Balancing Act
Rolls-Royce is focusing on a scaled derivative of the Trent 900 as its baseline propulsion offering for Boeing's proposed 7E7 transport, but the new technologies incorporated into the powerplant will depend on when the aircraft enters service and whether they have demonstrated their affordability and cost-effectiveness.
"We will not put in technology for technology's sake," said Mike Howse, the engine maker's director of engineering and technology. "[In today's environment], we can't put in technology that will prevent us from achieving Boeing's payload, range, fuel burn, environmental and cost-of-ownership targets for the aircraft. Technology must pay its own way."
General Electric and Pratt & Whitney are relying on the same philosophy in their 7E7 engine proposals. Reinforcing that view, Mike Terrett, Rolls' Civil Aerospace president, says environmental issues and life-cycle costs are key parts of the 7E7 engine equation. "In the U.S., potential customers are telling us they want a focus on economics. In Europe, the priority is the environment. In Japan, it's economics surrounding long- and short-range flights. But regardless of whom we talk to, all say they want a Trent available for the aircraft and they want it to be a low-risk engine."
According to Howse, his company's current baseline 62,000-68,000-lb.-thrust powerplant for the 7E7 is a machine with a bypass of about 11 to help decrease noise and boost fuel efficiency; a core scaled from the Trent 900; and a fan, about 112 in. in diameter, also scaled from the Trent 900 (AW&ST Feb. 3, p. 48). Aerodynamic improvements also are planned for the engine, including turbine blade endwalls that cut tip losses and increase component efficiency. Since these components are costly, Rolls' 7E7 engine team has not yet determined exactly which of the powerplant's three turbines should incorporate the improvement.
Rolls officials stress that only "incremental" technologies are being eyed owing to potential costs and risks. However, more advanced technologies could be incorporated if they are demonstrated and validated in time for the 7E7 propulsion program. "How fast Boeing needs the engine will decide the level of technology that's incorporated into the powerplant. In fact [from an engine maker's point of view], the 7E7 is probably the first program in which the business aspect is more difficult than the technical approach," Terrett said.
Many of these advanced technologies are being explored in the affordable, near-term low emissions (Antle) project, a European Union initiative led by Rolls-Royce and also supported by the U.K.'s Dept. of Trade and Industry. To date, Antle-related rig tests have examined a high-workload, high-pressure-ratio compressor with a reduced number of stages and integrally bladed disks; advanced, low-emissions combustors; and a contrarotating intermediate-pressure turbine. No complete powerplant incorporating the Antle technologies has run yet, but officials have already decided that a modified Trent 500 will serve as the Antle technology engine platform.
Three builds of the Antle engine are planned in 2004 and 2005, with each one incorporating increasingly complex technologies. The first build of the Antle engine, which will have some advanced features, should begin functional testing next spring in Spain. The second is scheduled to run in the fourth quarter of 2004 at Rolls' facilities in Hucknall, England. It will have the Antle turbomachinery plus noise treatments. The third build, which should go to test during the second quarter of 2005, will be run in Spain.
Howse cautions, however, that the Antle project needs to be kept in its proper perspective. "Antle is not a whole engine demonstrator; it's a component demonstrator using a whole engine." As a result, the company will not be simulating different bypass ratios or architectures with the Antle platform.
"We're not doing engines with fundamentally different cycles. The Trent 500 has a bypass of about 7.5 and that's what it will be in the Antle engine," Howse explained.
The ultimate Antle high-pressure compressor is expected to be a five-stage, high-workload unit that features integrally bladed disks. It should demonstrate "a significant boost in pressure ratio, and will have one fewer stage to do the work now accomplished by similar six-stage units," he said.
The combustor now under investigation is a lean pre-mix, direct-injection, single annular design. It substantially increases the flow of air through the burner to lean the fuel-air mixture and, in the burner, effectively accomplishes the staging that's typically found in multiannular combustors.
Rig tests have already demonstrated the ground-level ignition and pattern factors the company desires in the unit. Flame stability also has been "good," and Rolls expects to demonstrate the full functionality of the combustor throughout its flight envelope prior to testing the unit in the Antle engine. To meet this objective, Rolls engineers will continue high-pressure combustion rig tests and stability rig tests through this year.
