In mid-1955 the Soviet Union and the United States separately announced intentions to orbit satellites as part of the 1957 International Geophysical Year. Nevertheless, when the Soviet Union launched the first Earth artificial satellite-Sputnik (later called Sputnik 1)-on 4 October 1957, the event created a stir among the popular press. The seeming lack of response by President Dwight D. Eisenhower further antagonized the fourth estate and soon the American people as well. However, it was the 1,100-pound Sputnik 2 that ultimately caused the administration to take action, since it graphically portrayed the capability of Soviet launch vehicles and, directly, their ICBM program.

The Soviet achievements damaged American scientific and technological prestige, and the satellite was widely regarded as a threat to national security. Robert Gilruth later wrote, "I can recall watching the sunlight reflecting off the Sputnik 1 carrier rocket as it passed over my home on the Chesapeake Bay, Virginia. It put a new sense of value and urgency on the things we had been doing."1130!

Over a year before, the Air Force had begun Project HYWARDS (Hypersonic Weapon and Research & Development Supporting System) to design a successor to the X-15. Researchers considered this round III of the research airplane program. Round I had been the X-1 and D-558 series, while round II consisted of the X-15. The goal of round III was to design a vehicle capable of achieving at least Mach 12 and perhaps as much as Mach 18. HYWARDS is outside the scope of this history, but it created an enormous debate between researchers at Ames and Langley, and between the NACA and the Air Force. The Air Force soon combined HYWARDS with the remaining work on BoMi/RoBo and other projects into the Boeing X-20 Dyna-Soar program. Ultimately, the experimental research conducted for HYWARDS and Dyna-Soar, combined with the flight results from X-15, formed the technical foundation for the development of a space shuttle.

Although HYWARDS was the next logical step in the progressive effort to fly a man into space, other programs, such as Project 7969, were under way concurrently. The organizations that proposed these programs intended then to put a man into space as soon as possible, mainly as a publicity ploy, and offered little in the way of a long-term solution to space flight. The X-15 figured into some of these programs, and at least two proposals for orbital X-15s were made during 1957 and 1958 (see the "X-15B" section for more details).

In the meantime, the NACA Executive Committee met in its regular annual session on 10 October 1957, less than a week after the launch of Sputnik. Interestingly, the committee did not discuss the Soviet satellite at any length. But the NACA Committee on Aerodynamics met on 18-20 November 1957 aboard the aircraft carrier USS Forrestal (CVA-59) and paid a great deal of attention to crafting a response to Sputnik. The committee noted that "[t]he big question to be answered now is how can these views [on accelerating space research] be put across to the NACA and to the Government in order that the NACA be recognized as the national research agency in this field, and be provided with the necessary funds… the NACA should act now to avoid being ruled out of the field of space flight research." The committee suggested highlighting the hypersonics program in general and the X-15 program specifically in order to make that case.-1131!

This threw a great deal more attention onto the X-15 program than it was ready for. North American was making good progress with its development effort, but the first airframe was still almost a year away from being completed. The XLR99 engine was much further away.

Nevertheless, the media-and indeed, some within the NACA and military-saw the X-15 as the most promising American response to Sputnik. The North American plant in Inglewood, which was clearly visible from the Los Angeles International Airport, soon sported a huge "Home of X-15" neon sign and articles began to appear in periodicals ranging from popular newsstand magazines to serious industry journals. It was a spotlight the X-15 program was ill prepared to handle.-1132

Nevertheless, the publicity probably made some aspects of the X-15 program somewhat easier, particularly securing funding at a time when the program was seriously over budget. In his essay on the 1961 Collier trophy, W. D. Kay wrote:232

After the launch of Sputnik 1 in 1957, interest in the [X-15] project on the part of the military, political leaders, and the public at large grew rapidly… media coverage of the first flights was the most intense ever seen at Edwards, and even led to some public relations mix-ups between NASA and the Air Force. Once the first Mercury flights were underway, public attention shifted to the events at Cape Canaveral. This might, however, have ultimately worked to the [X-15] program’s benefit. A major contributor to the X-15’s success over the long run was its emphasis on incremental development and its use in highly specialized scientific and technical research. As experience with many later space projects… has shown, the general public tends to lose interest in such "routine" undertakings rather quickly. In short, it appears the X-15 got a needed boost of public fanfare at precisely the right point in its history-the later development and early flight test stage-and then became regarded as a low-key effort worthy of only occasional interest just as it was entering its less "flashy" research phase. These shifts in external perception probably could not have been planned any better.

Scott Crossfield might not completely agree that the program wanted the publicity, especially as he spent too many hours in an uncomfortable MC-2 full-pressure suit in the hot desert sun providing encouragement for the technicians working to get the X-15 ready for its first glide flight. Overall, however, events probably turned out as well as anybody could have expected.

