NASA considers next steps in green airliner concept study as budget squeeze looms
When asked the three largest U.S. airframe makers to study advanced concepts for next-generation ultra-efficient airliners so quiet they could barely be heard beyond the airport boundary, it did not count on so many surprises.
But surprises there were when, and presented their final reports to 's Environmentally Responsible Aviation (ERA) program last week. All submitted preferred system concepts which either met, or closely matched, NASA's stringent noise, emissions and fuel-burn targets for airliners entering service in the late 2020s. Although the fact that each concept scored high marks was not unexpected, it was the array of unanticipated technologies, innovations and system attributes used by the teams that surprised the agency.
In addition to an unconventional flying-wing design from Northrop Grumman, and an innovativeengine with an extremely large fan powering Lockheed Martin's box-wing concept, the studies unexpectedly underlined the significant benefits that would accrue from flying advanced airliners within the 's NextGen airspace system, NASA says.
All too expected, however, is the looming budget squeeze that is forcing the agency to reconsider its ambitions for a flying demonstrator, or scaled testbed vehicle (STV). Planned as a roughly half-sized test version to provide realistic aero-acoustic data, the STV was to have been based on one of the three preferred concepts, and was penciled in for first flight in 2018. The test program, based at Edwards AFB, Calif, would have helped raise relevant technologies to a development-ready level by 2019 in time for service entry by 2025.
To maximize the value of STV, NASA asked companies for designs with a 10,000-hr., 20-year research life. Initially aimed at flying for the ERA program up until 2020, the-sized STV was also to be provisioned to fly autonomously as part of plans to test unmanned aircraft systems in national airspace in 2020-25. Following this, a third career was envisioned for the STV as a testbed for ERA's sister effort, the subsonic fixed-wing (SFW) fundamental research program, from 2025-30.
Although details are scant, the ERA program is looking at fresh options to enable some form of STV to proceed. The revised vehicle would likely be less ambitious in both scale and technology to reduce costs, and would potentially involve partnerships with the U.S. Air Force and industry. Unlike the original STV—planned purely as a commercial demonstrator—the revised vehicle could also be a multirole transport technology testbed with military-airlift potential. ERA insiders say the viability of this depends on gaining high-level advocacy within NASA and possibly other government agencies.
Begun in fiscal 2010, the ERA program is at the midway point as plans are completed for a second phase, which will take it through fiscal 2015. Program manager Fayette Collier says progress on the broader aspects of Phase 1 has been substantial and adds that technology road maps and priority targets for Phase 2 will be shaped partly by the results of the vehicle concept studies. NASA has “kicked off a tiger team for planning Phase 2,” he says, adding that the agency “hopes to get authority to proceed in August.”
Key targets for the program include reducing noise by a cumulative 42 db compared to current Stage 4 levels, cutting emissions of nitrous oxides by 75% on takeoff and landing and 70% in cruise, and slashing fuel consumption by 50% relative to a 1998 technology baseline. Phase I research includes efforts to lower carbon emissions via drag, weight and fuel-burn reductions, and to make airport operations quieter by tackling sources of airframe and propulsion noise. Combustor and fuel-system work is focused on reducing emissions.
Details of the aircraft concepts, and the proposed demonstrators, were revealed at the American Institute of Aeronautics and Astronautics Aero-sciences conference in Nashville, Tenn., last week. Each company also was asked to study how their design could best fit into the FAA's NextGen airspace system as well as meet specific performance requirements. For the passenger model, 224 seats, a 50,000-lb. payload and an 8,000-nm range at Mach 0.85 were called for, while the freighter was required to carry a 100,000-lb. payload over a range of 6,500 nm. Each team was also asked to sketch out a 15-year technology maturation road map and propose critical technology demonstrations for the second half of the ERA program.
A Boeing-led team including Pratt & Whitney, Rolls-Royce, the Massachusetts Institute of Technology and Cranfield Aerospace, developed a preferred concept based on a blended wing body (BWB) with twin geared turbofans (GTF). Although a parallel version with triple open-rotor engines showed better fuel burn, it was 8 db noisier than the GTF-powered version. The geared-turbofan design achieved a 52% fuel-burn reduction, beating the target, but it still fell short of NASA's noise goal by realizing only a 34-db reduction.
