For more than 20 years, has successfully built up its widebody market on a series of big engines evolved from a common, three-shaft architecture. But facing an inevitable future of bigger fans and smaller cores, the company has wrestled with how to combine this successful formula with the new technologies it needs to power the airliners of the next decade.
Now Rolls believes it has come up with the answer. While it currently focuses on development of thefor the and the next version of TEN for the , the engine maker has unveiled its strategic road map for a new generation of turbofans for entry into service from 2020.
The ambitious plan centers on a two-phase evolution of the three-shaft architecture that is designed to position Rolls for new applications in the widebody market. Because the technology is scalable, Rolls believes the strategy could also provide a launch platform for new medium-thrust engines, possibly allowing it to reenter the narrowbody market ceded to Pratt & Whitney with its withdrawal from International Aero Engines in 2013. The road map also sees Rolls introducing composites on a wider scale in new areas such as fan blades and casings and, in its second phase, embraces geared turbofan technology for the first time. In the longer term, the plan also keeps the door ajar for potential open-rotor engine derivatives.
With the continuing growth of bypass ratios to attack fuel burn and noise, Rolls-Royce Civil Large Engines President Eric Schulz says there is an inevitable need for design innovation. “Our three-shaft technology has protected us as bypass ratios have grown over the years, but we will reach a limit at some point. Today we are at [a bypass ratio] of 11:1, and it doesn't take a rocket scientist to see that one day we will be approaching 15:1. With very, very high bypass ratios, the fan and low-pressure (LP) turbine become bigger, creating a weight problem. That means we have to come up with better solutions. That is why we are pushing hard for a composite fan system, because when the fan continues to grow it's a way to take the weight out.”
The option of a geared fan “seems to be an attractive solution to reducing weight,” adds Schulz. Although Rolls once publicly derided Pratt's pursuit of geared turbofans for mid-thrust engines, the U.K.-based manufacturer concurs on its use for higher thrust engines.
The first engine, dubbed the Advance, will be tested in initial demonstrator form in 2015 based on a re-cored XWB. Although no specific application has been identified or announced, Rolls says the new engine could enter service starting in 2020. The Advance configuration was the basis for the company's bid for the, for which 's GE9X was ultimately selected after an engine competition last year. Advance is outlined with 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 .
Advance also forms the basis for a more ambitious follow-on concept dubbed the UltraFan that Rolls revealed in concept form in early 2012 as part of's Environmentally Responsible Aviation (ERA) study with . The engine could be ready for service in 2025 and is targeted at a fuel-burn improvement at least 25% over the Trent 700. The UltraFan will incorporate a gear system that drives a variable pitch fan and is outlined with a 15:1 bypass ratio and overall pressure ratio of 70:1.
The multi-phase designs address mega trends in the engine business, which has seen a constant improvement in propulsive efficiency, mostly through increases in fan diameter and bypass ratio, as well as shrinking cores and improvements in thermal efficiency. “It would be so easy to get complacent, and there have been plenty of examples of people who took their eye off the ball. But we're not going to do that,” says Simon Carlisle, executive vice president and head of strategy and future programs for Rolls-Royce's large civil engine business.
“We have strong and capable competitors who we respect, and we want to maintain our position in the large engine business. So we continue to invest in technology, and last year we spent just over £1 billion [$1.7 billion] in technology funding. We need to keep doing that because the demands from the industry keep getting stronger. We know fuel prices are going to stay high and will continue to go in only one direction, so we're investing to address that,” Carlisle says.
“What we are announcing are the next two major steps in the evolution of the family,” says Alan Newby, Rolls Commercial Engines advanced projects chief engineer. The Advance, which is a generic collective term for the raft of new technologies, is aimed primarily at improving thermodynamic efficiency. The follow-on UltraFan will build on the Advance by introducing a geared architecture aimed largely at boosting propulsive efficiency.
