By Aviation Week & Space Technology, Aug 25, 2014
Earlier this year Rolls-Royce took the unusual step of publicly laying out its strategic vision for developing a new series of large turbofans for the next decade and beyond. Now the company is beginning to detail the first steps it will take to turn this vision into reality.
The launch of its next-generation road map comes at a good time for the company. Buoyed by growing volumes of business in the widebody airliner market with its three-shaft Trent engine family, Rolls is in the midst of its biggest production ramp-up ever to support expanding fleets of Trent 1000-powered Boeing 787s and XWB-powered AirbusA350s. At the same time, it is developing the Trent variants for the later derivatives of both these airliners, as well as beginning work on the Trent 7000 for the newly launchedA330neo.
Banking on the notion espoused by President John F. Kennedy that “the time to repair the roof is when the Sun is shining,” Rolls is acting now to ensure its competitiveness for the next round of airliner developments from the end of the decade and beyond. Ric Parker, director of research and technology at Rolls, says that thanks to the A350 and 787 engine programs, “we are in an amazing position today.” At the American Institute of Aeronautics and Astronautics Joint Propulsion Conference in Cleveland in late July, Parker also noted that the company’s strategy closely monitors the point “where evolution gives up and revolution takes over.” For the near term, the plan remains focused on the former, and the next steps will therefore be based on two more evolutions of the well-proven three-shaft heritage. “At Rolls-Royce we say, ‘invent once and use many times,’” adds Parker.
Rolls-Royce’s fundamental product plan, as first unveiled in February (AW&ST March 3, p. 20), is a two-phase evolution from today’s Trent XWB. The first engine, the Advance, is aimed at entry into service around 2020 and will have a bypass ratio in excess of 11:1, overall pressure ratio of more than 60:1 and fuel-burn level at least 20% better than the current Trent 700. The second, more ambitious follow-on engine is called the UltraFan, which Rolls first revealed in concept form in early 2012 as part of NASA’s Environmentally Responsible Aviation (ERA) study with Lockheed Martin. The engine could be ready for service in 2025 and is targeted at fuel-burn at least 25% better than the Trent 700. UltraFan drives a variable-pitch fan through a gear system and is outlined with a 15:1 bypass ratio and overall pressure ratio of 70:1.
Step 1 of the evolution involves fundamentally changing the traditional architecture of the Trent core to off-load the work performed by the intermediate-pressure (IP) spool and split it more evenly with the high-pressure (HP) system. To understand the significance of this, it is useful to note the basic architectural differences between the Trent family and the competing two-shaft designs produced by General Electric and Pratt & Whitney. Unlike these two-shaft engines, in which the fan and low-pressure (LP) compressor are driven by the LP turbine, the fan alone is driven by the LP turbine in the Trent. In place of the conventional LP compressor, the three-shaft design has an IP compressor which is driven by an IP turbine. Both two- and three-shaft engines have similar high-pressure spools, though there are fewer stages in the three-shaft compressor and turbine.
In previous evolutions of the Trent, Rolls has grown engine capability by expanding the work done by the IP compressor and turbine. “As we grew the Trent family IP compressor, we grew the pressure ratio and gradually supercharged the engine, always keeping the high-pressure spool very similar,” says Alan Newby, Rolls commercial engines advanced projects chief engineer. “The big change from the core point of view is that the Advance reverses that, so we will put more on the high-pressure spool,” he adds. The new Rolls engine will have a relatively larger high-pressure compressor with up to 10 stages (compared to six on the Trent XWB) and a greater pressure ratio, and it will be driven by a two-stage turbine against the single-stage used today. At the same time, the IP compressor will shrink from the eight stages of today’s XWB to around four, while the IP turbine count will be cut to one from two stages.
The new configuration “provides a very lightly loaded high-pressure spool, which gives good efficiency and, more importantly, significant commonality with the follow-on core of the UltraFan,” says Newby. “So we are laying down an architecture which we think is enabling the future.” In addition, for the first time on any Rolls engine, Advance will have a lighter composite-titanium fan, composite fan casing and lighter LP turbine system.
Beyond Advance, the next big architectural change is the inclusion of a gear system to drive what Rolls believes will be a new generation of larger, higher-bypass-ratio fans, as well as the introduction of variable pitch fan blades and complete elimination of the LP turbine. The move effectively means the engine is no longer a true three-shaft design but rather a “two-and-a-half” configuration, notes Newby. However, Parker adds, “Even with three shafts, we still haven’t reached the tipping point where the weight and complexity of the gearbox outweighs the weight and complexity of lower-speed, lower-efficiency components. That’s primarily because we don’t have a low-speed booster on the fan shaft. Instead, we have an IP compressor that finds its own speed.”
Besides major architectural changes, both Advance and UltraFan will see the wholescale introduction of new technologies. In some cases these will be key enablers to the new configuration and in others they will help augment overall performance. “Arguably, the Advance is mainly about the core, though we are introducing the lightweight low-pressure system, so it is a bit of improving propulsive efficiency and lot about improving thermal efficiency,” Newby says. The lightweight fan and casing will have “a novel system of embedding harnesses and pipes in a composite ‘raft’ attached to the casing,” he adds. Other technologies will include a more advanced low-emissions combustor, lightweight compressor and turbine blade designs, improved blade cooling, dynamic sealing and adaptive cooling systems to optimize bleed off-take cycles. It will also feature new hybrid ceramic bearings to support the lighter core in positions farther aft in cooler, more benign locations, away from hotter locations faced by current bearings.
