Rolls-Royce is working on designs for future engines which it hopes will power the aircraft of the 2020s and beyond. BILL READ FRAeS reports from Dahlewitz in Germany.
Advance demonstrator engine under construction. (Rolls-Royce)
One of the leading aero engine manufacturers in the world, Rolls-Royce currently supplies engines to over 35 different types of commercial aircraft with more than 13,000 engines currently in service. These include the Trent 1000 for the Boeing 787 Dreamliner, the Trent XWB for the Airbus A350 and the Trent 7000 which will be used to power the new Airbus A330 neo. However, Rolls-Royce is not just working on designs for the present generation of airliner but it looking to the future with the development of new engines for the next generation of civil aircraft.
“We spend between £1.2 to 1.3bn each year on R&D,” explained Caroline Day - Head of Future Programmes. “Our Future Technology Programme is looking simultaneously ahead over five, ten and 20 years. Vision 5 is about applying technology, Vision 10 looks at next generation products and Vision 20 is about exploring new ideas.”
In 2014 Rolls-Royce launched its ‘large civil architecture strategy’ to develop new engine designs beyond its current Trent XWB engine for the early and mid 2020s called Advance and UltraFan which will be 20% and 25% more efficient than the earlier Trent 700 engine. “The Advance is in the Vision 5 space while the UltraFan is in the Vision 10 space,” said Phil Curnock, Chief Engineer, Civil Aerospace Strategy and Future Programmes.
Rolls-Royce’s future engine research is being conducted simultaneously in a number of different areas, starting from research into new technologies, through technology testing to engine demonstrators. “A key part of our technology research is through partnerships,” said Caroline Day. “We have partnerships with 31 university technology centres and 14 research centres and other partnerships. The research areas include additive layer manufacturing (ALM), the design and manufacture of advance materials, advanced alloys and ceramic matrix composites (CMCs).
Rolls-Royce also has partnerships with manufacturing research centres and is conducting research into volume manufacturing. “While it’s great to have new technology, you also have to make it and then sell it,” pointed out Caroline Day. “You can develop new products in laboratories but you can’t just jump straight into manufacturing, you need to work out how to make them in volume.”
Rolls-Royce is currently working on a number of engine demonstrators concentrating on different aspects of technology. (Rolls-Royce)
“Our engine development progresses through a number of stages, starting with testing the capabilities of components and technologies, then testing parts of the engines individually and then integrated with other sub systems,” explained Phil Curnock “This is followed by the assembly of whole engine demonstrators which are tested in different conditions on the ground and then in the air. Once an engine has been proved to be robust then the final stage is to target products that it could be used on.”
As part of this ongoing research, Rolls-Royce is testing a number of full-scale demonstrators, each of which concentrates on a particular technology or part of an engine. Tests on two demonstrators, E3E and EFE (Environmentally Friendly Engine) which concentrated on environmental efficiency, have now been completed while work is underway on eight others, including the advanced low-pressure system (ALPS) and advanced low-emissions combustion systems (ALECSys) demonstrators.
The Advance 3 demonstrator engine under construction in Derby. (Rolls-Royce)
The three-shaft Advance 3 demonstrator, together with a four-stage intermediate pressure (IP) compressor and ten-stage high pressure (HP) compressor aero rigs which were tested at Anecom in Germany, are paving the way towards the new Advance engine design. Currently under construction at Bristol, the Advance 3 is due to go on test in Derby in mid 2017. “With the Advance 3 we’re changing the architecture in the engine by combining the Trent XWB fan, the back of the Trent 1000 and the all new HP core in middle, plus other new technology,” said Phil Curnock. “We’re also making use of 3D printing to make components quicker.”
“The Advance concept is about redistributing the workload between the IP and HP compressors and turbines,” he continued. “The XWB has quite a lot of IP compression going on, with a two-stage IP turbine, a shorter, smaller core, a single-stage HP and a shorter compressor. As you get to higher temperatures and higher pressure ratios, keeping the bearings and other systems cool with a short core becomes a problem. Therefore, what we’re doing with the Advance is to put more work back on the HP spool.”
Business jet engine
Advance 2 future business jet engine. (Rolls-Royce)
In addition to the Advance engine for larger commercial aircraft, Rolls-Royce is also working on a future new engine for business jets called Advance 2. Based on the BR725 engine design, the Advance 2 features improved fan and core technology with a ten-stage blisk HPC, two-stage HPT, Ti fan-blisk, lightweight containment system and high efficiency, low noise LPT. The Advance 2 will have a take-off thrust between 10-20klbf, together with improved SFC and thrust-to-weight ratio.
