February 17, 2014
Credit: Lockheed Martin Concept
Traditionally, performance drives military-aircraft design decisions and the energy implications of those choices are secondary. But as fuel costs eat into reduced budgets, the balance is shifting. Energy is fast becoming a critical constraint on operations, and the results could reshape aircraft design.
For now, the U.S. Air Force's efforts to cut fuel bills are focused on its transport and tanker fleet, which consumes two-thirds of the aviation fuel the service burns each year. While near-term retrofits—such as formation flying, winglets and other drag-reduction devices—can reduce the fuel consumption of existing aircraft, they will not provide the scale of savings sought in the long term.
The name of the Air Force Research Laboratory's (AFRL) Revolutionary Configurations for Energy Efficiency (RCEE) program says it all: Dramatic changes in aircraft design may be required to achieve significant reductions in fuel consumption.
The goal of RCEE Phase 1, which ran from 2009-11, was to define a next-generation mobility fleet that would use 90% less fuel than today's transports and tankers. Under Phase 2, which began in 2011 and will run until 2015, companies are taking a closer look at specific configurations.
In Phase 1, Boeing defined a mixed fleet that met the 90% savings target: an all-electric truss-braced-wing design with 20-metric-ton payload; a 40-ton-payload distributed-thrust hybrid-electric design; and a 100-ton payload hybrid-electric blended wing-body (BWB). In Phase 2, the company is taking a closer look at the distributed-thrust, hybrid-propulsion design.
Lockheed Martin, meanwhile, studied a wide range of configurations and technologies in Phase 1 in search of the 90% goal, concluding a hybrid wing-body (HWB) offers the most potential. In Phase 2, the company is further refining the concept, which combines a blended wing and forebody for aerodynamic and structural efficiency with a conventional aft fuselage and tail for compatibility with current airlift missions, including airdrop.
The twin-engine HWB is designed to take off in less than 6,500 ft. and fly 3,200 nm carrying 220,000 lb. of payload, including all the outsize cargo now airlifted by the Lockheed C-5. Lockheed calculates the aircraft will burn 70% less fuel than the Boeing C-17 through a combination of better aerodynamics, newer engines and lighter structures. “We use mature technologies to be affordable and could build it today,” says Rick Hooker, an aeronautical engineer at Lockheed Martin Aeronautics