X-15 Research Results

Chapter 9

A Flying Laboratory



FROM ITS INITIAL, broad-scale attacks on hypersonic and space-equivalent flight, the X-15 program shifted to an increasingly detailed probing of airflow and aerodynamic forces. The precise knowledge gained enabled researchers to explore the limits of the flight corridor with understanding and confidence.

As the X-15's primary role neared its conclusion, scientists both within and outside the aerospace disciplines expressed interest in making use of the aircraft's unmatched research capability. Some of them were involved in the expanding scientific assault upon space. Others were hoping to develop lighter, simpler, or more versatile aircraft to fly in the same realm as the X-15, and wanted it as a testing ground for their ideas. Because of these various interests, the X-15 program began to take on a new character.

The hypersonic thoroughbred has become a workhorse, dutifully carrying a weird variety of equipment and experiments and repeatedly exposing the payloads to high-temperature airflow, hypersonic aerodynamic force, or the space-equivalent region. Some of these experiments change the X-15 from a research airplane to a kind of space probe, such as Vanguard or Pioneer. Other experiments are pertinent to the development of supersonic transports and Mach 10 aircraft. The changing research program is perhaps best exemplified in the X-15-2, a modified version of the original craft, which may ultimately extend flight in the corridor to Mach 8.

Several tests are underway and many more are planned. Included in this program are high-temperature structural components, ranging from cermet (protectively coated) skids for the landing gear to special, detachable wingtips. Another study will include tests of heat exchangers under weightless conditions to verify performance analysis. A new type of supersonic decelerator for the recovery of payloads from space will also be evaluated.

The X-15 program is also capable of opening some windows in the atmosphere that shrouds the Earth. Satellites are thoroughly exploring the region above 100 miles. Balloon-borne instruments continue to probe the region below 20 miles. But many difficulties face an experimenter who is interested in measurements between those two altitudes. Prior to the X-15 program, rocket probes filled in some of this gap in information, but the recovery of a rocket payload is uncertain, and a rocket passes through the region in question in a very short period of time. The X-15, on the other hand, can stay appreciably longer in this area that is so difficult to explore by other means. In addition, it provides a controlled platform for relatively large experiments, and returns each payload to Earth intact.


photo of the x15 Here's the X-15-2 (rebuilt following the McKay accident) with jettisonable fuel tanks attached to its side fairings. Those tanks carry an extra 13 500 lb. of propellants and will boost the plane's top speed to Mach 8 or provide longer flights. The plane's surface must be covered with an ablative coating to protect its structure from the 4000-deg. air temperatures of Mach 8 flight.


To date, it has carried out five high-altitude experiments, and seven others are programed for the future. These experiments include the collection of micrometeorite particles, the measurement of sky brightness at high altitude, and efforts to find out how accurately special instruments can determine the Earth's horizon. The latter measurements are aiding in the design of instruments to be used to navigate the Apollo spacecraft to the Moon.


photo of the X15 fastened to the underside of a B53 wing.

An X-15 with pods fastened to its wingtips for the collection of micrometeorite
particles at high altitude is seen here attached to a B-52 drop plane just prior
to takeoff on one of its most recent research missions.


The X-15 has also made radiation measurements in the visible, infrared, and ultraviolet spectra. While the results have not upset any scientific theories, they have provided invaluable information on the background-noise level for the design of satellite and manned-spacecraft instrumentation systems. A key asset that the X-15 provides for this work is its ability to carry out detailed post-flight instrumcnt calibrations after systems have been exposed to a new environment. Such calibration is denied to most space experiments. Even the common solar cell has yet to be calibrated in the laboratory, operated in space, and then recovered for final laboratory recalibration. Plans are underway for the X-15 to provide this desired capability.

Following serious damage to one of the original X-15's during McKay's emergency landing, in late 1962, North American Aviation engineers proposed to rebuild that airplane into an X-15 with Mach 8 capability. The data obtained during the research program had given them a detailed picture of the problems, so they could design for higher speeds with greater precision.

