Photo Gallery: The First Eight X Airplanes: From Mach 1 to Almost 7

Stephen Mraz Jun 5, 2014

The Bell X-1 (1946-58)

Built by Bell Aircraft Corp., the rocket-powered Bell-1 was flown by a host of pilots as U.S. engineers explored speeds around Mach 1, the speed of sound. Air Force Capt. Chuck Yeager became the first pilot to break the sound barrier when he flew a Bell-1, The Glamorous Glennis (named for his wife) to Mach 1.06 at 43,000 ft in Oct. 1947. The rocket engine did not have a throttle. Pilots varied the plane’s speed by turning ignition on or off in one or more of four ignition chambers. This let pilots bump up thrust in increments of 1,500 lbf. The planes were taken aloft and launched from beneath the bellies of modified bombers such as the B-29 and RB-50 Superfortresses.
The X-2 Starbuster (1952-56)

The X-2, a swept-wing plane, was another rocket-powered aircraft designed and built by the Bell Aircraft Co. But it was meant to investigate the problems of high-altitude flight and handling at speeds in excess of Mach 3. The fuselage consisted of K-monel, a new alloy of copper and nickel, and the wings were stainless steel. The airplane’s nose, along with the cockpit, were ejectable in case of an accident as ejections seats had yet to be accepted. The X-2’s ejection canopy was flight tested on a German V-2 rocket left over from WWII. The Starbuster set many records, but its fastest speed was Mach 3.2 (2, 094 mph) and its highest flight topped out at 126,000 ft.
The X-3 Stiletto (1952-56)

This needle-nosed jet-powered aircraft from Douglas Aircraft Co., was supposed to explore the regime of Mach 2 cruise speeds. Unfortunately, an underpowered Westinghouse J34 turbojet powerplant could barely get the plane past Mach 1.2, and that was in a dive. The fact the plane routinely lost its small tires during landings led to a redesign of tires used on high-speed planes. Data from the plane’s flights also led to a clearer understanding of inertial coupling (or roll divergence), a phenomenon several test pilots had grappled with in other high-speed X planes, including the X-1. It is caused by the inertia of a relatively heavy fuselage overpowering the wing and tail’s stabilizing forces.
X-4 Bantam (1948-53)

Engineers at Northup Aircraft Co. built two of these planes that were designed not to have rear horizontal stabilizers. They were used to gather data on the stability of semi-tail-less aircraft at high subsonic and transonic speeds. It turns out that horizontal stabilizers were not just dead weight on supersonic aircraft; they proved critical for control at low and transonic speeds.
X-5 (1951-58)

Engineers at Bell Aircraft Co. took the design for a variable-sweep wing from the P.1101, a plane built at the Messerschmitt Co. and found after WWII. But while the sweep of the P.1101’s wings could only be changed on the ground, Bell engineers developed a set of electric motors that could adjust the Bantam's sweep-angle in flight. Data from flight and wind-tunnel tests on the X-5 went on to be used for the Air Force F-111 Aardvark and Navy F-14 Tomcat, two swing-wing jets.
X-13 Vertijet (1955-57)

The U.S. contracted with Ryan Aeronautical to build a plane that could take off vertically, hover, transition to horizontal flight, then return to vertical flight to land. The ultimate goal was to see if a plane could be stowed, then launched and recovered onboard surfaced submarines. Pitch and yaw control were done using vectored engine thrust. “Puffer” jets, or jet reaction controls, handled roll control.
X-14 (1957-81)

To further explore vertical take-off and launches, as well as hovering and going from vertical to horizontal flight and back, engineers at Bell Aircraft Co. cobbled together the X-14 out of the wings, ailerons, and landing gear of a Beech Bonanza and the tailcone and tail of a Beech T-34 Mentor. The open-cockpit plane had two jet engines mounted at the plane’s center of gravity. A series of pilot-controlled vanes directed the thrust, letting if fly vertically or horizontally. The aircraft’s flight controls were similar to those proposed for the Apollo lunar module, and Neil Armstrong flew the X-14 as a lunar-lander trainer. The plane was used to explore VTOL flight up through 1981.
The X-15 (June 1959-68)

By 1959, engineers wanted to really push the envelope, making planes that could fly over Mach 5 and above 250,000 ft. So a team at North American Aviation designed and constructed the X-15. Their efforts were right on the mark as it set a speed record of Mach 6.7 (and flying at Mach 6 means covering a mile every second), and as high as 354,200 ft, high enough to qualify for astronaut wings in the Air Force. Data from X-15 flights went on to be used in developing the Mercury, Gemini, and Apollo spacecraft, as well as the Space Shuttle.

X (experimental) airplanes were built by aviation companies but were flown and maintained by NASA (or its predecessor, NACA), and the military. These planes, most of which were one-of-a-kind, were used to explore high-speed, high-altitude flight, as well as vertical take offs and launchings, new materials such as titanium, and variable-geometry wings. Here’s a look at the first 13 years of X airplanes, going from 1946 through 1959.

For more information and images of the X-15, click here.