A version of this article appears in the July 14 edition of Aviation Week & Space Technology.

Engine development has always been the pacing item in aircraft development, but as powerplant manufacturers strive to meet the increasingly ambitious performance targets of the next-generation Boeing twins, the pressure is greater than ever for early test success.

This is no more keenly felt than by General Electric, which is developing the GE9X as the sole-source engine for Boeing’s new long-haul 777X derivative family. The engine is targeting a 10% fuel-burn improvement over the GE90-115B, the engine for the current 777-200LR/300ER series, and meeting this goal while delivering as much as 105,000-lb. takeoff thrust is pivotal to achieving the aircraft’s overall performance aims.

To get there, GE is testing a wide range of new designs and technologies in readiness for the first engine run in 2016. The engine architecture is all-new because a scaled-up derivative of GE’s latest engine family, the GEnx, would not in itself be sufficient to meet the targets. “If we took the GEnx and grew it out to 100,000 lb. thrust, we would have got about 5 percent of that fuel-burn improvement,” says GE90 and GEnx program manager Bill Millhaem. “The other 5 percent comes from technology programs in the fan, the high-pressure (HP) compressor and new materials in the HP turbine.”

From an external perspective the most distinguishing feature of the GE9X will be its enormous 133.5-in.-dia. fan. Settling out at the larger size after earlier hovering in the 132-in. range for much of 2013, the enlarged fan is key to boosting propulsive efficiency, and confirms the GE9X in physical terms as the largest turbofan ever developed. Although producing around 10,000 lb. less thrust than the GE90-115, the fan will be 5.5 in. wider than the 777 engine but will have only 16 blades, rather than the 22 of the current engine.

“When you invest in developing this technology you size it for the thrust required, so a number of trade studies have been completed to firm up conceptual configuration,” says Millhaem. Although both versions of the 777X, the -8 and 9, are aimed at the same maximum takeoff weight of 750,000 lb. as the current 777-300ER, they will be able to fly farther and more efficiently due largely to the wider-span, advanced composite wing of the new model. This enables the thrust requirement to be decreased from that of the GE90-115, and following the request late last year from Emirates for a final thrust bump, this has now settled at around 105,000 lb.

Although the GE9X blades will resemble those of the GE90, they will be constructed from a new fiber-resin composite and have aerodynamic changes, with more sweep and wider chord to improve pumping efficiency. “We have developed a new fan blade system that is stronger. It has gone through impact tests for the blades which have a thinner leading edge,” explains Millhaem. The changes also include a new metal leading edge sheath. Testing of one-fifth to one-sixth-scale versions of the GE9X blades is being conducted in Boeing’s universal propulsion system (UPS) aero-acoustic propulsion simulator test rig in Seattle. The rig, which runs at the same representative tip speed as the full engine, includes outlet guide vanes (OGV) and a core simulator with a 1.5 low-pressure compressor (booster) stage to allow for bypass ratio and generate the right acoustics.

Initial tests focused on runs of a baseline 18-blade set similar to the GEnx to verify results against known performance characteristics from this 747-8/787 engine, and subsequently moved on to evaluate two GE9X 16-blade designs with different aerodynamic features. 

“They have some shape and aerodynamic differences to give us the opportunity to iterate on different approaches,” Millhaem says. Testing on each set includes evaluations of low-speed fan/OGV interaction and flow properties, operability and crosswind testing. “We’ve mapped the aeromechanics and operability and now we are getting the fan acoustical data, so from that we will downselect the aero-acoustic configuration of the fan,” notes Millhaem. “We will downselect an aerodynamic configuration based on this testing then go back and redo impact tests with this final design. Then we will start into a more advanced test program before going to blade-out testing as we get closer to the final design size.”

Further tests of an 80%-scale version of the GE9X 11-stage HP compressor are planned for later this year at the company’s Massa, Italy, facility following initial evaluations early this year. “We met all our aeromechanical targets for durability and performance, so we feel that a pressure ratio of 27:1 is clearly within our capability. We put it through its paces and ran up and down the stall line at partial and full power and completed detailed mapping of the stage-to-stage loading. Overall we exceeded all our expectations. For the next phase we have put some very nuanced changes into the front end of it and we will go back and look at additional optimization of the efficiency of the compressor,” Millhaem explains.

The GE9X compressor pressure- ratio growth from 19:1 on the GE90-115 and 23:1 on the 10-stage unit in the GEnx-1B has also raised turbine entry temperature by around 100F. The engine will therefore incorporate a new disc alloy in the last stage of the HP compressor and first stage of the two-stage HP turbine. Both stages will be air-cooled, while the second stage will incorporate a new blade design. Heat-resistant ceramic matrix composite (CMC) will be used for the first- stage nozzle and shroud, as well as the second-stage nozzle. “We will run CMC -9X style combustion liner, shroud and nozzle parts in a GEnx development engine by year-end,” says Millhaem.

Full-scale GE9X components will run in two builds of GEnx-1B engines in 2015 in the run-up to first full GE9X engine runs in 2016, and the launch of certification tests. First flight on GE’s 747 flying testbed is set for 2017, in readiness for first flight of the 777-9X the following year.

Rolls-Royce’s immediate focus for Boeing is on development of the Trent 1000-TEN for the 787. The engine, which made its first run at Derby, England, late May, will be certified to 78,000 lb. thrust for all versions of the 787 by the end of 2015. The development is attracting more interest than usual because it also forms the basis for a variant which could be used to reengine the Airbus A330, as well as possibly the A380.

Launched at the 2012 Farnborough air show, the Trent 1000-TEN incorporates features from the Trent XWB developed for the Airbus A350, as well as the company’s Advance technology program. Versions of the engine destined for the 787 will be built in Singapore and enter service on the 787-8/9 in 2016. Service entry on the stretched 787-10 is scheduled to follow in 2018.

The 78,000-lb.-thrust rating is “slightly beyond Boeing’s requirement of 76,000 lb. but we think it is prudent,” says Trent 1000 project director Gary Moore. Initial demonstrations of some TEN features have already been completed, including advanced seals, a new fan case dressing system, a revised Trent XWB-based disc architecture and cyclic tests of the new case dressing on a Trent 1000. The integrated dressing system is like something “you might find in Formula 1 [motor racing], with pipes and harnesses woven in. It is very elegant technology and has been operating well in the demo program,” says Moore.

The engine incorporates a new eight-stage, higher-flow intermediate-pressure compressor and a new six-stage HP compressor derived from the XWB engine. The configuration was originally tested in the European New Engine Core Concepts (Newac) technology program and includes blisks in the front three stages. “It also has a new single-stage HP turbine to match the new compressor in a more efficient way,” adds Moore. The entire HP section will be cooled by an adaptive system which modulates the volume of bleed air from the HP compressor using a “vortex amplifier” that relies on an air-based valve system to control the flow. By modulating the cooling air volume off-take, the system only uses what is needed at specific times, thereby increasing efficiency.

Eight engines are scheduled to take part in the certification program, which includes flight-test campaigns on Rolls’s 747-200 flying testbed and a 787 in mid-to-late 2015, culminating in engine certification at the end of next year.