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First flown in 1959 from the NASA High Speed Flight Station (later renamed the Dryden Flight Research Center), the rocket-powered X-15 was developed to provide data on aerodynamics, structures, flight controls, and the physiological aspects of high speed, high-altitude flight. Three were built by North American Aviation for NASA and the U.S. Air Force.

The North American X-15A-2

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Hypersonic Research at the Edge of Space


This joint program by NASA, the Air Force, the Navy, and North American operated the most remarkable of all the rocket research aircraft. Composed of an internal structure of titanium and a skin surface of a chrome-nickel alloy known as Inconel X, the X-15 had its first, un-powered glide flight on June 8, 1959, while the first powered flight took place on September 17, 1959. Because of the large fuel consumption of its rocket engine, the X-15 was air launched from a B-52 aircraft at about 45,000 ft and speeds upward of 500 mph. The airplane first set speed records in the Mach 4-6 range with Mach 4.43 on March 7, 1961; Mach 5.27 on June 23, 1961; Mach 6.04 on November 9, 1961; and Mach 6.7 on October 3, 1967. It also set an altitude record of 354,200 feet (67 miles) on August 22, 1963, and provided an enormous wealth of data on hypersonic air flow, aerodynamic heating, control and stability at hypersonic speeds, reaction controls for flight above the atmosphere, piloting techniques for reentry, human factors, and flight instrumentation. The highly successful program contributed to the development of the Mercury, Gemini, and Apollo piloted spaceflight programs as well as the Space Shuttle program. The program's final flight was performed on October 24, 1968.

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 The X-15, designed to provide data on material and human factors of high-speed, high-altitude flight, made the first manned probes into the lower edges of space. It was built for speeds of up to 4,000 mph and altitudes of 50 miles, but these goals were exceeded on numerous occasions. Several X-15 pilots earned "astronaut" rating by attaining altitudes above 50 miles. The X-15 flight program contributed significantly to the Mercury, Gemini, and Apollo projects.

The X-15 was carried aloft by a B-52 and was released at about 45,000 feet and 500 mph. Its rocket engine then fired for the first 80 to 120 seconds of flight. The remainder of the 10 to 11 minute flight was powerless and ended with a 200 mph glide landing on a dry lake bed.

The first powered X-15 flight was made on Sept. 17. 1959, and 199 flights were made between 1959 and 1968 by the three X-15s which were built. The No. 1 X-15 is at the National Air and Space Museum and the No. 3 X-15 was destroyed in a crash. The No. 2 aircraft was retired to the U.S. Air Force Museum in October 1969.

Span: 22 ft. 5 in.
Length: 52 ft. 5 in.
Height: 14 ft.
Weight: 56,132 lbs. (at launch with ram jet test engine)
Armament: None
Engine: Reaction Motors YLR-99 rocket engine of over 50,000 lbs. thrust
Serial Number: 56-6671
C/N: 240-2

Maximum speed:
4,520 mph. (un-official record)
Range: Over 250 miles (flight path distance)
Service Ceiling: 354,200 ft. (un-official record by X-15 No. 3)


The XLR99 Rocket Engine


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The XLR99 was the first large, "man rated," throttle-able, restart-able liquid propellant rocket engine. The throttle setting could be varied from about 50 percent to 100 percent of thrust, and the restart capability allowed it to be shut down in flight with the assurance that power would again be available later, if needed. The XLR99 was one of the rocket engines used in the X-15 manned research aircraft which was capable of propelling man to the fringes of space.

Developed and built by Reaction Motors Division of Thiokol Chemical Company, the XLR99 could deliver up to 57,000 lbs. of thrust, or the equivalent of about 500,000 horsepower. The propellants for the XLR99 were liquid oxygen (LOX) and anhydrous ammonia, fed into the engine by turbine pumps at a flow rate of more than 10,000 lbs. per minute.

The XLR99 engine had a rated operating life of one hour, after which it could be overhauled and used again, though operating times twice that long were demonstrated in tests. Since the basic X-15 carried fuel for about 83 seconds of full-power flight, and the X-15A-2 carried fuel for more than 150 seconds of full-power flight, each XLR99 was theoretically capable of between 20 and 40 flights before overhaul.

In common with other large scale liquid fueled rocket engines, the walls of the XLR99's thrust chamber were constructed of hollow tubing so that fuel could be routed through the tubes to cool the chamber walls before being burned in the engine. The basic weight of the engine is 910 lbs.



June 1952
NACA Committee on Aerodynamics recommends increase in research dealing with flight to Mach 10 and to altitudes from 12 to 50 miles.
September 1952
Preliminary studies of research on space flight and associated problems begun.
February 1954
NACA Research Airplane panel meeting discusses need for a new research airplane to study hypersonic and space flight.
July 1954
Proposal for new research airplane presented to the Air Force and Navy.
December 1954
Invitations issued by the Air Force to contractors to participate in the X-15 design competition.
September 1955
North American Aviation, Inc., selected to develop three X-15 research airplanes.
February 1956
Reaction Motors, Inc., awarded development contract for XLR99 rocket engine,
December 1956
X-15 mock-up completed.
September 1957
Design configuration set. Construction starts.
October 1958
Factory rollout of No. 1 airplane.
June 8, 1959
First glide flight, No. 1 airplane.
September 17, 1959
First powered flight, No. 2 airplane.
March 25, 1960
First NASA flight in an X-15 aircraft. Pilot is Joe Walker.
November 15, 1960
First flight with XLR99 engine.
March 7, 1961
First flight to Mach 4.
June 23, 1961
First flight to Mach 5.
October 11, 1961
First flight above 200,000 ft.
November 9, 1961
First flight to Mach 6.
December 20, 1961
First flight of No. 3 airplane.
July 17, 1962
First flight above 300,000 ft.
August 22, 1963
Unofficial world altitude record of 354,200 ft.
January 28, 1964
100th flight in series.
October 3, 1967
World's absolute speed record for winged aircraft, 4,520 mph
October 24, 1968
Last X-15 flight, 199th mission.


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On November 9, 1962, an engine failure forced Jack McKay, a NASA research pilot, to make an emergency landing at Mud Lake Nevada, in his X-15 aircraft. The aircraft's landing gear collapsed and the X-15 flipped over on its back. McKay was promptly rescued by an Air Forced medical team and eventually recovered to fly the X-15 again.

X-15A-2 The No. 2 X-15, damaged in Nov. 1962, was re-designated the X-15A-2 and modified for testing hypersonic ram-jet engines. Modifications included adding external fuel tanks to increase engine burn time by about 70% and a 29-inch increase in fuselage length. Two flights were made with a white ablative coating to keep aircraft skin temperatures within structural limits of 1,200 degrees Fahrenheit.