Look - no LP turbine
The evolution of the Trent XWB to the UltraFan. (Rolls-Royce)
Meanwhile, work is also progressing on the Vision 10 UltraFan. “The UltraFan uses the same core as Advance but has a bigger fan,” said Phil Curnock. “That gives us a more efficient engine with a higher bypass ratio but it makes low pressure (LP) turbine larger as well. Now, as an engineer, I don’t like large turbines; they’re heavy and need more stages. I’d rather have a smaller more compact version. What we’ve done is to extend the IP turbine by a couple of stages. However, in order to do that, I have a fan which wants to turn slowly and a turbine that wants to turn faster. The solution is to use a powered gearbox to link the IP turbine to the fan, allowing deletion of the LP turbine.
“The reason for using a gearbox is to increase the propulsive efficiency of the engine,” said Mike Whitehead, Chief Engineer & Head of Programme - UltraFan. “It’s all about the fan. We want to shift lots of air but not too much. The turbine is operating hot and fast while the fan is big and slow. So, there’s a contradiction. If we make the fan more efficient then the turbine is less efficient. We need to combine the technology - which is where the gearbox comes in.”
Second powered gearbox being prepared for testing. (Rolls-Royce)
The PGB consists of a central cog surrounded by five rotating ‘planet’ cogs. “The planetary gearbox design is nothing new, the technology is used in electric drills,” remarked Mike Whitehead. “Nor is it new to us, as Rolls-Royce have been making them for 60 years. What is different about this one is its size and power.” The new PGB is described by Rolls-Royce as ‘the world’s most powerful aerospace gearbox’. It has a potential power of up to 100,000hp. “There are huge loads in the gearbox,” said Christian Seydel - Product Development Manager - Power Gearbox “Each tooth is the size of a little finger and two gear teeth can transmit the same power as the entire grid of a Formula 1 race. “Or, to put it another way, each planet gear can hold the force of a Trent XWB-84 engine at full thrust.”
Located south of Berlin, Rolls-Royce’s Dahlewitz site is the headquarters of Rolls-Royce Deutschland, specialising in two-shaft engines, high pressure compressors and nacelles, as well as providing on-wing support for 9,000 civil engines. (Rolls-Royce)
The PGB has been developed by an international team of research, industrial and funding partners from the UK and Germany, as well as a joint production venture called Aerospace Transmission Technologies with Liebherr-Aerospace in Friedrichshaven.
A virtual reality version of the powered gearbox. (Rolls-Royce)
The PGB was designed and built using virtual reality (VR) techniques to reduce development time. The testing of the gearbox is being carried out by a team of 100 engineers based at Rolls-Royce’s site in Dahlewitz in Germany. “No one has built gearboxes this size for aerospace before,” declared Christian Seydel. “There was no gearbox test facility, so we had to create one. We started construction in March 2015, tested our first PGB in September 2016 and conducted the first PGB power test in May.”
First run of the PGB attitude testing rig. (Rolls-Royce)
The Euro84m custom-built test facility at Dahlewitz includes an oil storage room, water cooling system and a power rig and an attitude rig in which the gearbox can be tested in different conditions and positions. The PGB attitude rig will be capable of simulating altitudes of up to 40,000ft and aircraft manoeuvres with a pitch of +/-45deg and roll of +/-35deg. The 35m long power rig will be capable of 150,000hp and can add up to 100MW of dynamic torque with a collar simulating inflight loads.
High power testing on the PGB began in Dahlewitz on 25 May. (Rolls-Royce)
“We are exploring the boundaries of gearboxes,” said Christian Seydel. “We can run the PGB at full speed and see what happens when we tip it about at different oil pressures or temperatures or what happens if the fan blades are windmilling when the engines aren’t being used. One issue that we’ve been looking at is that of heat management. We’re got lots of testing planned in which we get oil in and out so that it can be used both for lubrication and removing heat.”
Advance key technologies. (Rolls-Royce)
The two new engine designs also include other new technologies into which Rolls-Royce is also conducting research. “Changing the engine architecture is only part of the process,” continued Phil Curnock. “We’re also introducing new materials technology, lean burn technology and composite fan technology. The Advance engine has dynamic sealing, composite fan, smart adaptive systems, high torque density shafts, a lean burn combustor, a lightweight CTi system, lightweight high efficiency compressors and an adaptive cooling system.
UltraFan key technologies. (Rolls-Royce)
“In addition to the powered gearbox, there’s a whole suite of technologies going into the UltraFan, including a low speed fan system and multi-stage IP turbine system. The UltraFan will also feature advanced carbon/titanium (CTi) fan systems, hybrid ceramic bearings and ceramic matrix composites (CMCs).”
Icing tests on the ALPS demonstrator. (Rolls-Royce)
“We have to provide the aerodynamics of a fan turning slower and we’ve got some programmes looking into that,” said Phil Curnock. “A larger fan weighs more, as does its fan case, so we need to reduce their weight using composites,” “We’re also going to use composites for the annulus fillers between the blades. Our initial version of the UltraFan will have fixed pitch blades but part of the roadmap for future versions may include variable pitch. To get the efficiencies we need, we need future systems and future controls.”
