X-15 Research Results
A Hypersonic Structure
PERHAPS NOWHERE ELSE are the broad, interdiscipiinary
facets of hypersonic and reentry flight so apparent as in a close
examination of the X-15 structure. The basic effect of any change
in airflow, aerodynamic heating, or maneuvering loads is to alter
the stresses within each structural element. In some places, the
combination of stresses has permanently marred the once-sleek lines
of the wings and fuselage. Some scars are penalties for incomplete
understanding of the aerodynamic and thermal forces of airflow
from subsonic to hypersonic speeds. Others were left by the
oscillatory airflow that superimposed dynamic forces on already severe
static-load conditions. The deepest scars are found where the interplay
among these varied stresses intensified the effects of each. Yet,
these scars are superficial, of the engineering-fix type. The basic
structure has withstood repeated flights into the high temperatures
of hypersonic flight.
Although many details of the stresses within a heat-sink structure
were uncovered during the flight program, the major questions had
to be answered during design and construction. The problem for
the structural engineer would be relatively simple if weight were of
little importance. For example, the essential difference between the
weight of a diesel train and that of an airplane is that sufficient metal
is used in the former to maintain uniformly low stress levels throughout
the structure, while an airplane, in order to achieve minimum
weight, maintains uniformly high stress levels. For the X-15, it
was essential to achieve uniformly high stress levels within each
load-carrying element for the many uneven and fluctuating load
conditions of flight anywhere within the corridor, and with a reasonable
margin of safety.
The compounding factor was the effect of aerodynamic heating.
It required a reorientation of the structural designers' thinking,
because the many interactions of a hot structure impose further
stresses on a pattern already made complex by airloads. The designer
must analyze and sum the individual stresses from static airloads,
dynamit airloads, aerodynamic, and their interactions.
Since the structure responds dynamically as well as statically, a
complex chain of reaction and interaction faces the analyst.
Surprisingly, the force from aerodynamic lift that sustains or
maneuvers the X-15 is not a major stress problem. The total lift
force on the wings of the X-15 during reentry could be carried by
the wings of the Spirit of St. Louis, in which Charles Lindbergh
crossed the Atlantic. But this statement neglects the distribution
of that force, the added stresses from airloads that twist the wing,
and the dynamic loads. When these effects are included, the wing
of the Spirit of St. Louis would be as incapable of withstanding the
total airload during reentry as it would be vulnerable to aerodynamic
The effects from aerodynamic heating are twofold: reduction in
the strength of Inconel X as temperature increases, and distortion
of the structure from uneven thermal expansion. A new element
was also added to structural design, for with the heat-sink concept,
the time of exposure became the critical parameter that established
the amount of heat flow into the external structure when exposed
to a 2500° F airflow. In areas that carry only small aerodynamic
loads, Inconel X can withstand considerably more than 1200° F,
perhaps 1600° F. The sharp leading edge on the vertical fin has
withstood 1500° F, and one non-load-carrying section of the wing
skin has successfully endured 1325° F. These temperatures are
experienced for only brief periods of time, however. Prolonged
exposure would eventually cause these temperatures to be conducted
to load-carrying members, and thus impair the structural integrity
of the X-15.
The structural design requires a careful balancing between the
amount of material required to carry the load and that needed to
absorb the heat flow. On a typical flight, the structure near the
nose experiences 20 times as much heat input as the aft end. In
regions of high heat input -fuselage nose, wing leading edge, tail
leading edge- solid bars of Inconel X are required to absorb the
A factor important to design balance is that the maximum load
and maximum heating temperature do not occur simultaneously.
In actual practice, high temperatures have been explored in
essentially level flight, with low aerodynamic loads; the high loads of
reentry were encountered at relative low temperatures. But whenever
Mach numbers greater than 4.5 are achieved, the thermal
potential of the airflow can drastically affect the plane's strength.
Structural failure could occur at even low load levels during prolonged
flights at Mach 5 at low altitude where the heat flow is at
The structural engineer is faced with another formidable design
task in dealing with aeroclastic and aerothermoelastic problems.
The root cause is the flexibility of the structure and the deflection
that accompanies each stress. Although the X-15 isn't as flexible
as the wing of a jet transport, the effects on it of even minute
distortion can be far-reaching. The difficulty is that though structural
deflection is not objectionable, it induces additional aerodynamic
forces from the change in angle between the structure and the airflow.