The Crash Of  X-15A-3

15 November 1967


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Above is the X-15A-3 on its way to a test flight.

The X-15A-3 just after being dropped from the B-52.

Major Adams in the cockpit of an X-15A at Edwards AFB

Major Michael J. Adams in front of the X-15A

As had happened in some other research aircraft programs, a fatal accident signaled the end of the X-15 program. On 15 November 1967 at 10:30 a.m., the X-15-3 dropped away from its B-52 mother ship at 45,000 feet near Delamar Dry Lake. At the controls was veteran Air Force test pilot, Major Michael J. Adams. Starting his climb under full power, he was soon passing through 85,000 feet. Then an electrical disturbance distracted him and slightly degraded the control of the aircraft. Having adequate backup controls, Adams continued on. At 10:33 he reached a peak altitude of 266,000 feet. In the Dryden Flight Research Center (DFRC) flight control room, fellow pilot and mission controller Pete Knight monitored the mission with a team of engineers. Something was amiss. As the X-15 climbed, Adams started a planned wing-rocking maneuver so an on-board camera could scan the horizon. The wing rocking quickly became excessive, by a factor of two or three. When he concluded the wing-rocking portion of the climb, the X-15 began a slow, gradual drift in heading; 40 seconds later, when the craft reached its maximum altitude, it was off heading by 15°. As the plane came over the top, the drift briefly halted, with the plane yawed 15° to the right. Then the drift began again; within 30 seconds, the plane was descending at right angles to the flight path. At 230,000 feet, encountering rapidly increasing dynamic pressures, the X-15 entered a Mach 5 spin.


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An aerial view of the crash.

The remains of X-15-3 lies on the desert floor. 

In the flight control room there was no way to monitor heading, so nobody suspected the true situation that Adams now faced. The controllers did not know that the plane was yawing, eventually turning completely around. In fact, control advised the pilot that he was ”a little bit high,” but in ”real good shape.” Just 15 seconds later, Adams radioed that the plane ”seems squirrelly.” At 10:34 came a shattering call: ”I'm in a spin, Pete.” A mission monitor called out that Adams had, indeed, lost control of the plane. A NASA test pilot said quietly, ”That boy's in trouble.” Plagued by lack of heading information, the control room staff saw only large and very slow pitching and rolling motions. One reaction was ”disbelief; the feeling that possibly he was overstating the case.” But Adams again called out, ”I'm in a spin.” As best they could, the ground controllers sought to get the X-15 straightened out. They knew they had only seconds left. There was no recommended spin recovery technique for the plane, and engineers knew nothing about the X-15's supersonic spin tendencies. The chase pilots, realizing that the X-15 would never make Rogers Lake, went into afterburner and raced for the emergency lakes, for Ballarat, for Cuddeback. Adams held the X-15's controls against the spin, using both the aerodynamic control surfaces and the reaction controls. Through some combination of pilot technique and basic aerodynamic stability, the plane recovered from the spin at 118,000 feet and went into a Mach 4.7 dive, inverted, at a dive angle between 40 and 45 degrees.
Adams was in a relatively high altitude dive and had a good chance of rolling upright, pulling out, and setting up a landing. But now came a technical problem that spelled the end. The Honeywell adaptive flight control system began a limit-cycle oscillation just as the plane came out of the spin, preventing the system's gain changer from reducing pitch as dynamic pressure increased. The X-15 began a rapid pitching motion of increasing severity. All the while, the plane shot downward at 160,000 feet per minute, dynamic pressure increasing intolerably. High over the desert, it passed abeam of Cuddeback Lake, over the Searles Valley, over the Pinnacles, narrowing on toward Johannesburg. As the X-15 neared 65,000 feet, it was speeding downward at Mach 3.93 and experiencing over 15 g vertically, both positive and negative, and 8 g laterally. It broke up into many pieces amid loud sonic rumblings, striking northeast of Johannesburg. Two hunters heard the noise and saw the forward fuselage, the largest section, tumbling over a hill. On the ground, NASA control lost all telemetry at the moment of breakup, but still called to Adams. A chase pilot spotted dust on Cuddeback, but it was not the X-15. Then an Air Force pilot, who had been up on a delayed chase mission and had tagged along on the X-15 flight to see if he could fill in for an errant chase plane, spotted the main wreckage northwest of Cuddeback. Mike Adams was dead and the X-15 destroyed.
 Afterwards, Michael Adams was laid to rest at the Memorial Park Cemetery in Monroe, Ouachita Parish, Louisiana.



The Crash Site Today


Very little remains at this site.  In fact, aside from a small American flag posted in the center of the site, the average person would been totally unaware of the historic nature of their surroundings.  Small, scattered pieces are sprinkled there, and finding anything of the aircraft at the site is nearly impossible (however, portions of the X-15A-3 such as an engine access panel, a reaction control rocket for maneuvering in the upper atmosphere, a piece of the horizontal stabilizer, and a section of vertical stabilizer that had the numerals '72' on it, were found as late as 1992).  Rumor has it that the left wing has never been found

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Here are three photo of the site where the main fuselage came to rest.  Unlike most other crash sites, there is virtually nothing to be found of the aircraft laying on the surface.

  In the background you can see the same cleft on the hill as in the crash photo.



The X-15 Program In Retrospect





The X-15 program was the first major investment of the United States in manned aerospace flight technology. During the long 15-year lifetime of the program, hundreds of people have contributed importantly to its success. It is a great privilege for me to represent this outstanding team at this meeting of so many distinguished members of your society, including Frau Dr. Sänger-Bredt. We sincerely appreciate the award of the Sänger medal.

Professor Sänger's pioneering studies of long-range rocket-propelled aircraft had a strong influence on the thinking which led to initiation of the X-15 program. Until the Sänger and Bredt paper (ref. 1) became available to us after the war we had thought of hypersonic flight only as a domain for missiles. The concept of manned rocket aircraft flying efficiently at hypersonic speeds for very long ranges was new and highly stimulating. The remarkably detailed analyses of many aspects of their new concept which Sänger and Bredt undertook in their paper gave real substance to the idea. From this stimulus there appeared shortly in the United States a number of studies of rocket aircraft investigating various extensions and modifications of the Sänger and Bredt concept. These studies provided the background from which the X-15 proposal emerged.

