In 1910, Ralph, Clifford and Effie Corum built a homestead at the edge of Rogers Dry Lake in California. As other settlers came to live in the area, the community grew to the point that it needed a post office, and the new station bore the name Muroc, a backwards spelling of the Corum family name that was chosen to avoid confusion with the northern California mining town of Coram. Located at the western edge of the Mojave Desert, the 44 square mile dry lake, the largest of its kind in the world, soon drew the attention of Lt. Col. Henry “Hap” Arnold, who needed a place for his aircrews to practice bombing and, by 1939, the US Army Air Forces were firmly established at Muroc Amy Air Field. With 20,700 square miles of restricted air space, Muroc became the primary flight test facility for the US military, and benefitted from its location far from large population centers—and far from prying eyes. In 1946, when the US began its efforts to break the sound barrier with the Bell X-1, thirteen engineers and technicians from the National Advisory Committee for Aeronautics (NACA) moved from the Langley Memorial Aeronautics Laboratory in Virginia to Muroc (renamed Ewards Air Force Base in 1949) and set up the Dryden Flight Research Center. Soon, the quiet desert air over Muroc was shattered with the roar of rocket and jet engines and sonic booms as experimental X planes broke speed and altitude records and tested cutting edge aircraft designs. The Dryden Flight Research center was renamed the Armstrong Flight Research Center in 2014, though the more familiar Dryden name is still used by many, and the research center, with access to 16 runways on the Air Force base and on the dry lake, remains the primary testing location for US military aircraft today.
Pictured at the top, in a photo taken at Dryden in 1952, are six of those famous X planes. Starting at the back left of the photo and moving clockwise, the planes are the Douglas D-558-2 Skyrocket (the Navy, being the Navy and not the Air Force, chose not to use the X designation for its experimental aircraft), the Convair XF-92A, the Bell X-5, Bell X-1, Northrop X-4 Bantam, and Douglas D-558-1 Skystreak. Each one of these aircraft served as important landmarks on the road to modern supersonic flight, though some were more successful than others. Here is a snapshot of each of these historic aircraft, in the order of their maiden flight.
Bell X-1 (January 19, 1946): The X-1 was the first of a long line of experimental X planes, and was built to be the fastest aircraft in the world at the time. Bell Aircraft designers looked at bullets that were known to travel supersonically, and the X-1 was designed to mimic the shape of a .50 caliber bullet, an object that remained stable at supersonic speeds. Test flights of the rocket-powered X-1 began in 1946 with a series of glide tests piloted by Bell’s chief test pilot Jack Woolams, and, following Woolams death in an air racing crash, the test flights were taken over by another Bell test pilot, Chalmers Goodlin, who made a further 26 flights before interservice squabbling resulted in the project being taken over by NACA. US Air Force Captain Charles “Chuck” Yeager assumed flying duties and piloted the X-1 past the sound barrier for the first time on October 14, 1947. The aircraft, nicknamed Glamorous Glennis in honor of Yeager’s wife, was dropped from a modified Boeing B-29 Superfortress and reached a speed a Mach 1.06 (700 mph), making Yeager and the X-1 the first pilot and plane to exceed Mach 1 in level flight. Ultimately, Bell produced five variants, each testing different aspects of supersonic flight, as well as materials and systems for the manufacture and control of high-speed aircraft. The final variant, the X-1E, reached a top speed of Mach 2.21 in 1958.
Douglas D-558-1 Skystreak (April 14, 1947): A somewhat more conventional design than the X-1, the Skystreak was powered the US Air Force’s first axial flow turbojet, the Allison J-35, and was designed to investigate transonic and supersonic flight. The Skystreak, developed primarily for the US Navy, was envisioned as the first of a three-part program, and was followed by the jet-and rocket-powered D-558-2 Skyrocket. The fuselage was constructed largely from magnesium, and the straight wings were made from aluminum. Originally planned to experiment with different air intake configurations, the three test aircraft were instead all constructed with a single intake in the nose. The Skystreaks were initially painted red, earning them then nickname “crimson test tube,” but, along with all future X planes, it was painted white to improve visibility and photographic tracking (the X-1 was bright orange). The three D-558-1 aircraft completed 228 test flights, and reached a top speed of Mach .99. Though never quite breaking the sound barrier, and overshadowed by the supersonic X-1, the Skystreak helped NACA engineers gain valuable data in the understanding of longterm transonic flight.
Douglas D-558-2 Skyrocket (February 4, 1948): As the second phase of the Douglas D-558 program (the third phase, which would have been similar to the hypersonic North American X-15, was never built), the Skyrocket was much more streamlined than its predecessor and featured wings and tail swept at 35 degrees. The first flights were made using a Westinghouse J34 turbojet, and the three test aircraft undertook numerous flights to fine tune the airplane’s flight characteristics in transonic flight. Once the control systems were refined, the turbojet was removed and replaced with a Reaction Motors LR8-RM-6 liquid-fuel rocket engine, the same engine that powered the X-1. Test flights carried the rocket-powered Skyrocket very near Mach 2, and also set unofficial altitude records (official FAI records required an aircraft to take off and land on its own). Though there was great pressure to break the Mach 2 barrier, NACA, which was administering the tests, refused, saying that they were not in the business of setting records. However, they finally relented, and the Skyrocket was fitted with modified rocket nozzles, filled with chilled alcohol fuel so more could be placed on board, and the fuselage was waxed to help reduce drag. On November 20, 1953, after disengaging from a Boeing B-29 mothership, test pilot Scott Crossfield took the Skyrocket to 72,000 ft then pushed over into a shallow dive. He reached Mach 2.005 (1,291 mph), the one and only time that the Skyrocket exceeded Mach 2, and became the first to reach that speed milestone. All told, the three Skyrockets made 313 test flights, gathering significant data on swept wing flight in the transonic and supersonic regimes that would be used to help design the first generation of swept wing fighter aircraft.
