This article first appeared in the April 1982 Flight Test News.

X-29A, Grumman’s high performance, supersonic, forward swept wing concept demonstrator is the latest addition to the “X” series of experimental aircraft.
X-1: Originally designated XS (for Sonic)-1. Rocket-powered aircraft. Flown to Mach 1.06 on Oct. 14, 1947–first supersonic manned flight. Three built by Bell Aircraft Corp. A, B, C, D and E versions. The shock waves violently shook the X-1 at Mach .98. Pilot Chuck Yeager’s teeth were chattering. He fought to control the ship. Then, the 24 year old test pilot fired up the plane’s fourth rocket engine and blew right by the sound barrier. The jolting stopped.
X-2: Rocket-powered, swept wing aircraft. Set 2,094 mph and 126,200 feet records in 1956. Two built. Bell.
X-3: “Stiletto,” to explore problems of sustained high-speed (Mach 3) flight. No recorded above Mach 1 flights. One built. Douglas Aircraft.
X-4: “Bantam”- small, twin-jet. Basically a wing, short fuselage and vertical stabilizer, based on German Messerschmitt Me-163. Two built. Northrup Aircraft.
X-5: First successful variable sweep wing plane. Sweeping mechanism successfully tested July 27, 1951, plane’s fifth flight. Design resembled Messerschmitt P-1101. Two built. Bell.
X-6: Nuclear propulsion aircraft concept. None built. However, one B-36H modified for program to carry reactor to test radiation factor.
X-7: Supersonic ramjet engine test-bed. Air-breathing record Mach 4.31. Lockheed Missiles and Space Company built 61 in X-7 A-1, -2, -3 versions.
X-8: Upper-atmosphere, rocket-powered research vehicle to succeed captured V-2s. 1951 carried two mice and a monkey 75 miles into space, first time living creatures recovered from space. Aerojet-General.
The launching of a monkey and two mice into near space on an X-8 and their safe recovery thrilled many people. But not everyone. The British Royal Society of the Prevention of Cruelty to Animals wasn’t happy with the program at all.
X-9: “Shrike” missile built to research stand-off weapon concept. Bell.
X-10: Supersonic cruise missile prototype for ICBM research. First unmanned vehicle with autonavigation system. North American Aviation.
X-11: Rocket, similar to V-2, for ICBM research. Today’s gimballed rocket engine designs developed from this program. Convair.
X-12: Jet version of X-11. None known built. Convair.
X-13: Full-scale vertical takeoff and landing jet (VTOL) aircraft. First hovering flight May 28, 1956. Two built. Ryan Aeronautical.
X-14: First aircraft to fly using jet thrust diverter system for vertical lift. One built. Bell.
X-15: Manned, rocket-powered vehicle for high-speed, high-altitude tests. Altitude record: 354,200 feet, Aug. 22, 1963; speed record: 4,534 mph. Oct. 3, 1967. Three built. North American Aviation.
X-16: High altitude, twin-jet reconnaissance plane for Air Force and CIA. One prototype and 22 production craft on order when A.F. and CIA cancelled and opted for U-2. None built. Bell.
X-17: Three-stage, solid propellant research rocket to test re-entry nose cone designs for nuclear missiles. At least 26 built. Lockheed.
X-18: Tilt-wing, twin turboprop VTOL concept for cargo planes. One jet engine for pitch control. One built. Hiller Air- craft Corp.
X-19: VTOL craft. Two turboshaft engines driving four propellers in tilting, wingtip nacelles. Unique propellers provided vertical lift even in horizontal positions. Originally Curtiss-Wright Model 200. Two built.

This article first appeared in the April 1982 Flight Test News.

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