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> MTU PROJECTS EVOLUTIONARY NEXT-GENERATION GEARED TURBOFAN > THE ENGINE-MAKER’S CEO COMMITS TO DUCTED DESIGN FOR NEXT GTF > WATER INJECTION TRIALS END IN SETBACK
Guy Norris,Jens FlottauMarch 102025TOUGH CHOICES
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Pratt & Whitney's GTF Advantage cleared for A32Oneo p.18
> MTU PROJECTS EVOLUTIONARY NEXT-GENERATION GEARED TURBOFAN > THE ENGINE-MAKER’S CEO COMMITS TO DUCTED DESIGN FOR NEXT GTF > WATER INJECTION TRIALS END IN SETBACK
Guy Norris
Jens Flottau
Colorado Springs and
Munich
As the aerospace industry prepares for the next generation of narrowbody aircraft slated to enter service in the second half of the 2030s, basic design choices must be made, ranging from radical proposals, such as the GE AerospaceSafran open-fan studies, to iterative advances. The first player has now publicly committed to a defined path that could have big ramifications for its competitors.
MTU Aero Engines CEO Lars Wagner said at the company’s annual results presentation on Feb. 19 that MTU and its partner Pratt & Whitney are committed to developing a ducted second-generation geared turbofan (GTF) engine as a powerplant for the successors to the Boeing 737 MAX and Airbus A320neo. “We will continue in an evolutionary way,” Wagner said, noting that “there will be revolutionary elements, such as hybridization,” and future conversion to hydrogen combustion is under consideration.
Wagner said that while the consortium has looked at different concepts, such as the open rotor GE Aerospace and Safran have studied as part of the Revolutionary Innovation for Sustainable Engines (RISE) program, it has decided the next engine will feature the same ducted architecture as the current GTF family.
Wagner’s narrowing down of MTU’s and Pratt’s design choices to a ducted conventional engine could have significant implications for aircraft manufacturers that seek to offer their customers a choice between two engine-makers. Ground clearance will be a key design driver for the large-diameter open fan, and Boeing, for one, has expressed concerns that this could force aircraft designers to consider different wings for ducted or open propulsor concepts on low-wing airliner configurations.
“The market wants a dual engine [choice],” Wagner said. “We are entering discussions on both platforms with great optimism,” he noted, referring to future Airbus and Boeing narrowbodies. In addition to the open-rotor studies, GE and Safran are also looking into options for a more efficient ducted engine.
Wagner, who will leave MTU by yearend to become CEO of Airbus’ Commercial Aircraft business, forecast at the presentation that Boeing will launch a new narrowbody before Airbus does.
The next-generation GTF is facing its own technical challenges and setbacks. One main issue: MTU has dropped the water-enhanced turbofan (WET) concept that was part of its broader Sustainable Water-Injecting Turbofan Comprising Hybrid-Electrics (Switch) project with Pratt. As a key feature, water vapor would have been recovered from the engine exhaust and reinjected into the combustion chamber, targeting considerably lower fuel burn, reduced nitrogen oxide emissions and much less contrail formation.
Wagner said the design required for WET would have been too complex without generating the efficiency savings that MTU had anticipated. The company continues to work on heatrecovery technology, which was part of the WET concept. But the discontinuation of WET leaves MTU without a credible path toward tackling contrails, although Wagner referred to operational measures to avoid their creation as a near-term option.
Hints that the WET concept might not move forward began to emerge last year, when German aerospace center DLR’s analysis of the concept revealed shortcomings in overall weight and complexity as well as heavy reliance on the performance of key technology elements, such as the condenser. Although the studies concluded that the concept was feasible and even recommended further detailed evaluation, the analysis suggested that overall efficiency gains might not be as great as MTU had anticipated.
When comparing a notional WET engine sized for an A320-type application with an advanced GTE, the DLR predicted the alternate-cycle engine would be 36% longer overall and 84% heavier in bare engine mass. The research agency found that the overall pressure ratio (OPR) of the WET engine would be limited to moderate values of up to 30:1 by the blade height of the last compressor stage, and it recommended detailed studies of bypass channel designs and system integration to avoid high levels of pressure loss.
Although the water injection increased the core’s specific power, allowing a higher bypass ratio for the same fan diameter, the benefits were counteracted by the need to incorporate large heat exchangers. The studies also found that the large condenser in the bypass duct required integration space and additional engine length. Although the axial length of the evaporator and condenser was comparable to an advanced GTE, the positioning of the condenser closer to the fan meant the intermediate duct had to bridge a large axial offset to connect both heat exchangers.
The studies found that the nacelle length also increased by 49% compared with the reference GTE architecture, and that presented challenges for integrating the engine under the wing. However, the DLR added that the nacelle radius could be maintained by using the WET concept. On the negative side, the center predicted that the larger nacelle would increase drag and overall aircraft fuel burn.
Although the absence of a low-pressure compressor—or booster—and an intermediate frame reduced the mass, that also lowered pressure at the inlet of the high-pressure compressor. To compensate for the lower air density, the high-pressure compressor had to be enlarged, making it 19.5% heavier. The DLR analysis also indicated that the combustor would need to be larger, and therefore heavier, due to the lower OPR for the WET concept.
So without WET, how can Pratt & Whitney’s ducted GTE compete with the efficiency gains targeted by the GE-Safran RISE open fan? The company has outlined plans to develop an enhanced second-generation GTE targeted at up to 25% lower fuel burn at an integrated aircraft level than today’s engines, potentially incorporating a higher gear ratio and some hybridization.
Alongside these targets, Pratt has laid out plans to move to a largerdiameter composite fan contained within a composite case and shrouded within a compact, shorter duct nacelle. The engine also would incorporate a revised, smaller core with a higher-OPR, ultra-low-emissions combustor—and, more than likely, hybridelectric motor-generators.
Although the configuration would retain the same baseline geared architecture, the next-generation version would make more of the gear system, according to Geoff Hunt, senior vice president of engineering and technology at Pratt. “If you want propulsive efficiency, you’re driving up bypass ratio, and that’s going to keep pushing on gears,” Hunt noted at the International Society for Air-Breathing Engines conference in Toulouse last September. “We’re currently running a little over 3.5:1 gear ratio, but in a ducted configuration, you must push up to four or so.”
At the same time, the newer configuration will “take more work off the low-pressure spool and reduce the work off the high-pressure spool,” Pratt & Whitney Chief Sustainability Officer Graham Webb says. “That enables us to increase overall pressure ratio while improving durability.” To increase bypass ratio, fan diameter also will increase to 85-90 in. from the 81 in. of the PW1100G. Rig tests of GTE Gen 2 combustors, compressors, high-pressure turbines and composite fan blades are underway as well, he adds.
Tests of the hybrid-electric elements are under development as part of the Switch engine research program. RTX company Collins Aerospace is supplying two motor generators, the hybrid-electric element of which will be tested in a modified PW1100G. Both are identical 1-megawatt machines, but the unit connected to the high-pressure spool is derated to 500 kW, while the motor generator attached to the aft of the low-pressure shaft is rated to 1 megawatt. RTX says that in an operational version, the electrical system will be used for taxiing and boosting power during takeoff as well as other transient phases.
Pratt’s advanced GTE might not be the only ducted turbofan option for Airbus. Rolls-Royce is studying the development of an UltraFan version for single-aisle applications under a strategic plan launched by CEO Tufan Erginbilgic in 2023. The study is partially backed by Heaven (Hydrogen Engine Architecture Virtually Engineered Novelly), a project under Europe’s Clean Aviation research initiative.