Convair XF-92 (April 1, 1948): As World War II drew to a close, more than 1,500 German scientists, along with a trove of data and experimental aircraft, were brought to the United States as part of Operation Paperclip. Among the aircraft was the Akaflieg Darmstadt/Akaflieg München DM1, a fully delta wing glider designed by Alexander Lippisch. The glider’s design was studied by NACA engineers, who passed their data to Convair for the development of a point-defense fighter which adopted the full-delta design inspired by the DM1. Though the Air Force contract was eventually canceled, development of the XF-92 continued, and it was fitted with an Allison J33 turbojet for flight testing, becoming the world’s first aircraft to fly with a full delta wing configuration. Convair built just one XF-92, and it was flown by a number of test pilots, including Scott Crossfield and Chuck Yeager, who managed to nudge the XF-92 past Mach 1. However, the aircraft was notoriously difficult to fly, and showed some serious problems with pitch-up. Very few test pilots wanted to fly it. Nevertheless, the XF-92 provided critical data on delta wing aircraft design which was used in the development of the F-102 Delta Dagger, F-106 Delta Dart and B-58 Hustler, all Convair designs, and which all had significantly more successful careers.
Northrop X-4 Bantam (December 15, 1948): In many ways, German research and development in aviation was far ahead of the Allies during the Second World War, and the Convair XF-92 was not the only American postwar aircraft design to benefit from the data collected from Germany by Operation Paperclip. During the latter stages of the war, Germany put another Lippisch design into production, the Messerschmitt Me 163 Komet, a rocket-powered fighter that was revolutionary for its swept wing and lack of a horizontal tail surface. The Bantam was of a similar design, though it was powered by two Westinghouse J30 turbojets instead of a rocket motor. The theory behind the semi-tailless design was that it might avoid the controllability problems experienced by aircraft as they approached the speed of sound. However, the X-4 displayed serious problems with pitch up which often led to porpoising flight. While the X-4 did not prove to be a successful design, it proved that such a tailless design was unsuitable for transonic flight, though the Vought F7U Cutlass that followed was of a similar design and was capable of supersonic flight, though it led a rather checkered career as a US Navy fighter and also suffered from dangerous handling problems. It was not until the advent of computer-aided fly-by-wire control surfaces that the semi-tailless design was able to reach its full potential.
Bell X-5 (June 20, 1951): The capture of German aircraft design and aeronautic data continued to be of use to American aircraft designers, and one of the more advanced aircraft under development in Germany was the Messerschmitt P.1101, a jet fighter designed near the end of the war that featured aviation’s first variable-sweep (variable geometry) wing. While the P.1101's wing sweep had to be preset on the ground and could not be changed in flight, it provided the inspiration to engineers at Bell Aircraft who used the idea in the design of the X-5, the world’s first aircraft capable of changing the sweep of its wings in flight. The idea behind variable geometry is that the wings could be swept forward for low-speed flight and swept back for high-speed flight. The X-5's wings could be positioned at 20°, 40° or 60°, and a full sweep of the wings could be accomplished in 30 seconds. However, the aircraft’s design led to serious problems with spin, and the second of two prototypes was lost in a crash, killing Air Force test pilot Ray Popson when he failed to recover from a flat spin. While the X-5 was ultimately a failure, due in large part to its poor aerodynamics, it paved the way for future swing-wing aircraft such as the General Dynamics F-111 and Grumman F-14 Tomcat.
The late 1940s and 1950s was one of the most exciting eras of aircraft development, a time when the slide rule was king and technical drawings were made by draftsmen with pencil and paper. Cold War budgets were seemingly bottomless, and engineers created aircraft that had to be flown to discover whether or not they would work. Test pilots, many of whom gave their lives, were some of the true heroes of the era. In our modern age, computer aided drafting, computational fluid dynamics and computer modeling, which streamline the design process and make the job of the test pilot much less dangerous, have also removed much of the mystery, and even romance, from the creative process, for better or worse. The role that these and other X planes played in the advancement of aviation technology cannot be understated. For a little perspective on how these aircraft accelerated the development of flight, consider this: The Wright Brothers made the first flight in 1903 at a ground speed of 34 mph. It took 44 years to reach Mach 1, but just 6 more years to reach Mach 2, and only 3 more years to reach Mach 3. By 1967, the North American X-15 achieved a top speed of 4,520 mph, Mach 6.72, and reached the edge of space, setting a record that still stands today. And it all started with the X-1.
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