Transiting from Air to Space
The North American X-15
EDITOR'S INTRODUCTION
The first call for an X-15-class research vehicle came from
Robert J. Woods, a colleague of Walter Dornberger at Bell, during a
meeting of the prestigious NACA Committee on Aerodynamics on
October 4, 1951. He reiterated his support for such a vehicle
during subsequent meetings and, as a result, the NACA committee
passed a motion on June 24, 1952 that charged the agency to expand
its research aircraft program to include studying the problems of
manned and unmanned flight at altitudes between 12 and 50 miles, and
velocities of Mach 4 to Mach 10, as well as devoting "a modest
effort" to study exoatmospheric flight from Mach 10 to escape
velocity. The major NACA field centers exchanged various paper
plane proposals. NACA engineers L. Robert Carman and Hubert Drake
of the High-Speed Flight Station drew up configurations for Mach 3+
launch aircraft carrying small hypersonic research aircraft
including, in August 1953, a five-phase proposal culminating in the
design of an orbital air-launched hypersonic boost-glide winged
vehicle. The NACA shelved this bold proposal as too futuristic,
which it was; its advocacy of a "two-stage to orbit" research
vehicle was one of the earliest of the "piggyback" concepts
predating the current Space Shuttle. The NACA, like other federal
and private organizations, favored a more modest approach. In
October 1953, the Air Force's Scientific Advisory Board recommended
development of a Mach 5-7 research aircraft, and at the same time,
the Office of Naval Research had funded the Douglas Aircraft
Corporation to study the feasibility of a Mach 7+ rocket-propelled
research airplane, informally referred to as the D-558-3 1.
Figure 1
AN EARLY AMERICAN AIRCRAFT/ORBITER PROPOSAL:
THE DRAKE-CARMAN COMPOSITE RESEARCH AIRCRAFT PROPOSAL OF 1953.
During 1954, the NACA, in partnership with the Air Force and
Navy, further explored the hypersonic aircraft concept. The
agency's Langley laboratory (later NASA's Langley Research Center)
had formed a hypersonic study team comprised of chairman John V.
Becker, Maxime Faget, Thomas Toll, N. F. Dow, and J. B. Whitten, and
this group subsequently evolved a baseline design that closely
resembled the ultimate X-15 configuration. Their conception
incorporated Inconel alloy heat-sink construction, had a cruciform
tail configuration, a wedge vertical fin for increased directional
stability, and similar weights and specifications as the final
aircraft. In December 1954, the NACA, Air Force, and Navy agreed to
undertake joint development of the proposed hypersonic research
aircraft, and in January 1955 it received the designation X-15.
That same month, the Air Force (which administered the design and
construction phases of the project) held the first briefings for
potential contractors. This culminated in a competition between
North American, Bell, Douglas, and Republic, which North American
won on September 30, 1955. The Bell entry, which featured a novel
form of "double-wall" construction, reflected the firm's obsession
with Sänger-like boost-gliders (indeed, in April 1952, Bell's
Dornberger had journeyed to France in a vain attempt to convince
Sänger and his wife to join the company), and had no real hope of
winning. The subsequent technical development of the North American
X-15 went smoothly, with the exception of its rocket powerplant,
which generated great concern before it, too, reached fruition 2.
The X-15, "Round Two" in the parlance of the NACA, had many
features that separated it from the previous rocket research
aircraft and placed it at an intermediate level between the purely
supersonic aircraft (such as the X-1) and the purely winged reentry
vehicles (like the proposed "Round Three" Dyna-Soar and the eventual
Space Shuttle). For example, it incorporated a reaction control
system of hydrogen peroxide rocket thrusters for keeping the
aircraft under control at high altitudes; the pilot wore a full
pressure pilot protection suit (the Clark MC-2) having provisions
for physiological monitoring. It was the first flight vehicle to
blend the application of hypersonic aerodynamic theory to an actual
aircraft. It incorporated high temperature seals and lubricants,
and had a "Q-ball" flow direction sensor capable of operating with
stagnation air temperatures of 3500° F. The pilot relied on
inertial flight data systems developed especially for operation
under space-like conditions. The X-15's Inconel structure was the
first reusable super-alloy structure capable of withstanding the
temperatures and thermal gradients of hypersonic reentry.
Subsequently, during its flight program, the X-15 spawned
development and application of a refurbishable ablative heat
protection system (the Martin MA-25S) 3.
The X-15 spanned 22 ft. 4 in., and had a length of 50 ft. 9 in.
It utilized a Thiokol (Reaction Motors Division) XLR-99 throttleable
rocket engine, burning a mixture of anhydrous ammonia and liquid
oxygen. (Delays in the development of this engine forced North American to install
two XLR-11 engines in the X-15s during 1959, before beginning the
research program, for purposes of checking out the aircraft and its
systems; the first XLR-99 flight did not come
until November 15, 1960). The three X-15 aircraft quickly
established a number of speed and altitude marks, which often
obscured the less glamorous but occasionally more important work
they accomplished in mapping out the frontiers of' hypersonic flight.
