The aviation research community was aware of problems relative to high-speed flight before World War II. However, the explosive development of aircraft capabilities after 1939 quickly forced aviation researchers to deal with compressibility-the physical phenomenon caused by the altered flow of air over an object in motion. The closer the aircraft's speed approached Mach 1 and the more dynamic the airflow over an airfoil, the more serious the compressibility effects on aircraft control surfaces. In severe cases, aircraft control is lost, with dangerous results for pilot and plane.
Prior to World War II, the NACA maintained an excellent record of research on basic aeronautical issues. The NACA's careful, some might say cautious, attention to detail resulted in numerous successes in aviation research. With the outbreak of fighting in 1939, the NACA research focus was changed to correcting specific developmental problems for combat aircraft. As part of its wartime responsibilities, the NACA was charged with discovering how to deal with compressibility issues. However, NACA's wind tunnel testing procedures were physically constrained from recreating natural conditions in the transonic speed ranges (Mach 0.75 to Mach 1.2). Thus, as early as 1941, NACA researcher John Stack was advocating that the agency pursue the construction of an overstrength, high-speed research aircraft to examine compressibility issues.
During 1943-44, the NACA's High-Speed Panel and the Army Air Forces Materiel Command at Wright Field, Ohio, developed a program to explore the flight behavior of aircraft in the transonic range. While existing NACA wind tunnel data and wing-flow research provided information on flight characteristics in the lower transonic region, Stack emphasized that there was no substitute for flight testing. In April 1944, the NACA proposed to the AAF and the Navy at a conference on compressibility that they inaugurate a high-speed research aircraft program. As outlined by Stack, this program would include the AAF/Navy procurement of several overstrength aircraft only for NACA research. These planes would be capable of studying buffeting problems, stability and control issues, and design requirements in the transonic region. The planes would be heavily instrumented by the NACA to secure flight data to provide the basis for new production aircraft for the military.
Concurrent with the discovery of the extensive German aeronautical research programs in 1944-45, both the AAF and NACA proposed expanded aviation research programs to Congress. For the NACA, this was a logical outgrowth of its established role as the premier aviation research agency of US government. For the AAF, the proposed new role was founded on its extensive contribution to victory in the war, its growing desire for independence from the Army, and its sometimes expressed doubts as to the efficacy of relying on any other agency to do its flight research and development. Both proposed expanded (and expensive) wind tunnel facilities for the post-war period. The oft-discussed high-speed research plane was to be an integral part of each program.
Over the summer of 1944, the AAF and NACA held numerous meetings to discuss transonic aircraft design. In keeping with their historic missions, the NACA proposed a more conservative transonic, turbojet-powered, ground-takeoff-capable research aircraft, while the AAF suggested a more daring rocket-powered airplane capable potentially of supersonic flight. NACA objections were based upon the unreliability of rocket engines and the dangerous fuel required for flight as well as the high wing loading required for ground take-off. However, the AAF rightly gauged the higher potential for rocket power to rapidly pierce the transonic region providing a margin of safety in the zone of the unknown.
Although Bell Aircraft offered on November 30, 1944, to build a high-speed research aircraft, the AAF chose not to issue a sole-source contract and secured two bids as a result of its request for proposals. The McDonnell Aircraft bid (MCD-520) incorporated a diving flight technique and a mother ship air-launch requirement. The Bell proposal (MCD-524) featured testing in level flight and conventional take-off procedures.
At a joint AAF/NACA meeting at Wright Field in mid-December 1944, Stack pointed out the dangers of the diving technique for research. He stated that the airlaunch technique was troublesome and not consistent with production aircraft. Stack desired that the aircraft possess jet engines as these were safer and less troublesome to operate. The AAF insisted on rocket engines even though it acknowledged them as dangerous and less reliable. Straight wings were selected for the new design since current and proposed production aircraft had straight wings. The Bell proposal was selected for further development.
During the follow-up AAF consultations with Bell, it was agreed that the research aircraft would be capable of ten minutes of powered flight; the research tests would be conducted in level or climbing flight, would allow the pilot to be seated rather than prone and would be constructed within a year of the contract award.
During March 15-17, 1945, Bell presented its design proposal at Wright Field to the AAF and NACA. The AAF representatives expressed serious doubts about the utility of the performance capabilities in the proposal. NACA representative John V. Becker stated that the plane met agency design criteria. He believed the aircraft was capable of transonic speeds and urged that the Army accept the design as it was a significant advance over any airplane currently flying. The AAF agreed to purchase the plane.
The contract (W-33-038-ac-9183) to build a transonic aircraft was signed between Bell Aircraft and the Air Technical Services Command (ATSC) on March 16, 1945. Bell Aircraft was tasked to construct three experimental airplanes capable of exploring transonic research issues. Total estimated cost plus change orders was $4,278,537. The AAF assigned three serial numbers to the aircraft (46-062 to airplane #1, 46-063 to XS-1 #2, and 46-064 to the #3 aircraft). The X-1 program was initially designated MX-524, then changed to MX-653. It retained the designation MX-653 until the fall when the aircraft were labeled as XS-1 (Experimental Supersonic Contract #1) and remained so-designated throughout the initial life of the project. By 1948, internal Air Force designations had changed and the program has since been identified simply as X-1. The MX-653 program was classified confidential and all X-1 performance data were labeled secret.
Bell would do the detailed design, build and fly the plane in the acceptance testing of the two-phase contract. Contract requirements mandated a plane that could achieve speeds of Mach 0.8 and pull 8g's. A follow-on contract would explore transonic issues. The ATSC and NACA stipulated 20 powered flights as the minimum for execution of the Bell contract. The ATSC and NACA agreed that for the follow-on contract to supersonic flight, consideration would be given to hiring a civilian pilot regardless of whether NACA or the Air Force maintained the planes. The NACA agreed to cooperate with the program by developing the design specifications, planning the general flight research program, and doing the flight data research collection and analysis.
Under separate contract Reaction Motors Inc. (RMI) designed and built the XLR-11 rocket engine for the X-1. RMI was also initially tasked to develop a new turbine-driven dual-propellant fuel pump to work with the engine and provide longer flight times than the pressure-fed system in the first two X-1s. This work was later transferred to General Electric (GE). Due to delays in the construction of the RMI rocket engine, no motors were ready for the initial tests. The failure by RMI, and later GE, to develop an adequate turbine pump also caused the suspension of work on the #3 aircraft in August 1946. This technical suspension was extremely prolonged due to program costs and the success of the first two X-1 aircraft.
Although the NACA wanted to test the X-1 at its Langley research facility's airfield, Bell and the ATSC wanted another location. Bell examined numerous sites nationally before recommending Pinecastle Army Air Field in Orlando, Florida. Pinecastle was chosen for its remote location and its 10,000 feet long runway. Bell's first choice, Muroc Army Air Field, was flooded during the proposed initial testing period. The ATSC and Bell did not want to postpone the tests until the normally dry lakebed was again useable.