WHAT WAS ACHIEVED?

Initially the primary justification for a manned research airplane was the choking problems of the wind tunnels, but, as it turned out, this limitation disappeared prior to the beginning of high­speed flight tests. Although this largely eliminated the need for the X-planes, it is unlikely that the progress in developing transonic ground facilities would have occurred without the stimulus begun by the X-1 and D-558. Clearly, there was an important two-way flow of benefits. Stimulated by the problems encountered by the research airplanes during flight, researchers created new ground facilities and techniques that in turn provided the data necessary to develop yet faster airplanes. Comparing the results of flight tests at ever-increasing speeds allowed the wind tunnels to be refined, producing yet better data. It was a repetitive loop.-122

The programs proceeded remarkably rapidly, and the first supersonic flights showed nothing particularly unexpected, much to the relief of the researchers. The most basic result, however, was dispelling the myth of the "sound barrier." The fearsome transonic zone became an ordinary engineering problem, and allowed the designers of operational supersonic aircraft to proceed with much greater confidence.-1231

When people think of X-planes, record-setting vehicles like the X-1 generally come to mind. In reality, most X-planes investigated much more mundane flight regimes, and there were only a handful of high-speed manned experimental aircraft, built mainly during the late 1940s and early 1950s. Specifically, there were five designs (only three of which carried X" designations) intended for the initial manned assault on high-speed flight: the Bell X-1 series, the Bell X-2, the Douglas D-558-1 Skystreaks, the Douglas D-558-2 Skyrockets, and the North American X-15. Of the five, one probed high subsonic speeds, two were supersonic, and one pushed the envelope to Mach 3. The fifth design would go much faster.-124

The X-planes gave aviation its first experience with controlled supersonic flight. On 14 October 1947, Air Force Captain Charles E. Yeager became the first human to break the sound barrier in level flight when the XS-1 achieved Mach 1.06 at 43,000 feet. It took six additional years before NACA test pilot A. Scott Crossfield exceeded Mach 2 in the D558-2 Skyrocket on 20 November 1953. The Bell X-2 proved to be the fastest and highest-flying of the "round one" X-planes and the most tragic, with the two X-2s logging only 20 glide and powered flights between them. Nevertheless, Captain Iven C. Kincheloe, Jr., managed to take one of the airplanes to 126,200 feet on 7 September 1956. Twenty days later, Captain Milburn G. Apt was killed during his first X-2 flight after he reached Mach 3.196 (1,701 mph), becoming the first person to fly at three times the speed of sound, albeit briefly.1251

The contributions of the early high-speed X-planes were questionable, and the subject of great debate within the NACA and the aircraft industry. Opinions on how successful they were depend largely on where one worked. The academics and laboratory researchers, and a couple of aerospace-industry designers, are on record indicating the contributions of the X-planes were minimal. On the other side, however, many of the hands-on researchers and pilots are certain the programs provided solid, real-world data that greatly accelerated progress in the design and manufacture of the Mach 1 and Mach 2 combat aircraft that followed.-1261

For instance, the X-1 was the first aircraft to purposely break the sound barrier in level flight, but other aircraft were doing so in shallow dives soon afterwards.1271 The first combat type designed from the start as a supersonic fighter—the Republic XF-91 "Thunderceptor’—made its maiden flight only 19 months after Yeager’s flight. How much the X-1 experience contributed to Alexander Kartveli’s design is unknown.1281 The same thing happened at Mach 2. By the time Scott Crossfield took a D-558-2 to twice the speed of sound, Kelly Johnson at Lockheed had already been developing what would become the F-104 Starfighter for over a year. It is unlikely that the rocket-powered X-planes actually assisted Johnson much—something he would make clear during later deliberations.1291

The X-1E complemented the heating research undertaken by the X-1B, but the F-104 was already flying and could more easily acquire data at Mach 2. Even at the Flight Research Center (FRC), there was debate over how appropriate this exercise was. FRC research engineer Gene Matranga later recalled, "We could probably fly the X-1E two or three times a month, whereas Kelly [Johnson] was flying his F-104s two or three times a day into the same flight regimes, so it really didn’t make sense for us to be applying those kinds of resources to [obtain] that kind of information." However, it is unfair to judge the X-1E program too harshly since its major purpose was simply to keep a cadre of rocket-powered experience at the FRC in anticipation of the upcoming X-15.1301

Even John Becker recognized the dichotomy represented by the experience: "[T]he cooperative research-airplane program pursued by the Air Force, NACA, and Navy had not been an unqualified success…. Some had lagged so seriously in procurement that their designs had become obsolescent before they were flown. In a few cases tactical designs superior to the research aircraft were in hand before the research aircraft flew." It was not anybody’s fault— technology was simply changing too fast. Trying to sort out the detailed story is nearly impossible and well beyond the scope of this book.1311

Nevertheless, although most believed that the concept of a dedicated research airplane still held promise, researchers decided that the next design would need to offer a significant increment in performance to leapfrog the combat types then in development. Chuck Yeager’s October 1947 assault on the sound barrier had ignited a billion-dollar race to build ever-faster aircraft, and directly affected every combat aircraft design for the next two decades. However, a few

aeronautical researchers had always been certain that the sound barrier was simply a challenge for the engineers, not a true physical limitation. The X-1 had proven it was possible for humans to fly supersonically. The next goal was so much faster.

The X-1E was the last rocket-powered X-plane at the NACA High-Speed Flight Station until the arrival of the three X-15s. There is considerable debate over the economics of flying the X-1E given that some jet-powered aircraft could attain the same velocities, but the primary purpose of the X-1E was to maintain a cadre of rocket experience at the HSFS pending the arrival of the X – 15. (NASA)