The X-48C is a rebuilt twin-engined derivative of the three-engine X-48B tested between 2007 and 2011 in 92 flights. In this new configuration, the engines are mounted further forward from the trailing edge and shielded between vertical tails which are moved inboard and replace wingtip-mounted tails. The low-noise design is “a configuration of the BWB that represents a vehicle we are actually studying,” says Boeing X-48B/C project manager Mike Kisska.
The sub-scale aircraft is being studied as a potential future military and commercial airlifter/tanker, as well as for possible strike and reconnaissance roles. The design, which is also under evaluation by Boeing as part of NASA's Environmentally Responsible Aviation (ERA) program, has shown the potential for up to 50% lower fuel burn and 40 dB less noise than a similarly sized tube-and-wing aircraft.
Initial testing of the first-generation X-48B explored the low-speed stability and control characteristics of its unusual design over 92 flights. Boeing was particularly keen to demonstrate that the BWB design was viable and safe, and not prone to stalls at high angles of attack. An advanced flight- control system was tested to its limits during the early phases with the X-48B and showed it was possible to maintain control at both high angles of attack and high sideslip angles.
The upcoming testing with the new configuration “gives us the opportunity for back-to-back testing,” says Kisska, who adds that Boeing “is looking to start flying toward the middle or late next month [July].” The test effort will run to December and include around 25 flights.
Boeing originally hoped to develop purpose-designed miniature turbofans to power the X-48C but was forced to adopt turbojets developed by Advanced Microturbo (AMT) of Geldrop, Netherlands, when this proved problematic. The AMT turbojets replace the X-48B's original trio of JetCat P200s, and are enclosed in the larger nacelle ducts representative of the higher-bypass engines which were to have powered the X-48C.
The company has therefore shelved plans to validate the noise characteristics of the low-noise configuration and Kisska confirms that the use of the low- bypass AMTs rules out any acoustic evaluation. Longer term, Boeing hopes to use the X-48C as a stepping stone to tests of a scaled BWB demonstrator vehicle “large enough for a pilot to fly.”
The advanced technology unit is trying to secure other flight-test work, with its eyes on a NASA downselect in the Reusable Booster System program. The agency is expected to name one company to build a pathfinder demonstrator to validate the boost technology around 2015.
Boeing also recently submitted a proposal for NASA's solar-electric propulsion demonstration program and, next year, expects to compete for the next phase of the cryogenic propellant storage and transfer program NASA is running, notes Steve Johnston, director for advanced space exploration at Phantom Works.