NASA is now focusing on cheaper alternatives than an X-plane to demonstrate how new propulsion concepts and a radical departure from today's "tube-and-wing" airframe design can reduce fuel burn by 50%.
As a five-year campaign called "environmentally responsible aviation" (ERA) winds down by Fiscal 2015, NASA expects to have completed ground tests of several key components and validated a type of flying-wing airliner called a "Hybrid Wing Body" (HWB) in simulation.
But the agency's aeronautics research branch still has no funding to take the critical next step and develop a subscale flying prototype, says Fay Collier, ERA programme manager, speaking at the joint propulsion conference organised by the American Institute for Aeronautics and Astronautics.
An X-plane "is a difficult thing to discuss in this austere environment", Collier says. The focus instead has shifted to considering cheaper alternatives to an X-plane, but Collier did not elaborate.
Less than two years ago, however, Collier's team was preparing to launch such a follow-on flight demonstrator to take the ERA-funded technologies, including the pairing of a HWB airframe with a high-bypass ratio turbofan or open rotor, to a technology readiness level of six, which qualifies the integrated system to enter a commercial development programme.
By January 2012, Collier acknowledged that NASA had "taken our foot off the gas" of such a sub-scale test vehicle, estimated to be about the size of a Boeing 737.
NASA is continuing to consider funding X-plane flight demonstrators, but the priority appears to have shifted away from subsonic technology. Two months ago, Collier's colleagues in the agency's high-speed research programme said they were preparing an internal proposal for NASA to fund a supersonic X-plane project starting in FY2015.
The absence of a subsonic flight demonstrator will make it harder for NASA to achieve the ERA programme's goals, which is to introduce technology by 2020 with the capability of reducing fuel burn by 50% and noise levels by 42db below the Stage 4 standards published by ICAO.
Michael Winter, Pratt & Whitney's chief engineer for technology, said a flight demonstrator is critical to move the integrated HWB airframe and propulsion technology beyond the laboratory stage. Winter cited the example of the P&W PurePower PW1000G engine series. It wasn't until Airbus began flying the geared turbofan engine on an A320 that new technology was truly accepted by the market.
At the same time, it is not clear that there is a viable future for the HWB concept. Boeing Commercial Airplanes has been adamant that it sees no role for the HWB as a passenger aircraft, despite the fact that Boeing owns the intellectual property since the idea was invented by McDonnell Douglas engineer Robert Liebeck in the early 1990s.
One possible option is a military transport to replace the Lockheed Martin C-130 fleet, although the US Air Force has no plans to introduce such an aircraft before 2030.