There were a variety of proposals (some legitimate, most not) to use the X-15 to put a man into orbit before the Soviets. However, there was a flaw with all of these ideas: the lack of a suitable booster. The ICBMs then under development had two significant problems. First, none had the "throw weight" to launch a complicated lifting reentry vehicle, be it an X-15 derivative or one of the round III concepts under study at Ames, Langley, or Wright Field. Second, the early ICBMs did not work very well; they tended to blow up.

Pilot in the Loop

During the late 1950s and early 1960s, the value of a human pilot and redundant systems in space vehicles was a matter of some controversy. There was, and continues to be, a great debate on the relative merits of piloted vehicles versus automated ones. Because it had many similarities to early spacecraft, the X-15 became the subject of several evaluations and studies to determine whether the general design approach taken concerning redundancy and control were appropriate.-1146!

The AFFTC conducted one of these evaluations during late 1961. The basic approach of the study was to perform a detailed flight-by-flight engineering analysis of each problem or failure that occurred for the 44 free flights, plus an additional 2 captive flights and 30 aborts, made through 1 November 1961. For each problem, researchers assessed the action taken by the pilot or redundant system with regard to its impact on mission success and vehicle recovery. The researchers then compared the results with those that would be obtained on a hypothetical unmanned and/or non-redundant X-15.147

The researchers strictly adhered to several important ground rules during the evaluation and documented every problem, whether it seemed significant or not. The researchers conservatively assessed the benefits of the pilot-in-the-loop and redundancy to avoid any glorification of either of these elements.-148! The researchers also attempted to minimize conjecture, especially in the case of the hypothetical unmanned X-15. For instance, the researchers did not credit a pilot with detection or corrective action that some other element would definitely have provided in his absence. Likewise, he was not marked down for detrimental effects that would have been the same without a pilot. The study used a similar assessment scheme for redundancy. Finally, for the hypothetical unmanned X-15, the study assumed no changes to systems or components other than removing all redundant systems and substituting relatively simple and reliable present-day systems in their place.149

The results were not surprising. Of the 44 free flights conducted up to that time, researchers considered 43 successful as flown.150! Computed as an airplane that carried a pilot but no redundant systems, only 27 would have been successful. The number fell to 24 with redundancy but without a pilot, and to only 23 with no redundancy or pilot. The study noted that 19 flights were completely trouble – free, so they would have been successful in any of the three configurations. Significantly, the evaluation showed that the majority of times the mission was not

successful, the aircraft would have been lost. In fact, in the case of a piloted but non-redundant X-15, the study showed that 14 aircraft would have been lost in 44 missions.-1151

Around the same time, The Boeing Company conducted a similar study that analyzed the first 60 flights of the Bomarc surface-to-air missile. This large unmanned missile was designed to be relatively non-redundant in order to keep its manufacturing costs low.-1152 The Boeing study compared the actual flight results with a theoretical piloted Bomarc that incorporated a level of redundancy mostly equivalent to the X-15. Thus, the Boeing study was roughly the inverse extrapolation of the AFFTC X-15 evaluation, and the results bore an amazing similarity. For the X-15, the total mission success rate had been approximately 98%, which compared well to the computed 97% rate for a piloted and redundant Bomarc. Conversely, for both the actual Bomarc and the theoretical unmanned, non-redundant X-15, the total mission success rate was an identical 43%. This lent credibility to the idea that, with the current state of the art, it was still important to include a pilot in the loop.-153

Final Dispositions

In November 1968, William P. Albrecht and Vincent N. Capasso inspected X-15A-2, which had been in storage at the FRC since the completion of the thermal tests. The engineers determined what work would be needed to prepare the aircraft for display in a museum, or to return it to flight status if necessary. As the airplane stood, it was missing control surfaces, most of its cockpit displays, and the removable right outer wing panel. All of the pieces were stored nearby.[393]

A cursory inspection of the airplane showed signs of minor corrosion in unprotected areas, and the engineers believed the aircraft needed "a thorough inspection for corrosion with cleaning and repainting as required. A lubrication would be accomplished at the same time to protect moving surfaces…. This would take two men approximately 2 to 4 weeks to accomplish."*394

To restore the vehicle to flight status, the engineers believed three to four months of work would be required, including installing the engine plumbing, control surfaces, actuators, and SAS pump. All of the wiring would have to be checked and the hydraulic system would need servicing. In addition, the instrument panel would have to be installed and the landing gear made flight-ready.

If the airplane went to a museum, the engineers thought that some items (mainly bars to replace the control surface actuators) would have to be fabricated. A rough estimate included three people for a month to prepare the airplane for display, plus time to paint and stencil the exterior.-1395!