The basic aerodynamic configuration has not been altered, since it has adequate stability for flight to Mach 8. To achieve increased performance, however, an additional 13 500 pounds of propellants are carried in two external tanks. These propellants will accelerate the X-15-2 to about Mach 2, when the tanks will be jettisoned. Other modifications have added compartments in the center section of the fuselage, and at the aft ends of the side fairings and vertical tail, for carrying extra test equipment and scientific experiments.

The major obstacle that confronts the modified X-15-2 is the increased aerodynanic heating for Mach 8. Not only does the airflow temperature rise to 4000 F but the aircraft will be exposed to high temperatures for considerably longer periods than before. This combination increases heating for some areas of the structure by a factor of eight over a Mach 6 flight. Since the heat-sink structure can withstand only a small fraction of this heating, the solution comes from adding a protective coating to the outer surface. This coating is similar to the ablative materials that protect ballistic-entry capsules.

Ablative materials have never before been applied to aircraft. The entire external surface of the X-15-2 must be covered, yet if the coating were applied in thick layers, it would produce a prohibitive increase in weight. Thus, while the forward surfaces may require as much as three-quarter of an inch of coating, most of the airplane will be protected by much less - .050-inch in some areas. The ablative material must be reapplied after each flight.

This program should provide much useful information about the use of ablative materials on lifting surfaces. If they prove to be practical for repeated use, the airplane may find a new role in testing ramjet or turbo-rocket propulsion systems. At present, the development of advanced propulsion systems is greatly hindered by lack of suitable ground-test facilities for speeds above Mach 6. The X-15-2 is being studied as one potential means of overcoming this deficiency. A program to mount test engines in place of the lower vertical tail is underway, though as yet its feasibility is still under study. Any such engine will be too small to provide additional performance for the X-15, but it will provide valid test results that can be applied to full-scale engines for future hypersonic craft.

The X-15-2 represents a significant change in the research program. Enabling the craft to achieve Mach 8 has required not only new materials but new components and new operating procedures. The scientific experiments that the X-15-2 carries have grown in scope to include complex, astronomical equipment, which occupies one-half of the instrument compartment. It comprises a stellar tracking instrument for photographing the ultraviolet radiation from selected stars. Its use will demand flights for that purpose alone and force the pilot to perform an intricate space-control maneuver. He must precisely align the airplane with specific stars (by instrument, not by sight) during flight into the space-equivalent region.

Eventually, the X-15 seems certain to add a host of new roles to its lengthy list of research accomplishments. It has already underscored one fundamental fact - the difficulty of determining in advance what may be learned from a research program of this nature. Certainly, it has filled one role envisioned by its pioneers - that of stimulating research.

rear view of the X15 tail section. An infrared horizon-scanner, with cover plate removad, is seen here in its compartment behind the upper speed brakes of an X-15 before a research flight to high altitude. The instrument helped measure background noise for the design of satellite instruments. Those bundles of stainless-steel pressure tubes on the aft end of the upper vertical tail lead to pressure rakes on the sides of the tail.


Perhaps the only goal the program has not achieved is that of stimulating work on a successor. Since the initiation of the U.S. research-airplane program, in 1946, aircraft speeds have doubled every six years. A projection of this pace past that set by the X-15 predicts flight to Mach 12 by 1967. But the space age has largely eclipsed aerodynamic flight, and no plans are as yet underway for a follow-on research airplane. Since such developments typically take about five years, from feasibility study to first flight, the X-15 seems destined to hold its place as the world's most advanced airplane for many years. And who can foresee what technology may bring during that period to end, or to extend, the X-15 program?

X15 drawing

Above are outline drawings of two structural modifications of the X-15
for further research. Both involve a 29-inch extension of the fuselage.
The topmost profile reveals the plane with underwing tanks and
additional propellants for probing speeds to Mach 8. The lower profile
above shows the X-15 modified for in-flight study of small ramjet engines,
carried in the area usually occupied by the ventral fin. The drawing below
shows how a modified X-15 will make leading-edge (1) and panel (2)
experiments, and environmental tests with detachable wingtips (3).

X15 drawing



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