The Legacy of the X-15

The year 1999 marked the 40th anniversary of the first flight of the X-15; this anniversary occurred more than 30 years after the program ended. The X-15 was the last high-speed research aircraft to fly as part of the research airplane program. The stillborn X-30 of the 1980s never took flight, and the verdict is still out on the fate of the Lockheed Martin X-33 demonstrator. Neil Armstrong, among others, once called the X-15 “the most successful research airplane in history.”1

Twelve men flew X-15. Scott Crossfield was first; William Dana was last. Pete Knight went 4,520 mph (Mach 6.70); Joe Walker went 67 miles (354,200 feet) high. Five of the pilots were awarded Astronaut Wings. Mike Adams died. What was learned? What should have been learned?

In October 1968 John V. Becker enumerated 22 accomplishments from the research and development work that produced the X-15, 28 accomplishments from its actual flight research, and 16 from experiments carried by the X-15. Becker’s comments have been well documented elsewhere, but are quoted here as appropriate.2

Nearly ten years after Becker’s assessment, Captain Ronald G. Boston of the U. S. Air Force Academy’s history department reviewed the X-15 program for “lessons learned” that might benefit the development of the X-24C National Hypersonic Flight Research Facility Program, an effort that was cancelled shortly afterwards. Boston’s paper offered an interesting perspective on the X-15 from the vantage point of the mid-1970s?

In 1999, the historian at the Dryden Flight Research Center, J. D. “Dill” Hunley, wrote a lessons-leamed paper on the X-15. Drawing heavily but not uncritically upon Becker’s and Boston’s insights, it too pro­vides an interesting perspective, and is quot­ed several times in the pages that follow.4

Lessons Learned (or not)

The X-15 was designed to achieve a speed of Mach 6 and an altitude of 250,000 feet to explore the hypersonic and near-space envi­ronments. More specifically, its goals were:

(1) to verify existing (1954) theory and wind tunnel techniques;

(2) to study aircraft structures under high (1,200 degrees Fahrenheit) heating;

(3) to investigate stability and control problems associated with high-altitude boost and reentry; and

(4) to investigate the biomedical effects of both weightless and high-g flight.

All of these design goals were met, and most were surpassed. The X-15 actually achieved Mach 6.70, 354,200 feet, 1,350 degrees Fahrenheit, and dynamic pressures over 2,200 pounds per square foot.5 In addition, once the original research goals were achieved, the X-15 became a high-altitude hypersonic testbed for which 46 follow-on experiments were designed.

Unfortunately due to the absence of a subse­quent hypersonic mission, aircraft applica­tions of X-15 technology have been few. Given the major advances in materials and computer technology in the 30 years since the end of the flight research program, it is

unlikely that many of the actual hardware lessons are still applicable. That being said, the lessons learned from hypersonic model­ing, simulation, and the insight gained by being able to evaluate actual X-15 flight test results against wind tunnel and predicted results, greatly expanded the confidence of researchers during the 1960s and 1970s.

In space, however, the X-15 contributed sig­nificantly to both the Apollo and Space Shuttle programs. Perhaps the major contribu­tion was the final elimination of a spray-on ablator as a possible thermal protection sys­tem for the Space Shuttle. This would likely have happened in any case as the ceramic tiles and metal shingles were further developed, but the operational problems encountered with the (admittedly brief) experience on X-15A-2 hastened the departure of the abla­tors. Although largely intangible, proving the value of man-in-the-loop simulations and pre­cision “dead-stick” landings have also been invaluable to the Space Shuttle program.

The full value of X-15’s experience to designing advanced aircraft and spacecraft can only be guessed at. Many of the engi­neers (including Harrison Storms) from the X-15 project worked on the Apollo space­craft and the Space Shuttle. In fact, the X-15 experience may have been part of the reason that North American was selected to build later spacecraft. Yet X-15’s experience is overshadowed by more recent projects and becomes difficult to trace as systems evolve through successive programs. Nonetheless, many of those engineers are confident that they owe much to the X-15, even if many are at a loss to give any concrete examples.