Facing the Heat Barrier - NASA's History Office

Facing the Heat Barrier: A History of Hypersonics
The plane’s designers also benefited from a stroke of serendipity. Like any airplane,
the X-15 was to reduce its weight by using stressed-skin construction; its
outer skin was to share structural loads with internal bracing. Knowing the stresses
this craft would encounter, the designers produced straightforward calculations to
give the requisite skin gauges. A separate set of calculations gave the skin thicknesses
that were required for the craft to absorb its heat of re-entry without weakening.
The two sets of skin gauges were nearly the same! This meant that the skin could
do double duty, bearing stress while absorbing heat. It would not have to thicken
excessively, thus adding weight, to cope with the heat.
Yet for all the ingenuity that went into this preliminary design, NACA was a very
small tail on a very large dog in those days, and the dog was the Air Force. NACA
alone lacked the clout to build anything, which is why one sees military insignia
on photos of the X-planes of that era. Fortuitously, two new inventions—the twinspool
and the variable-stator turbojet—were bringing the Air Force face to face with
a new era in flight speed. Ramjet engines also were in development, promising still
higher speed. The X-15 thus stood to provide flight-test data of the highest importance—and
the Air Force grabbed the concept and turned it into reality.
Origins of the X-15
Experimental aircraft flourished during the postwar years, but it was hard for
them to keep pace with the best jet fighters. The X-1, for instance, was the first
piloted aircraft to break the sound barrier. But only six months later, in April 1948,
the test pilot George Welch did this in a fighter plane, the XP-86. 3 The layout of the
XP-86 was more advanced, for it used a swept wing whereas the X-1 used a simple
straight wing. Moreover, while the X-1 was a highly specialized research airplane,
the XP-86 was a prototype of an operational fighter.
Much the same happened at Mach 2. The test pilot Scott Crossfield was the
first to reach this mark, flying the experimental Douglas Skyrocket in November
1953. 4 Just then, Alexander Kartveli of Republic Aviation was well along in crafting
the XF-105. The Air Force had ordered 37 of them in March 1953. It first flew in
December 1955; in June 1956 an F-105 reached Mach 2.15. It too was an operational
fighter, in contrast to the Skyrocket of two and a half years earlier.
Ramjet-powered craft were to do even better. Navaho was to fly near Mach 3. An
even more far-reaching prospect was in view at that same Republic Aviation, where
Kartveli was working on the XF-103. It was to fly at Mach 3.7 with its own ramjet,
nearly 2,500 miles per hour (mph), with a sustained ceiling of 75,000 feet. 5
Yet it was already clear that such aircraft were to go forward in their programs
without benefit of research aircraft that could lay groundwork. The Bell X-2 was
in development as a rocket plane designed to reach Mach 3, but although first
thoughts of it dated to 1945, the program encountered serious delays. The airplane
did not so much as fly past Mach 1 until 1956. 6
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The X-15
Hence in 1951 and 1952, it already was too late to initiate a new program aimed
at building an X-plane that could provide timely support for the Navaho and XF-
103. The X-10 supported Navaho from 1954 to 1957, but it used turbojets rather
than ramjets and flew at Mach 2.There was no quick and easy way to build aircraft
capable of Mach 3, let alone Mach 4; the lagging X-2 was the only airplane that
might do this, however belatedly. Yet it was already appropriate to look beyond the
coming Mach 3 generation and to envision putative successors.
Maxwell Hunter, at Douglas Aircraft, argued that with fighter aircraft on their
way to Mach 3, antiaircraft missiles would have to fly at Mach 5 to Mach 10. 7 In
addition, Walter Dornberger, the wartime head of Germany’s rocket program, now
was at Bell Aircraft. He was directing studies of Bomi, Bomber Missile, a twostage
fully reusable rocket-powered bomber concept that was to reach 8,450 mph,
or Mach 12. 8 At Convair, studies of intercontinental missiles included boost-glide
concepts with much higher speeds. 9 William Dorrance, a company aerodynamicist,
had not been free to disclose the classified Atlas concept to NACA but nevertheless
declared that data at speeds up to Mach 20 were urgently needed. 10 In addition, the
Rand Corporation had already published reports that envisioned spacecraft in orbit.
The documents proposed that such satellites could serve for weather observation
and for military reconnaissance. 11
At Bell Aircraft, Robert Woods, a co-founder of the company, took a strong
interest in Dornberger’s ideas. Woods had designed the X-1, the X-1A that reached
Mach 2.4, and the X-2. He also was a member of NACA’s influential Committee on
Aerodynamics. At a meeting of this committee in October 1951, he recommended
a feasibility study of a “V-2 research airplane, the objective of which would be to
obtain data at extreme altitudes and speeds and to explore the problems of re-entry
into the atmosphere.” 12 He reiterated this recommendation in a letter to the committee
in January 1952. Later that month, he received a memo from Dornberger
that outlined an “ionospheric research plane,” capable of reaching altitudes of “more
than 75 miles.” 13
NACA Headquarters sent copies of these documents to its field centers. This
brought responses during May, as several investigators suggested means to enhance
the performance of the X-2. The proposals included a rocket-powered carrier aircraft
with which this research airplane was to attain “Mach numbers up to almost
10 and an altitude of about 1,000,000 feet,” 14 which the X-2 had certainly never
been meant to attain. A slightly more practical concept called for flight to 300,000
feet. 15 These thoughts were out in the wild blue, but they showed that people at least
were ready to think about hypersonic flight.
Accordingly, at a meeting in June 1952, the Committee on Aerodynamics
adopted a resolution largely in a form written by another of its members, the Air
Force science advisor Albert Lombard:
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