The Flight Program

By January 1958, everything had moved into high gear and North American was assembling the three model NA-240 airplanes at its facility in Inglewood, adjacent to the Los Angeles International Airport. The company had released over 6,000 engineering drawings-including one that was 50 feet long-by the end of 1957, although it continued to make minor changes to the configuration. North American subcontracted about 200 items to various vendors, but manufactured the majority of the airplane on the premises.-11


On 15 October 1958, North American rolled out the first X-15 (56-6670) in Inglewood to great pomp and circumstance. It was ironic, in a way. The NACA had given birth to a concept and had nurtured the X-15 for over four years, but two weeks earlier the committee itself had ceased to exist. In its place, the National Aeronautics and Space Administration (NASA) was created effective 1 October 1958. The X-15 had been the largest development program at the NACA; it would soon be one of the smallest at a moon-destined NASA.

The Flight Program

X-15-1 was presented during the roll-out ceremony. The air-data boom on the nose would be

used until the ball nose was ready. The bug-eye camera ports located behind the canopy and under the fuselage in the center-of-gravity compartment would provide some breathtaking views of the early flights courtesy of National Geographic. (North American Aviation).

The master of ceremonies at the rollout was Raymond H. Rice, vice president and general manager of the Los Angeles division of North American Aviation. The keynote speakers included Major General Victor R. Haugen, deputy commander of the ARDC; Brigadier General Marcus F. Cooper, commander of the AFFTC; Walt Williams, chief of the HSFS; and Harrison Storms, chief engineer for the Los Angeles division. Also in attendance were six future X-15 pilots: Neil A. Armstrong, A. Scott Crossfield, John B. McKay, Captain Robert A. Rushworth, Joseph A. Walker, and Captain Robert M. White.[2]

Congressmen and senators sat in the grandstands and Vice President Richard M. Nixon was on hand to proclaim that the X-15 had "recaptured the U. S. lead in space." There were special exhibits featuring a David Clark full-pressure suit and a mockup of the Reaction Motors XLR99 engine. Guests could sit in the X-15 fixed-base simulator, and attend a gala luncheon where everybody praised the efforts of all involved. For the X-15 team it was a moving occasion and a much-needed respite from the years of hard work.[3]

Bob White, the man who felled every Mach number and altitude milestone in the X-15, later remembered eloquently, "The X-15 … was in the public eye from its inception and grew almost asymptotically from the day of its manufacture. Witness the presence of the vice president of the United States at the X-15 rollout ceremony. The X-15 was not controversial; it was audacious. It literally vibrated the imagination that this aircraft would double the fastest speed by more than three whole Mach numbers and… fly out of the atmosphere, into space, and back again to an on – Earth landing. The X-15 did these things and many more….,,[4]

The space race had already begun, and the United States was eager to show any progress toward besting the Soviet accomplishments. Newspapers, magazines, and newsreels all heralded the X – 15 as the American entry in the space race. Considering that only four years earlier researchers had wanted to remove the "space leap" from the X-15 concept, it was ironic that many now portrayed this small black airplane as America’s response to the Soviet threat. It may have been small, but it had not come cheaply. Despite the original $12,200,000 estimate prepared by the WADC in 1954, at the time of the rollout Major General Haugen estimated that the government had spent nearly $120 million-and the airplane had not yet flown.-15

Haugen also pointed out that the X-15 rollout was taking place two weeks ahead of the schedule established in June 1956, calling this "a tribute to all of the government and industry team." The general then summed up the spirit of the program: "It has been said that there are two extremes to research or exploratory flying-the approach that, for example, would strap a man on an ICBM and see what happens, and the super safe approach that would have us take tiny steps into the unknown and be absolutely sure of each step. We believe the solution is neither of these but rather a bold step into the future within the known technical capabilities of our engineers. We believe that the X-15 represents such a bold step… that will help us build better air and space vehicles in the future."[6]

Perhaps because his base would host the flight testing, Brigadier General Cooper was a little more cautious in his remarks: "I wish to point out here that this research program will be one of long duration, and the type of flights which excite the imagination and make newspaper headlines are many, many months away." However, the general could not resist riding the space bandwagon, at least a little, by calling the program "the first major breakthrough in sustained piloted space


The Flight Program

Given the recent successes of the Soviets with Sputnik, the rollout of the X-15 was considered sufficiently important for the vice president of the United States to show up. Richard M. Nixon presided over the ceremonies along with distinguished speakers from the Air Force and the state of California. (North American Aviation)

Not to be outdone, Stormy Storms was even more direct: "The rollout of the X-15 marks the beginning of man’s most advanced assault on space. This will be one of the most dramatic, as in the X-15 we have all the elements and most of the problems of a true space vehicle." Describing the potential performance of the airplane, Storms said, "The performance of the X-15 is hard to comprehend. It can out fly the fastest fighters by a factor of three, a high-speed rifle bullet by a factor of two, and easily exceed the world altitude record by many times."81

Following the conclusion of the official ceremonies, North American moved the first X-15 back inside and prepared it for delivery. On the night of 16 October, covered completely in heavy-duty wrapping paper, X-15-1 traveled overland by truck through the Los Angeles foothills to Edwards for initial ground-test work.