Research on CTi composite fan systems is being conducted using different types of blades. “A composite blade behaves differently under impact to a titanium blade,” said Phil Curnock. “We’ve been conducting tests for a while now, including indoor and outdoor testing, blade untwist, blade clearance, flutter mapping, nozzle calibration, crosswind testing, ice testing, noise, damaged blades and over 50 bird strikes. We’re utilising different methods of testing, such as the single arm bird strike test which looks at the effect of a bird strike on a single fan blade.”
Pre-production work on composite fan blades, casing and annulus fillers has been carried out from the Rolls-Royce-GKN Aerospace joint venture at the CTAL facility in the Isle of Wight (now a part of the Rolls-Royce group) but manufacturing is expected to be relocated to Bristol this year.
Other areas of research include the use of new materials, including the introduction of hybrid ceramic bearings. To save weight, titanium aluminide will be used in the Advance concept's LP turbine and UltraFan IP turbine, together with lightweight high efficiency compressors using one-piece blisks instead of assemblies.
High temperature and lean burn research
Lean burn combustor. (Rolls-Royce)
Another ongoing research project is the lean burn combustor which can turn streams of fuel on and off, optimise the performance of combustor and tune systems to reduce emissions and noise. The lean burn combustor, which has completed sea-level rig tests in Derby and altitude rig testing in Stuttgart, will help reduce emissions to conform with future environmental requirements. Rolls-Royce is also looking at smart adaptive systems which can vary the flow of cooling air, as well as second generation nickel alloys to cope with higher temperatures for the disks inside the engine.
Ceramic matrix composite (CMC) HP seal segments. (Rolls-Royce)
Rolls-Royce has used an advanced manufacturing process known as ‘CastBond’ to create metallic turbine blades with reduced wall thicknesses and more intricate features which do not need so much cooling air. In 2016, Rolls-Royce opened a new facility in California to produce CMCs for combustor components, turbine seal segments, blades and vanes. Made from a continuous silicon carbide fibre reinforcement with a fibre-matrix interface coating surrounded a ceramic matrix primarily of silicon carbide, the CMC components enables weight reductions and higher temperature capabilities. The high temperature capabilities of both CMCs and advanced manufactured metallic components will be tested in the HT3 (High Temperature Turbine Technology) demonstrator based on a Trent XWB-97.
Rolls-Royce's virtual engine allows engineers to test new concepts. (Rolls-Royce)
To speed the development of these new engines, Rolls-Royce is making a greater use of computerised modelling using a digital ‘virtual engine’ which can be used by engineers to experiment with new concepts. “We can do more and more analysis in the computer,” said Phil Curnock. “Engine testing is expensive and the more we can do on computers to refine the design the better. Using the virtual engine will enable us to introduce products faster. The reason for this is not just about creating better designs but also about removing tests from some of our programmes because we can do a lot more work through computational analysis. For example, we can now do simulations of fan blade off tests. Eventually we hope to be able to do without actual physical tests for certification which are expensive as they write off a whole engine. However, that not happening just yet though.”
A busy year
Advance 3 compressor. (Rolls-Royce)
Work on all the different projects is proceeding. “This year, there’s lots happening,” Phil Curnock explained. “We’ve got the gearbox power rig test in Q2, Advance 3 demonstrator ground test by mid-year, ALPS demonstrator ground test and ALECSys combustor ground test in Q3 and the assembly of the HT3 demonstrator in Q4. The UltraFan programme is on track with the concept freeze gate later in 2017. Low speed fan tests are completed, the first trial blades have been cast for the second generation TiAl IP turbine and initial PGB testing is now underway. The first flight test is scheduled for 2021 with a potential entry into service from 2025.”
Chicken or egg?
Peering into the future. Will an OEM decide to use Rolls-Royce's new designs to power a future aircraft? (Rolls-Royce)
What comes first - the engine or the aircraft? With all this work going on future engine designs, the question being asked is what aircraft might they be used to power? Rolls-Royce’s answer to this is to admit that no platform has yet been identified but the designs are scaleable and could be adapted for both wide-body and narrow-body designs. The fact that such engines will be available in the future may spur aircraft manufacturers into designing new aircraft that can best use them. The UltraFan technology suite could even be used as a building block to an open-rotor engine should a manufacturer decide to build such an aircraft.
Even if no orders have yet been forthcoming, the research is not being wasted, as the work is being used to support the UltraFan development programme. “This isn’t just about future products,” added Caroline Day. “Not only will this new technology be useful for other future engines but some of it will feed back into our current portfolio to refresh our existing engines.”