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What might future commercial airplane designs look like 25 to 30 years from now?
Two teams led by Boeing Research & Technology have completed 18-month studies on that question and have submitted their findings to NASA under a program called N+3, which denotes three generations beyond the current transport fleet.
After examining various subsonic and supersonic concepts, the teams have come up with potential configurations that may offer dramatic improvements in operational and environmental performance over the aircraft of today to meet aggressive goals set by NASA.
The Boeing subsonic team, which includes BR&T, Boeing Commercial Airplanes, General Electric and Georgia Tech, has looked at five concepts as part of the SUGAR (Subsonic Ultra Green Aircraft Research) project. The concepts include two conventional reference configurations, similar in appearance to a 737 (nicknamed SUGAR Free and Refined SUGAR), two versions of a new design high span, strut-braced wing aircraft (referred to as SUGAR High and SUGAR Volt), and a hybrid wing body configuration (called SUGAR Ray).
The team’s report provides detailed benefits and drawbacks as well as recommendations for further study, but doesn’t show favorites. “No single concept met all of the study goals, so we did not pick a preferred concept,” said team leader Marty Bradley of BR&T.
For the subsonic concept, hybrid electric engine technology "is a clear winner because it can potentially improve performance relative to all of the NASA goals."
However, the team has found that the SUGAR Volt concept (which adds an electric battery gas turbine hybrid propulsion system) can reduce fuel burn by greater than 70 percent and total energy use by 55 percent when battery energy is included. Moreover, the fuel burn reduction and the ‘greening’ of the electrical power grid can produce large reductions in emissions of life cycle CO2 and nitrous oxide. Hybrid electric propulsion also has the potential to shorten takeoff distance and reduce noise.
The SUGAR team’s report concludes that hybrid electric engine technology “is a clear winner, because it can potentially improve performance relative to all of the NASA goals.”
However, Bradley said, in order for the hybrid electric concept to be competitive, battery technology “needs to improve many, many times over what we have today. Battery technology is being worked around the world, especially in the auto and electronics industries. We need to leverage that work to see if we can get the improvement we need in an aviation compatible package.”
The SUGAR team identified hybrid electric engine technology as a “high-risk high-payoff technology,” Bradley said. “At this point, the SUGAR Volt is only a concept configuration that we are using to assess the potential of hybrid electric engine technology.”
For conventional propulsion, a combination of improvements to air traffic management, airframe and propulsion could reduce fuel burn by 44 to 58 percent, the SUGAR team’s report says. Other improvements include use of sustainable biofuels, which could reduce CO2 emissions even more and use of advanced combustor technology, which could reduce nitrous oxide emissions by 75 percent.
For noise reduction, “the best performing concept is the SUGAR Ray (the hybrid wing body), which achieved a 37 decibel reduction relative to today’s aircraft,” said Bradley. “That’s well short of the NASA goal, so more work needs to be done in this area.”
The Boeing SUGAR team was one of four that received contracts from NASA in 2008 to study subsonic concepts for the 2030 to 2035 timeframe. The other teams were led by GE Aviation, Massachusetts Institute of Technology and Northrop Grumman.
All four teams have submitted proposals for a second phase of studies to begin developing new technologies that will be necessary to meet the national goals related to an improved air transportation system with increased energy efficiency. Contract award for Phase II, which will start later this year, is expected in the next few months.
The Boeing supersonic team, which includes BR&T, BCA, Pratt & Whitney, Rolls Royce, General Electric, Georgia Tech, Wyle and M4 Engineering, has focused on four concepts that include a low fuel burn / low boom swing-wing “arrow” configuration, a low sonic boom concept with a V-tail to shield noise and control the sonic boom, a joined wing alternate concept and an oblique “scissor” wing alternative concept.
Based on conceptual design studies, the team has recommended to NASA a fixed wing configuration (nicknamed Icon II) with V-tails and upper surface engines as the technology reference concept plane for N+3, said team leader Bob Welge of BR&T. The Icon II concept can carry 120 passengers in a two-class, single-aisle interior, and can cruise at Mach 1.6 to Mach 1.8 with a range of about 5,000 nautical miles.
The study acknowledges that supersonic aircraft inherently have less fuel efficiency than subsonic aircraft, but points out they offer offsetting productivity benefits because of speed. The study concludes that advanced technologies can reduce fuel burn enough that a supersonic aircraft could be viable, economically and environmentally, in multiple markets.
The study also indicates that these efficiencies can be achieved while meeting the same community noise certification limits as subsonic aircraft – with a reduction of the sonic boom noise en route to 65 to 75 decibels. “That may make it possible for a supersonic transport to operate at maximum cruise speed -- even over land,” Welge said.
The Boeing-led team was one of two that received contracts from NASA to study supersonic concepts. The other was led by Lockheed Martin.
The NASA N+3 supersonic program does not provide the option for a Phase II system study, but Welge explained that technology development research announcements are anticipated in the near term.