By 1954 we had reached a definite conclusion: the exciting potentialities of these rocket-boosted aircraft could not be realized without major advances in technology in all areas of aircraft design. In particular, the unprecedented problems of aerodynamic heating and high-temperature structures appeared to be so formidable that they were viewed as "barriers" to hypersonic flight. Thus no definite requirements for hypersonic vehicles could be established or justified. In today's environment this inability to prove "cost-effectiveness" would be in some quarters a major obstacle to any flight vehicle proposal. But in 1954 nearly everyone believed intuitively in the continuing rapid increase in flight speeds of aeronautical vehicles. The powerful new propulsion systems needed for aircraft flight beyond Mach 3 were identifiable in the large rocket engines being developed in the long-range missile programs. There was virtually unanimous support for hypersonic technology development. Fortunately also, there was no competition in 1954 from other glamorous and expensive manned space projects. And thus the X-15 proposal was born at what appears in retrospect as the most propitious of all possible times for its promotion and approval.

The broad objective of the X-15 was to take a long step forward in developing the new technologies needed for rocket-boosted hypersonic aircraft (ref. 2). Fortunately, it was not proposed as a prototype of any of the particular concepts in vogue in 1954, which have since largely fallen by the wayside. It was conceived rather as a general tool for manned hypersonic flight research,, able to penetrate the new regime briefly, safely, and without the burdens, restrictions, and delays imposed by operational requirements other than research. The merits of this approach had been convincingly demonstrated by experiences with the previous research air planes, notably the X-1 series.

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Figure 1. Typical X-15 research flight paths.

The plan called for two different types of flight profile. The first consisted of a variety of constant angle-of-attack, constant altitude, and maneuvering flights within the altitude corridor of interest to hypersonic gliders. In the second type of trajectory it was proposed that the vehicle should explore for the first time some of the problems of manned space flight by making long leaps out of the sensible atmosphere (fig. 1). Our analyses of 1954 (ref. 2) showed that such excursions into space were feasible provided that an Inconel X heat-sink structural concept was used together with employment of high lift and low L/D during the reentry pull-up maneuver. This latter maneuver in itself was recognized as a prime problem for manned space flight from both the heating and the piloting viewpoints.

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Figure 2. Velocity and altitude of the X-15 flights

While the hypersonic aeronautics aspect of the X-15 proposal enjoyed virtually unanimous approval, it is interesting to note that the space flight aspect was viewed in 1954 with what can best be described as cautious tolerance. There were' few then who believed that space flight was imminent, and even the most sanguine believed that manned space flight was many decades in the future, probably not before the 21st century (ref. 3). Several of the senior research consultants who reviewed the proposal counseled that the space flight maneuver was premature and should be eliminated. Fortunately it remained in the program as a prime objective, not because of superior vision on the part of the X-15 planners, but rather because of their recognition that the problems involved were basic and would have to be solved before true manna space flight could be achieved, and that it was now possible in the X-15 Project to take the first steps toward their solution.

An idea of the scope of the program is obtainable from figure 2. The dots show the speed and altitude of all of the flights. Of the 199 flights accomplished in the program 109 exceeded Mach 5, and four exceeded Mach 6. The highest speed, Mach 6.7 (2020 meters/sec), was reached in 1967. In altitude, it can be seen from figure 2 that the space-trajectory type flights constituted a major part of the program with a peak altitude of 108,000 meters, well above the 82,000 meters vhich was the goal set in the original prospectus.

The results of the program have been widely disseminated and previous reviews have been given periodically (refs. 4, 5, and 6, for example). There is no need for a detailed review here. It will be of interest, however, now that the program has been completed, to examine in retrospect its principal accomplishments against a background of the original hopes and aims of 1954.


Piloting Aspects


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NASA pilot Neil Armstrong is seen here next to the X-15 aircraft #1 after a research flight.

One of the major initial goals of the program which has been most richly achieved was to explore the capabilities, and the limitations, of the human pilot in an aerospace vehicle. There were those in 1954 who speculated that man had no place in hypersonic or space flight. And there were others who believed that he would prove indispensable. In either event, the space trajectory and reentry maneuver which the X-15 pilot was asked to negotiate were guaranteed to provide a convincing test.

From the outset simulators of all kinds were used to an unprecedented extent in pilot training., flight planning, and also in vehicle design. There was no two-seated version of the X-15 in which pilots could be taught to fly. Twelve pilots trained on the simulators with outstanding success. These experiences paved the way for similar all-out use of simulators in the space program.

It is well known that for greatest effectiveness the use of simulators requires careful correlation with flight testing. In the early stages of X-15 design, of course, flight data were not available, and some of the design features decided upon on the basis of the simulated experiences alone proved to be wrong and had to be altered. One of these was the large ventral tail employed during the first phase of the program. In the original vehicle configuration developed by RACA in 1954 it had been found that this arrangement suffered at high angles of attack at hypersonic speeds from a nearly complete loss of effectiveness of the upper tail, and a large increase in effectiveness of the laver tail, leading to very high and undesirable negative dihedral effect. Thus, our original proposal suggested that only a small ventral tail should be used. The early simulator studies, however, revealed that the large ventral tail was necessary for law angle of-attack controllability to cope with feared thrust misalignment effects of the rocket engine. Furthermore, as shown in figure 3, left side, the simulator studies with the large ventral indicated that the machine could be controlled without dampers at high angles of attack in spite of the negative dihedral. Thus the decision was made to use this symmetrical tail configuration.

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Figure 3. Handling characteristics of X-15 with dampers inoperative.

This condition of "dampers-off" controllability was an essential design requirement because of the doubtful reliability of the damper system. In the first flights of the program, contrary to the simulator results, the machine was found to be unflyable at angles of attack above about 8° with dampers inoperative (fig. 3,, center). This discrepancy was traced, in part, to the influence of secondary aerodynamic effects (such as trim, for example) on the stability derivatives, effects which were not included in the original simulation. In addition, the pilots naturally felt less secure in flight than in the simulator and were not willing to accept vehicle motions which they had rated "acceptable for emergency" on the early simulator. With flight "calibration" of this kind together with a continuing program of other improvements, the fixed-base simulator eventually achieved satisfactory simulations of instrument flight.

Early in the flight program when the state of affairs shown in figure 3, center graph, had been established there was serious doubt as to whether the high altitude "space flight" missions of the X-15 could be flown safely. These missions typically required angles of attack in excess of 17° on reentry. One of the major constraints in the problem was eliminated when operational experience with the XLR99 rocket engine revealed that it had no significant thrust misalignment as originally feared. Thus the underlying reason for the large ventral disappeared, the ventral rudder was removed., and the problem was solved by a return to a tail configuration similar to that recommended by NACA in the original 1954 study (fig. 3. right graph). As an added safety measure, a back-up damper system was installed to provide high reliability. With this system the "uncontrollable" region above 20° could be safely penetrated, and reentry trajectories up to 26° were flown.

And so it vas that the absence of flight "calibrations" of the early fixed-base simulator, together with -unfounded worries over thrust misalignment led to a costly excursion in configuration design. A consoling thought in retrospect is that more was learned than if this mistake had somehow been avoided.

