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In the 1940s and 1950s, the Air Force built a series of “X-planes” for high-speed, high-altitude research. One requirement was for research into the heat generated by air friction in sustained periods of Mach 2 flight.
In 1949 Douglas won the competition for such an aircraft with the X-3, one of the most visually striking aircraft ever built. Quickly dubbed the “Stiletto,” the X-3 had a very long (67 feet), slender fuselage with a very sharp “needle” nose. It was made with a large amount of unpainted, heat resistant (and expensive) titanium, notably on the underside of the tail boom and the slab tail planes.
The X-3 had very short (22 feet) low aspect ratio straight wings for high speed but, unlike earlier X planes, it did not have to be loaded under a carrier aircraft but would take off under its own power, climb to high altitude and fly at a sustained cruise speed of Mach 2.
The very long, slender nose was filled with test equipment and kept the X-3’s frontal area to a minimum. The semi-buried pressurized cockpit and windscreen was designed to minimize the expected thermal heating that would have melted a normal canopy.
The pilot had special flight suit and helmet for a supersonic ejection and sat on a downwards-firing ejection seat, which also served as an elevator to lift him to the cockpit from the ground.
The fuselage was painted gloss white — more heat reduction — but the wings were highly polished aluminum. For data gathering, the X-3 used 850 pinholes spread over its structure to record pressures and 185 strain gauges to record air loads. There were 150 temperature recording points spread across the aircraft, and a total of about 1,200 pounds of test equipment was carried so after landing details of every flight could be completely analyzed.
Alas, the engine technology of the day could not match the airframe design. The X-3 was originally to be powered by two Westinghouse J46 turbojets with afterburners that would produce 7,000 pounds of thrust each, but the J46 never came close to meeting the thrust and size requirements and Douglas had to substitute Westinghouse J34 turbojets with only 4,900 pounds of thrust. Airframe weight also increased and the result was that X-3 was significantly underpowered.
The sole X-3 was delivered to Edwards Air Force Base, Calif., for tests by Douglas. The first flight was made on Oct. 20, 1952, but tests quickly showed that the underpowered X-3 could only break the sound barrier in a dive, no different from any operational fighter of the day and useless for research.
Douglas completed its test program and turned the X-3 over to the USAF, which flew it only six times before giving it over to NASA’s predecessor, the National Advisory Committee for Aeronautics. In late October 1954, NACA used the X-3 to explore a phenomena knows as “roll inertia coupling,” in which a maneuver in one axis caused an uncommanded maneuver in one or two others, but in the tests the X-3 was so badly damaged it was grounded and soon given back to the Air Force.
Since the X-3 was the first aircraft that used titanium on a large scale, it provided useful data on using that material for the later SR-71 family. It also showed that tire technology required improvements, because on the short winged X-3’s need for very high takeoff and landing speeds — 260 mph and 200 mph, respectively — often caused its tires to disintegrate.
The sole X-3 was transferred in 1956 to the National Museum of the U.S. Air Force at Wright-Patterson Air Force Base, Ohio, where its exotic design makes it one of the museum’s main attractions.
(For questions or comments, contact Dr. Michel at marshall.michel@spangdahlem.af.mil.)