This redistributes the airload and results in a further change
in pressure forces and deflection, which continues until the aerodynamic
forces and structural resistance are in equilibrium. Thus the rigidity
of the structure appreciably affects the load it is subjected
to. While rigidity influences fuselage design to some extent, it was
a prime factor in the design of the thin wings and tail surfaces. For
they must have not only adequate resistance to bending but also
adequate torsional rigidity to resist twisting.
At high speeds, the large forces acting on surfaces require the
designer to analyze more and more exactly these elastic deformations.
Yet the solution for complex flow patterns and deflection from
thermal expansion often does not yield to analysis. Another consequence
of flight to speeds above the transonic region is that the airflow
is characteristically fluctuating, and causes buffeting and vibrations.
In some instances, resonances, or self-excited oscillations between
airflow and structure, are encountered. This phenomenon,
called flutter, is extremely complicated, since resonances are possible
in any combination of bending and torsional oscillations.
Aeroelastic problems began to play a prominent role in high-speed
aircraft design soon after World War II. Prior to that time,
aircraft structures usually were sufficiently rigid and speeds
sufficiently low to avoid most aeroelastic problems. But such problems
had been encountered frequently enough during flight
-often disastrously- to stimulate many studies into the phenomena.
By the early 1950's, much had been learned about the interactions
of aerodynamic-elastic-inertial forces through theoretical analysis
and experiment. But much remained vague and unknown. Each
increase in speed seemed to compound the problems. Even simplified
theories to account for interactions required such complicated
systems of equations as to preclude their practical use in the era before
modern, high-speed digital computers. Designers relied upon wind-tunnel
tests with dynamically-scaled models to study the aeroelastic
response of the structure. They sometimes obtained final verification
only through slow and tedious flight tests.
The X-15's extension of flight conditions to Mach 6 and large
aerodynamic forces represented a step into many new aeroelastic
areas. At the time of the design, there were no experimental flutter
data for speeds above Mach 3, and an adequate aerodynamic theory
had not been established. To this perplexity was added the question
of the effects of heating the structure to 1200° F. This high
temperature not only reduces the strength but the stiffness of Inconel X,
lessening its resistance to deflection.
tubes in the leading edge of the X-15's upper vertical tail measure
The thermal expansion of a hot structure reduces stiffness more
markedly, however. The uneven heating of the structure produces
large differences in the expansion of its various elements.
The distorsion caused by this uneven expansion seriously increases
the aeroelastic problems, for it can reduce stiffness as much as 60
airflow conditions in the wake of the fuselage. The craft's instrumentation,
elaborate despite weight and volume restrictions, measured pressure at
Although some aeroelastic problems could be scaled for wind-tunnel
testing, no facility existed for combined testing of aerothermoelastic
problems during the design period. (Later, some full-scale
tests were made in a new NASA facility to proof-test the vertical tail
at Mach 7 and design temperature and pressures.)
A rather novel test program was undertaken to overcome this
potentially serious lack. Small dynamic models were tested in the
"cold" condition, with their stiffness reduced to simulate the
hot-structure condition. The amount of reduction in stiffness was
determined from laboratory tests of structural samples subjected to the
anticipated load-temperature variation with time during flight. A
very extensive test program was carried out, including tests in eight
different wind tunnels, at speeds to Mach 7.
From these various design conditions and procedures, a structure
developed that bears many similarities to, as well as differences from,
those of previous aircraft technology. The basic structure is a
conventional monocoque design, in which the primary loads are carried
in the external skin of the fuselage and wing. The fuselage skin
also forms the outer shell of the propellant tanks. Thus, it must
withstand the stresses from propellant weight as well as from internal
tank pressurization. To absorb heat input, skin thicknesses on the
forward fuselage are about three times those near the tail section.
Fifteen feet aft from the nose, skin thickness is sized by load, rather
than by heating, and is comparable to that of aluminum structure.
An important feature of the structural design is that only a small
amount of the heat absorbed by the external skin is conducted, or
radiated, to the internal structure. Consequently, much of the
internal structure of the fuselage is of titanium and aluminum.
Extensive use is made of corrugations and beading, to allow for uneven
thermal expansion between external skin and internal structure.