The capabilities demonstrated by the pilots in the principal areas of interest are summarized briefly as follows:


Exit Phase

The program shows clearly that, given precise displays, the pilot can fly rocket-boosted vehicles into space with great accuracy (refs. 7, 8). He cannot do any better than completely automated systems, however. Perhaps his best role will be as a monitor of automatic system able to contend with malfunctions or to make trajectory changes as needed.


Attitude Control In Space

This was considered a major research problem area in 1954. Development of a workable reaction control system was achieved with the aid of a ground-based simula or and flight tests at low dynamic pressure in the X-lB airplane. As a result of this program it became clear that attitude control without aerodynamics and with threshold aerodynamics were skills readily acquired by pilots, and the X-15 high-altitude flights fully confirmed this finding.


Maneuvering Reentry

The steep reentries of the X-15 with flight path angles up to -38°, Mach numbers approaching 6, and angles of attack up to 26° presented a more difficult piloting problem than the shallow entries of lifting manned vehicles returning from orbital or lunar missions. The prime requisite, of course, is a flyable vehicle, which means in general for hypersonic flight a vehicle incorporating artificial damping systems. When the X-15's damping systems were operative the pilots could perform the reentry maneuver readily (refs- 7, 9). The "self-adaptive" damping system was preferred over the simple rate-responsive dampers. (Footnote: The basic feature of the "self adaptive" system is its automatic gain changer which maintains the desired dynamic response characteristics of the airplane for a wide range of dynamic pressures. Added capabilities of the installation in the X-15-3 airplane were dual redundancy, integration of aerodynamic and reaction controls, and automatic stabilization in pitch, roll and yaw. The system was developed under sponsorship of the USAF, Aeronautical Systems Division, and it represents one of the noteworthy advances associated with the X-15 program.) Transitions from reaction to aerodynamic controls were made without difficulty and a control mode in which the two systems were blended was also developed satisfactorily.


Gravity Effects On The Pilot

With a few exceptions these proved small - essentially negligible. Weightlessness, which was one of the largest fears of the unknown in the early system studies, produced no difficulties in the few minutes it existed in the high altitude flights. This result, of course, shortly lost its impact after the first Mercury flight in 1962 involving a much longer period of weightlessness.


Pilot Plus Redundancy

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This photo illustrates how the X-15 rocket-powered aircraft was taken aloft under the wing of a B-52. Because of the large fuel consumption, the X-15 was air launched from a B-52 aircraft at 45,000 feet and a speed of about 500 miles per hour.

An analysis of the first 44 flights showed that 13 would have failed in the absence of a human pilot together with the various redundant systems provided in the vehicle (refs. 5, 7). Against these' figures in favor of the pilot there were only a few examples where the pilot's error degraded the mission performance, and only one catastrophic accident out of 199 flights. The NASA-USAF board investigating this accident reported that in its judgment, the pilot confused roll and yaw indicators and inadvertently yawed the airplane to 90° or more at the start of the reentry, possibly as a result of display misinterpretation, distraction, or vertigo. This condition apparently lead to complete loss of control and subsequent breakup of the X-15-3 airplane (ref. 10).

The broad positive finding of the program, however, is clear: the capability of the human pilot for sensing, judging, coping with the unexpected, and employing a fantastic variety of acquired skills remains essentially undiminished in all of the key problem areas of aerospace flight. It is equally clear that there are many new areas in aerospace flight in which the pilot's capabilities must be supplemented. The need for artificial damping of hypersonic vehicles is one example.



Hypersonic Aerodynamics


Hypersonic aerodynamics was in its early infancy in 1954. The few small hypersonic wind tunnels then in existence had been used almost entirely for fluid mechanics studies. They were unable to simulate either the high temperatures or the high Reynolds numbers of flight. Because of strongly interacting flow fields, viscous interactions with strong shocks, and possible real gas effects, it was generally feared that testing in these limited wind tunnels would not produce valid results. And it was expected that the X-15 would reveal large discrepancies between flight and ground test data (ref. 2). Our inability to devise ground facilities capable of true-temperature simulation was in fact in 1954 regarded as a sort of "facility barrier". All-out efforts were launched during that period to try to develop high-temperature facilities.

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Figure 4. Typical comparison of wind-tunnel and flight aerodynamic data.

The X-15 program helped to expose the fallacies of this "facility barrier". Virtually all of the flight pressures and forces were found to be in excellent agreement with the low-temperature wind-tunnel predictions (refs. 11, 12, 13, and 14; fig. 4 shows two typical examples). Prior to the start of flight operations it was learned by analysis that the "real gas" high-temperature effects in themselves were for the most part negligible below Mach 10. Thus the agreement noted above implies primarily an absence of any important scale effects on the pressures and forces (other than skin friction) for the X-15 configuration. (Footnote: Other configurations, notably the highly swept delta wing with trailing-edge flaps, have been found to exhibit important scale effects, not only hyper sonically but throughout the speed range.)

Concurrent with the first years of the X-15 flight program, a number of missile and space vehicle configurations were also successfully developed in small low-temperature hypersonic wind tunnels, and in a few cases limited flight data were obtained which provided some additional confirmation of the wind-tunnel results. With this broad general validation, the bulk of which came from the X-15 results, the conventional low-temperature hypersonic wind tunnel became the accepted tool for configuration development. The "true-temperature" hypersonic aerodynamics tunnel on the other hand, with its enormous operational and interpretational difficulties, has proved useful only for limited special problems where full temperature simulation is mandatory (ramjet combustor development, for example).


Laminar Flow


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Figure 5. Boundary-layer transition location on X-15 wing compared with wind-tunnel prediction, and roughness Reynolds number of X-15 leading-edge joint compared with critical roughness Reynolds number .

In the early studies of hypersonic aircraft, there was a widely held belief that the hypersonic laminar boundary layer would prove infinitely stable because of the heat flow out of the boundary layer. It was thought likely that hypersonic vehicles would be the first true laminar flow aircraft. Actually there was little substance to this belief other than an early theoretical indication of a stabilizing effect due to heat flaw from the boundary layer. Nevertheless it was the hopeful practice in the early fifties to compute performance and heating for the all-laminar case. One respected research group even suggested that hypersonic research to reduce laminar friction deserved top priority.

This technical superstition has now almost entirely disappeared, largely as a consequence of the X-15 findings (ref. 15; fig- 5). At the Mach 6 flight Reynolds number, which was about 2.7 million/meter, wind tunnel data for a smooth model, not including any benefits from heat transfer indicate extensive laminar flows. But only the leading-edge region was found to be laminar in flight. A "step" irregularity existed behind the leading edge which, although very small, was sufficient according to low-speed criteria to trigger transition. Thus the small surface irregularities which have proved the nemesis of laminar flaw at lower speeds, are apparently equally adverse on the blunt-edged wing at Mach 6.