Maximum Altitude

Joe Walker would fly the maximum altitude flight (3-22-36) of the program on 22 August 1963, his second excursion above 300,000 feet in just over a month. The simulator predicted that the X-15 could achieve altitudes well in excess of 400,000 feet, but there was considerable doubt as to whether the airplane could successfully reenter from such heights. A good pilot on a good day could do it, but if anything went wrong, the results were usually less than desirable. In order to

provide a margin of safety, NASA decided to limit the maximum altitude attempt to 360,000 feet, providing a 40,000-foot pad for cumulative errors. This might sound like a lot, but the flight planners and pilots remembered that Bob White had overshot his altitude by 32,750 feet. The X – 15 was climbing at over 4,000 fps, so every second the pilot delayed shutting down the engine would result in a 4,000-foot increment in altitude. The XLR99 also was not terribly precise – sometimes the engine developed 57,000 lbf, while other times it developed 60,000 lbf. An extra 1,500 lbf for the entire burn translated into an additional 7,500 feet of altitude. A 1-degree error in climb angle could also result in 7,500 feet more altitude. Add these all up and it is easy to understand why the program decided a 40,000-foot cushion was appropriate.12051

Walker had made one build-up flight (3-21-32) prior to the maximum altitude attempt in which he overshot his 315,000-foot target by 31,200 feet through a combination of all three variables (higher-than-expected engine thrust, longer-than-expected engine burn, and a 0.5-degree error in climb angle). Walker commented after the flight, "First thing I’m going to say is I was disappointed on two items on this flight, one was that I was honestly trying for 315,000, the other one was I thought I had it made on the smoke bomb on the lakebed. I missed both of them." Although he missed the smoke bomb on landing, it was well within tolerance. As for missing the altitude, the 40,000-foot cushion suddenly did not seem very large.12061

The flight was surprisingly hard to launch, racking up three aborts over a two-week period mainly because of weather and APU problems. On the actual flight day, things began badly when both the Edwards and Beatty radars lost track on the NB-52 during the flight to the launch lake, but both reacquired it 4 minutes prior to the scheduled launch time. The launch itself was good and Walker began the long climb to altitude. Although this was the first flight for the altitude predictor,

Walker flew the mission based on its results, changing his climb angle several times to stay within a predicted 360,000 feet. When the XLR99 depleted its propellants, X-15-3 was traveling through 176,000 feet at 5,600 fps. It would take almost 2 minutes to get to the top of the climb, ultimately reaching 354,200 feet.12071

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It was not uncommon for X-15 altitude flights to miss their expected altitude. The X-15 climbed at over 4,000 fps, so every second that the pilot delayed shutting down the engine resulted in a

4,0- foot increment in altitude. The XLR99 also was not terribly precise – sometimes the engine developed 57,000-lbf; other times it developed 60,000-lbf. An extra 1,500-lbf for the entire burn translated into an additional 7,500 feet of altitude. A 1-degree error in climb angle could also result in 7,500 feet more altitude.

To give the reader a sense of what reentry was like on this flight: The airplane was heading down at a 45-degree angle, and as it descended through 170,000 feet it was traveling 5,500 fps-over a mile per second. The acceleration buildup was non-linear and happened rather abruptly, taking less than 15 seconds to go from essentially no dynamic pressure to 1,500 psf, then tapering off for the remainder of reentry, and reaching a peak acceleration of 5 g at 95,000 feet. Walker maintained 5 g during the pullout until he came level at 70,000 feet. All the time the anti-g part of the David Clark full-pressure suit was squeezing his legs and stomach, forcing blood back to his heart and brain. Walker said that "the comment of previous flights that this is one big squeeze in the pullout is still good." The glide back to Edwards was uneventful and Walker made a perfect landing. The flight had lasted 11 minutes and 8 seconds, and had covered 305 ground miles from Smith Ranch to Rogers Dry Lake. Although Walker had traveled more than 67 miles high, well in excess of the 62-mile (100-kilometer) international standard supposedly recognized by NASA, no astronaut rating awaited him. This was apparently reserved for people who rode ballistic missiles at Cape Canaveral, and it would take 42 years to correct the oversight.-1208!

