Facing the Heat Barrier - NASA's History Office

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The X-15
Across almost half a century, the X-15 program stands out to this day not only
for its achievements but for its audacity. At a time when the speed record stood right
at Mach 2, the creators of the X-15 aimed for Mach 7—and nearly did it.* Moreover,
the accomplishments of the X-15 contrast with the history of an X-planes
program that saw the X-1A and X-2 fall out of the sky due to flight instabilities, and
in which the X-3 fell short in speed because it was underpowered. 1
The X-15 is all the more remarkable because its only significant source of aerodynamic
data was Becker’s 11-inch hypersonic wind tunnel. Based on that instrument
alone, the Air Force and NACA set out to challenge the potential difficulties
of hypersonic piloted flight. They succeeded, with this aircraft setting speed and
altitude marks that were not surpassed until the advent of the space shuttle.
It is true that these agencies worked at a time of rapid advance, when performance
was leaping forward at rates never approached either before or since. Yet
there was more to this craft than a can-do spirit. Its designers faced specific technical
issues and overcame them well before the first metal was cut.
The X-3 had failed because it proved infeasible to fit it with the powerful turbojet
engines that it needed. The X-15 was conceived from the start as relying on
rocket power, which gave it a very ample reserve.
Flight instability was already recognized as a serious concern. Using Becker’s
hypersonic tunnel, the aerodynamicist Charles McLellan showed that the effectiveness
of tail surfaces could be greatly increased by designing them with wedge-shaped
profiles. 2
The X-15 was built particularly to study problems of heating in high-speed
flight, and there was the question of whether it might overheat when re-entering
the atmosphere following a climb to altitude. Calculations showed that the heating
would remain within acceptable bounds if the airplane re-entered with its nose high.
This would present its broad underbelly to the oncoming airflow. Here was a new
application of the Allen-Eggers blunt-body principle, for an airplane with its nose
up effectively became blunt.
*Official flight records are certified by the Federation Aeronautique Internationale. The cited
accomplishments lacked this distinction, but they nevertheless represented genuine achievements.
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