Turbulent Heat Transfer


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Figure 6. Turbulent heat-transfer data for wing
of X-15 compared with prediction method.

Fortunately the X-15 structure was designed conservatively to cope with turbulent heating. Hypersonic turbulent heating rates were recognized from the outset as an area of special importance for flight measurements because of the weakness of the available semi-empirical prediction methods and the almost complete lack of reliable hypersonic wind-tunnel data for turbulent flow. It should not have been any cause for real surprise, therefore, when the X-15 flight results of 1961 (ref. 16) showed a marked departure from the predictions available at that time, averaging about 35 percent below the van Driest and the Eckert T' predictions (refs. 17, 18) for the low wall temperature conditions of these flights (fig. 6).

At first this result was received with disbelief by most fluid mechanics specialists, who are by nature skeptical of flight measurements. The result has been thoroughly substantiated., however, by repeated measurements at several locations on the airplane together with local flow field surveys to aid in analysis of the data. The important highlighting of this weakness of the prediction methods by the X-15 stimulated comprehensive studies in a variety of ground facilities. New cold-wall data have been obtained which fully confirm the X-15 results. Improved prediction methods are being sought and new investigations of the structure of the cooled turbulent boundary layer are in progress (ref. 19, for instance). This is an excellent example of one of the greatest values of an exploratory research airplane - the highlighting of an important problem and the stimulating of ground-based research for its solution.




The X-15 was the first manned aircraft in which aerodynamic heating was the dominant problem of structural design. It was, of course, out of the question in 1954 to hope to be able to identify an optimum structural concept representative of distant future developments and no attempt was made to do so. The approach taken was to utilize the concept best suited to the short duration X-15 mission itself, with its particular requirements for heat-transfer research for drastically different trajectories ranging from equilibrium glides to steep high-angle-of-attack space reentries. A thick-skinned heat-sink approach was adopted. This has proved satisfactory both as a structure and as a calorimeter for heat-transfer measurements. Aside from its relatively thick. heat-sink skin, the X-15 structure contains many features representative of current advanced concepts for Mach 6-8 cruise aircraft, such as corrugated shear webs, combined use of super alloys and materials of a lower expansion coefficient such as Titanium, and the use of segmented leading edges. In retrospect, the major advances in the area of primary structure occurred during design and development. Great reliance was placed on the results of structural tests in which heat was applied electrically and loads mechanically according to schedules representing actual flight environmental histories. Measurements of the behavior of the primary structure in actual flight have verified these ground simulations. Thus the X-15 confirmed that ccMplex high-temperature structures can be reliably developed with ground-based "partial simulation" techniques using only rather simple and inexpensive equipment. Here again, early notions that a true-temperature aerodynamic type of structures test facility would be required were dispelled by the X-15 findings.

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Figure 7. Wing skin buckle on flight to Mach 5.3 and corrective modification. Figure 8. Temperature distribution aft of leading-edge expansion slot.

Although no surprises were found in flight for the primary structure, many unanticipated problems came to light in the secondary structures. Early in the program the pilot reported a rumbling noise at high dynamic pressures. This turned out to be panel flutter of large areas of the skin on the side fairings and tails. It was found to be related to certain design features incorporated to reduce thermal stress. Solution of this problem produced important advances in flutter prediction criteria (ref. 20).

Unanticipated leading-edge distortions and buckling of the adjacent wing skin developed during the first flight to Mach 5.3 (fig. 7). The slots, perhaps through vortex action and partly through triggering transi tion, cause intense local heating. Covering the slot and providing an internal shear tie between segments solved this problem. Evidence of the local heating is seen in figure 8 (ref. 5).

Although the soda-lime glass windshields of the X-15 were designed conservatively from the standpoint of glass temperatures, a failure occurred, fortunately involving only the right outer pane (fig. 9). Close examination of the failure revealed that it was initiated by buckling of the retainer frame. This problem was solved by replacing the Inconel X frame with Titanium, for which buckling does not occur because of the lower expansion coefficients of Titanium (fig. 10). The aft portion of the frame was removed because it was apparently causing a shock-induced hot spot on the aft part of the window (ref. 7). These fixes have proved successful.

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Figure 9. Cracked windshield glass in flight to Mmax= 6.04. Figure 10. X-15 windshield retainer modifications.

One could argue that these local problems could have been solved by more comprehensive ground testing and analysis of the local flow field and heating mechanisms. True, if someone had had the foresight to identify these new problems in advance of their disclosure by the flight tests. The really important lesson here is that what are minor and unimportant features of a subsonic or supersonic aircraft must be dealt with as prime design problems in a hypersonic airplane. This lesson was applied effectively in the precise design of' a host of important details on the manned space vehicles.

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Figure 11. Wing covered with spray-on ablator prior to sanding.

During the past year one of the airplanes has been flown with a radical change in the structural concept (ref. 10). The airplane was covered with ablative insulation designed to permit flights to Mach 7.4 (fig. 11). A silicone electrometric ablator was sprayed on in variable thickness appropriate to the local heat loads. Leading edges were protected by a related erosion-resistant material applied in preformed sections. This program was of interest to lifting reentry vehicle development, because the spray-on technique has often been advanced as a possible means of refurbishing the incremental protection required for metal reentry structures. In the initial phases of a lifting reentry where the heating rates are beyond the limits of the metallic radiation cooled structure, the material would ablate. Then, as the speed reduces the bare metal would be exposed and radiation cooling would take over.

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Figure 12. Telemetry antenna showing effect of bow shock heating on fuselage, Mmax= 6.7.

The X-15 experience rather clearly shows that this approach to a refurbishable structure is impractical. Some 5 weeks time and over 2000 hours were required for the total job including special treatment of removable hatches, covers, etc., hardly a practical operation for a refurbish able logistics vehicle.


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Figure 13. Cavity heating revealed by charring of ablator, Mmax= 6.7. Figure 14. Tail-fuselage juncture heating, Mmax= 6.7.
Figure 15. Local heating on wing due to impingement of fuselage bow shock and side-fairing shock. Figure 16. Dummy ramjet engine installation.

Performance of the material was generally satisfactory on flight which reached Mach 6.7. Charring occurred only in regions of highest heating, and a few local failures occurred where the thickness of the material was inadequate or where bond failures occurred due to back surface heating.

A most interesting by-product of the ablative insulation flights was the revelation of a number of heating phenomena not detectable on previous flights with the metal heat-sink structure. The ablator tended to show up areas of intense heating by charring. Many of these areas did not become very hot in the bare-metal flights because of heat-sink cooling, but the ablation material blocked the cooling effect and showed up the hot spots. An example is the high-pressure heating zone behind the detached bow shock of the telemetry antenna (fig. 12).

