Engine maker on brink of ditching single-stage architecture in favour of two-stage spool as it pursues higher fuel efficiency
CFM International's highly profitable, 37-year-old association with a single-stage architecture for turbofan engines may be nearing an end.
The single-stage CFM56 engine family has been the civil turbine market's best-seller, dominating the market for narrowbody airliners since the early 1980s. However, as the General Electric/Snecma joint venture contemplates how long-term fuel prices will drive demand for a next-generation, Leap-X engine, the CFM56's proven and reliable - but less efficient - single-stage architecture appears increasingly irrelevant.
Ron Klapproth, Leap-X programme manager, confirms that CFM's three-year-old next-generation technology studies have shifted focus radically from a single-stage to a two-stage spool. The shift is a function of the trade-off between the higher fuel efficiency of the two-stage architecture versus the maintenance cost advantage of a single-stage system.
"When we started the Leap56 programme in 2005-6, fuel prices were relatively low. And when we look at the balance between fuel price and maintenance cost and solve for the lowest cost of ownership, it pointed to the single-stage architecture - the CFM56 architecture - as being the lowest-cost architecture because of the reduced maintenance cost," Klapproth says.
"As fuel prices escalated, we crossed over to the point where the fuel burn advantage of the two-stage high-pressure turbine offset its higher maintenance cost," he adds. "So if you do cost of ownership at $4 per gallon, say, it pointed to the two-stage high-pressure turbine architecture as being the lowest cost of ownership."
LEAP IN SOPHISTICATION
If accepted, the proposed two-stage, high-pressure spool would represent a leap in the technical sophistication for the CFM joint venture, although GE's half of the partnership is well-schooled in the complexities of multi-stage engine architectures, albeit for significantly larger and more powerful engines, such as the GE90 that powers the Boeing 777.
In the early 1980s, GE partnered NASA in the E-cubed energy-efficient engine programme to develop the class of compressors that would enter service in the mid-1990s with the GE90. If the single-stage architecture is abandoned, CFM can expect to rely heavily on that development experience, as well as the hundreds of millions of in-service hours on GE's two-stage engines, for Leap-X.
"Since 1995, we've run eight builds of the GE90 family core and each time we run it we yield improvements in efficiency and stall margin," Klapproth says. "We feel confident we understand the architecture - we can make a world-class core with that architecture. We're pushing all of those frontiers."
Even so, the CFM entity has no track record for inventing or developing an all-new engine core. When GE and Snecma agreed to develop a turbofan in 1971, the architecture for the new powerplant was a given. GE paired a core of the F101 engine, which was designed for the Rockwell B-1A supersonic bomber, with an off-the-shelf Snecma fan.
The partnership has since greatly improved the efficiency and performance of that initial core, adding the 3-D aerodynamic fan blade and the twin-annular premixing swirling (TAPS) combustor.
Despite this record, CFM will still face an altogether new challenge in developing an all-new core based on a two-stage architecture - a concern company executives acknowledge.
"We like the single stage because we have so much experience with it," Klapproth says. "It's extremely reliable. It really sets the standard out in the industry for durability and reliability. But when we look at the future in fuel price it looks like the two-stage architecture is the right architecture. And again, it's not a matter of execution to us. We can go either way. It's just making the right selection for a long-term problem."
Maintaining the CFM56's current level of reliability with the initial release of an all-new, two-stage engine will be one of the key challenges. It does not help that a two-stage architecture creates an inherently harsher environment in the engine core.
"It will have some maintenance cost headwind," says Klapproth, "because it's got more parts in the hot section. So that's really the decision, the trade study that gets executed."
CHALLENGE
The engineering challenge to leap from a single- to two-stage architecture is not trivial. One of the key differences involves raising the maximum high-pressure ratio allowed in the compressor chamber, a key measure of the engine's fuel efficiency.
The single-stage architecture of the latest version of the CFM56 offers a high-pressure ratio of 11:1. Previous Leap-X studies focused on a single-stage turbine proposed raising this level by about 50%, to about 15:1 or 16:1.
It is unclear, however, if the single-stage architecture provides any room for future growth, even as Pratt & Whitney introduces the all-new geared turbofan architecture for the same sector of the market.
By switching to a two-spool engine, CFM could resolve concerns about the growth potential of the Leap-X architecture in a stroke. The high-pressure ratio for the initial version of the engine would be set at 22:1 to 23:1 minimally, and CFM could then define a roadmap to future refinements for the all-new engine.
Although CFM officially remains undecided about the Leap-X design, the arguments in favour of the two-stage architecture appear to have already taken hold. "There used to be a lot of debate about single-stage versus two-stage," Klapproth says. "Today, there's not much with the focus of Leap-X and the fuel prices. We're really mobilising the two-stage."
Accordingly, CFM also has decided to modify the Leap-X demonstration plan. A single-stage demonstrator engine built for acquiring engineering data is to enter tests in June.
In addition, the engine maker intends in 2010 to develop a second mascot engine designed around a two-stage core. "The single-stage core is going to go to test in June. That compressor will be used as the building block for the 22 or 23:1 compressor," Klapproth says. "So everything we learned on that test from the compressor flows directly into the design of the ultimate higher-pressure ratio."
The design for the second demonstrator engine - "corebuild Two"- is scheduled to be frozen by the end of 2008, and CFM is on track to meet that milestone. The decision has already been made to make the second test rig a two-stage engine, Klapproth adds. "So it's really a decision about how much pressure ratio to put on high pressure compressor," he says.
Meanwhile, CFM is close to testing its first piece of hardware for the ultimate Leap-X design. A Leap-representative fan module, featuring resin transfer moulded (RTM) composite blades, is now being installed on the front of a CFM56-5C engine. The latter was chosen because it shares the 183cm (72in) fan diameter chosen for the Leap-X concept.
By January or February, the fan module will complete a first block of testing at Snecma's facilities in Villaroche, France. These initial tests will focus on gathering initial performance and endurance data.
The same module will be shipped to GE's facility in Peebles, Ohio later in the first quarter for operability, crosswind and acoustic testing. "It's not the end of development," Klapproth says. "It's a major milestone in the development of the RTM fan, but it's by no means the end. We'll continue with development."
Perfecting the Leap-X fan design will continue, but the main effort will be devoted to the defining the configuration for the core. CFM is preparing to take a radical step by moving to an all-new architecture. But there is likely to be no more important decision for an engine maker to make correctly, and the competitive stakes are high.
"If you don't have a world-class core to build off, you're just not going to be competitive. So you have to get the core right early so you can get it on test and mature the technology and have that to build on," Klapproth says.
Source: Flight International
