The “hot section” of jet engines—i.e., combustor and turbine areas—has long been the domain of materials such as superalloys, but GE Aviation is working hard at substituting these with durable, lightweight composite components.
The GE Rolls-Royce Fighter Engine Team’s F136 development engine for the Joint Strike Fighter (JSF) contains third-stage, low-pressure turbine vanes made by GE from ceramic matrix composites (CMCs). The company is hopeful that this will lead to the first commercial use of CMCs in the hot section when an F136-powered JSF begins flight testing in 2010.
CMC development, according to GE Aviation, plays a key role in several of its private and government-funded engine demonstrator programs currently underway. CMC components are also integral to GE’s eCore program, which was launched last summer at the Farnborough Air Show, as are advanced cooling technologies, next-generation TAPS (twin-annular, premixed, swirler) combustors, and 3-D aerodynamic design airfoils. The goal of eCore, which is dubbed “the technology cornerstone” for next-generation jet engines powering narrow-body, regional, and business jets, is to offer aircraft operators up to 16% better fuel efficiency compared to GE’s current crop of engines.
“Under a NASA-led program, GE Aviation worked closely with industry partners to select silicon carbide (SiC) as the most suitable ceramic material for high-temperature structural applications,” Jim Steibel, Subsection Manager for Advanced Ceramic Technologies, GE Aviation, told Aerospace Engineering & Manufacturing. “GE's efforts to successfully design and develop a manufacturing process to produce a composite with SiC fibers [resulted in] a highly dense SiC-based matrix and was the key to realizing the full technical capability of this material.”
Manufactured through a “highly sophisticated” process, CMCs are made of SiC ceramic fibers and ceramic resin, and they are enhanced with proprietary coatings. “The manufacturing process involves coating the silicon carbide fibers in a chemical-vapor-deposition operation, creating a tape-like material via matrix impregnation, fabricating the component shape using traditional polymer matrix composite technology, then forming a ceramic matrix while densifying the component using a molten silicon infiltration process,” Steibel explained.
These CMC manufacturing processes have been scaled up to low-rate production at GE’s Ceramic Composite Products facility in Newark, DE.
One reason they are desirable for jet-engine components should come as no surprise: their weight-reduction potential. CMCs are one-third the density of nickel alloys, Steibel noted. “For a midsized engine, weight savings in the range of 200 lb is possible,” he said.
Another reason CMCs are being pursued is their durability and high heat resistance, which leads to less cooling air being required, which in turn allows a jet engine to run at higher thrust and/or more efficiently.
The company’s history with composites includes using polymeric matrix components made of carbon fiber and epoxy resin in the “cold section” of jet engines. GE claims to have introduced in 1995 the first carbon-fiber-composite front fan blade in an airline engine with its GE90, which powers the Boeing 777. For the new GEnx engine, which will power Boeing’s 787, GE will introduce composite fan blades that use the same fibers, resin, and manufacturing processes as the GE90 blade, as well as a fiber-braided composite fan case.
For more than 15 years now, GE Aviation and GE’s Corporate Research Center have worked to advance CMC technology. Several years ago, GE Aviation ran a government demonstrator engine with a combustor liner and low-pressure turbine blades made of CMC.
“Developing new jet-engine materials takes many years of investment and commitment,” said Robert Schafrik, GE Aviation’s General Manager of Materials and Process Engineering. “But the benefits can provide a considerable competitive advantage. CMCs are a new frontier that will raise the bar in jet-engine performance.”
Schafrik believes that eventually CMC components will populate many areas in the engine’s hot section, including high- and low-pressure turbine vanes and blades, turbine shrouds, and combustor liners. CFM International, a 50:50 joint company of Snecma and GE, will run a Leap-X demonstrator turbofan engine in 2010 with CMC components. Leap-X, which could be certified by 2016, according to CFM, will power future replacements for current narrow-body aircraft.
CMC combustor liners are also under consideration for future GEnx production models.
“GE continues to work closely with the technology experts within the U.S. Air Force, Navy, and Army to address application-specific challenges for advanced military engines,” said Steibel, “for example, the development of customized test methods and structural sub-elements to understand the CMC material capability for key component features, such as airfoil trailing edges and blade platforms. The resulting data is ultimately used to finalize component configuration prior to freezing the design and manufacturing engine test hardware.”
Beyond the technical challenges of readying CMCs for hot-section use, Steibel admitted that perhaps the “most significant” challenge “is establishing and qualifying a supply chain that produces the material constituents and components to aerospace quality standards at an affordable cost. It must be a viable supply chain,” he continued, “which means that everyone involved can make a reasonable profit without becoming unaffordable.”