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Figure 17. Failure of pylon due to interference
heating from dummy ramjet.

Cavity heating is evident in figure 13. On this flight the reaction controls were not used and the nozzles were filled with recessed plugs. Tail-body juncture heating (fig. 14) and shock-intersection heating (fig. 15) have been the subject of many wind tunnel studies but have not been seen previously in flight. A far more serious example of impingement heating occurred under the airplane where a dummy ramjet nacelle was mounted (fig. 16).

The supporting pylon was subject to strong shock fields from the spike and from the closed cowl inlet. Heat protection material was applied to the leading edge of the pylon in roughly the same thicknesses as for the wing leading edge but the complex local heating phenomena on the pylon were not accounted for in the design. A serious failure of the system occurred with actual burn-through of the pylon skin (fig. 17). Pylon leading-edge heating rates of the order of seven times those without interference were estimated. This is the closest the X-15 ever came to a major structural failure in flight due to heating. Again these results underscore the need for maximum attention to aerothermodynamic detail in design and preflight testing.


Operational Problems With Subsystems


In order to achieve maximum penetration into the realms of hypersonic and space flight, the X-15 necessarily had to use many newly conceived, partially developed subsystems fabricated of new materials by new processes. The XLR99 rocket engine, the auxiliary power units (APU), and the stability augmentation (damper) system are examples. A host of new problems were encountered with the subsystems after the start of flight operations, and flight schedules were subject to innumerable delays (refs. 5, 7, and 21).

In retrospect, a great deal of money and lost time could have been saved through more imaginative and more elaborate ground testing of all major subsystems. This sounds like a serious general criticism. But one must remember that the only precedents available in the mid-fifties were those based on experience with ordinary aircraft and with the much simpler previous research airplanes. As it was, the initial environmental testing of the X-15 subsystems far exceeded that of any previous vehicle. In this area perhaps more than any other, the X-15 brought to light innumerable "real" problems which could not have been foreseen by any means other than involve ment with an actual flight vehicle. The program revealed the true nature of the difficulties encountered in an aerospace environment and identified many specific new requirements for developmental testing that were of value in subsystem development for the space program



Follow-On Scientific Experiments


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Figure 18. Solar spectrum instrumentation in wing-tip pod
Figure 19. Saturn insulation on X-15 dive break showing flow-field rake and photographic reference grid..

It became apparent early in the program that the X-15's could perform a valuable function, not foreseen in the original planning, as reusable carriers for a wide variety of scientific experiments. Some 15 such experiments which have returned useful data are tabulated below:

A quick scan of the list reveals that all but a few of the experiments deal with space problems.

Multiple experiments were carried on many flights, and the X-15 system, unlike space rocket testing, permitted full recovery of the equipment, recalibration, and repeat runs where needed. In nearly all cases use of the X-15 was the least costly and quickest means of achieving the desired data.

An illustration of the solar spectrum equipment is shown as an example in figure 18. The equipment is mounted in a wing tip pod, and is exposed after the aircraft reaches the high altitude environment. A second type of experiment involved testing the insulation of the Saturn booster on the dive brakes of the X-15 where the severe real-environmental heating situation of the Saturn could be duplicated (fig. 19).



Other Contributions


One of the important general claims of the original prospectus for the program was that it would stimulate aerospace research and development. A measure of how well this claim was realized is seen in the following tabulation of technical documents:

Principal Technical Documents Associated with the X-15 Program
Development of X-15 system and supporting R and D 276 documents
Flight test results 290 documents
General research inspired by X-15 program 200 documents
Total 766 documents

The total production of over 700 technical documents is equivalent to the output of a typical 4000-man federal research center for a period of some 2 years. Of special interest and significance are the 200 papers reporting general research inspired by the X-15. These latter papers were identified by personal knowledge of various members of the X-15 team and by contacts with-the authors. They are reports of research that would not have been undertaken had it not been for the inspiration provided in one way or another by the X-15. Thus we see statistical confirmation of the massive stimulus and the focus provided by the program.

It is most important to note that this dispute of new technology represents, in addition to specific research results, the acquisition of new manned aerospace flight "know how" by many teams in government and industry. They had to learn to work together, face up to unprecedented problems, develop solutions, and make this first manned aerospace project work. These team were an important national asset in the ensuing space program.

At the start of the Mercury program there was, of course, a significant foundation of missile technology that was heavily utilized for launch propulsion, blunt-body aerodynamics, and heat protection. However, the innumerable added constraints and requirements of a manned aerospace system were present and in this area the X-15 team and the X-15 developments were the principal technology sources. The later developments of the USAF X-20 Dyna-Soar Program, particularly the large advances it provided in radiation-cooled structures and materials, also became available for post-Mercury systems.

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The X-15 was configured with a mammoth XLR99 rocket engine providing 57,000 pounds of thrust. The airplane's skin surfaces were fabricated from a special chrome-nickel allow that would enable it to withstand the searing 1200-degree Fahrenheit temperatures predicted in the hypersonic flight environment.

If one takes a broad look at all of the contributions of the program and considers relative values based on the actual applications that have been made of the results, it is quite clear that the space-oriented results have been of greater value than the hypersonic aeronautics contributions. This is the reverse of what was expected in the beginning and is primarily a consequence of the unanticipated early arrival of the space age. It is interesting and important to note that any attempt at a cost-effectiveness evaluation of research aircraft in 1954 would almost certainly have either ignored or grossly undervalued the space flight aspects. At the same time, the boost-glide missions which have since been displaced by unmanned space systems, would have been exaggerated in value in any 1954 assessment. Who could have foreseen in 1954 that within 5 years the top priority goal of Western technology would be to put a man in orbit in space without delay?

Without the advantage of X-15 technology lead time, would Project Mercury and the subsequent manned space projects have been delayed? What misfortunes might have occurred? What losses in national prestige might have resulted? Haw much was it worth to have X-15 technology in these critical times? No specific answers are possible, of course. But the existence of intangible and initially unforeseeable values is undeniable.



Implications For Future Research Aircraft


A large new area of potential interest for exploration by future research aircraft has been opened up by the advent of hypersonic air breathing propulsion systems. These systems, which were not generally believed feasible in the early years of the X-15 program, employ some combination of the ramjet or scramjet for cruise, with the turbojet or rocket for acceleration and climb. The systems of interest include space launch vehicles, military strike or reconnaissance vehicles, and commercial transports, with speeds extending upward to about Mach 12. Many new problems not covered by X-15 research are found in the propulsion system, cryogenic fuel tank, lightweight radiation-cooled structures, and in the piloting and operations areas. While it is not clear at present that a new air-breathing research airplane system will prove justifiable, it is desirable to explore and evaluate some of the possibilities.