“At 50 paces, the Advance looks like a Trent, but it is not,” says Newby. “We've changed the core architecture and the work split. Significantly, we also see the use of a carbon fan for the first time.” The change in core configuration means that for the first time in the lineage of its three-shaft designs, the high-pressure (HP) turbine will have two stages instead of the usual single stage, while the intermediate-pressure (IP) turbine is reduced from the two stages introduced with the Trent XWB, back to one. “For us this is quite a big deal,” Newby says.
Unlike two-shaft designs produced by General Electric and Pratt, in which the fan and LP compressor are driven by the LP turbine, the fan alone is driven by the LP turbine in the three-shaft engine. In place of the conventional LP compressor, the three-shaft design has an IP compressor that is driven by an IP 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 look to the future, it has caused us to look at what that work split should be for growth and efficiency,” says Newby. The revised configuration enables a higher-pressure-ratio HP compressor while reducing the relative load on the IP spool.
The revised architecture means “we can do different things with the power offtake,” Newby adds. “It gives us a robust air system and a lighter core.” This means that the structure for the shafts can be supported on bearings placed further aft in cooler, more benign locations, away from the more challenging bearing sites in current engines that have to work in hotter environments.
The large-scale use of composites will make the largest contribution to the weight reduction. A composite fan made from third-generation carbon-titanium (CTi) will spearhead the initiative. Currently under evaluation in the advanced low-pressure system (ALPS) test program, the CTi fan will be flight tested later this year on a modified Trent 1000 on Rolls's-200 flying testbed based in Tucson, Ariz. The flights will mark the first time a three-shaft Rolls engine has been tested with a composite fan since the company's pioneering HyFil material was dropped from the baseline RB.211 engine in 1970.
Composites also will be used for the fan containment system, radial drive shaft, rear casing and the “rafts” that will house the engine accessories. Other components of Advance will include titanium-aluminide (ti-al) turbine blades, an improved method of disk shielding to stop hot air entering the engine annulus and high-aspect-ratio blades.
Other features will include dynamic sealing; hybrid ceramic bearings; smart, adaptive systems; fifth-generation three-dimensional aerodynamic turbine blades; and fluidic switches to actively control cooling air. The engine will also incorporate an advanced combustor design, ceramic bearings and ceramic matrix composites (CMC) “first in the shrouds, then in static parts and ultimately rotating parts,” Newby says.
“Advance lays down the architecture we're going to take forward to the next phase” Newby says, referring to the UltraFan. The concept will be the first Rolls large turbofan to incorporate a fan-drive gear system or “power gearbox,” as the company calls it, and is the first significant change to the traditional three-shaft layout since the advent of the RB.211 in the 1960s. The feature “earns its way” onto the UltraFan because of the engine's relatively large diameter fan, Newby says. “This gives the LP turbine a significant challenge and to mitigate that we have a reduction gearbox,” he adds. The revised configuration retains an effectively unchanged HP spool and a conventionally connected intermediate spool. However, unlike any previous Rolls design, the UltraFan will interpose a gear system between the IP compressor and the fan. The design therefore does not include an LP turbine.
To maintain the stability of the large-diameter, low-pressure-ratio fan, Rolls also is studying either a variable-area nozzle like that evaluated—but not adopted—by Pratt for the-powered and Airbus . An alternative and possibly more likely option is a variable-pitch blade system that will have adequate rigidity and would enable a thrust-reverser system to be eliminated, Newby says.
Other features targeted for UltraFan include blisked stages and metal matrix composite “bling” (bladed-ring) compressor stages, further applications of CMCs, next-generation nickel alloys, hybrid ceramic bearings, high-aspect-ratio ti-al, and integrated heat exchangers to “cool” cooling air. The larger engine, which in the Lockheed ERA design was a 63,000-lb.-thrust UltraFan with a 174-in.-dia. fan, will also have to be enclosed within a low-drag, slimline nacelle. c