Boeing 787 Dreamliner - Mark Wagner [63]
Although not immediately apparent, one of the clever design features of the Trent 1000, as the RB262 was to become, was its method of increasing overall bypass ratio without making the fan diameter any larger. The design originally helped solve a conundrum facing engineers who were trying to meet the tough Sonic Cruiser engine performance goals by making the bypass as large as possible within the tight confines of the design space inside the Cruiser’s wing.
Once freed from high subsonic targets and the cramped dimensions of the Cruiser’s inlet duct, Rolls-Royce swiftly adapted the same baseline design—already tailored as part of propulsion studies for Boeing’s “reference design”—to meet the needs of the 7E7. The trick, as always with turbofans, was to find the right ratio of bypass. Too little, and the engine fails to meet its noise targets; too large, and the fan diameter is so big that it creates excess drag, which drives up fuel burn.
Judged by some to be technically ahead of its competitors, Pratt & Whitney’s proposed PW-EXX was based around the core of the F119 developed for the F-22 Raptor and later F-35 Lightning II. Incorporating integrally bladed rotors, the engine would have had a core-mounted gearbox for electrical power generation, and a compressor pressure ratio of 20:1 versus 11:1 on the PW4000. Pratt & Whitney
The answer was to insert a series of highly swept fan blades into a smaller-radius hub. Not only did this satisfy the airflow requirements, but it also kept the dimensions of the nacelle small enough to enable the engine to be transported in a 747 freighter in one piece.
Behind the new fan, the Trent 1000 incorporated an eight-stage IP compressor; a six-stage counterrotating HP compressor; an advanced low-emissions combustor; and HP, IP, and LP turbines, each with six stages.
The unique three-shaft configuration gave Rolls-Royce a new design opportunity for the increased electrical load requirements of the 787. Unlike its predecessors, the Trent 1000 power off-take was from the aft of the IP compressor rather than the usual front of the HP compressor, allowing a greater stability margin and lower flight and ground idle thrust.
The design evolution of the 7E7 was meanwhile set to throw more challenges at the engine companies. As they prepared to submit “Phase 3” proposals toward the end of 2003, Boeing revealed that the payload/range performance spread of the 7E7SR, base model, and stretch was too great for a single engine to handle in an optimum way. Boeing was anxious to avoid compromising the performance of the 7E7SR by tying its structural weight to the other longer-range and stretch versions just for production commonality. It therefore made the crucial decision to take significant weight out of the design by reducing wingspan, and other structural tweaks.
The fallout was a potential six-month slip in the engine decision, and the first major schedule hiccup for the 7E7. Senior Vice President Mike Bair said the concerns of the engine manufacturers drove the decision. “They were getting a bit nervous with the timing in front of us,” he commented in November 2003. The slide bought the company “a bit of time at the front end of the program” and enabled more options to be considered. These ranged from simple derating to more fundamental solutions, such as different fan sizes, or even common fans paired with scaled core sizes. Bair remarked that studies also included “one engine more specialized for the SR marketplace, or maybe one for the SR and base and another for the stretch.”
Tests on a modified Trent 500 under the affordable near-term low-emissions (ANTLE) technology program helped pave the way for several technology features of the Trent 1000. Royce’s proprietary powder nickel alloy, RR1000, was tested in the ANTLE compressor and was later used in the last two stages of the Trent 1000 HPC drum and HP turbine disk for benefits in cycle operating temperature and component life. Here the ANTLE is