By the end of 1963, the program had gathered almost all of the data researchers had originally desired and the basic research program was effectively completed. The program would now push into the basic program extensions phase using X-15A-2 to gather similar data at increasingly high speeds while the other two airplanes continued the follow-on experiments. The basic program extensions were a set of experiments that had not been anticipated when the Air Force and NASA conceived the basic program. Some were truly follow-ons to issues that were uncovered during the basic program; others were the result of new factors, such as the increased capabilities of the modified X-15A-2. In general, researchers continued all of the original research into aerodynamics, structures, and flight controls for the rebuilt airplane. The FRC paid for many of the experiments from general research funds, not from a separate appropriation from Headquarters or Congress.-1209

Despite the progress and future plans, there was no uniform agreement that the X-15 program should continue. At least privately, several officials (including Paul Bikle) argued that the value of the projected research returns was not worth the risk and expense, and that the program should be terminated at the conclusion of the basic research program, or as soon as the X-15A-2 had completed its basic program. This body of opinion was the same that had initially led NASA to argue against modifying X-15-2 into the advanced configuration. However, by this time the X – 15A-2 was well under construction and researchers had proposed an entire series of follow-on experiments, making it unlikely that the program would be terminated any time soon.-129

Although the point would be moot after the Department of Defense canceled the Dyna-Soar on 10 December 1963, at the end of 1963 the X-15 program was making plans for four unnamed future X-20 pilots to make high-altitude familiarization flights in X-15-3 during the first half of 1964. Several of the Dyna-Soar pilots had already flown the X-15 simulator in preparation.-121


When the Air Force approved the X-15 program in December 1954, the aviation medical community in the United States had already embarked on research into the physiological effects of weightlessness in anticipation of technological developments that would permit manned space flight. This research consisted largely of obtaining physiological measurements from human subjects while they flew Keplerian trajectories in a modified Lockheed F-94C Starfire and attempted to perform certain psychomotor tasks. The maximum duration of weightlessness in these early experiments was 30-40 seconds, and it was difficult to separate the subject’s responses to the weightless state from his responses to the pre – and post-trajectory accelerations. Nausea and vomiting, for example, occurred in the majority of test subjects in these experiments, probably due to the relatively rapid transitions between hypergravic and hypogravic states. Despite the amount of uncertainty surrounding the experiments, some medical researchers nevertheless concluded that weightlessness would induce nausea and vomiting in most people.

The Air Force widely reported this conclusion, which greatly influenced several early manned space studies.-161

Because the X-15 flight profiles would provide longer periods of weightlessness than were possible with lower-performance aircraft, acquiring physiological data became an early objective of the program. As North American finalized the X-15 configuration, there were additional reasons for monitoring biomedical data from a safety standpoint. Since the cockpit of the X-15 was engineered for a 3.5-psi differential between the inside of the cockpit and the outside atmosphere (or lack thereof), it was not considered feasible to use a breathable atmosphere. At sea level, oxygen accounts for approximately 20% of the normal atmospheric pressure of 14.7 psi. A breathable atmosphere therefore requires 20% of the normal 14.7 psi-about 3 psi of oxygen in the 3.5-psi cockpit.-171 Engineers considered the problems of combustion and fire associated with this 86% oxygen atmosphere to be insurmountable. As a result, North American pressurized the X-15 cockpit and ventilated the full-pressure suit with nitrogen, supplying breathing oxygen to the helmet area only. A neck seal (MC-2 suit) or a face seal (A/P22S-2 suit) separated the breathing space from the remainder of the suit. In order to prevent the leakage of nitrogen into the breathing space, technicians adjusted the breathing-oxygen regulator to deliver oxygen at a pressure 1 inch of water higher than the suit pressure.-181

The maintenance of this helmet-suit pressure differential was vital, since otherwise the pilot could develop an insidious hypoxia that would lead to serious impairment or unconsciousness. The suit pressure regulator maintained 3.5 psi in the suit, the same pressure maintained in the cockpit. In the event of loss of cockpit pressure, the suit would automatically inflate, thus preventing a catastrophic decompression of the pilot. It was not particularly unusual for the suit to inflate partially as the X-15 cabin differential changed during exit or reentry. The suit/cabin-pressure differential provided an indication of the proper functioning of the suit and suit-pressure regulator, and the cabin-pressure regulator. In the event of failure of the suit regulator or breathing-oxygen regulator, the pilot could select the emergency oxygen system that pressurized both the suit and helmet with oxygen; this was the same emergency system used during ejection.-1191

North American designed and fabricated the original biomedical instrumentation system as part of the basic X-15 contract. The system monitored eight parameters, including the ECG, oxygen flow rate, helmet/suit-pressure differential, cabin/suit-pressure differential, and pilot skin temperature, from four locations. The helmet/suit-pressure differential also served as an excellent respirometer; since the breathing space in the helmet was relatively small, the pilot’s respiration produced pressure fluctuations that the 0-0.5-psi transducer could follow, providing a real-time indication of respiratory rate. The ECG, oxygen flow rate, and skin temperatures were recorded using an onboard oscillograph recorder. The two pressure differentials were sent to the ground via PDM telemetry and displayed in the control room, along with cabin pressure (obtained via vehicle, not biomedical, instrumentation) on a heated stylus stripchart recorder. The first physiological and environmental data recorded on the X-15 program were obtained during flight 1-6-11 on 6 May 1960 with Bob White at the controls.-1291

