For more than 65 years, NASA (and its predecessor, NACA), the Pentagon, and various defense contractors have collaborated on the experimental X-Plane program, testing innovative technologies and looking at cutting-edge engineering concepts that push the boundaries of flight.
This week, at a test range over the Pacific Ocean, aerospace engineers had hoped that a new generation of X-marked aircraft, the unmanned X-51A Waverider, would be able to fly at Mach 6 -- six times the speed of sound, or roughly 4,000 miles per hour -- for a sustained 300 seconds.
That was not to be. The Air Force said today that yesterday's bid by the X-51A ended abruptly about 31 seconds into free flight after "a fault was identified with one of the cruiser control fins."
Yesterday's test wasn't the first free flight for a scramjet-powered X-51A. In May 2010, an X-51A flew for just over three minutes, getting up to nearly Mach 5.
Heading aloft attached to a B-52, as in this picture, the X-51A would have been released about 50,000 feet above the Pacific Ocean, and its solid rocket boosters were to accelerate the vehicle to about Mach 4.5. If things had gone according to plan yesterday, the scramjet engine would have been activated and the X-51A would have climbed to 70,000 feet and to Mach 6.
Boeing built four X-51A aircraft for the Air Force. After yesterday's test flight, and the two before that, just one remains. The Air Force said today it has not yet decided whether that remaining airframe will fly.
The X-Plane program, of course, has had a long record of success, along with inevitable failures. The X-1 aircraft, in which Chuck Yeager first broke the sound barrier, in 1947, pioneered many structural and aerodynamic advances proving that humans could go beyond the speed of sound, and reinforcing the idea that technological barriers could be overcome.
This 1955 photograph shows the D-558-2 "Skyrocket," an all-rocket-powered aircraft, at Rogers Dry Lakebed at Edwards Air Force Base. This aircraft set a speed record in 1953 when it was the first to reach Mach 2. (A handful of experimental aircraft, the D-558-1 and -2 among them, did not bear the "X" label but, says NASA, they were clearly intended for the same purpose.)
The rocket-powered X-2 was the first aircraft to reach Mach 3, recording a Mach 3.2 speed in September 1956. That same flight destroyed the plane and killed pilot Mel Apt, as the result of a potentially lethal phenomenon known as inertial coupling. The X-2 was also the first aircraft to fly higher than 100,000 feet, reaching 126,200 feet about three weeks before that final, fateful flight.
During 199 flights between 1959 and 1968, the X-15 was one of the most remarkable of all the rocket research aircraft -- and the fastest piloted aircraft ever. The X-15 first set speed records in the Mach 4 to 6 range with Mach 4.43 in March 1961; Mach 5.27 in June 1961; Mach 6.04 in November 1961; and Mach 6.7 on October 3, 1967.
The X-15 also set an altitude record of 354,200 feet in August 1963, and gathered enormous amounts of data on hypersonic air flow, aerodynamic heating, control and stability at hypersonic speeds, reaction controls for flight above the atmosphere, piloting techniques for re-entry, human factors, and flight instrumentation of relevance not only to aeronautics but to spaceflight.
Not every X-Plane is built for speed. The X-37B, for instance, is giving the Air Force a chance to see what it might be able to get out of an unmanned space plane. One clear ability: endurance. On June 16 of this year, an X-37B returned to Earth after spending 469 days (a year and a quarter) in orbit, easily topping the record set by a sibling vehicle in December 2010.