Snecma Silvercrest core to run later this year

 - April 30, 2007, 5:52 AM
French engine manufacturer Snecma is making progress with plans to develop a turbofan in the 10,000-pound-thrust class. The manufacturer revealed plans last October to enter the business jet engine market. Design of the Silvercrest powerplant is now well under way, and the core is scheduled to run later this year.

With 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 an mtow of between 45,000 and 60,000 pounds.

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

The Silvercrest consists of a one-stage fan, no booster, a five-stage high-pressure compressor, a direct-flow combustor, a one-stage high-pressure 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 the Snecma engineers designed. For these they employed the same 3-D aerodynamic tools they use for their larger CFM56 engines. Fan diameter will be 40 inches, and the bypass ratio will be 4.5.

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

The final stage of the compressor will be centrifugal, which is possible only because of the engine’s 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, centrifugal compressors are unsuitable because of the stresses imposed on the parts.

When the engine’s thrust is below this threshold, however, the 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.

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 below a given value. This value becomes relatively great as the blades decrease in size. Thanks to its different shape, a centrifugal compressor avoids the problem.

The main challenge with this design, Cojan said, is high temperature at the 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 design 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.

Now that environmental impact has become second only to reliability as the design driver in the engine industry, the combustor will have a direct-flow configuration. Italian Snecma partner Avio will supply such a design instead of a reverse-flow combuster. “It yields lower emissions,” Cojan said.

In terms of noise, Snecma is targeting 15 to 20 dB less than the current Stage 4 standard (25 to 30 dB below the Stage 3 standard). Emissions should be 50 percent lower than currently allowed by International Civil Aviation Organization CAEP6 standards.

The engine’s specific fuel consumption is expected to be 15 percent better than that of existing engines in the class.

Snecma has been well known in the commercial air transport sector, mainly with the CFM56 engine line that it produces with General Electric. It is starting to explore lower segments, such as regional airliners with the SaM146 and business aviation with the Silvercrest.

With the aging of the General Electric CF34-3B (the only production engine in the class) Snecma is eyeing future large business jets, in the category of the Bombardier Challenger 600 series, as potential applications. According to Cojan, Snecma is still in the ongoing competition to power the new Dassault super-midsize design, but he declined to comment further.

For customer service, Snecma will capitalize on the infrastructure it already has in place for the worldwide CFM56-powered airliner fleet. The engine manufacturer operates a 24/7 call center and has a Web portal that allows customers to check the status of parts orders or warranty claims, but Cojan said the company has no illusions about the differences in dealing with business aviation customers as opposed to airlines.

“We have an established expertise in customer support today that is basically applied to airlines, but we understand that the world of business aviation is not the same as airline customers,” he said. In business aviation, “you have not much more than one aircraft per customer,” whereas one airline might operate up to 500 aircraft.

Asked about the name, Cojan told AIN that Silvercrest was chosen because Snecma wanted to follow market traditions. “In business aviation, aircraft tend to have names [rather than just numbers and acronyms],” he said. 
AIN staff editor Curt Epstein contributed to this article.