In the middle: The turbine center frame for large aircraft engines
Turbine center frames for widebody engines.
11.2015 | Text: Patrick Hoeveler
Patrick Hoeveler has been a member of the editorial team at aero-space magazine FLUG REVUE for some 15 years. His responsibilities there include the engines, regional aviation and history sections.
GE Aviation widebody engines have a key German-made component in their turbines: MTU Aero Engines develops and manufactures turbine center frames for the GP7000, the GEnx and, since recently, also the GE9X.
High temperatures, enormous pressures, high speeds: the most challenging conditions in an aircraft are concentrated in the engine. Right in the middle of it all is the turbine center frame (TCF), which plays a key role in every turbofan. Its name may sound a little dry and technical, but its position in the engine reveals its importance: the TCF is situated between the high-pressure turbine and the low-pressure turbine, where it fulfills two important functions. It connects the high-pressure shaft’s rear bearing with the housing and forms an aerodynamic transition duct between the high-pressure and low-pressure turbine. “This area is subject to very high stresses, because bearing loads are conducted to the outer casing through the TCF structure. In the event of faults, such as a broken fan blade, the turbine center frame must be able to withstand the resulting loads in terms of mechanical integrity,” explains Dr. Martin Metscher, Head of Development for the GE9X at MTU Aero Engines. In addition, the component has to permanently withstand temperatures in excess of 1,000 degrees Celsius.
TCFs essentially consist of two main component groups. The first of these is the hub strut case, which is the load-bearing structure and takes the form of a casing with several struts assembled around a hub with an integrated bearing. The second main group is the struts’ panels and fairings—also known as the flowpath hardware—which form the channel for hot gas flowing from the high-pressure turbine. In addition, there are various seals, and finally, oil lines and cooling air channels, through which oil and air are conveyed through the TCF to the turbines and the bearing.
High-tech manufacturing assembly of a GEnx turbine center frame.
High-tech manufacturing for best seller GEnx
MTU gained its foothold in this field with the engine for the Airbus A380, the Engine Alliance GP7000. The TCF for this engine builds on the design used in GE Aviation’s GE90 and was adapted and optimized by MTU engineers. To date, the company has delivered over 400 units. “MTU wanted to position itself as a center of expertise for GE turbine center frames,” says Metscher. The final breakthrough came in 2008, when MTU was also entrusted with responsibility for this component for the GEnx, which powers the Boeing 747-8 and 787. Again, the German company’s specialists in Munich were able to optimize the center frame and quickly ramp up production. GE received the first segment in late August 2011, and in May 2012, Cargolux put a GEnx with an MTU turbine center frame into service in a Boeing 747-8. Since then, over 700 units have left the production facilities at MTU’s headquarters. On average, one turbine center frame is manufactured per day.
The high production rate is no walk in the park, and not only because of the almost 3,000 individual parts that make up each TCF. “Manufacturing TCFs is a complicated business,” explains Metscher. “We work with very large parts that are difficult to machine due to the high requirements they have to meet in terms of temperature levels and strength values.” Assembly is largely carried out using classical methods, but it still poses challenges due to the very high accuracy requirements. After all, the housing defines the position of the bearing, which is important for maintaining component clearance in the entire high-pressure area. Optimum clearance, in turn, remains indispensable for the high efficiency of the entire engine, which is vital for ensuring low fuel consumption. Although some components have a diameter of around 1.5 meters, even a mere one-millimeter deviation would be too much. The two production lines are characterized by their high degree of automation. “We’re defining the state of the art here with new production machines that drill and then screw components together. We’re trying out partly automated manufacturing, and we work virtually around the clock. Our strength lies in the fact that manufacturing, assembly and engineering are strongly intermeshed in one location,” says Metscher. The company’s U.S. partner agrees: “MTU brings a wealth of experience and technological expertise to the table, in both manufacturing and maintenance. We’re delighted to have MTU as a partner for this and for future engines,” says Tom Levin, General Manager GEnx at GE Aviation.
New manufacturing techniques
The latest turbine center frame, which MTU is now developing for the Boeing 777X’s GE9X engine, promises further progress. “Particularly with regard to the struts in the hub strut cases, we’re considering whether we could, in the future, manufacture them using additive methods.” In this innovative manufacturing technology, a laser fuses components and builds them up layer for layer from a metallic powder bed. Compared with casting parts, additive methods significantly reduce manufacturing costs. Metscher also points to improvements in design: “The TCF for the GE9X had to be even lighter than the design for the GEnx. Thanks to our experience and improved design methods, we were able to further optimize the components through classic design evolution.” Assembly of the first housing begins in Munich at the end of the year. It will then be tested in an engine for the first time at GE Aviation in 2016. The GE9X is due for commissioning in 2020; over 800 engines have already been sold.