If we look to the X-15 experience as a guide, what prime features does it suggest? What features were most vital to the role it has played in aerospace history? Clearly the decision in 1954 to proceed quickly with a general research tool as opposed to a configuration fully optimized with respect to the 1954 vision of the feature mission was vital. If the optimized pseudo-prototype route had been followed one can see with the advantage of hindsight that we would have picked the wrong mission, the wrong structure, the wrong aerodynamic shapes, and the wrong propulsion.

More important, in the 3 or 4 years which all of this optimization might have consumed we would have dissipated our technology lead time, and with the start of the space age in 1958, in all probability the X-15 would never have been attempted. A second basic feature of the X-15 that proved vital was the design of the system with great latitude in performance so that it would reach well beyond the hypersonic aerodynamic corridor into simulated space flight.

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Figure 20. Air breathing research airplane study configuration.

A possible new research airplane system conforming to these and other X-15 guidelines is illustrated in figure 20. (Footnote: This is one of several systems receiving preliminary study in the United States.) It is a lifting-body cruise configuration designed for Mach numbers up to 12. Its acceleration engine is a hydrogen fueled J-2S rocket adapted from an upper-stage engine of the Saturn vehicle. Integrated into the lower surface is a research scramjet engine sized to power the airplane in cruise. Following guidelines from our X-15 experience the vehicle is kept as small as possible, about 25 meters in length, and it remains in the Mach 12 environment only long enough for research purposes, about 5 minutes. As we have leaned from the X-15, a new hypersonic research airplane system is likely to have a long lifetime of perhaps 15 years, during which many new unsuspected ideas for research and changes in configuration are likely to appear as the program develops. Accordingly, we are proposing here actually three different vehicle arrangements. We would start the program with the lifting-body rocket glider without the air-breathing research engine. Later, a delta-winged version using the same subsystems would be flown. And, finally, the integrated scramjet research engine shown here would be installed. Provision for structural cooling schemes including direct fuel cooling, air-film cooling, and other schemes likely to appear in these vehicles might also be made.

The case for approval of such a new research airplane is no longer simple as it was in 1954. A major difference is the high level of confidence now enjoyed by the partial-simulation techniques of ground-based research and development. The X-15 program itself, together with the successfully developed reentry vehicle systems, has thus tended to eliminate a major justification which existed in 1954. In the author's opinion no new exploratory research airplane can ever again be successfully promoted primarily on the grounds that it will produce unique flight data, without which a successful technology cannot be achieved. Thus there is not likely to be a future research airplane unless a high valuation is placed on the other vital but less tangible contributions - the focusing of the countless detailed efforts in many areas, the revelation of new and unsuspected problems for research, the overall stimulation of technology development, and the early availability of new technology for important but initially unforeseeable new applications. The X-15 experience affirms that the exploratory research airplane is a most effective device for producing these values.


The X-15 Photo Gallery


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X-15 Pressure Suit


 The pressure suit on the left is an improved David Clark Co. XMC-2 suit, helmet, and boots used by Maj. Robert A. Rushworth. The suit integrated the various components of earlier X-15 suits into a single garment with a pressure-proof zipper to facilitate dressing and undressing the pilot. A parachute harness was built into the outer coverall. Major Rushworth made his first X-15 flight on Nov. 4, 1960 and ultimately flew more X-15 missions (34) than any other pilot.



The suit on the right is an early example of an XMC-2-DC full-pressure suit developed in the mid-1950s jointly by Wright Field personnel and the David Clark Co. for X-15 pilots. It represented a major advancement in pressure suit technology, serving as a prototype for those later used by Mercury and Gemini astronauts. It allowed the pilot freedom of movement while keeping him comfortable and protected in the event of cabin pressure failure or emergency ejection from the aircraft. Components of the suit incorporated a ventilation layer to cool the pilot. Rubberized airtight garments containing inflatable anti-gravity bladders restrained by an innovative link-net material, and an outer heat resistant layer. Suit controls were contained in the back mounted control unit. The helmet, a variation of the MA-1, was built by the Bill Jack Scientific Co. and contained oxygen equipment, microphone and earphones, and an anti-fogging feature. This suit was tailored for North American Aviation pilot Scott Crossfield who made the first eight X-15 test flights.
 An example of an XMC-2 full pressure suit developed for use in the Mid-50s jointly by Wright Field personnel and David Clark Company for X-15 pilots. It represented a major advance in pressure suit technology serving as prototype for those used later by Mercury and Gemini astronauts. It allowed the wearer freedom of movement while keeping him comfortable and protected in the event of cabin pressure failure or emergency ejection from the X-15 at extreme altitudes. The suit incorporated a ventilation layer to cool the user and an outer heat resistant layer. The helmet was built by the Bill Jack Company and contained oxygen equipment, microphone and earphones, and an anti-fogging feature.








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First flight of the X-15

B-52 flyby after landing

After landing
X-15 mounted on mother-ship ready for takeoff

The X15 as displayed at the Air Force Museum

Window detail Right forward fuselage Nose gear detail
Detail of Ventral Fin area Detail of ground carriage assembly Detail of LOX and Hydrogen Peroxide Jettison

Detail of rocket exhaust As crew members secure the X-15 rocket-powered aircraft after a research flight, the B-52 mothership used to launch this unique aircraft does a low flyby overhead. Shock waves festoon a small scale model of the X-15 in NASA¡s Langley Research Center¡s 4 x 4 Supersonic Pressure Tunnel.

National Air and Space Museum



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The North American X-15A-2, serial 56-6671, is displayed at the Air Force Museum. Its markings have been restored and it has been fitted with its external propellant tanks. They are painted as they appeared on the first flight of the X-15A-2 with the tanks mounted. That flight occurred on November 3, 1965.




From the Pilot's Seat

by William H. Dana
From Science News, 24 February 1968


Before describing the specific sensations of flight in the X-15, let me first put these comments in context. The X-15 is the most marvelously engineered piece of hardware I have ever flown, and by listening to the remarks of my fellow pilots who have flown it, I conclude that they share my sentiments. The X-15 operates in an environment that varies from the pure vacuum of near space to the heavy air loads of 2,200 pounds per square foot near the earth. Shock waves strike on its control surfaces from the time it crosses Mach 1 on its way up to its maximum speed of seven times the speed of sound and until it decelerates to subsonic speeds again. During no two seconds of supersonic flight are these shock impingements identical. Yet the X-15 remains totally responsive to the commands of the pilot from launch to landing.

So flying it is a piece of cake, right?

