuel-saving improvement on the 777X comes from the General Electric GE9X turbofan engine, and here the risk levels become more complicated.

For the 777X family, Boeing asked GE to deliver a new engine that consumes 10% less fuel compared with the GE90-115B, which set an industry standard earlier this decade on the 777-300ER. It also must be 5% more fuel-efficient than the GEnx-1B that GE delivered for the 787 family, but without that engine's fuel-saving, bleedless architecture.

As a result, the GE9X will feature an exotic new material called ceramic matrix composites (CMCs), that have not entered service on any other aircraft engine so far. Able to survive in temperatures 20% hotter than metal, but twice as strong at one-third the weight, CMC's have been in development by GE for decades and are finally coming of age.

Although CMCs account for up to 30% of the predicted fuel savings of the GE9X, GE ­executives feel comfortable that the new ­material is ready to be used extensively throughout the hottest parts of the engine.

"We've got thousands of hours on these CMC's already," says GE Aviation chief executive David Joyce. "It's not like yesterday we pulled them out of the drawer and said we were going to start working on them."


At least some of the likely 777X customers agree with Joyce's assessment. Boeing and GE have briefed John Plueger, president and chief operating officer of Air Lease, on the CMC materials planned for the GE9X engine.

"I don't see any huge leap there," Plueger says. "I just see a normal evolutionary ­development on the engine side."

Indeed, GE first replaced a nickel-based alloy with CMCs in the turbine shrouds of the 7FA industrial gas turbine in 2003, says GE9X programme manager Bill Millhaem.

Shrouds help control the tip clearance ­between the ­engine case and the turbine blades spinning directly aft of the combustor. Nickel-based shrouds must be cooled or melt in the extreme temperatures by extracting pressurised, cooler air from the compressor section and piping it aft.

"That is very expensive air because you've spent a lot of time pressurising it, and then instead of letting it go through the engine you have to actually use it to cool the shroud," Joyce explains. After running 14,000h of ­uncooled CMC shrouds in the industrial turbine over the last decade, GE next ­decided to incorporate the same material in the first-stage turbine shroud of the CFM International Leap-1 narrowbody engine, which is scheduled to enter service five years ahead of the GE9X in mid-2015.

The GE9X, however, could be designed to use CMCs far more extensively than the Leap-1 engine. In addition to the stage 1 shrouds, the GE9X will also use CMCs in the lining of the combustor and the nozzle for the high pressure turbine.

But the real leap for using CMCs in the GE9X involves a proposal to use the material to make the blades on the stage 2 rotor of the high pressure turbine - one of the most complex and sensitive rotating components in any turbine engine.

"At this point we are looking at doing an uncooled CMC stage 2 turbine blade," Millhaem says. "So we'll be doing testing on that to determine feasibility for that."


GE plans to begin testing the CMC materials in a GEnx demonstrator engine in 2014. As GE expects the uncooled turbine blade to deliver one-fifth of the overall fuel burn reduction for the GE9X, compared to the GE90, passing those tests are critical for the programme to achieve success.

But GE also has a plan B, in case the tests reveal that CMCs are not ready to be applied as a turbine blade.

"We have a back up plan that would allow us to keep the impact small," Millhaem says.

At the same time, GE is clearly proud of its heritage of successfully introducing new materials in commercial turbofan engines. After being introduced on the GE90, composite fan blades have survived nearly 200 bird strikes without losing a blade, Millhaem says. Joyce also cites the example of introducing titanium aluminide blades in the last stages of the low pressure turbine on the GE90. Introducing ­titanium aluminide and CMCs is part of a long-term strategy to replace standard metals and alloys throughout the hot section of ­aircraft engines.

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Half of the fuel-saving improvement on the 777X will come from the GE9X

"If you look back in the low pressure ­turbine, we have titanium alumnide that we have in the GEnx in the last two stages, and that's very, very lightweight," Joyce says. "We'd like to push titanium aluminide as far forward as we can until it runs out of temperature capability, and then push CMCs as far aft as we can. And somewhere in the middle those two will meet."

The strategy only works if CMCs can be produced reliably and affordably at expected levels of demand. CMC is not a simple ­material - it requires a multi-step process to produce, starting with spools of fibre. The fibre is coated and transformed into sheets, then a laser cuts the sheets into pieces that are hand laid-up, then inserted into a furnace. Finally silicon is injected into the base material and a machine carries out finishing work.

It is a process that must be repeated on each of the 18 shrouds designed for every Leap engine, with more than 4,600 already on backlog. The same system also will be used to make parts for the GE9X, which will include much more than just shrouds, so how will GE build the parts affordably?

"It's volume," Joyce says. "For us it will be volume. When you commit a shroud like this to a next generation CFM [engine], there's already 4,600 engines on order. Think of how many pieces are going to be in the GE9X."

GE also is building new factories to support the CMC ramp-up, as part of a $2 billion investment in the GE9X programme over the next seven years. Joyce announced at the Paris air show that GE will open a plant in Asheville, North Carolina to build CMC parts for the Leap engine. The facility also will produce parts for the GE9X engine, he says.

It is also clear that GE's strategy for CMCs stretches beyond next-generation engines. As the company folded GEnx-derived performance improvements into the CF6, so will it develop CMC-based improvements for the GEnx engine family which powers the 787 and 747-8.

Running the first CMC turbine blade test in the GEnx demonstrator in 2014 is part of that strategy. The experience will help GE develop spin-offs for the GEnx engine quickly.