Facing the Heat Barrier - NASA's History Office

Facing the Heat Barrier: A History of Hypersonics
had taken form with considerable dispatch, and the key was the feasibility study
of Becker’s group. An independent review at WADC confirmed its conclusions,
whereupon Air Force leaders, both in uniform and in mufti, embraced the concept.
Approval at the Pentagon then came swiftly.
In turn, this decisiveness demonstrated a willingness to take risks. It is hard today
to accept that the Pentagon could endorse this program on the basis of just that one
study. Moreover, the only hypersonic wind tunnel that was ready to provide supporting
research was Becker’s 11-inch instrument; the AEDC hypersonic tunnels
were still several years away from completion. But the Air Force was in no mood to
hold back or to demand further studies and analyses.
This service was pursuing a plethora of initiatives in jet bombers, advanced fighters,
and long-range missiles. Inevitably, some would falter or find themselves superseded,
which would lead to charges of waste. However, Pentagon officials knew that
the most costly weapons were the ones that America might need and not have in
time of war. Cost-benefit analysis had not yet raised its head; Robert McNamara
was still in Detroit as a Ford Motor executive, and Washington was not yet a city
where the White House would deliberate for well over a decade before ordering the
B-1 bomber into limited production. Amid the can-do spirit of the 1950s, the X-15
won quick approval.
X-15: The Technology
Four companies competed for the main contract, covering design and construction
of the X-15: Republic, Bell, Douglas, and North American. Each of them
brought a substantial amount of hands-on experience with advanced aircraft.
Republic, for example, had Alexander Kartveli as its chief designer. He was a highly
imaginative and talented man whose XF-105 was nearly ready for first flight and
whose XF-103 was in development. Republic had also built a rocket plane, the XF-
91. This was a jet fighter that incorporated the rocket engine of the X-1 for an extra
boost in combat. It did not go into production, but it flew in flight tests.
Still, Republic placed fourth in the competition. Its concept rated “unsatisfactory”
as a craft for hypersonic research, for it had a thin outer fuselage skin that
appeared likely to buckle when hot. The overall proposal rated no better than average
in a number of important areas, while achieving low scores in Propulsion System
and Tanks, Engine Installation, Pilot’s Instruments, Auxiliary Power, and Landing
Gear. In addition, the company itself was judged as no more than “marginal” in
the key areas of Technical Qualifications, Management, and Resources. The latter
included availability of in-house facilities and of an engineering staff not committed
to other projects. 53
Bell Aircraft, another contender, was the mother of research airplanes, having
built the X-1 series as well as the X-2. This firm therefore had direct experience
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The X-15
both with advanced heat-resistant metals and with the practical issues of powering
piloted aircraft using liquid-fuel rocket engines. It even had an in-house group that
was building such engines. Bell also was the home of the designers Robert Woods
and Walter Dornberger, with the latter having presided over the V-2.
Dornberger’s Bomi concept already was introducing the highly useful concept of
hot structures. These used temperature-resistant alloys such as stainless steel. Wings
might be covered with numerous small and very hot metal panels, resembling shingles,
that would radiate heat away from the aircraft. Overheating would be particularly
severe along the leading edges of wings; these could be water-cooled. Insulation
could protect an internal structure that would withstand the stresses and forces of
flight; active cooling could protect a pilot’s cockpit and instrument compartment.
Becker described these approaches as “the first hypersonic aircraft hot structures
concepts to be developed in realistic meaningful detail.” 54
Even so, Bell ranked third. Historian Dennis Jenkins writes that within the proposal,
“almost every innovation they proposed was hedged in such a manner as to
make the reader doubt that it would work. The proposal itself seemed rather poorly
organized and was internally inconsistent (i.e., weights and other figures frequently
differed between sections).” 55 Yet the difficulties ran deeper and centered on the
specifics of its proposed hot structure.
Bell adopted the insulated-structure approach, with the primary structure being
of aluminum, the most familiar of aircraft materials and the best understood. Corrugated
panels of Inconel X, mounted atop the aluminum, were to provide insulation.
Freely-suspended panels of this alloy, contracting and expanding with ease,
were to serve as the outer skin.
Yet this concept was quite unsuitable for the X-15, both on its technical merits
and as a tool for research. A major goal of the program was to study aircraft structures
at elevated temperatures, and this would not be possible with a primary structure
of cool aluminum. There were also more specific deficiencies, as when Bell’s
thermal analysis assumed that the expanding panels of the outer shell would prevent
leakage of hot air from the boundary layer. However, the evaluation made the flat
statement, “leakage is highly probable.” Aluminum might not withstand the resulting
heating, with the loss of even one such panel leading perhaps to destructive
heating. Indeed, the Bell insulated structure appeared so sensitive that it could be
trusted to successfully complete only three of 13 reference flights. 56
Another contender, Douglas Aircraft, had shared honors with Bell in building
previous experimental aircraft. Its background included the X-3 and the Skyrocket,
which meant that Douglas also had people who knew how to integrate a liquid
rocket engine with an airplane. This company’s concept came in second.
Its design avoided reliance on insulated structures, calling instead for use of a
heat sink. The material was to be a lightweight magnesium alloy that had excellent
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