ANALYSIS: GE opens five-year development effort for 777X engine

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Boeing's selection of the General Electric GE9X for all three proposed variants of the still-unlaunched 777X begins a five-year campaign for the engine manufacturer to test and certificate a new product featuring several new advances in gas turbine technology and capability.

GE's preliminary development plan for the GE9X calls for certification in May 2018 on a common core, with a slightly more than 100,000lb-thrust variant to power the 777-9X, a roughly 90,000lb-thrust variant to power the smaller 777-8X and another variant to power the ultra-long-range 777-8LX, says Bill Millhaem, the GE9X programme manager.

That development schedule means the first 777X is unlikely to enter service before mid-2019, as there is usually at least a one year gap between engine certification and aircraft certification. Millhaem's comments also clarify that the 777-8LX concept, featuring a 17,550km (9,480nm) range, remains in Boeing's long-term plans for the re-engined and re-winged widebody.

Boeing is still refining the design and business case for the 777X ahead of asking the company's board for authority to launch the programme. Technically, GE was selected as Boeing's engine partner on 777X studies.

"We are aggressively moving forward per our plan and working with our customers on the requirements," Boeing says. "While we haven't set a firm timeline or launched the programme, we've consistently talked about a potential [entry-into-service] around the end of the decade."

Along with a new composite wing and a stretched fuselage, the GE9X is among the three key design changes aimed at improving the efficiency and the competitiveness of the venerable 777 family, which faces a new challenger in the market with the debut of the Rolls-Royce Trent XWB-powered Airbus A350-900 and -1000.

Boeing has only one chance to get the 777X design right, and its patience is clear in the GE9X configuration, which has evolved even since September. At that time, GE revealed a configuration with a 129in (3.28m) fan diameter. On 15 March, GE announced the fan diameter had grown to 132in, although Millhaem clarified in an interview that the diameter is really 131.5in.

"Boeing wanted a larger [fan] that will give us a little more thrust for the engine," Millhaem explains. "The core will grow a small amount as well. It's a relatively small change that we're making on the engine."

Millhaem says the new fan diameter establishes the GE9X bypass ratio as 10.3:1, a 14% improvement compared to the 9:1 bypass ratio of the GE90-115B that powers the 777-300ER. Bypass ratio measures the air that flows through the inlet fan and around the core compared to the air that enters the combustion flowpath.

The bypass ratio is one of three changes that are intended to give the GE9X a 10% fuel advantage compared to the GE90-115B. GE also has included several changes that reduce the weight and increases the thermal efficiency of the new engine.

The engine is lighter for several reasons, including a switch to a set of fewer, composite fan blades and a composite fan case, an innovation GE launched on the GEnx engine family. The GE9X inlet fan will be comprised of only 16 thinner and wider blades, or six fewer than on the GE90-115B.

The fan diameter of the GE9X, however, is 20in wider than the GEnx-1B, meaning that the composite fan case must be able to survive significantly higher loads in the event that a blade is ripped loose.

"We think the technology is sufficiently mature for where we want to go with the fan diameter," Millhaem says.

Another key change to reduce weight is the incorporation of lighter and thermally stronger ceramic matrix composites (CMC) in new areas of the engine.

GE is already introducing CMCs in the shrouds of the first stage high pressure turbine of the Leap narrowbody engine, which it is developing with CFM International joint venture partner Snecma.

In the GE9X, GE is expanding the use of CMCs dramatically, replacing metallics with the new material in the inner and outer linings of the combustor, the first and second stage turbine nozzles and the second stage turbine blades, Millhaem says.

Using CMCs in the combustor linings also helps GE deal with the higher temperatures created by the third key design change on the GE9X, which is the significant improvement in thermal efficiency.

A single kilogram of air entering the combustion flow path at the fan inlet will be squeezed to weigh 61kg by the time it enters the combustor, which is known as 61:1 overall pressure ratio (OPR). The state of the art in the industry today is about 50:1, and the nearly 20-year-old GE90 achieves only a 40:1 OPR. As the same kilogram of air exits the fan and enters the first stage of compressor, it will weigh 27kg by the time it reaches the last of 11 stages, giving the GE9X a 27:1 compressor pressure ratio. That is also significantly higher than engines today, which can achieve a 24:1 compressor ratio at best.

Such increasing pressures drive higher temperatures inside the compressor. Using CMCs in the combustor linings mean that GE can reduce the amount of cooling airflow to prevent the linings from melting, Millhaem says. The air exiting the 11th stage of the compressor, meanwhile, will be about 26.7°C (80°F) hotter than the same stage of the GEnx, which forces GE to use what it describes as a fourth-generation of power-based alloys that can survive higher temperatures.

GE will test these advanced compressor technologies for the time in July or August at the company's oil and gas facility in Italy. That will be followed by a test of the entire engine core in the middle of 2015, Millhaem says. The first complete engine to test milestone is scheduled in mid-2016.

"That will put us really into a formal flight test and certification programme," he says.