The wing presented a difficult design problem, to account for uneven
heating from leading edge to trailing edge and between lower
and upper surfaces. At high angles of attack, inconsistent heating
typically subjects the wing's lower surface to temperatures 400° F
higher than those of the upper surface. The result of higher heating
at the leading edge and lower surface is that these two surfaces
try to expand faster than the rest of the wing. Thus, the wing
structure had to be designed to allow for this expansion without
deforming to a large extent, while, at the same time, carrying rather
large airloads. A balance was achieved by allowing some expansion
of skin to alleviate a part of the thermally induced stresses, and by
the use of titanium internal structure, which has a higher elasticity
than Inconel X. The internal structure provides enough restraint
between attach points to give the hot wing surfaces a tufted-pillow
appearance as they try to expand. Corrugations in the internal
structure allow it to flex enough to keep skin stress within tolerable
The movable horizontal tail presented another knotty structural-design
problem. Aerothermoelastic effects were severely complicated,
since the movable surface could not be rigidly attached to the
fuselage along the length of the inboard end. All loads had to be
carried through the single pivot point, which made much more difficult
the problem of maintaining adequate torsional rigidity. This
problem was so predominant, in fact, that it was the basic factor
governing the design of the horizontal tail. In order to achieve
adequate stiffness, the external surface here is restrained much more
than the wing surface, and the pillowing effect at high temperature
is quite marked. These are transient effects, however; no permanent
deformation has been observed.
paint strikingly reveals the uneven heating
Despite the general information gained during design and construction,
several interesting additional problems were uncovered
during flight. It is not unusual that these problems occurred in
regions of large aeroelastic and aerothermoelastic interactions, or in
regions of large thermal stress.
to which the X-15's heat-sink wing structure was subjected during a
high-heating mission. Dark areas indicate the higher temperature.
Light areas reveal internal structure.
A classical example of the interaction among aerodynamic flow,
thermodynamic properties of air, and elastic characteristics of
structure was the local buckling at four locations, just aft of the
leading edge of the wing, during the first significant high-temperature
flight to Mach 5. This buckling occurred directly back of the expansion
slots that had been cut in the leading edge of the wing. The slots
induced transition to turbulent flow, with an accompanying large
increase in heat flow to the surrounding structure. The resulting
thermal stresses in the skin because of hot spots and uneven
expansion produced small, lasting buckles in the wing surface. From
this one flight, the problem of even small surface discontinuities was
revealed, and the mechanism of the problem analyzed. Fortunately,
the buckles could be removed, and relatively minor modifications
were made to eliminate a recurrence. Additional expansion slots
were cut, and thin cover plates were made for all slots, to
prevent turbulent flow.
drawing shows a typical buckle in the wing skin of the X-15,
Another problem from turbulent flow has been the cracking of
the canopy glass. The canopy protrudes into the airflow behind the
nose shock wave, and, in combination with the flow around the
fuselage, produces an unpredictable tangle of turbulent flow conditions.
Although initial analysis indicated that the glass would be
subjected to maximum temperatures of 750° F, more detailed
studies revealed that the glass would be heated to the same maximum
temperatures as the Inconel X structure. Structural integrity was
seriously threatened, in consequence. Although the solution was a
dual glass design, with an outer pane of high-temperature
alumina-silica glass, both inner and outer panes have cracked in
the course of the flight program. Fortunately, they have never cracked
simultaneously on both sides of the canopy, nor have both panes cracked
on one side. The failures were due to thermal stresses in the
glass-retainer ring. Several changes in its shape and material to
minimize hot spots have eliminated the problem. It has served to
emphasize the difficulty of predicting thermal strsses for this condition.
It remains an area of deficiency in research information.
caused by uneven expansion between the leading edge and the area
directly behind it in hot airflow at hypersonic speed. Bottom drawing
shows how covering the slots with small Inconel tabs and adding a
rivet prevented recurrence of the buckling.
noteworthy scar of the X-15's first flight to Mach 6 was this cracked outer
The aeroelastic-model program carried out during design successfully
eliminated surface flutter. However, the lightweight design
resulted in some very thin skins, which have proved susceptible to a
variety of vibration, noise, and peculiar flutter problems. Most of
these were overcome during extensive ground-testing and captive
B-52 flight tests. But one of the many unusual facets of flutter still
plagued the flight program. This was the fluttering of individual
external skin panels rather than an entire surface. It was first
encountered on the fuselage side fairings, later on the vertical tail.
