The “X” classification in research and development simply marks an invention’s current use as “experimental.” Some such inventions make it beyond that stage; the majority of those that follow did not. Whatever their contributions to aeronautics in the United States, these X-planes can be appreciated for their interesting designs.
1. The Avrocar
Actually Canadian in origin, the goal of such a UFO-looking craft lay in its potential for vertical take-off and landing (VTOL). When the Canadian government dropped the program due to expense, the USAF took over the project. The “plane” could lift and land, but experienced severe instability just a few feet off the ground, making the hope for supersonic speeds an impossibility. So the project moved on to the Army’s initial plans for a subsonic craft. But the flying saucer was destined to remain a fixture of science fiction, as further tests proved it incapable of sustained, high-speed flight.
2. Northrop’s “Tacit Blue”
Nicknamed “The Whale,” this 1980s-era plane looks somewhat like a Beluga. The craft flew over 100 times, primarily to test new stealth technology. Though discontinued itself, it had a great influence on the design of further stealth crafts by proving a few significant things: stealthy planes could have curved surfaces, rather than geometrically-complex and severely-angled surfaces, leading to later development of the B-2; with the right design, a plane could so greatly diminish its heat signature as to pass undetected, even when directly above or behind an ongoing battle; and that effective, undetected communication could still occur with such a stealthy design.
3. The Lockheed D-21
Lockheed Martin’s famous “Skunk Works” contingent (a group within the company dedicated to producing cutting-edge aircraft) created this remotely piloted aircraft (RPA) in the 1960s. If it looks similar to the SR-71 Blackbird, it is because it aimed to accomplish the same high-speeds, but unmanned. The D-21 flew four highly-classified missions over communist China, none of which “succeeded.” The Air Force scrapped the program in 1971. Originally designed to launch off the back of an M-21, the D-21 moved to release below the wing of a B-52 when it collided with an M-21 after take-off, resulting in one fatality.
4. North American X-15A-2
This plane has the distinct honor of being the first piloted craft to achieve hypersonic speeds. It was meant to test the absolute limits of human flight, bringing pilots closer to space than ever before, reaching the very edge of our planet. When developed, all of those involved had no idea what effects such conditions might have on the body. The goal was to discover these effects in order to prepare American men and women for space flight. Manned by future astronauts like Neil Armstrong, these planes were launched from the wing of a B-52 at 45,000 feet. Streaking upwards at incredible speeds and gliding back down to land, flights lasted about 10 minutes on average. Pilots who reached 50 miles in altitude earned astronaut wings.
5. McDonnell X-85 Goblin
This tiny plane was a “parasite” fighter launched from a B-36 with the intent of protecting the bomber, should enemy fighters appear. Once the little Goblins fought off the attackers, they were to be recovered mid-air by the same bombers. Test flights proved recovery impossible, and so the program ended four years later in 1949 in favor of focusing efforts on mid-air refueling of traditional fighter aircraft.
6. Lockheed Martin RQ-3 DarkStar
This stealthy RPA never made it to full production, with only four total having been built. It could complete all its tasks autonomously, but the craft itself proved far too unstable aerodynamically. The program started in 1996 and ended within three years.
7. Douglas X-3 Stiletto
The early-1950s “Stiletto” was one of the Air Force’s earliest attempts at a high-speed jet with very short wings. Only one was ever built. Though it failed as a craft in its own right, its wing design led to later success in fighter development.
8. Lockheed YF-12A
Very similar in many ways to the SR-71 Blackbird, the YF-12A led directly to the Blackbird’s development. The USAF primarily intended that it act as a successful interceptor against supersonic bombers. It set altitude and speed records at the time, reaching just beyond Mach 3 and flying up to 80,000 feet. Lockheed built only three; one could not be repaired after a bad landing, another crashed after a fire that caused the crew to eject, and the remaining craft now sits at the museum.
9. Martin X-24B
This small craft directly led to space shuttle development. It proved what no other plane could before–that a craft with a “lifting body” (that is, a body that produces lift, rather than or in addition to the wings) could land successfully on a runway. Like the X-15A-2, the X-24 launched from a B-52 at 45,000 feet, and like the future space shuttles, it glided without power into a landing.
10. Ryan X-13 Vertijet
The Vertijet aimed to–and did–prove that a jet could take-off vertically, fly horizontally, and land, as it started, vertically. Its second model successfully completed this entire cycle in April 1957, with the assistance of a mobile trailer which it used for take-off and landing. It also demonstrated that success isn’t everything; what matters most is functionality. The craft had little potential for military operations at that time. Today, the CV-22 somewhat accomplishes this same flight cycle.
11. Bensen X-25A Gyrocopter
During the Vietnam war, the USAF searched for various ways to improve the chance of a downed pilot’s escape from enemy hands. This gyrocopter was one contractor’s approach to the problem. The intent was to stow this craft in the plane, and it would eject with the pilot. As it fell, it would essentially catch flight, and the airman would pilot it to safety. Developed at the end of the war, it was soon canceled over doubts of its ability for operational use, much like the Vertijet.
12. North American XB-70 Valkyrie
This sleek aircraft could reach Mach 3.1 at 73,000 feet. The X-B70 program started in the 1950s, and was canceled as an official Air Force project in 1961 before any planes were even built. The contracted company, however, still built the plane and the USAF eventually bought two in the mid-60s to study supersonic flight dynamics. They intended to buy three, but after the second crashed in a mid-air collision, the third was never completed.
13. American Helicopter Co. XH-26 Jet Jeep
Another aircraft of the miniature persuasion, this helicopter had no internal engine. Instead it operated via pulsejets on the ends of its rotor blades. Its allure lay in its simplicity; it weighed under 300 pounds, and was easily disassembled and could be reassembled in 20 minutes by two men. In addition, it could run on gas fit for a Jeep. The unbearable noise from the pulsejets led to this craft’s discontinuation.
14. Convair NC-131H Total In-Flight Simulator (TIFS)
A truly unique aircraft, the TIFS allowed pilots and engineers to test how an aircraft would handle in-flight before developing a full prototype. It was particularly useful in simulating take-off and landing of large plane designs. One of the ways it accomplished this was through vertical fins located on the plane’s wings that allowed the simulation of crosswinds. During its near-40-year use, the TIFS simulated the B-1, B-2, Tacit Blue, Space Shuttle, and C-17, among other aircraft. It also served as a trainer for test pilots.
For more interesting planes and Air Force artifacts, visit the National Museum of the USAF website here, or visit the museum in-person at Wright-Patterson Air Force Base near Dayton, Ohio.