High-speed heritage: X-1 to X-15

There’s a heroic fraternity of research pilots who mounted a rare fleet of experimental aircraft, monitored by savvy engineers, in the Mojave Desert in California for more than two hectic, heady decades. Starting with the first U.S. purpose-built X-plane, the Bell X-1 in 1946, and culminating in the still-to-be surpassed Mach 6+ X-15, these high-speed aircraft were helped along their paths to glory by NASA, and its precursor, the National Advisory Committee for Aeronautics (NACA).
 
X-1
 
The Bell X-1 (originally called XS-1) brought together the talents of Air Force, industry, and NACA minds to create, operate, and evaluate the world’s first supersonic aircraft. The X-1 evolved along lines recommended by the NACA. Paid for by the Army Air Forces (later the U.S. Air Force) and flown by Air Force Capt. Charles E. Yeager for the first supersonic flight on Oct. 14, 1947, the Bell X-1’s historic feat reflected deserved credit to the Air Force for its vision in pursuing the program. But before it went beyond Mach 1, the X-1 incorporated advances promulgated by the research minds of the NACA. The problem with supersonic flight centered around dramatic and punishing shock waves that disrupted air flow over wings and tail surfaces as an aircraft neared the speed of sound. The NACA came up with the idea of making the X-1’s horizontal stabilizer thinner than the wing section, to ensure the wing and tail did not enter the grip of transonic shock waves at the same time. Perhaps of greater significance, the NACA pioneered the use and rationale for a movable horizontal stabilizer, permitting the X-1 to alter how its tail surfaces met the shock wave in flight. (This has since become a standard feature on many aircraft, civil and military, that enter the realm of transonic flight.)
 
With the X-1 set to make its flights with destiny over Rogers Dry Lake in California’s Mojave Desert, five NACA engineers traveled there in the fall of 1946, leading to creation of the NACA High-Speed Flight Research Station, further evolved into what is now the NASA Dryden Flight Research Center, located on Edwards Air Force Base. The message was clear: speed was king.
 
D-558-I and D-558-II
 
Even as Bell and the Air Force capitalized on the 1, Douglas Aircraft, the U.S. Navy, and the NACA perfected the slightly slower, but
longer-endurance, D-558-I Skystreak to provide a wealth of transonic flight data unobtainable from most wind tunnels of the era. In fact, the D-558-Is helped validate new wind tunnel designs by flying mission profiles to gather data for comparison with predictions garnered in new NACA-developed tunnels.
 
Flying the required four passes over a timed course, Navy Commander Turner F. Caldwell set a speed record of 640.663 miles an hour in the D-558-I on Aug. 20, 1947, only to be bested five days later by Marine Corps Maj. Marion Carl who made it official at 650.796 miles an hour in the sister ship to the Skystreak flown by Commander Caldwell.
 
Then, high over Rogers Dry Lake, the sweptwing Douglas D-558-II Skyrocket wrote a new page in high speed flight when NACA pilot Scott Crossfield became the first person to reach twice the speed of sound on Nov. 20, 1953. In six short years since Yeager’s first supersonic flight, the boundary had been pushed back that far.
 
The three highly-streamlined Skyrockets advanced the level of
understanding of transonic and supersonic phenomena on swept wings — data of significance to subsequent generations of production jet fighters for the U.S. Air Force, Navy and Marines. The Skyrockets also zoomed into public recognition and fancy, serving as the prototype for the high-speed aircraft flown by Captain Midnight in 1950s television adventures, and as the subject of models and toys.
 
X-2
 
The NACA worked with the Air Force to exploit the Bell X-2, another sweptwing rocket plane probing the limits of speed and altitude. Air Force pilot Capt. Mel Apt lost his life immediately after entering the books as the first pilot to breach Mach 3 when the X-2 went out of control due to the effects of inertial coupling.
 
The X-2 is significant for more than reaching triple-sonic speed first; its test program also was first to feature a simulator "used for the various functions of flight test planning, pilot training, extraction of aerodynamic derivatives, and analysis of flight data," notes NASA Dryden historian Dr. J.D. Hunley.
 
X-15
 
Intervening X-planes contributed to the body of aeronautical knowledge in many ways, including advances in swept wings, movable wings,
remotely piloted aircraft and other facets of use to industry. But not until the X-15 of 1959 was the realm of high-speed flight energized as it had been by the revolutionary X-1 back in 1947.
 
NASA was the lead organization, in league with the U.S. Air Force, the U.S. Navy, and contractor North American Aviation, in the ambitious X-15 program. Three X-15s brought to the NASA research facility on Edwards Air Force Base used new alloys, coatings, powerplants, controls, and techniques to obliterate previous obstacles to high speed, high altitude flight. The X-15s handily claimed Mach 4, then 5, 6, and ultimately Mach 6.7 — that’s 4,520 miles an hour. Maximum altitude reached was 354,200 feet, or 67 miles above the earth; more than sufficient to earn some X-15 pilots astronaut wings.
 
The X-15 program led to greater understanding of hypersonic (Mach 5 and above) heating phenomena, subsequently aiding in design of the Space Shuttle. Navigation equipment pioneered on the X-15 later helped guide Apollo moon rockets; insulation tests also bore fruit for Apollo.
 
Today, indicative of their significance to the progress of flight, the two remaining X-15s are enshrined in the National Air and Space Museum in Washington, D.C., and the Air Force Museum in Dayton, Ohio.
 
Speeding Into the Future
 
As you read this, engineers and technicians at the NASA Dryden Flight Research Center are readying a new unpiloted craft for the return to hypersonic research flights. Not since Bill Dana last flew an X-15 past Mach 5 on Oct. 24, 1968 have aircraft ventured that fast. The air-breathing X-43 is being groomed at Dryden for launch from a B-52, at the head of a Pegasus missile, for an assault on Mach 7 and Mach 10 that promises to yield significant data about new hypersonic-capable powerplants.
 
Speed is back in fashion, this time as an avenue for the development of hypersonic transport aircraft. And NASA Dryden’s brain trust in the high Mojave Desert is once again the focus for this flight research effort, to bring to life the hypersonic X-43 program managed by NASA’s Langley Aeronautical Research Center in Virginia.