One of the first things that researchers assessed on the X-15 was whether the cockpit environment adequately protected the pilot. The requirement was to keep the pilot’s skin temperature below 100°F even though the aircraft’s outer skin could heat to nearly 1,000°F. In fact, researchers found that the cockpit outer wall was reaching 750°F, but the cockpit itself remained in a temperature range between 36°F and 81°F. The Air Force removed the skin- temperature sensors after the first round of flights when it became obvious that the mission was not exposing the pilot to any significant thermal stress.1211

The original biomedical signal-conditioning package, mounted in the instrumentation compartment behind the cockpit, was 5 by 6.5 by 11.5 inches in exterior dimensions and weighed 11 pounds. In general, this instrumentation functioned well, but as researchers gained experience they corrected minor deficiencies and simplified the system. For example, the original ECG used five electrodes that simulated the clinical I, II, III, and V4 leads. However, since the ST segment and T(minus) wave changes are essentially uninterpretable under dynamic conditions, a one-channel ECG gave just as much information as a multi-channel system. Therefore, the ECG was simplified to a three-electrode configuration (two 0.75-inch-diameter stainless-steel screen mid-axillary leads and a reference electrode on the lower abdomen). A silicone potting compound ring surrounded the metal mesh electrode, a conductive paste assured good contact with the skin, and pressure-sensitive adhesive secured a plastic cap over each electrode to keep it in place.

Beginning with flight 2-18-34 on 12 September 1961, the Air Force installed a Tabor Instruments amplifier on the ejection seat. Amplifying the signals closer to the pilot resulted in much better data.1221

Researchers found the pilot’s heart rate during flight usually increased from the normal 70-80 beats per minute to 140-150 per minute, but with no apparent physiological effect. One interesting finding, later confirmed on Mercury flights, was that the pilot’s heart rate decreased during the period of zero g. The reduction, however, was not great (to about 130 beats per minute). The respiration rate followed similar trends, increasing to three or four times the resting rate, but researchers considered this less meaningful because talking influences the respiratory system and has a poor dynamic response rate in any case. On almost all flights, there was a large peak in respiration rate during the powered portion of the flight when the pilots tended to breathe rapidly and shallowly.[23]

During the initial phases of the flight program, researchers only installed the biomedical package in the X-15 on a non-interference basis. As a result, it frequently did not work correctly since technicians had not allocated sufficient time to its installation and checkout. Most of the difficulties were traced to shorts and broken wires. Although the biomedical team coordinated with the Air Force and NASA, the next flight frequently suffered the same problems. As a result, in early 1961 the Air Force and NASA assembled a dedicated team to work on biomedical issues, and the system became much more reliable.[24]

The researchers requested that the biomedical instrumentation package fly on all XLR99 flights that expanded the envelope. In a meeting held at the FRC on 2 December 1960, the Air Force and NASA agreed that the acquisition of physiological data was important from both a flight-safety and research perspective. However, NASA did not make the acquisition of biomedical data mandatory, mainly because it did not want to have to cancel a flight because the biomedical package failed.-1251

After Paul Bikle approved the concept of installing an FM-FM telemetry system dedicated to the biomedical package, the Air Force awarded a $79,000 contract to the Hughes Aircraft Company to develop and manufacture the system.-126 The system included an FM radio transmitter rather than a hardwire link to the aircraft telemetry transmitter. This was done to demonstrate the feasibility of the radio link in order to permit the mobility expected on future spacecraft. The final Bendix TATP-350 unit measured 9.30 by 3.95 by 0.70 inches and weighed 0.84 pound. The first unit was delivered in March 1962, and the third and final unit was delivered in July 1962. Researchers demonstrated the system during flights of the JTF-102A using MC-2 suits, and in an unusual test the system received a telemetered ECG from a free-falling parachutist.-1271

NASA installed the new FM-FM system in all three X-15s during the summer of 1962. This installation permitted the ECG to be telemetered and displayed in the control room along with the helmet/suit – and cabin/suit-pressure differentials, cabin pressure, and two axes of aircraft acceleration (vertical and longitudinal). The system multiplexed these signals onto a single FM channel and then displayed them on a six-channel Sanborn stripchart recorder in the control room.-126

In 1962, the AFFTC began providing biomedical system expertise to the Dyna-Soar program. Because of severe space and weight limitations on the X-20, miniaturization was an absolute necessity. Based on its success in providing the X-15 FM-FM system, the Air Force awarded Hughes Aircraft a contract to fabricate two prototype signal-conditioning units designed around X-20 requirements. Researchers first used the system to monitor environmental and physiological data during dynamic simulations on the centrifuge at NADC Johnsville. When North American rebuilt X-15-2 into its advanced configuration, the company installed a Hughes signal­conditioning system to test the new system prior to its use on the X-20. After the cancellation of the Dyna-Soar program in December 1963, Hughes modified the design to incorporate interchangeable modules that could meet a variety of requirements. Hughes repackaged the system so that all of the modules were a common size and used identical connectors.-1291

