The global aerospace sector has always represented the cutting edge of practical technology advancement. When the first military jet engines emerged in the post-war 1940s it was clear that commercial applications would soon follow. The leap in performance, payload capability, maintainability, and speed compared to the best that turbo-supercharged piston-engines could offer was truly revolutionary.
Subsequent decades saw jet air travel becoming the normal, preferred way to travel, and when the first big-fan-powered wide-body Boeing 747 entered service the way was open for the world to become an interconnected network of safe, reliable air routes.
Thanks to the birth of mass air transport and subsequently a relentless growth in market demand, the technology needed to deliver the means to satisfy that demand has proceeded in steady increments. New families of wide-body transport airplanes were developed, and these regularly incorporated technological advances in aerodynamics, materials, and propulsion until today when it might be thought that evolutionary development had almost reached the end of the trail, with such impressive products as the Airbus A380 and A350, and the Boeing 777-X and 787.
The more futuristic proposals for airplanes powered by prop-fans, or open rotors, are continuing as active research and development programs on both sides of the Atlantic, but customer-power is still the dominant factor in launching new engines, and what airline chiefs are now demanding is evolutionary engines that will be a more risk-free stepping stone toward even more efficient products that can be in service a lot sooner than the super-advanced designs.
Open rotors still have a long way to go before such important factors as noise, airframe integration, and maintenance accessibility— and blade-off safety issues— are fully de-risked. So although the potential +25% reduction in fuel consumption and overall operating efficiency might be a very attractive incentive, what airlines really want is a new generation of big fan engines suitable for powering later developments of today’s generation of airplanes, including the A380 and 787, and also possibly any replacement 250-seat design that Boeing might introduce to replace its successful but out-of-production 757. The sales success of both the new Boeing 737 Max and Airbus A320neo, with orders for an incredible 6280-plus aircraft yet to be delivered, demonstrates the willingness of airlines to adopt new engine options where the extra efficiencies can save them significant sums of money.
New Engine Options
Despite a recent dramatic drop in world oil prices, the familiar efficiencydriven demand for new engines is still building up in the market for application on existing wide-body airplanes. High oil prices were seen as a prime motivator for the switch to newer, more efficient engines, but the overall benefits, including lower emissions and noise as well as better fuel economy, have kept up the sales momentum.
Last year Airbus made the decision, under pressure from some key customers, to launch the A330-800 and -900neo (new engine option) models featuring the latest Rolls-Royce Trent 7000 engines, even though the company was simultaneously introducing a similar size all-new wide-body, the A350, which everyone assumed would replace the A330 in production. Airbus claims the neo versions of the A330 will save 14% fuel consumption per seat compared to the existing -200 and -300 aircraft.
While R-R has been frozen out of the new 777-X program as a result of an exclusive supplier arrangement between Boeing and General Electric, it has taken some of the best design aspects of its previously proposed RB3025 engine to incorporate these with other new technology design features to create a two-stage product route-map toward the next generation of large-fan production engines.
The first new engine, named “Advance” is aimed at a 2020 timeframe, offering a 20% fuel-burn reduction over the earliest Trent, with the second, named “UltraFan,” delivering a 25% reduction, to become available around 2025.
To arrive in the highly competitive marketplace within these timescales, the new engines draw on experience gained from its latest two products, the Trent 1000 powering the 787 Dreamliner, and the Trent XWB powering the A350. Innovation has been at the heart of all Trent engines to date, with the three-shaft architecture a common feature.
Advance introduces a new core architecture. This redistributes the workload between the intermediate- and high-pressure shafts to achieve the highest commercial turbofan overall pressure ratio of more than 60:1 and a bypass ratio of more than 11:1. As well as reducing the parts count and weight, Advance will incorporate a lean-burn low-NOx combustor and advanced heat-tolerant materials, which will further improve thermal efficiency and improve component life.
The most noticeable feature of the new engine will be the use of the new R-R carbon-titanium (CTi) fan blades (replacing hollow titanium blades) and the associated integrated composite engine casings, which have built in electrical harnesses and pipework.
Amongst the other performance gains these features provide a major weight saving of some 1500 lb on a twin-engine aircraft, equivalent to carrying another eight passengers. These new features are the result of extensive design work over recent years and much R&D testing and evaluation, which is now continuing at an increasing pace.
The company’s hollow titanium fans have established a reputation for performance and reliability, but a series of successful technology demonstrator programs has pointed toward composite materials as the next step, incorporating specific technology innovations. Many years of looking at how advances could be gained from the weaving of the material used led to the adoption of a CTi design that delivered a lighter, thinner fan blade that was very robust and offered high performance.
