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Rolls-Royce's Engine for the More Electric 787 Dreamliner Takes Flight
Joseph Ogando, Senior Editor -- Design News, June 29, 2007
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With the Boeing 787 rollout just days away, one of its engine designs has started to soar — literally.
The Trent 1000, Rolls-Royce’s engine for the 787, took its first test flight two weeks ago, turning in a performance that its chief engineer, Andy Geer, describes as “flawless.” This test bed flight put the engine through its paces on a Boeing 747-200 and helped validate the engine’s interfaces with the airframe and intake.
The first set of Trent 1000 engines has already been delivered to Boeing, and this test bed flight brings Rolls Royce that much closer to the conclusion of its testing program. Geer points out that the engine has already passed a battery of ground tests — including high altitude simulations as well as fan-blade-release and bird-ingestion tests. “This is a really exciting time for us,” he says, adding that the Trent 1000 has only a few weeks of testing left in its certification program.
Geer isn’t surprised that the engine has sailed through its tests. He explains that two of Boeing’s key design goals for the 787 — its ‘more electric’ architecture with bleedless engines and a mandate to reduce total cost of ownership — resulted in technology decisions that brought engine efficiency and robustness to new highs.
As an example of the interplay between these two design goals, Geer points to the evolution of the Trent 1000’s IP Power Offtake. This unique system extracts power for the 787’s starter-generators from the three-spool engine’s intermediate pressure shaft. Geer argues that this arrangement improves the engine’s fuel burn by about 6 percent compared to designs whose power extraction comes from the high-pressure shaft. His argument comes down to the fact that the IP shaft’s speed better matches the demands demands of the 787’s starter generators, particularly during descents. “The main benefit is at low power conditions,” Geer notes.
GE, which takes power from the high-pressure compressor of its two-spool GEnx engine for the 787, would no doubt disagree.
But what’s interesting for engineers not versed in the intricacies jet engine design is how Rolls Royce has over the past months simplified the design of its power offtake system. The system at first included a relatively complex coupling between the high- and intermediate-pressure spools. It would lock the HP and IP spools together during start-ups. Yet as Rolls engineers optimized the engine, they found they could meet all of Boeings start-up requirements without the coupling. So they got rid of it. “We just hook the generator to the IP now,” Geer says. He estimates that eliminating the coupling saved about 100 lb of weight and more than 100 components.
Numerous other design decisions on the Trent 1000 also support the low-cost-of-ownership goals. “We picked the technological solutions for the engine with total cost of ownership in mind,” Geer says.
Some of these picks obviously relate to fuel economy. To take one example, the Trent 1000 features a titanium containment case for the sake of weight savings — and corrosion resistance too. The engine’s 20-blade fan system also features a swept blade design with a lower hub-to-tip ratio than previous models. Geer says the fan design helps create a “big aerodynamic area in as small a volume as possible,” which in turn leads to reductions in drag. He adds that the design also contributes to a flat fuel burn profile, or consistent efficiency across different power levels.
Rolls engineers also made some technical choices that go beyond improving aerodynamics while reducing weight and mechanical complexity. “Not all the improvements involve bits of metal and springy things,” Geer says. For example, Geer points out that Trent 1000 will go into service with more engine monitoring functionality than the company’s previous engines. Engine management is nothing new, and the Trent 1000 has the usual complement of electronics and sensors related to this task. But the Trent 1000 also has about ten additional sensors — for pressures, temperatures, speeds and vibration — that provide diagnostic and predictive maintenance data. “We’ll be able to monitor engine health in addition to managing its performance on wing,” Geer says.
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