Previous supersonic research had made it known, but it was not predicted
to be a problem for the X-15. However, it was encountered
at moderate supersonic speeds, and restricted flight operations over
much of the corridor until a solution for it was found.
on the right side of the windshield. Investigators found that thermal stresses
than expected in the metal retainer holding the glass had caused the cracking.
An extensive wind-tunnel and analysis program was carried out
in conjunction with X-15 flight tests. By the time the program was
completed, 38 panels on the airplane had been found susceptible
to flutter. By good luck, relatively minor modifications, which
stiffened the panels and increased their resistance to fluctuating airflow,
eliminated the problem. Since this was the first occurrence of panel
flutter to be well documented and explored, it stimulated much research
into the basic mechanism.
More than 75 flights of the X-15 to high temperatures have demonstrated
the soundness of the basic load-thermal-stress analysis.
Much remains unknown about the magnitude of the individual airload
and thermal stresses and deflections within the structure, however.
For design, these unknowns were overcome through ingenuity
and judgement in introducing assumptions for a simplified model of
the structure. Sometimes, a simple beam suffices as a model. But
researchers continue to try to develop models that will yield exact
solutions for the distribution of load stresses and thermal stresses.
For complex structure such as the X-15, it is a very difficult analysis
problem trying to match actual responses to their model. It requires
the use of high-speed digital-computer techniques.
Structural loads at the very lowest end of the flight corridor, the
landing, have also received much study. The X-15 represents a
new class of reentry vehicles, for which the externally stored landing
gear must be able to withstand high temperatures from aerodynamic
heating, in addition to normal landing loads. The landing gear
developed to meet these requirements for the X-15 is unusual. On
a normal airplane, primary impact loads of landing are absorbed by
the main gear, located close to the plane's center of gravity. But the
extreme-aft location of the main landing skids on the X-15 produces
dynamic-response characteristics during landing that are as unusual
as the gear itself.
The primary cause of the unconventional response is the craft's
downward rotation onto the nose gear immediately following the
main gear's touchdown. Significantly, this movement onto the
nose gear causes a subsequent rebound onto the main gear, providing
a much higher load there than that at initial touchdown. In addition,
the nose gear encounters loads that are two to three times greater
than at either of the main-gear skids. Another unique feature is
that the gear loads achieve about the same maximum level whether
the pilot "greases-it-on" or lands with a high rate of descent. These
new gear characteristics have not been without problems. Much
study and analysis of the dynamic response of the airplane during
landing has led to strengthening the gear and back-up structure and
modifying the nose gear so as to provide greater energy absorption.
The concept represents a distinct state-of-the-art advance for
high-temperature, lightweight landing gears.
The landing-gear research information may have more lasting
significance than the heat-sink structural development. A new
concept of radiation cooling has been developed for flight to Mach
10 or 20, which limits structural temperatures to 3000° F yet
requires no more structural weight than the X-15 has.
While the heat-sink concept now appears to have limited future
application, it has admirably served a vital function for the X-15
program. The successful development of the concept has made it
possible to explore hypersonic-airflow conditions of 2500° F with
confidence. Certainly, the early philosophy that more could be
learned from a hot structure has borne fruit. And, of course, much
information pertinent to the response of the structure to airload
and thermal stress has universal application. Deficiencies in research
information have been pinpointed for the canopy, panel flutter,
and aerothermoelastic effects. Although some details are still
obscure, engineers have a clearer understanding of the complex
interactions between local airflow and structural response.
The success of this structural development is shown by the fact
that speeds of Mach 6 and temperatures of 1200° F have been
probed repeatedly. In addition, flights have been made to the
high-air-pressure conditions at the lower boundary of the flight corridor
between Mach 5 and Mach 6, which produced a maximum temperature
of 1325° F. Thus, the full speed and temperature potentials
of the X-15 have been achieved.
While the design-altitude goal of 250 000 feet has also been
achieved (and actually exceeded by 100 000 feet), the full altitude
potential of 400 000 feet has not been attained. The limit for flight
above the corridor, however, is a compounding of many factors other
than airloads and thermal effects. In fact, relatively low temperatures
are encountered during a high-altitude flight, and thermal
effects are of only minor importance. The primary limiting factors
are the conditions encountered during reentry. These include
consideration of over-all airplane response to the effects of structural
load, aerodynamic flow, control system, and pilot control. Since
these effects are transient in nature, reentry flight represents a
difficult compounding of the dynamic response to flight to extreme