The final package measured 4.0 by 3.5 by 0.7 inches and weighed only 0.6 pound. Like the earlier

Hughes system, this package used an FM transmitter link to the aircraft telemetry system, although a hardwire link could also be used if needed. For X-15 use, the package provided the ECG, helmet/suit – and cabin/suit-pressure differentials, Korotkoff sounds, and partial pressure of oxygen in the breathing space. When required, researchers could substitute modules that provided the partial pressure of CO2 and an impedance pneumogram.[30]

During the summer of 1963, the Air Force again modified the biomedical instrumentation, this time by adding a blood-pressure monitoring system developed by the Air Force School of Aerospace Medicine at Brooks AFB, Texas. The system used an occlusive cuff crystal microphone to determine arterial pressure in the upper arm. An electro-pneumatic programmer that cycled once per minute automatically inflated the cuff. During deflation of the cuff, a microphone detected the Korotkoff sounds, and a display showed these sounds simultaneously with the cuff pressure trace and the ECG. The system could be turned on or off by the pilot and had a fail-safe feature that could dump the pressure in the cuff in the event of a power failure, preventing a tourniquet effect from the cuff. Surprisingly, the first few flights of the package did not yield meaningful blood-pressure information, since the pilot’s pressures exceeded the maximum reading available on the instrumentation. The school modified the cuff inflation pressure to allow up to 240 millimeters of mercury to obtain useful systolic pressure end-points.-131

The school had used the blood-pressure monitoring system for in-flight studies using conventional jet aircraft for some time, and it had been very reliable in service. However, the initial use in the X-15 was not completely satisfactory because of a generally inadequate signal-to – noise ratio. The X-15 environment proved to be particularly severe; the combination of frictional noise from the suit, vibration, acoustic noise, and pilot movement artifacts produced a high background noise and a low signal-to-noise ratio. Researchers experimented with several filters and amplifiers, and eventually found a satisfactory combination.-32

In addition to the ECG and various cockpit and suit pressures and temperatures, the researchers added blood pressure, skin temperature (on the calf, abdomen, forearm, and axilla), respiratory rate, radiation, and partial pressures of oxygen and carbon dioxide in the helmet. A $47,000 Bendix Aviation mass spectrometer determined the partial pressure of oxygen, carbon dioxide, carbon monoxide, nitrogen, and water vapor.[33] A linear pneumotachometer provided by Spacelabs, Inc., under a $23,000 Air Force contract341 furnished a rapid method for determining the total oxygen consumed by measuring changes in the pilot’s breathing rate.35

As researchers evaluated the biomedical data during the flight program, it became apparent that the initial objective of obtaining data on the physiological response to weightlessness was not feasible using the X-15. The duration of weightlessness (3-4 minutes) was too short and the pilot’s responses were conditioned by too many uncontrollable variables that occurred simultaneously for any conclusions to be made concerning the physiological response to weightlessness. In addition, the manned space programs initiated shortly after the X-15 began its flight program provided a much longer weightlessness duration without the attendant stresses of having to fly the airplane; this portion of the X-15 data was instantly obsolete. Nevertheless, the X-15 data provided researchers a unique opportunity to observe the basic physiological responses of pilots in manned vehicles flying exit and reentry profiles-something that Mercury did not, since the astronaut was simply along for the ride during those periods.-1361

Saturn Insulation

The "Saturn insulation" experiment exposed various types of insulation material from the Saturn launch vehicles to the hypersonic environment. Some documentation shows this as experiment #41. The X-15-3 made at least five flights with pieces of Saturn insulation material. By flying the material on the X-15, researchers could examine it after a flight, which was not possible with the expendable Saturn boosters. Generally, researchers installed variable-thickness panels on the upper speed brakes with two thermocouples on the left side, and seven thermocouples, two static-pressure transducers, and one pitot probe on the right side. They also installed additional constant-thickness insulation panels on the lower speed brakes with two thermocouples on the right side and seven thermocouples on the left side. NASA installed a camera in the right wing-tip pod to look at the upper speed brakes, and a second camera in the left pod pointed at the lower speed brakes. Some of the tests were decidedly unsuccessful. For instance, NASA applied Saturn insulation to the upper left speed brake on the aborted flight attempt on 31 October 1967 (3-A – 96); the bond failed and the insulation came off during the captive flight. Researchers replaced the insulation before Mike Adams’s fatal flight (3-65-97).[204]

Saturn Insulation

The test of insulation for the Saturn launch vehicles is usually heralded as one of the X-15’s contributions, but in reality the tests were minimal and concentrated more on the adhesives behind the insulation. Here the Saturn insulation is installed on the upper speed brakes of X-15-3 on 31 October 1967, just before its last flight. Note the tail cone box behind the speed brakes. (NASA)

After X-15-3 was lost, NASA transferred the experiment to X-15-1. Researchers installed 16-

mm movie cameras in each wing pod to photograph the insulation on the upper speed brakes, and installed 18 thermocouples in the speed brakes themselves. Several flights carried the insulation until the end of the program, although sometimes it was on the lower speed brakes (and other times on both).-1205!

