Snecma is progressing with its plans to develop a turbofan in the 10,000-pound-thrust class. Last October, the French manufacturer revealed plans to enter the business jet engine market. Design of the Silvercrest powerplant is well under way and the core should run later this year.

With the Silvercrest, the Safran subsidiary is offering thrust levels between 9,500 and 12,000 pounds. According to the company, better temperature margins will result in improved reliability and maintenance cost standards. The typical application will be a large-cabin, long-range business jet with a max takeoff weight between 45,000 and 60,000 pounds.

“The full-scale program is likely to be launched by the middle of 2008,” Jean-Pierre Cojan, Snecma’s executive vice president for commercial engines, told EBACE Convention News. Before then the company will evaluate a demonstration core engine under a $100 million technology program. This core should run by the fourth quarter of this year, probably in October. Engine certification is penciled in for the end of 2010 or early 2011, depending on the schedule of the airframe applications found.

Commercial Reliability
The Silvercrest consists of a one-stage fan, no booster, a five-stage high-pressure (HP) compressor, a direct-flow combustor, a one-stage HP turbine and a three-stage low-pressure turbine. “We want business aviation to reap benefits from commercial aviation,” Cojan said.

This is the thinking behind the titanium, solid, wide-chord, swept fan blades designed by the Snecma engineers, who employed the same 3-D aerodynamic tools they use for bigger CFM56 engines. Fan diameter will be 40 inches and the bypass ratio will be 4.5.

Cojan is not impressed with existing business jet engines. “Current bizav turbofans are below commercial reliability standards,” he commented. He expects the Silvercrest to provide time-on-wing performance much improved over that of existing engines.

The final stage of the compressor will be centrifugal, which is possible only because of the relatively low thrust. The maximum thrust for a turbofan to accommodate a centrifugal compressor is between 12,000 and 16,000 pounds. Above this limit, the stress the parts undergo makes centrifugal compressors unsuitable, but under this threshold, advantages are significant. “The pressure ratio with this single stage is very good. In our case, it translates into a pressure ratio greater than 17 on the entire five-stage compressor,” Cojan said. The engine’s overall pressure ratio is 27.

Also, blade tip clearance is an issue on the final stages of an axial compressor. Clearance between blade tips and the internal side of the casing cannot be reduced beneath a given value. This value becomes relatively great as the blades decrease in size. Thanks to its different shape, a centrifugal compressor solves the problem.
The main challenge with this design, Cojan explained, is the high temperature at compressor exhaust stage. The answer should lie in cooling and high-quality hardware.

Turbomeca, Snecma’s helicopter turboshaft manufacturing sister company in the Safran group, helped in designing the Silvercrest’s centrifugal compressor. This part of the engine is a core skill for helicopter engine makers. According to Turbomeca vice president of engineering Jacques Brochet, one centrifugal stage can replace three or four axial stages.

As environment has become the first design driver in the engine industry, the combustor will have a direct-flow configuration. Rather than a reverse-flow combustor, Snecma’s Italian partner Avio will supply a design that is longer, “but yields lower emissions,” Cojan pointed out. The Silvercrest engine will be 74-inches long.

In terms of noise, Snecma (Booth No. 610) is targeting 15 to 20 db below the current Chapter 4 standard (that is, 25 to 30 db under the Chapter 3 standard, which operators are more familiar with). Emissions should be 50 percent lower than current International Civil Aviation Organization CAEP6 standards.

Specific fuel consumption is expected to be 15 percent better than that of existing engines in the class, and thrust at climb and cruise should improve by 25 percent.

So far, Snecma has been well known in commercial air transport mainly through the CFM56 engine family that it jointly produces with General Electric. Now, however, now it is starting to explore segments of the industry operating smaller aircraft, such as regional airliners, with the SaM 146 (to power the Russian-led Superjet 100 program) and business aviation, with the Silvercrest. With the aging of the General Electric CF34-3B (the only engine in this class in production), Snecma is eyeing future potential applications in large business jets around the size of the Bombardier Challenger 600.

Asked about the Silvercrest name, Cojan told EBACE Convention News that it is about exclusivity, altitude, clean mountain air and lineage. Entering a new market, Snecma also wants to follow market traditions. “In business aviation, aircraft have names [rather than just numbers and acronyms],” he explained. The project was known as the SM-X until it was christened Silvercrest at the National Business Aviation Association Convention last October.