Less than a month after the action of the Executive Committee,  Henry J. E. Reid, Director of the Langley Aeronautical Laboratory, appointed Clinton E. Brown, Charles H. Zimmerman, and William J. O'Sullivan, aeronautical engineers at the Virginia center, to work up a thorough proposal for research in upper-atmospheric and space flight. Specifically the Langley engineers were to suggest a suitable manned vehicle on which construction could be initiated within two years. Their proposal was to be reviewed by a board composed of representatives from the three NACA laboratories and NACA's High Speed Flight Station at Edwards Air Force Base, California.7
Throughout the next year and a half, the Langley study group, engineers at Ames and the flight station, and the review board worked on a plan for the new research instrument. There was wide divergence of opinion as to what should be the nature and objectives of the vehicle; some parties were even skeptical about the wisdom of any space-directed research. Reid, John Stack, and others at Langley favored modifying the X-2 research airplane, then under development by Bell Aircraft, to make it a device for manned flight above 12 miles.8 Smith J. DeFrance, one of the early Langley engineers who had become Director of the Ames Aeronautical Laboratory when it opened in 1941, originally opposed Woods' idea for a study group on space flight because "it appears to verge on the developmental, and there is a question as to its importance. There are many more pressing and more realistic problems to be met and solved in the next ten years." DeFrance had concluded in the spring of 1952 that "a study group of any size is not warranted."9
In July 1954, however, representatives of NACA disclosed to the Air Force and the Navy their conclusions regarding the feasibility of an entirely new rocket-powered research airplane and suggested a tripartite program for the manned exploration of the upper atmosphere. NACA's views were based mainly on the findings and proposals of the Langley study group, which had been working on the problem since 1952 and had made a more detailed presentation than research teams from Ames and the High Speed Flight Station. NACA envisioned an aircraft that would fly as high as 50 miles and whose speed would reach perhaps mach 7 (approximately 5,000 miles per hour). Such a craft would be especially valuable for studying the critical problems of aerodynamic heating, stability, and control at high altitudes and speeds. Data gathered on its flights "would contribute both to air-breathing supersonic aircraft . . . and to long-range high altitude rocket-propelled vehicles operating at higher Mach numbers." Realizing that the temperatures generated on its return into the heavier atmosphere would be greater than on any previous airplane, NACA suggested as a structural metal Inconel-X, a new nickel-chrome alloy "capable of rapid heating to high temperatures (1,200°F) without the development of high thermal stresses, or thermal buckling, and without appreciable loss of strength or stiffness."10
This long-range plan was shortly accepted by the Air Force and the Navy Bureau of Aeronautics and put into motion as the "X-15 project." In December 1954, NACA, the Air Force, and the Navy agreed to proceed with the project  under operating arrangements roughly similar to the previous "X" aircraft ventures. The Air Force had responsibility for finding a contractor and supervising design and construction; both the Air Force and the Navy would provide financial support; and NACA would act as technical director.11
As prime contractor for the X-15, the Air Force picked North American Aviation of Los Angeles. The performance specifications of the X-15 called for a rocket engine consuming anhydrous ammonia and liquid oxygen and providing some 57,000 pounds of thrust for as long as six minutes. This powerplant would be four times as big as that of the X-2. A highly sensitive flight-data system, thick upper and lower vertical stabilizers for aerodynamic control, small reaction jets burning hydrogen peroxide for control in the near-vacuum of the upper atmosphere, and a new structural material - these were some of the novel characteristics of the stub-winged craft.12
The X-15 would not fulfill its original design objectives until 1962, long after NACA had become NASA and in the same year that Project Mercury achieved its basic goals. Even so, the X-15 was by far the most ambitious, expensive, and publicized research undertaking in which NACA ever participated. Its eventual success stemmed largely from the imagination and ingenuity of the NACA engineers who had started planning for an advanced aerodynamic vehicle in 1952.