Since these tests were conducted fairly late in the Saturn development program (1966-1967), it is unlikely that any unexpected information was gained. More probably, the researchers just achieved final confirmation of the material’s ability to withstand high dynamic pressures without losing its thermal properties.


Hugh Latimer Dryden was born 2 July 1898 in Pocomoke City, Maryland. He earned his way through Johns Hopkins University, completing the four-year bachelor of arts course in three years and graduating with honors. Influenced by Dr. Joseph S. Ames, who for many years was chairman of the NACA, Dryden undertook a study of fluid dynamics at the Bureau of Standards while taking graduate courses at Johns Hopkins. In recognition of his laboratory work, the university granted him a doctor of philosophy degree in 1919.[12]

Dryden became head of the bureau’s aerodynamics section in 1920. With A. M. Kuethe, in 1929 he published the first of a series of papers on the measurement of turbulence in wind tunnels and the mechanics of boundary-layer flow. He advanced to chief of the Mechanics and Sound Division of the Bureau of Standards in 1934, and in January 1946 became assistant director. Six months later he became associate director.

In 1945 Dryden became deputy scientific director of the Army Air Forces Scientific Advisory Group. In 1946 he received the nation’s second highest civilian decoration, the Medal of Freedom, for "an outstanding contribution to the fund of knowledge of the Army Air Forces with his research and analysis of the development and use of guided missiles by the enemy."

In 1947 Dryden resigned from the Bureau of Standards to become director of aeronautical research at the NACA. Two years later the agency gave him additional responsibilities and the new title of director. Dryden held this post until he became deputy administrator of the new National Aeronautics and Space Administration (NASA) in 1958. The National Civil Service League honored Dryden with the Career Service Award for 1958. He served as the deputy administrator of NASA until his death on 2 December 1965.


The primary objective of the flight program was to explore the hypersonic flight regime and compare the results against various analytical models and wind-tunnel results. The physical X-15 configuration was of only passing interest and was not an attempt to define what any future operational aircraft might look like; it was simply a means to obtain the necessary thermal environment and dynamic pressures. The researchers wanted to understand heating rates, stagnation points, laminar and turbulent flow characteristics, and stability and control issues.

Later, the X-15 would become a carrier for various experiments, and the airplane configuration would be of even less interest.

and X-15-3 (56-6672). The second airplane became X-15A-2 after North American extensively modified it following an accident midway through the flight program. The two carrier aircraft were an NB-52A (52-003) and an NB-52B (52-008); although not identical, they were essentially interchangeable.-19!

The program used a three-part designation for each flight. The first number represented the specific X-15 ("1" was for X-15-1, etc.). There was no differentiation between the original X-15- 2 and the modified X-15A-2. The second position was the flight number for that specific X-15 (this included free flights only, not captive carries or aborts); the first flight was 1, the second was 2, etc. If the flight was a scheduled captive carry, the second position in the designation was a C; if it was an aborted free-flight attempt, it was an A. The third position was the total number of times that either NB-52 had carried aloft that particular X-15, including captive carries, aborts, and actual releases. A letter from Paul Bikle established this system on 24 May 1960 and retroactively redesignated the 30 flights that had already been accomplished.-119!

High-Temperature Loads Calibration Laboratory

The requirement to measure flight loads on aircraft flying at supersonic and hypersonic speeds led the FRC to construct the High-Temperature Loads Calibration Laboratory in building 4820 during 1964. The facility allowed researchers to calibrate strain-gage installations and test structural components and complete vehicles under the combined effects of loads and temperatures. The laboratory was a hangar-type structure with a small shop and office area attached to one end. A door measuring 40 feet high and 136 feet wide allowed access to an unobstructed test area that was 150 feet long by 120 feet wide and 40 feet high.222

High-Temperature Loads Calibration Laboratory

The High Temperature Loads Calibration Laboratory was established at the Flight Research Center to allow researchers to calibrate strain-gage installations and test structural components and complete vehicles under the combined effects of loads and temperatures. The facility was equipped with a programmable heating system that used infrared quartz lamps available in various lengths from 5 inches to 32 inches. Reflector arrangements were available for heating rates from 0 to 100 Btu per second per second and temperatures up to 3,000 degrees Fahrenheit. This photo shows an X-15 horizontal stabilizer being tested under the lamps. (NASA)

