X-PLANES

As it happened, John Stack had already considered other alternatives. The idea of a modern research airplane—one designed strictly to probe unknown flight regimes—came in a 1933 proposal by Stack. On his own initiative, Stack went through a preliminary analysis for "a hypothetical airplane which, however, is not beyond the limits of possibility" to fly well into the compressibility regime. Stack calculated that a small airplane using a 2,300-horsepower Rolls – Royce piston engine could obtain 566 mph in level flight—far beyond that of any airplane flying at the time. Ultimately, the NACA did not pursue the suggestion, and it would be another decade before the idea would come of age.-131

Ezra Kotcher at the Army Air Corps Engineering School at Wright Field made the next proposal for a high-speed research airplane. In 1939 Kotcher pointed out the unknown aspects of the transonic flight regime and the problems associated with the effects of compressibility. He further discussed the limitations of existing wind tunnels and advised that a full-scale flight research program would be an appropriate precaution. By early 1941 John Stack had confirmed that data from wind tunnels operating near Mach 1 were essentially worthless because of a choking problem in the test section. He again concluded that the only way to gather meaningful data near the speed of sound would be to build a vehicle that could fly in that regime. Again, no action resulted from either Kotcher’s or Stack’s suggestions and determining the effects of compressibility on airplanes remained a largely theoretical pursuit.-141

The real world intervened in November 1941 when Lockheed test pilot Ralph Virden died trying to pull a P-38 Lightning out of a high-speed dive that penetrated well into the compressibility regime. By 1942 the diving speed of the new generation of fighters exceeded the choking speed of the wind tunnels then in use. Researchers increasingly supported the idea of an instrumented airplane operating at high subsonic speeds. Those involved do not remember that any one individual specifically championed this idea, but John Stack soon became the chief Langley proponent.151

Interestingly, there was little interest within the NACA in flying through the sound barrier. It appeared that one of the early turbojet engines could push a small airplane to about Mach 0.9, but the only near-term way to go faster was to use a rocket engine—something that was considered too risky by the NACA.

A posed group portrait of early X-planes at the NACA High-Speed Flight Station in August 1953. Clockwise from the bottom are the Douglas D55-1, Douglas D-558-2, Northrop X-4, Convair XF-92A, and Bell X-5. This group represents a wide variety of research programs, and only the D558-2 was a true high-speed airplane. (NASA)

The Army, however, wanted a supersonic airplane and appeared willing to accept rocket propulsion. In fact, Ezra Kotcher had listed this as an option in his 1939 proposal, and it became increasingly obvious that a rocket engine represented the only hope for achieving supersonic speeds in level flight in the near future.-1161

Possible Navy interest in the undertaking also appeared during 1942-1944. However, significant differences of opinion came to the forefront during a 15 March 1944 meeting of Army, NACA, and Navy personnel. The NACA thought of the airplane as a facility for collecting high-subsonic speed aerodynamic data that were unobtainable in wind tunnels, while the Army thought it was a step toward achieving a supersonic combat aircraft. The Navy supported both views, wanting to dispel the myth of the impenetrable sound barrier, but was also interested in gathering meaningful high-speed data. Despite the NACA’s concerns, the Army soon announced its intention to develop a rocket-powered research airplane.-1171

As John Becker remembers, "The NACA continued to emphasize the assumed safety aspects and relatively long-duration data-gathering flights possible with a turbojet engine compared to the short flights of any reasonably sized rocket plane. Furthermore, the turbojet would have obvious applicability to future military aircraft while the rocket propulsion system might not. This apparently irreconcilable difference was easily resolved; the Army was putting up the money and they decided to do it their way.’1181

The beginning of supersonic flight research likely occurred when Robert J. Woods from Bell Aircraft met with Ezra Kotcher at Wright Field on 30 November 1944. After they discussed the basic specifications, Kotcher asked Woods if Bell was interested in designing and building the airplane. Woods said yes, and in late December Bell began contract negotiations with the Army to build the rocket-powered XS-1 research airplane.-119

Melvin N. Gough, the chief test pilot at Langley, dismissed the rocket-plane concept: "No NACA pilot will ever be permitted to fly an airplane powered by a damned firecracker." When it became clear in early 1944 that the Army was going to insist on rocket propulsion, John Stack began lobbying the Navy to procure the type of airplane the NACA wanted. The Navy was more receptive to the turbojet-powered airplane, and the Navy Bureau of Aeronautics (BuAer) began negotiations with Douglas Aircraft for the D-558 Skystreak in early 1945.[20]

These were the beginnings of the cooperative research airplane program. In reality, until the advent of the X-15 there were two distinct programs: one with the Army and one with the Navy. Just because the NACA did not agree with the path the Army had elected to pursue did not mean the Agency would not cooperate fully in the development of the XS-1. The Navy enjoyed the same level of cooperation for the D-558. John Stack noted in 1951 that "the research airplane program has been a cooperative venture from the start…. The extent of the cooperation is best illustrated by the fact that the X-1, sponsored by the Air Force, is powered with a Navy-sponsored rocket engine, and the D-558-1, sponsored by the Navy, is powered with an Air Force-sponsored turbojet engine." [21]