In September 2014, R-R completed ground testing of the CTi fan system at the Stennis Space Center in Mississippi. This involved crosswind testing on a Trent 1000 ALPS (Advanced Low Pressure System) technology demonstrator to verify the new fan design performance in advance of flight tests taking place.
The R-R Outdoor Jet Engine Test Facility at Stennis was expanded in 2013 to add a second test stand and it can now carry out specialist development engine testing including noise, crosswind, thrust reverse, cyclic, and endurance testing on all large R-R engine types.
Last October the new fan blades took to the air for the first time in Tucson aboard a company-owned Boeing 747 flying test bed. This aircraft had the ALPS demonstrator engine fitted in place of one of the regular RB211 engines. The demonstrator engine was from a donor Trent 1000, as fitted to the Boeing 787, but had been modified with the use of CTi fan blades. Six flights took place over 11 days.
“We look forward to testing the system even more rigorously in the next phase,” said Mark Pacey, the Rolls-Royce ALPS Chief Project Engineer. “We had two main aims in testing—proving flight dynamics and the performance of the fan. This involved approaching the limits of the aircraft’s operating envelope, recording data at altitudes up to 40,000 ft and speeds from Mach 0.25 to 0.35.”
He pointed out that the European Commission Clean Sky initiative has supported the ALPS program from the beginning and this has helped to progress the new technology.
In November noise testing of a second ALPS composite fan engine continued at Stennis. The work on the integrated composite engine case is also making progress, with manufacture of a complete unit underway and the coming together of new fans and a new casing is planned for mid 2015, which will allow ground testing to take place.
The Advance engine is likely to become the basis for a new-generation of Trent-derived big fan engines, though it will represent a significant technological advance, as the name suggests. Apart from the new CTi fans and integrated composite casing, it will have a new leanburn combustion system and use ceramic matrix composite material. Ground testing in the U.K. and Germany has validated emissions predictions and later this year flight test will begin and continue into 2016.
The key to the Advance design is the redistribution of workload between the intermediate- and high-pressure shafts. This core architecture, the conceptual design of which is completed, is scheduled to be incorporated into a demonstrator to run in 2016. Long-lead time components are in manufacture and this demonstrator will be based on a donor Trent XWB, as fitted to the Airbus A350, but minus its high- and intermediate-pressure spools. The investment in this rolling development, test, and evaluation program is up and running to ensure that all the advanced technology elements are in place and proven to meet the company’s own 2020 timescale. It will then be very well placed to respond rapidly to the expected demand for a new-generation big fan engine from the airframe companies.
The second new engine, the UltraFan, is also the subject of much investment in even more advanced techniques as the design is refined, components developed, and it is prepared for testing. This program may seem far from the target timescale, a decade into the future, but the company is determined, once again, to de-risk and prove the new technologies well ahead of the market demand to retain a competitive edge and to ensure a potential world-beating product is ready at the right time.
The UltraFan will incorporate all the new technologies used in the Advance engine, but will address new challenges involving larger fans and smaller cores to achieve even greater engine pressure ratios. When the bypass ratio increases to around 15:1 the low-pressure turbine system driving the fan gets disproportionately large and heavy and managing temperatures, pressures, and aerodynamics between the fan and smaller core becomes more of an issue, according to Alan Newby, R-R Chief Engineer, Future Programs and Technology, Civil Large Engines.
“Our solution with UltraFan is to eliminate the LP turbine in its current form and drive the larger fan through an enhanced intermediate-pressure turbine,” he said.
A power gearbox between the fan and IP compressor is a key element in this design as it enables the IP turbine to do that without running too fast for the low-speed fan.
“By doing this, we can continue to be confident our three-shaft architecture ensures our compressors and turbines run at optimum speed and deliver optimum performance, and also enables us to remove the need for a thrust reverser and we can introduce an integrated, slim-line nacelle,” said Newby.
Very large fan engines need greater integration with the nacelle and airframe, as weight, drag, and ground clearance can all be issues. For the UltraFan, R-R is developing a new power gearbox to deliver the 15:1 bypass ratio. Testing will begin at the end of this year at a new purpose-built R-R facility at Dahlewitz in Germany.
Gear units and oil systems will be tested at a variety of angles and this work is supported by the European Clean Sky 2 and German national LUFO programs. Continued work on geared designs will build on much company heritage experience from turboshaft and turboprop engines in the civil market and also the R-R LiftFan system that is an essential propulsion feature delivering VSTOL capability aboard the F-35B combat aircraft.
“One element of the Advance and UltraFan program that was critical to its success was informing the industry well in advance,” said Simon Carlisle, Executive Vice President, Strategy and Future Program-Civil Large Engines, R-R.
These are early days still, but it certainly looks like this important new engine design initiative is being built on a solid foundation.