By the end of 1961, the X-15 had achieved its Mach 6 design speed,
and had reached altitudes in excess of 200,000 feet. On
August 22, 1963, NASA research pilot Joseph Walker reached 354,200
feet in the third X-15 aircraft, still a record for winged vehicles.
X-15 testing revealed a number of interesting conditions about
hypersonic flight, including the discovery that hypersonic boundary
layer flow is turbulent and not laminar, that turbulent heating
rates were lower than predicted by theory, that supersonic skin
friction was likewise lower than predicted, that local surface
irregularities generated hot spots (in one notable case, aerodynamic
heating caused buckling of the wing skin behirid leading edge heat
expansion slots), and that the cruciform tail configuration created
a serious adverse roll problem at high angles of attack during
atmospheric reentry (NASA cured this by removing the jettisonable
lower half of the craft's ventral fin). The flights demonstrated
that a pilot could successfully transition from aerodynamic to
reaction controls and back again, function in a weightless
environment (which became an academic question after Vostok and
Mercury), control a rocket-boosted vehicle during atmospheric exit,
and use energy management techniques to make a hypersonic/supersonic
reentry and glide approach to a precision landing. The X-15
eventually made reentries at angles of attack up to 26 deg. and at
flightpath angles as low as -38 degrees at Mach 6 flight speeds 4.
Figure 2
THE X-15 ROCKET RESEARCH AIRPLANE
As with the previous "Round one" rocket research airplanes, the
X-15 was airlaunched, being dropped from a modified Boeing B-52 jet
bomber. The flights were made over a specially instrumented
485-mile-long 50-mile-wide flight test corridor stretching from
Nevada to Edwards Air Force Base in California. Following a landing
accident with the second X-15, the Air Force and NASA authorized the
manufacturer to modify it as a special testbed for NASA's planned
Hypersonic Ramjet Experiment. North American lengthened the
aircraft, making numerous modifications to it, and added provisions
for two large jettisonable external tanks. Thus equipped, the
aircraft, designated the X-15A-2, was capable of Mach 7 flight
speeds, if equipped with a pr3per thermal protection system. NASA
finally selected Martin to develop a suitable ablator, and that
company derived the MA-25S, an ablator mix consisting of a resin
base, a catalyst, and a glass bead powder. Hopes that such ablators
could enable designers to build refurbishable spacecraft that could
be stripped and recoated after each flight proved ill-founded,
however. On October 3, 1967, the X-15A-2 attained Mach 6.72 (over
4,520 mph), while piloted by Air Force Maj. William J. Knight.
Unfortunately, the plane landed in extremely worn condition -- a dummy
ramjet had separated off the craft, in fact -- and the ablator would
have required massive cleanup efforts prior to reapplication. North
American repaired the craft and returned it to NASA, but it never
flew again. The third X-15 made a number of notable high-altitude
flights above 50 miles. Unfortunately, this aircraft was lost,
together with pilot Michael J. Adams, on November 15, 1967. The
first X-15 completed its last flight, the 199th flight for the type,
on October 24, 1968 5.
Following awarding of the X-15 development contract, North
American had considered a so-called "X-15B" orbital spacecraft (even
before Sputnik), to be launched by two Navaho boosters and possibly
carry a two-astronaut crew. After Sputnik, it went through a cycle
of shelving and revival until finally overcome by the ballistic
blunt-body spacecraft approach as taken by the McDonnell Mercury
vehicle. The X-15 series itself, however, did perform a number of
"Shuttle" like missions, for after 1962, the X-15 program switched
concentration from hypersonic aerodynamics to using the vehicle as a
testbed carrying a wide range of applications and experiments, such
as insulation intended for the Saturn booster, and navigation
instruments under development for Apollo. By 1964, fully 65 percent
of all data returned from the X-15 related to follow-on programs,
and this figure continued rising until the conclusion of the program
in December 1968. NASA even briefly considered using the X-15 as a
launcher for Scout rockets carrying small satellite payloads, the
B-52/X-15/Scout becoming, in effect, one large booster, but after
examining the idea, NASA rejected it on grounds of safety, cost, and
practicality. Fittingly, in December 1968, the Deutsche
Gesellschaft fUr Raketentechnik und Raumfahrt awarded John Becker
and the X-15 team with the Eugen Sänger Medal, created to honor
individuals and groups who have-made special contributions to the
field of recoverable spacecraft 6.
The following case study of the X-15 was prepared by the late
Robert S. Houston of the then-Historical Branch, Office of
Information Services, Wright Air Development Center,
Wright-Patterson AFB, Ohio, in 1959. It has been expanded and
updated by the editor to treat the X-15's flight test program and
research legacy as well, with much of this supplementary material
drawing upon the editor's On the Frontier: Flight Research at
Dryden, 1946-1981 (Washington, D.C.: NASA, 1984), and then-Captain
Ronald G. Boston's "Outline of the X-15's Contributions to Aerospace
Technology", prepared in support of the National Hypersonic Flight
Research Facility effort in 1977. At the time, Captain Boston was
an instructor in the Department of History, Air Force Academy,
Colorado Springs, Colorado.
Figure 3
THE MODIFIED X-15A-2 ROCKET RESEARCH AIRCRAFT