Wrong. The very nature of the airplane's design mission dictates that an X-15 flight will be a challenge to the men who work with it, and ultimately, to the man who flies it. Hundreds of hours are spent in the precise planning stage that precedes every flight. Once the complicated flight plan is completed, another 50 hours are spent practicing the 10-minute flight in an electronic simulator. The last two hours prior to launch are spent in ground checks and inflight system checks that are so routine that they mock the forthcoming events.

The launch itself provides the first surprise of the flight. The act of leaving the wing of the mother ship in free fall gives the X-l5 pilot the sensation that he was fired off the mother ship's shackles by some hidden cannon. This sensation is not eased until the rocket engine lights one to two seconds after launch.

Then begins as busy a minute-and-a-half as most pilots ever want to experience. In that short time, the X-l 5 accelerates from subsonic speed to about six times the speed of sound. A myriad of events take place in this 90 seconds: the pull-up to climb altitude; the maintaining of this climb angle to the designated pushover point; monitoring of speed, altitude, and air loads to ascertain that the planned flight is being flown; manipulation of switches to trigger the experiments onboard; and finally shutdown of the engine at the prescribed maximum velocity. All of these events are performed under an acceleration that presses the pilot into the back of his seat with two to four times the force of gravity.

A time deviation of one second in the performance of any of these events can mar the quality of the important data being acquired.

Engine shutdown brings some relief of physical stress, but the workload continues. If the flight is an altitude mission, that is, if it leaves the atmosphere, the pilot must operate attitude control rockets to keep the X-15 upright and weathervaned along the flight path until the atmosphere is reentered.

Then come the loads of reentry; the pilot is forced down into his seat with five times his normal weight as the X-15's fall is broken. If the mission is a heating research flight, the craft remains within the atmosphere, and at the speed it travels the air loads are massive, usually half to three-quarters of a ton per square foot. Any maneuvering transmits these loads to the pilot; at Mach 5, for example, a simple twenty degree change of heading requires a 5 "g" turn for ten seconds.

Whatever its mission, eventually the X-15 decelerates and maneuvers to a position over the dry lake near NASA's Flight Research Center at Edwards, Calif., and the pilot finds himself once again in an environment familiar to him. The craft is now subsonic, and the pilot has practiced the X-15 approach pattern hundreds of times in an F-104 jet. The approach is relaxed and the landing is straightforward.

The flight has ended and the pilot invariably reflects that the mission was challenging indeed. Therefore, the satisfaction of its successful completion is great, and, more than at any other time, the pilot longs for the next flight.

by William H. Dana



Former NASA X-15 Pilots Awarded Astronaut Wings

By Robert Z. Pearlman
 23 August 2005


Today at NASA's Dryden Flight Research Center in California astronaut wings will be awarded to the three civilian research pilots who flew the X-15 into space in the mid-1960s.Between the years that NASA flew its first unmanned Mercury sub-orbital space flight and when its Apollo astronauts trained for their first mission to orbit the Moon, 12 test pilots flew the nation's first rocket plane to the edges of the atmosphere ... and beyond. Of the dozen, eight of those pilots flew the experimental X-15 to altitudes above 264,000 feet—50 miles—a height recognized by the U.S. Air Force as being in space. Of the eight, five pilots were employed by the Air Force and received their astronaut wings. The three others were NASA research pilots—William H. "Bill" Dana and the late John B. "Jack" McKay and Joseph A. Walker.

 In a private ceremony, scheduled to take place Tuesday afternoon before an audience of 200 invited guests and NASA Dryden employees the three civilian pilots will at long last receive their wings. The event is expected to include presentations by Kent Rominger, NASA's Chief of the Astronaut Corps and by Johnny Armstrong, an X-15 test engineer and current Deputy Director of the Access to Space Office at the Air Force Flight Test Center, Edwards Air Force Base, Calif.

 Among other guests expected to attend are several of the other X-15 test pilots, including Scott Crossfield and Joe Engle, who received USAF astronaut wings and flew the Space Shuttle as a NASA astronaut.

 Unlike the Air Force wings, which are physically a metal pair of wings, the distinction that Dana and the families of McKay and Walker will accept is to be more ceremonial in nature. Each will receive a certificate, a Velcro-backed badge of the type NASA astronauts adorn on their flight suits, and the actual flight logs from their now recognized space flights.

 The honor officially establishes Walker as only the 11th person to fly in space and the 7th American by his first wings-qualifying flight on January 17, 1963. McKay is the 26th space explorer (13th U.S. astronaut) by his flight on September 28, 1965. Dana ranks as the 35th human to leave Earth (and 21st American) by November 1, 1966.Of the three, McKay broke the 50 mile barrier only once, Dana twice and Walker three times.

Two of Walker's fights also exceeded 328,000 feet or 62 miles altitude, the internationally accepted boundary of space set by the Fédération Aéronautique Internationale.

In total, 13 of the 199 X-15 flights reached space. Aside from the six space flights achieved by NASA pilots, the seven others were flown by Air Force pilots Robert White, Robert Rushworth, Engle, William Knight and Michael J. Adams. Only three of the eight X-15 astronauts - White, Engle and Dana - are still living.

After flying the X-15 (including its last flight in October 1968), Dana served as a research pilot for the Air Force's X-20 Dyna-Soar program. He led NASA's lifting body program during the late 1960s and 1970s. In 1993, Dana was named Chief Engineer at NASA Dryden until 1998 when he retired.

McKay joined NACA (National Advisory Committee for Aeronautics), NASA's predecessor, in 1951 as a test pilot for the X-1 and D-558 and served as a project pilot on the F-100, F-102, F-104, and F-107 test programs before he was assigned to the X-15. He was seriously injured when the X-15 he was piloting crashed in 1962. McKay's death on April 27, 1975, stemmed from liver damage suffered in the accident.

Walker's altitude record of 67.1 miles set on the third of his X-15 space flights went unbroken by any other rocket plane until the privately-funded SpaceShipOne exceeded 69 miles in October 2004.

Walker made the first test flight of the X-15 in 1960 and went on to fly a total of 24 times. After the X-15, he tested the "flying bedstead," the Lunar Landing Research Vehicle, in advance of training NASA's Apollo astronauts how to touchdown on the moon. Walker was killed on June 8, 1966, when the F-104 chase plane he was piloting collided with the XB-70 "Valkyrie" he was chasing.

The X-15 explored the realm of hypersonic flight during a nine year joint program between NASA, the U.S. Navy, the U.S. Air Force and North American. The X-15 was air launched from a B-52 aircraft before igniting its liquid-fuel rocket engine. In addition to setting altitude records, the experimental aircraft also achieved speed milestones in the Mach 4 to 6 range. The X-15 program is credited with contributing to the development of the Mercury, Gemini, and Apollo spacecraft as well as NASA's Space Shuttle.


The X-15 Missions




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