1940s and 1950s
X-1
Bell Aircraft built three of the original X-1s, plus an X-1A and X-1B, an X-1D. There was also an X-1E rebuilt from the X-1 #2. They flew a total of 214 flights between 1946-1958. This was a joint program among the NACA, the Air Force, and Bell Aircraft. The bullet-shaped, rocket-powered aircraft became the first airplane to break the sound barrier on Oct. 14, 1947. Flight research by the NACA continued through such advanced models as the X-1B and X-1E, providing a wealth of information for use in correlating from the X-1 #2 wind-tunnel data with actual flight data and for designing later high-performance aircraft.
D-558-I
Douglas manufactured three airframes. They flew a total of 225 flights between 1947-1953. This was a joint program among the NACA, the Navy-Marine Corps,. and Douglas Aircraft. The straight-winged, turbojet-powered "Skystreak" collected data in the transonic region about stability, control, loads, buffeting, and handling qualities.
D-558-II
Douglas manufactured three airframes. They flew a total of 312 flights between 1948-1956. This was a joint program among the NACA, the Navy-Marine Corps,. and Douglas Aircraft. The swept-wing aircraft flown with both turbojet and rocket power set an altitude record of 83,235 ft. on Aug. 21, 1953, and a speed record on Nov. 20, 1953, when it became the first aircraft to reach Mach 2. The "Skyrocket" collected data about handling qualities, wing loads, and stability and control, especially pitch-up.
XF-92A
Convair manufactured one airframe. The XF-92A flew a total of 325 NACA flights between 1948-1953. Other flights were flown by Convair and the Air Force. This was a joint program among the NACA, the Air Force, and Convair to test the country's first delta-wing air-craft. Stability and control, pitch-up, and lift-over-drag measurements obtained from this program contributed to the technology used to develop the F-102, F-106, XF2Y-1 Sea Dart, and B-58 aircraft.
X-2
Bell manufactured two airframes. They flew a total of 17 flights between 1954-1956. This was a joint program with the Air Force, although the NACA never flew the swept-wing, rocket-powered aircraft designed to fly Mach 3. The NACA supported the Air Force with advice and data analysis. The X-2 did become the first aircraft to reach Mach 3, recording a Mach 3.2 speed on its last flight, which destroyed the aircraft and killed the Air Force pilot because of inertial coupling. It collected data on aerodynamic heating and stability and control effectiveness at high speeds and altitudes. The X-2 was also the first aircraft to fly higher than 100,000 feet on Sept. 7, 1956, when it reached 126,200 feet 20 days before the aircraft reached Mach 3.2
X-3
Douglas manufactured one airframe. The X-3 flew a total of 26 flights between 1952-1955. This slender, jet-powered aircraft tested such new materials as titanium and collected data on stability and control, pressure distribution, and flight loads. The X-3 failed to achieve the high speeds for which it was designed but pioneered in the use of titanium and contributed to the development of aircraft tire technology.
X-4
Northrop manufactured two airframes. They flew a total of 90 flights between 1948-1953. In a joint program with the Air Force and Northrop, the NACA conducted most of the flights in this semi-tailless aircraft (which had no horizontal stabilizer). Powered by two turbojet engines and featuring swept wings, the X-4 helped demonstrate that tail surfaces are important for proper control effectiveness but that a properly configured semi- tailless airplane was a viable platform for research on dynamic stability and also provided data (from tufts) on airflow anomalies.
X-5
Bell manufactured two airframes, only one of which was flown by the NACA. It flew a total of 133 Air Force - NACA flights between 1951-1953. The X-5 completed all of the research goals originally set for the first aircraft capable of variably sweeping its wings in flight. Demonstrating wing sweep from 20 to 60 degrees, the aircraft verified NACA wind-tunnel predictions of reduced drag and improved performance resulting from increased wing sweep as it approached Mach 1. Even vicious spinning characteristics of the X-5 yielded a wealth of data for determining poor aircraft spin design.
X-15
North American Aviation manufactured 3 airframes. They flew a total of 199 flights between 1959-1968. This joint program by NASA, the Air Force, the Navy, and North American operated the most remarkable of all the rocket research aircraft. Composed of an internal structure of titanium and a skin surface of a chrome-nickel alloy known as Inconel X, the X-15 first set speed records in the Mach 4-6 range with Mach 4.43 on Mar. 7, 1961; Mach 5.27 on June 23, 1961; Mach 6.04 on Nov. 9, 1961; and Mach 6.7 on Oct. 3, 1967. The airplane also set an altitude record of 354,200 feet (67 miles) on Aug. 22, 1963, and provided an enormous wealth of data on hypersonic air flow, aerodynamic heating, control and stability at hypersonic speeds, reaction controls for flight above the atmosphere, piloting techniques for reentry, human factors, and flight instrumentation of relevance not only to aeronautics but to spaceflight.
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