In 1954, the year of Major Arthur Murray's climb to about 17 miles in the X-1A, the idea of manned rocket flight to an altitude of 50 miles seemed exceedingly visionary. Most people in NACA, the military, the aircraft industry, and elsewhere assumed that over the years vehicles with substantial lift/drag ratios would evolve to higher and higher speeds and altitudes until, by skipping in and out of the atmosphere like a flat rock across the surface of a pond, they could fly around the world. Even then, however, there were those within NACA who took the Executive Committee's mandate for "research in space flight and associated problems" literally and who felt that the X-15 concept did not go far enough. They looked to the second part of the resolution adopted by the Committee on Aerodynamics and approved by the Executive Committee, which sanctioned "a modest effort" on the "problems associated with flight at altitudes from 50 miles to infinity and at speeds from Mach number 10 to the velocity of escape from the earth's gravity."
Some of the most "far out" aeronautical engineers working for NACA in the early fifties were employed at the Ames laboratory. As early as the summer of 1952, Ames engineers, experimenting at the supersonic free-flight, 10-inch-by-14-inch, and 6-inch-by-6-inch wind tunnels at the California site, had examined the aerodynamic problems of five kinds of space vehicles - glide, skip, ballistic, satellite, and interplanetary. They knew that the aerodynamic forces acting on a vehicle above 50 miles were relatively minor, as were problems of stability and control at such altitudes. They concluded, however, that a space vehicle should probably be controllable at lower altitudes, although it "may not be optimum from the point of view of simplicity, etc...."13
6 Minutes, NACA Committee on Aerodynamics, Wallops Island, Va., June 24, 1952, 19-21; memo, M. B. Ames, Jr., Acting Asst. Dir. for Research, to Langley Aeronautical Laboratory, "Research on Space Flight and Associated Problems," July 10, 1952; memo, John W. Crowley, Assoc. Dir. for Research, to Ames Aeronautical Laboratory, "Research on Space Flight and Associated Problems," Aug. 26, 1952; memo, Crowley to Langley Aeronautical Laboratory, "Research on Space Flight and Associated Problems," Aug. 31, 1952; minutes, NACA Executive Committee, Moffett Field, Calif., July 14, 1952, 15, NASA Historical Archives, Washington.
7 Memo, Henry J. E. Reid, Dir., Langley Aeronautical Laboratory, to NACA, "Research on Space Flight and Associated Problems," Aug. 5, 1952; NACA Research Authorization A73L95, Sept. 8, 1952.
8 Memo, Reid to NACA, "Meeting of Committee on Aerodynamics at Wallops Island on June 24, 1952," May 26, 1952.
9 Memo, Smith J. DeFrance, Dir., Ames Aeronautical Laboratory, to NACA, "Report on Research of Interest to Committee on Aerodynamics," May 29, 1952.
10 "NACA Views Concerning a New Research Airplane," NACA, Washington, Aug. 1954. On the development of nickel for use in aircraft construction see F. B. Howard-White, Nickel: An Historical Review (New York, 1963), 249-258.
11 Wendell H. Stilwell, X-15 Research Results (Washington, 1965), 11-16; Kenneth S. Kleinknecht, "The Rocket Research Airplanes," in Emme, ed., History of Rocket Technology, 205-208; Editors, Air University Quarterly Review, "The Spiral Toward Space," in Kenneth F. Gantz, ed., Man in Space: The United States Air Force Program for Developing the Spacecraft Crews (New York, 1959), 208-210; Myron E. Gubitz, Rocketship X-15 (New York, 1960); Jules Bergman, Ninety Seconds to Space: The Story of X-15 (New York, 1960). The X-2 was the last rocket-powered research airplane that flew before the X-15 went into operation, although the fifties also saw flights of jet-propelled research craft like the X-3, nicknamed the "Flying Stiletto," the X-4, and the variable-sweep X-5, as well as the rocket-powered X-1B, used by NACA for reaction-control and heating studies. X-15: Research at the Edge of Space, NASA EP-9 (Washington, 1964), 9.
12 Stilwell, X-15 Research Results, 17-31; Editors, Air University Quarterly Review, "Spiral Toward Space," 210-212; X-15, 11-15; Charles V. Eppley, The Rocket Research Aircraft Program, 1946-1962 (Edwards Air Force Base, Calif., 1962), 25-30; Gubitz, Rocketship X-15, 61-74; John V. Becker, "The X-15 Project: Part I: Origins and Research Background," Astronautics and Aeronautics, II (Feb. 1964), 52-61.
13 Memo, DeFrance to NACA, "Research on Space Flight and Associated Problems," Sept. 18, 1952.