A state-of-the-art control room was provided to operate the heating and loads equipment remotely, and a data acquisition system occupied the second floor over the office spaces. Large windows overlooked the hangar floor, and the room included a closed-circuit television system. A high-capacity hydraulic system could operate up to 34 actuators to apply loads to test specimen or entire aircraft. Perhaps more importantly for the X-15 program, the facility had a programmable heating system that used infrared quartz lamps available in various lengths from 5 inches to 32 inches. Reflector arrangements were available for heating rates from 0 to 100 Btu per second per second and temperatures up to 3,000°F.[223]

NASA used this facility for a variety of purposes during the remainder of the flight program. This included testing a set of X-15 horizontal stabilizers as part of the loads program undertaken late in the flight program, and the laboratory proved to be critical for solving the inadvertent landing- gear extension problem suffered by X-15A-2 when it began its envelope expansion program. NASA later used the laboratory to test portions of the XB-70A and Lockheed Blackbirds.

Some Results

In mid-1965 the X-15 program was roughly three-quarters of the way through its eventual flight program, and flight surgeons at the AFFTC published a report on their findings to date. At the time, nine different pilots had flown the X-15 (Adams, Dana, and Knight had not yet flown); however, researchers only collected data from the six who had flown a sufficient number of flights to be statistically relevant to the analysis (omitting Armstrong, Petersen, and Thompson). The researchers noted that a "potentially very useful comparison of pilot performance and concurrent physiologic response is not possible because the X-15 flight test program is not structured as a psycho-physiological experiment. The aforementioned variability in flight profiles and unpredictable aircraft malfunctions makes possible only a general, qualitative comparison, rather than a specific, quantitative one."[37]

It is important when considering the physiological data obtained during the X-15 program to keep in mind the conditions under which researchers collected the data. In addition to the normal variability of physiologic responses, no two X-15 flights were the same. Different flight profiles and random aircraft malfunctions varied the physiological and psychic stresses to which the pilots were exposed.[38]

During an altitude mission, immediately after launch, the X-15 rotated to a preplanned climb angle and accelerated at 3-3.5 g. The pilot experienced a front-to-back ("eyes-in") inertial force that increased the apparent weight of the body, particularly the chest area, and resulted in a prompt increase in respiratory rate that continued until the acceleration subsided. After approximately 40 seconds of acceleration, the pilot pushed over to a "zero-normal" acceleration. The pulse rate, which had been increasing up to and throughout the launch operation, tended to decline during this period. At engine burnout, approximately 80 seconds after launch, the longitudinal acceleration dropped abruptly to zero, followed by a variable period in which all accelerations were essentially zero. The pilot was in an essentially weightless state during this period and the respiration rate showed a prompt decline.[39]

Immediately after engine burnout, the pilot invariably immediately experienced an increased heart rate that tended to decrease during the zero-g period. The increased heart rate at this point was probably a psychic response to the abrupt transition from a hypergravic state (in the normal plane) to a hypogravic ("weightless") state. From this point to the landing phase, the pilot was busily engaged in usually complex flight maneuvers, including "accomplishing deliberate aircraft perturbations in roll, pitch, and yaw for the purpose of collecting stability and control information." Heart rates and respiratory rates tended to reflect the difficulties the pilot encountered in managing the flight.*40

After the aircraft passed over the top of its trajectory and was descending at a steep angle, the pilot had to pull out of the dive into level flight. This pull-up generated a positive normal acceleration that the pilot experienced as increased body weight. At this point, the anti-g portion of the David Clark full-pressure suit activated to counteract these forces. Nevertheless, during this maneuver, the blood tended to pool in the lower parts of the body, the carotid arteries experienced decreased pressure, and the cardio-accelerator reflex produced a prompt increase in heart rate. The heart rate lowered as the accelerations decreased. The landing maneuver produced only mild accelerations, and the small increase in both heart rate and respiratory rate during this phase was entirely a psychic response to the task of accomplishing the landing. Since steady-state conditions existed for only a few seconds at a time during the brief 8-10-minute flights, physical and psychic stimuli were usually occurring concurrently and independently. This meant that "only the grossest correlation with heart rate and respiratory rate responses [could] be made."*41

In most professions, including piloting, there is a general trend for heart rates and respiratory rates to decrease as an individual gains experience in performing a task. However, an analysis showed "no statistically significant difference" between early flights and later flights for each of the six pilots analyzed. Researchers believed a number of factors could explain this failure to adhere to the expected trend. The first, and probably most important, was that there were no "easy" X-15 flights. Trying to obtain the maximum amount of data on each flight kept the pilot very busy performing the required maneuvers at the proper time while maintaining the desired flight profile. This required intense concentration, and the pilot also had to monitor aircraft systems during this period.-*42*

