First all-new big core configuration in 50 years paves way for next-gen Advance and UltraFan
Just over 3.5 years after unveiling its future large civil engine development road map, Rolls-Royce is poised to run the first demonstrator core at the heart of the strategy. The core is the common element of the company’s plan to make a step change in efficiency with two engine families for the 2020s. The first of these, the Advance, is a direct-drive turbofan with an overall pressure ratio of more than 60:1 and fuel-burn level at least 20% better than the current Trent 700. The follow-on UltraFan marries a modified version of the same core to a large gear-driven fan to improve fuel burn by at least another 5% over the Advance.
Both will be three-shaft engines, with low-, intermediate- and high-pressure (HP) spools, though in the UltraFan the intermediate-pressure (IP) turbine will drive the fan through gear, allowing the low-pressure (LP) turbine to be deleted.
To suit the flow requirements of this Advance3 demonstrator, the core is sandwiched between the conventional fan system of a Trent XWB-84A350 engine and the LP turbine of a Trent 1000 engine. Even with this unusual hybrid arrangement, any similarity to the current three-shaft family ends there.
Instead of the single-stage HP turbine and six-stage HP compressor of today’s Trent XWB, the relatively skinny core packs in a two-stage turbine and 10-stage compressor. As the high-pressure spool has grown, so the IP spool has contracted. Instead of the usual eight-stage IP compressor and two-stage IP turbine, the new core has four and one stages, respectively. Together these changes, which lead to a relatively lightly loaded high-pressure rotor, represent a historic break with the trend of supercharging the intermediate spool with successive growth steps. It also marks the beginning of the end of the company’s five-decade-long association with the current RB211/Trent core architecture.
“The work split in the compression system and turbine is different, and we are going to have to prove that we picked the right optimization of that core work split,” says Andy Geer, chief engineer and head of program for UltraFan, Advance3 and civil technology programs. “We need to do that at a whole engine level, so that’s why we have Advance3.”
Although ostensibly paving the way for a potential direct-drive product, “since 2014 the light is shining much more strongly on UltraFan than on the direct-drive replacement,” Geer says. “That is a function of what the airframe manufacturers look like they want. Nonetheless, we have stuck with that overall shape of the program, because it is convenient for us in terms of staged derisking, and it is how we have built our demonstrators in that order,” he adds.
Despite the lean to the geared option, Rolls believes there is potential for a downstream application of Advance technology on airframes where, because of the bigger nacelle, the configuration is not suited to the high 15:1 bypass ratio of the UltraFan. “If you have product that cannot cope with that bypass ratio in terms of installation then you could use Advance technology,” says Geer, who adds that, as the UltraFan is developed some of the follow-on core technology could feed back into higher overall pressure ratio versions of Advance. It could therefore conceivably provide aircraft manufacturers with a reengining option such as the Trent 7000-powered Airbus, the first of which made its maiden flight on Oct. 19.
Rolls-Royce is also saying more about new technology areas that will be tested in the demonstrator, including lean-burn combustion, hybrid ceramic bearings, ceramic matrix composite (CMC) first-stage HP turbine seal segments and first-stage vanes made using a cast-bond process that introduces cooling passages during manufacturing. Although some of these technologies were originally developed for theJoint Strike Fighter program and operated briefly on the EFE (Environmentally Friendly Engine) demonstrator, “this is the first time they’ve been included from the outset as a key enabler for a new architecture,” says Geer.
The Advance3 combustor is based on the same technology that in 2018 will be flight-tested on the company’s Alecsys low-emissions combustion Trent 1000 demonstrator. However, for Advance3, “it will just be a function for the benefit of the rest of the engine,” says Geer.
The hybrid mainline bearings, which are ceramic rolling elements running on metallic races, “have been used in machine tools for a long time, but in this case what is new is the scale and the environment they live in,” says Geer. “The test is about understanding more about how they fit into the engine subsystem.”
Ceramic bearings are more tolerant of debris ingress and will be required in future engine designs to manage high bearing load environments inside cores that are getting progressively smaller for a given engine-thrust level.
CMC testing is similarly focused on assessing how the nonmetallic segments work in a whole-engine environment. “It is designed so it does not leak air at high temperatures, but how do you make sure it does not leak at low temperatures when it does not expand as much as the metals around it?” asks Geer. The material, provided by the company’s recently opened Southern California research and development facility, was partly selected because the temperature profile of the lean-burn combustion system is more aggressive to the outer wall than the rich-burn devices commonly used today. “A bit more temperature capability in the segment pays you back,” he adds.
As Advance3 begins tests, Rolls-Royce is simultaneously moving forward with testing of the second high-power gearbox (PGB) demonstrator, as part of the technology forming the other key element of its UltraFan plan. In early September, the company’s PGB for the initial version of the UltraFan demonstrator engine was tested at its maximum rating of 70,000 hp during runs in Dahlewitz, Germany. This rating is designed to enable the PBG to be mated with the Advance3 demonstrator, which is aimed at the 70,000-80,000-lb.-thrust bracket.
The milestone, which was announced on Sept. 4 at the International Society of Air Breathing Engines conference in Manchester, England, by Paul Stein, Chief Technology Officer at Rolls-Royce, came only four months after full-scale tests of the PGB began in the power rig test facility and less than a year since the gear first ran in an adjacent attitude test site in Dahlewitz.
The power tests will ultimately evaluate follow-on versions of the transmission system at levels up to 100,000 hp and beyond and are being conducted in a 35-m-long (115-ft.) rig capable of applying 100 megawatts of dynamic torque. The gear system is the largest ever developed for an aerospace application and, at about 80 cm (30 in.) in diameter, is sized to power the biggest versions of the approximately 25,000-110,000-lb.-thrust class UltraFan family.
The Rolls PGB is a planetary-style gearbox with a ring gear on the outside and five planet gears inside rotating around a central sun gear. The design, with a gear ratio around 4:1, drives the fan from a centrally mounted planet carrier, unlike the star-style gear system used in Pratt & Whitney’s geared turbofan.
The gear demonstrator was developed by Aerospace Transmission Technologies, a joint venture formed in mid-2016 between Rolls-Royce and Liebherr-Aerospace. Supported by UK and German national aerospace research funding, as well as the European Clean Sky 2 project and the local Brandenburg regional government, the PGB development team also includes British and German universities and research organizations.
“I can’t tell you exactly the levels of efficiency we achieved in this gearbox, but if you can imagine 53 megawatts at 99% efficiency, we’d have 500 kW of excess energy—which is a pretty sizable gas turbine in its own right,” says Stein. As a result, he adds a significant focus for the tests is evaluation of heat management technology. “That’s a lot of energy to get rid of, so the heat-to-oil and oil system is very significant.”
“We have been experimenting with different oil scavenge configurations,” says Geer. “[The first PGB] has tested well, and the second one is under evaluation,” he adds. “The latest unit carries a lot of instrumentation to measure internal loads and thermal data, and is moving closer to something that would be a production-standard configuration.” Rolls-Royce engineers are now stripping and inspecting the first gearbox.
EnviroTREC is very interested in technology road maps, having participated in two key related projects. The preceding is a road map report from Rolls-Royce which presents their progress in developing the next generation of gas turbine engines. We note from this report that turbofan ratios are ever increasing and in particular this report indicates a ratio of 60:1 which improves fuel efficiency considerably.
The technology roadmap for Manitoba’s aerospace community can be downloaded HERE.