Key technologies for UltraFan will build on the Advance developments in some cases and in others—such as the power gearbox, variable pitch blades and variable area nozzle—will be all-new. UltraFan will also have a new form of fully integrated, slim-line nacelle design. As the fan system is designed to vary pitch in all phases of flight, including landing, the nacelle will not include a thrust reverser. UltraFan will have a multi-stage IP turbine, the blades of which will be longer than any previous design. To reduce weight, titanium aluminide will be used for rotating parts and ceramic matrix composites (CMC) for static parts such as nozzles.
Some potential technology elements “may be longer-term and may not be in the UltraFan when we take it to market,” says Newby, citing cooled cooling air and “blings” (bladed rings) as examples. An actively controlled cooled cooling-air system holds the potential to enable higher pressure cycles and turbine exit temperatures because it not only removes bleed air at a later stage, but it reinjects significantly cooler air back into the turbine blades, stator vanes, rotor disk and possibly liners. The system works by taking bleed air from the back of the compressor, passing it through a heat exchanger system linked to the bypass duct, and routing it into the turbine section. Testing of basic cooled cooling-air systems has been undertaken as part of the European Newac program.
Blings are “the next evolution from blisks [bladed disks] today and this takes it further,” Newby says. The bling eliminates the need for a deeper ring by having a very strong, potentially reinforced metal-matrix ring with integral blades formed on the outside.
Validation of the composite fan for Advance is underway as part of the ALPS (Advanced Low-Pressure System) program. The new fan set has been mounted on a pair of Trent 1000 engines in a straight swap for the original hollow titanium fan. Following initial sea-level testing here, one ALPS test engine has been shipped to Rolls-Royce’s site at Stennis Space Center, Mississippi, for crosswind evaluation. A second engine is being readied for flight tests later this quarter on the company Boeing 747-200 flying testbed in Tucson, Arizona. “That’s really a final check to make sure there is nothing in terms of flight loads that we haven’t spotted in sea-level testing. We wanted to do the Stennis testing first to make sure we understand its behavior,” Newby says.
The projected weight savings of 750 lb. per engine from adopting the new material is “well worth having,” he adds. Although GE has been using composite fans on its large engines since the 1990s, Rolls said its hollow titanium blades have remained competitive. However, with increasing fan diameters, “the time is right because the manufacturing technology allows us to get the thickness [of a composite blade] right down,” says Newby.
More substantial tests of the Advance core technology at full engine scale are also planned using a Trent XWB donor engine. “We will take the HP and IP spool out of an XWB and replace it with an Advance core architecture,” Newby says. He notes that the “Stage 1” exit from concept definition is finished and design freeze is the next step. “We have already got the disk forgings in, and we are starting to machine those. We are looking at different supply chain options to get bits in quickly, and our plan is to run the first build by the end of next year and run a further build [for endurance testing] in 2016,” he adds. “A team has been created and it has got that buzz about it like a new project. It is quite exciting.”
The new core section will incorporate a four-stage IP compressor and 10-stage HP compressor for the testing, though Newby cautions that this “may or may not be the final production configuration,” adding: “It’s not just about the aerodynamics; it’s about how you bring it together as a system. It is those things we want to check as well as the basic aerodynamics.” The only stage counts that are guaranteed to carry through to the production standard will be the test unit’s single-stage IP and two-stage HP turbines.
The higher pressure ratios planned for Advance and UltraFan mean higher operating temperatures and increased generation of nitrous oxides. Several key technologies for coping with these—and to reduce fuel burn and emissions—will be validated during continuing runs of Rolls’s long-running Environmentally Friendly Engine (EFE) test unit. First run in 2010, the fourth build of the Trent 1000-based EFE recently completed a new series of evaluations of high-temperature-capable advanced turbine materials and a lean-burn combustor design. EFE was used to test CMC high-pressure turbine blade tracks in 2013 and more recently evaluated CMC shroud segments. Testing will continue through 2015, Newby says.
Trials of a robust lean-burn combustion system have “gone really well,” says Newby. “We have done rig and core engine testing and will run another Trent 1000 with a new combustion module. We’ve done the chemistry in the rigs and we know it works, so this is about putting into a systems environment to see how it behaves on relight, altitude relight, pull-away and when you put water down it. Again like ALPS, this Advanced Lean Combustion System (Alecsys) and EFE are all full-scale Trent 1000s, and Advance will be XWB.” Together with earlier work conducted in Germany under the European Clean Sky 1 research program and Alecsys, “we know this gives us good margin to the [emission regulations] CAEP 6 and 8, and gives us confidence we can meet targets even with more aggressive cycles,” Newby says. “So this allows us to grow these cycles and improve emissions which tend to go in the opposite direction.” Following ground tests, Rolls plans to flight-test the new combustor in 2015 on the company’s 747 flying testbed.
In readiness for UltraFan, Rolls is building a €65 million ($87 million) research and development facility in Dahlewitz, Germany, for testing power gearboxes. “This is a big program,” says Newby. “It will take the Advance core, which we are laying down through the XWB, and wrap a new low-pressure system around it. This will be one of the main parts of the Clean Sky 2 program and is partially funded through both U.K. and German national programs,” he adds. Although Rolls acknowledges the UltraFan gear will be a form of planetary device, it is reluctant to divulge more details. “We have a clear idea of what the baseline is, and we think we know what the right answer is; we just have to validate it. The gear ratio will be around 3:1, if not probably a bit more.” The rig will be adaptable to various engine sizes and capable of testing gear units and associated oil systems at various angles and attitudes. Power gearbox testing is scheduled to run through the end of 2015, with component testing running beyond that.
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