The goal of the combustor effort is to demonstrate an oxides of nitrogen (NOx) level that's just 40% of CAPE 2 emission limits. However, Howse is guardedly optimistic over how quickly this performance will be reached. "We might not achieve this goal with the first Antle engine. Regardless, the NOx levels should still be lower than those generated by current combustors," he observed.
Since 7E7 propulsion design decisions probably will be made in mid-2004, before the Antle combustor can demonstrate its readiness, engineers are unsure whether the unit will be a part of their 7E7 engine offering. In fact, it's still unclear whether the technology will be needed by the 7E7. "It will be a risk judgment that will have to be made at the time," Howse said.
Another technology being scrutinized under Antle is an intermediate turbine that rotates in a direction opposite that of the high-pressure turbine. With a contrarotating IP turbine "you can reduce the size of the nozzle guide vane, which cuts losses," Howse explained. The problem, however, is in modeling the complex flow in this area. "It's been difficult and we didn't incorporate this technology into the earlier Trent 900--which does have a contrarotating high-pressure turbine--because we were concerned we might disrupt the aerodynamics of the system."
Rolls engineers don't plan to add the contrarotating IP turbine to their Antle engine. Instead, the company expects to demonstrate the technology in rig tests, "because that's all that's necessary." Howse did note, however, that like the Trent 900, the Antle/Trent 500 test platform would have a contrarotating high-pressure turbine.
Other technologies to be explored in later builds of the Antle engine (but not expected to be ready in time for the 7E7) are a cooled, variable-capacity intermediate-pressure turbine and an active magnetic bearing. Engineers are looking to change the capacity of the intermediate turbine by varying the throat area of the IP turbine nozzle.
"This would offload the IP compressor and eventually lead to a reduction of stages in the unit. It also could lead to the elimination of variable stages in the IP compressor, which would reduce engine weight and complexity," Howse said.
ACTIVE MAGNETIC bearings also are being examined under Antle. In operation, the bearing's monitoring systems would be able to sense if a low-pressure engine shaft is moving out of alignment and, by applying magnetic energy, coax the errant shaft back into place. According to Howse, using the bearings also would help eliminate complex oil systems. Perhaps more importantly, the bearings would free up space, allowing engineers to add a motor generator in the low-pressure shaft, simplifying the mechanical complexity of today's power generating systems.
Current engines usually have a shaft that runs from the high-pressure spool to a gearbox. Mounted on the gearbox are engine accessories and a generator that provides AC power to the aircraft's electrical system. In the case of the 7E7, power demands are likely to be large, probably exceeding one megawatt.
An alternative to the gearbox-mounted generator is one located at the rear of an engine, either on the low-pressure or intermediate-pressure spool. Attaching the generator to the low-pressure spool would allow engineers to eliminate the redundant, expensive ram air turbines that are now utilized to generate electricity under emergency conditions. "If the generator is attached to the low-pressure spool, a windmilling fan could be used to drive the generator, " Howse said.
Similarly, connecting the generator to the intermediate-pressure spool--either mechanically or electrically--would provide another range of benefits. "This offloads the IP system so you get more capacity from the IP compressor. This allows you to reduce fan speeds, which leads to a reduction in blade count and an increase in efficiency," he explained.
Noise-reduction technologies are not a part of Antle, and Rolls has been pursuing them under two separate projects. The first, the Quiet Technology Demonstrator (QTD), was conducted with Boeing and examined serrated nozzles and advanced acoustic liners as ways to cut engine noise. Flight tests in October 2001 on a Boeing 777, and later tests run at Rolls sites in 2002, showed a jet noise reduction of 4 dB. and a fan noise reduction of 13 dB. Targets were 3 dB. and 7 dB., respectively.
Current noise-reduction work is being pursued under a European technology effort dubbed Silencer, signifying "significantly lower aircraft environmental noise community exposure." Airbus, Snecma, ITP, Hurel-Hispano and several research establishments and universities are participating in the project. Rig tests are underway, with flight tests expected to follow. Work is focused on low-noise, low-pressure turbines, nozzle treatments and innovative inlets.
Rolls is involved in some rig testing and is providing engine pieces, but much of the company's Silencer efforts are directed at a negative scarf inlet--one in which the lower portion projects out beyond the upper portion, directing inlet noise upward. Analytical work conducted by the engine maker on this inlet is aimed at minimizing aerodynamic losses caused by the nacelle. However, it's also important, Howse noted, that fan analysis be performed "to make sure the fan behaves properly in crosswinds."