It was the golden age of flight experimentation when Scott Crossfield arrived at Edwards AFB, California, in 1950. The soon-to-be legendary test pilot likened the atmosphere to an “Indianapolis 500 without rules”, as each flight attempted something – a speed, an altitude or some combination – that no human had ever before achieved.

In a 12-year span between the Bell X-1 and the North American X-15, the human speed record leaped from Mach 1.0 to Mach 6.7. Not surprisingly, commercial aviation roughly doubled in speed over the same time, as airline fleets transitioned from piston power to jet propulsion.

The goals of NASA’s newly announced plan to revive the manned X-plane tradition may not seem quite so ambitious as the X-15, but it is also no mere nod to nostalgia. For the time being, there is no need to expand the boundaries of the human flight envelope of altitude and speed, which were set almost entirely over the Mojave desert during the 1950s. NASA is instead focusing on a narrower set of achievable goals that might transform the air travel experience in decades to come.

The Anglo-French Concorde proved that a niche segment of the air transport market prizes speed over cost. Such a segment still exists, but supersonic speed may be unobtainable as long as sonic booms are outlawed over land.

Technology now available should be able to muffle the sound of the boom, but laws will not change until the scientific principle is proven using a flying demonstrator. NASA’s low-boom sonic demonstrator – if approved by Congress – could unlock the supersonic market for business jets after a decade. Another generation of aerodynamic optimisation may yield the same speed benefit for commercial airliners.

NASA’s proposal also includes a request to launch a subsonic X-plane. The candidates include a hybrid wing-body, a double-bubble fuselage and a strut-based wing. Nothing in these designs will make commercial air travel any faster, but they represent aviation’s best hope for making the biggest leap in fuel efficiency since the advent of the high-bypass turbofan engine in the late 1960s.

Of the three concepts, NASA’s scientists may prefer the hybrid wing-body, but the outlook for such a design in the commercial market is doubtful. As a commercial technology, the most promising candidate may be the strut-braced wing, but only if concerns about flutter and control can be overcome.

Source: Flight International