In psychological terms, the pilot had a fast-moving, intensive task to perform continuously during his 8-10-minute flight, plus a few minutes on each end. Added to this was the psychic stress of actual or potential system failures, which were not uncommon during the flight program. Another factor was the variation in flight profiles, which meant that the pilots had little or no opportunity to develop a routine for a familiar flight. Furthermore, there was often a considerable interval between flights flown by individual pilots. In the end, the researchers found that it was "not surprising that a rapid reduction in responsiveness" was not seen. The researchers found that, overall, "the spectrum of physiological response of the pilots to X-15 flights, in terms of heart rate, respiratory rate, blood pressure, and pulse pressure, has remained quite stable throughout the X-15 Program regardless of pilot experience level. This pattern of physiological response may be tentatively considered the norm for this type of operation."*431

In the end, the X-15 program was both a contributor to and a recipient of biomedical instrumentation. It was the first program to generate meaningful requirements in airborne biotelemetry and was the impetus for the development of several pieces of instrumentation that later found their way into standard clinical practice. Although Mercury and Gemini gathered better data, the X-15 nevertheless contributed to the physiological database that helped establish baselines for future programs. However, perhaps the most significant contribution of the X-15 program from a biomedical perspective was "the unequivocal, and at times dramatic, demonstration of the capabilities of the human pilot in managing a vehicle and a flight profile from launch to landing, which is a true space flight in miniature.’444*

Skid Materials

The ASD sponsored an experiment that was essentially a product evaluation program of materials selected for use on the Dyna-Soar. Researchers bonded cermet (ceramic-metallic composite) runners to the rear landing-gear skids on the X-15 for five flights using X-15-3 in early 1964. Two additional flights were conducted using X-15A-2 to evaluate Inconel X skids. Engineers compared these data with those obtained on five earlier flights that used standard 4130-steel skids but carried additional instrumentation to measure landing loads:*206!

































































































































Inconel X










Inconel X








One of the outcomes of the study was an evaluation of skid wear. The amount of skid wear depended on the speed of the sliding, the hardness of the skid material, the strength of the surface material, and the sliding distance. For this evaluation, engineers measured the thickness of the X-15 skids after each flight, generally near the point of attachment to the main strut. The difficulties involved in removing and reinstalling the skids in a timely manner precluded weighing them. The cermet skids experienced a considerable amount of wear during the first landing because of the soft outer layer of copper-nickel, but showed less wear on later landings because the tungsten-carbide chips were uncovered.-1207

The data for the 4130-steel skids showed an increasing amount of skid wear as the sliding distance increased beyond 6,400 feet. The wear characteristics of the Inconel X skids were not determined because of the difficulty of measuring the chemically milled areas inside the skid. However, preliminary data indicated a wear resistance superior to that of the 4130 steel, with or without a cermet coating.-208


Joe Engle has the unique honor of having flown the X-15 and the Space Shuttle, bringing lifting – reentry vehicles full circle. Engle flew the X-15 for 24 months from 7 October 1963 until 14 October 1965, making 16 flights with the XLR99 engine. Engle reached Mach 5.71, a maximum speed of 3,888 mph, and an altitude of 280,600 feet.

Joe Henry Engle was born on 26 August 1932 in Abilene, Kansas, and graduated from the University of Kansas at Lawrence with a bachelor of science degree in aeronautical engineering in 1955. After graduation he worked at Cessna Aircraft as a flight-test engineer before being commissioned through the Air Force ROTC program in 1956. Engle earned his pilot wings in 1958 and flew F-100s for the 474th Fighter Squadron (Day) and later the 309th Tactical Fighter Squadron at George AFB, California.

Engle graduated from the Test Pilot School in 1962 and attended the ARPS at Edwards for training as a military astronaut. He graduated from the ARPS in 1963 and became a project pilot for the X – 15 program in June 1963. Engle received an Air Force astronaut rating for making a flight above 50 miles in the X-15.-113

In 1966, at the age of 32 years, Engle became the youngest person selected to become an astronaut. First assigned to the Apollo program, he served on the support crew for Apollo X and then as backup lunar module pilot for Apollo XIV. In 1977 he was commander of one of two crews that conducted the approach and landing tests with the Space Shuttle Enterprise. In November 1981 he commanded the second flight (STS-2) of the Columbia and manually flew the reentry, performing 29 flight-test maneuvers from Mach 25 through landing rollout. This was the first (and so far only) time a pilot has flown a winged aerospace vehicle from orbit through landing. He accumulated the last of his 224.5 hours in space when he commanded Discovery during mission 51-I (STS-27) in August 1985.

Engle has flown more than 180 different types of aircraft and logged nearly 14,000 flight hours. Among his many honors, Engle has been awarded the Distinguished Flying Cross (1964), the AIAA Lawrence Sperry Award for Flight Research (1966), the NASA Distinguished Service Medal and Space Flight Medal, and the Harmon International, Robert J. Collier, Lawrence Sperry, Iven C. Kincheloe, Robert H. Goddard, and Thomas D. White aviation and space trophies. In 1992 he was inducted into the Aerospace Walk of Honor.-114