Boeing ERA program manager John Bonet says: “With an advanced landing gear and slat-noise reduction technology we can meet the goal of -42 db. We noticed that jet noise is so low on the BWB that other sources become dominant. Airframe noise reductions are the only ones that will allow us to meet the noise goals.” In addition, Boeing assumed a 14% fuel-burn benefit from detailed analysis of operating in the NextGen airspace.
Collier says: “There were a number of things I hadn't anticipated, and one was the benefit of [NextGen improvements to the] national airspace system. That's a low-hanging fruit maybe, and confirms a number that came out of our colleagues in the SWF program.”
Boeing's demonstrator, originally planned for assembly in 2017, is a 65%-scale version of the preferred concept. The 149-ft.-span, 83-ft.-long, 21-ft.-high vehicle is configured with twin 24,000-lb.-thrust Pratt & Whitney PW1000Gs. With a modular, stitched resin-infused composite structure, it would have commercial-off-the-shelf landing gear and a modified business-jet flightdeck with modular electronics. The wing would not be initially be equipped for drag-reducing laminar flow, but “would support spiral development,” says Bonet.
Lockheed Martin's team, which included Rolls-Royce and the Georgia Institute of Technology, proposed an unconventional box-wing concept with two ultra-high-bypass engines suspended from the aft wing. The 181-ft.-long, 171-ft.-span design is optimized for low drag and reduced fuel burn, says program manager Kenneth Martin. With potential military roles in mind, Lockheed says the design also offers “scalability from tactical to strategic, as well as reduced span for compatibility with the existing infrastructure.”
With an advanced composite structure, the lightweight airframe has a maximum takeoff weight of 365,900 lb. versus the 550,400 lb. of an equivalent 1998 technology-standard aircraft. Fuel weight is similarly reduced, being less than half that of the 250,000 lb. required by the baseline design.
The high-aspect-ratio wing design, the feasibility of which Lockheed acknowledges is wholly dependent on advances in composite structures, provides the capability of making steep, 6-deg. approaches to help contain noise within the airport boundary. Partially because of this, the design achieves -35 db, while engine emissions beat the target by coming in at -89% relative to current Committee on Aviation Environmental Protection (CAEP)/6 standards.
A major surprise of Lockheed Martin's concept is the Rolls-designed UltraFan engine. Conceived as a simple shrouded fan, the engine has developed into a hybrid between current advanced turbofans and open rotors, says Rolls. With an extremely large 138-144-in.-dia. fan, the engine will be geared and possibly configured with three shafts. Rated at 63,600-lb. takeoff thrust, the UltraFan is encased in a slimline, natural laminar-flow, nacelle without a thrust reverser. Collier says: “We thought we knew where things were going with the engine companies, but the UltraFan concept came out of the blue. That Lockheed had a concept that could handle this was also surprising.”
Further optimizations will include airframe noise-reduction technology including continuous-moldline flaps, landing-gear fairings, slat fillers and shape-memory alloy serrations on the engine bypass-duct exit.
Lockheed's proposed demonstrator is a 50%-scale vehicle, measuring 125 ft. in length and 99 ft. in span with a maximum takeoff weight of 162,500 lb. Powered by unspecified 45,000-lb.-thrust engines, the aircraft would have a-based cockpit with open-architecture mission systems to support avionics and autonomous system upgrades.
Northrop Grumman's flying-wing concept, proposed by a team including Rolls-Royce, Wyle Laboratories and Iowa State University, is based directly on Northrop's B-2A bomber heritage. The passenger version has a 260-ft. span and a wide centerbody cabin, while the freighter variant has a 230-ft. span. Both are 119 ft. long.
With a flying wing, the cabin layout “drives the centerbody, and the propulsion system is integrated with the flow path and side clearances,” says Northrop Grumman ERA program manager Aaron Drake.
“Where we really get the big benefits is in noise, which is substantially better than the baseline,” says Drake. Noise reduction is predicted to be around -74.7 db, emissions 88% below current levels and fuel burn 41.5% below 1998 levels. “The biggest benefit is in the advanced propulsion, which provides 20% of overall improvement; second is swept-wing laminar flow, which contributes around 8.3%,” he adds. Key design concepts include composite wing, embedded high-bypass engines, advanced inlets as well as maneuver load alleviation and carbon-nanotube data cables.
Northrop's 55%-scale STV design is 143 ft. in span, 65.7 ft. in length, and powered by fourTech X future regional-jet engines.