Officially, the X-15 program simply "expired" at the end of its authorized funding on 31 December 1968. After the New Year holiday, things began to happen quickly. Between the Apollo program and the increasing tempo of the air war in Southeast Asia, neither NASA nor the Air Force seemed particularly interested in the small black airplanes that were stored in the High – Temperature Loads Calibration Laboratory at the FRC.

On 4 January 1969, officials at Edwards formally requested reassignment instructions for the two remaining X-15 airplanes. A response came on 20 February directing that the "number one X-15 be made available for display in the Smithsonian Museum. The Smithsonian is prepared to receive the X-15 and it may be transported to Andrews as soon as it is ready for shipment. In order to protect the option of any future flight test program, extreme care should be taken in handling the X-15 so that it will not be altered or damaged. The Air Force Museum should retain accountability for the aircraft and reassign it to the Smithsonian for display." A later message directed NASA to transfer X-15A-2 to the Air Force Museum at Wright-Patterson AFB.!396

A meeting on 7 January 1969 at North American discussed how best to dispose of the remaining program assets for which North American still had responsibility. Unlike many programs that require the contractor to account for every pencil purchased with government funds, the X-15 ended much more casually. For instance, the "contractor was advised … that no physical inventory of X-15 assets will be required," and that the North American working inventory would be "accepted by the Air Force as the formal X-15 inventory record." The Air Force justified this casual attitude by noting that "(a) it is a research type program, (b) the last physical inventory was taken less than a year ago, (c) [it is] in the interest of program economy." Nevertheless, North American had to assign class codes (indicating how to dispose of the item) to some 9,000 line items having a monetary value in excess of $6,000,000.[397]

On 18 March 1969, the public affairs officer at the FRC wrote to his counterpart at NASA Headquarters. The opening paragraph was telling: "Sometime this spring, probably May, the number one X-15 will go into the Smithsonian. Because of the haste of the announcements and Apollo 8, the world didn’t seem to care much when the program was concluded last December. I’d like to see if we can’t remind the world about the X-15 and use the Smithsonian as an excuse."!3981

The Smithsonian Institution’s National Air and Space Museum (NASM) had begun its efforts to acquire an X-15 as early as 1962, but nothing was likely to happen as long as the flight program continued. After the funding expired, the Air Force agreed to lend X-15-1 to NASM for two years. NASA partially disassembled X-15-1, loaded it onto a flatbed trailer, and flew it to Andrews AFB inside an Air Force transport. On 13 May 1969, a truck moved X-15-1 from Andrews to the Silver Hill Facility (later the Paul E. Garber Facility) in Maryland. After some minor refurbishment, the Smithsonian installed the X-15 near the original 1903 Wright Flyer on the floor in the north hall of the Arts and Industries building, which housed the NASM at the time. On 7 July 1971, the Air Force officially transferred ownership of X-15-1 to the NASM, which subsequently loaned the airplane to the FAA for display at Transpo 72 in the spring of 1972. The airplane then traveled to the FRC to help commemorate its 25th anniversary. The NASA loan was effective for one year beginning in August 1972, but ultimately was extended until the summer of 1975. The X-15 returned to the Smithsonian for installation in the new NASM building on the mall before it

opened to the public on 1 July 1976. X-15-1 currently hangs in the Milestones of Flight Gallery at the NASM in Washington, D. C.[399]

X-15A-2, completely refurbished after its unhappy experience with the ablative coatings, became the property of the National Museum of the United States Air Force at Wright-Patterson AFB, Ohio. However, prior to the airplane arriving in Ohio, the museum loaned it to the Alabama Space and Rocket Center in Huntsville (now the U. S. Space & Rocket Center). The airplane arrived on a one – year loan on 27 March 1970, although for some reason the Air Force did not installed the right wing while the airplane was in Huntsville. The airplane now sits—in black Inconel finish—in the Presidential and Research & Development Galleries of the National Museum of the United States Air Force. A set of external tanks and a dummy ramjet are part of the display.

The Air Force buried the remains of X-15-3 at an undisclosed location on the Edwards reservation. In 1991 the Astronaut Memorial at the Kennedy Space Center, Florida, added Mike Adam’s name, a tacit reminder of an oft-forgotten manned space program.*400

The two NB-52s remained at Edwards to support the heavyweight-lifting-body program. Not long after the end of the lifting-body program, NASA retired the NB-52A to the Pima Air & Space Museum outside Tucson, Arizona. The NB-52B continued to serve as a carrier aircraft, launching the X-38 and X-43 vehicles, through the beginning of 2005. NASA finally received a B-52H in mid-2005 to serve as a carrier aircraft, but a lack of requirements resulted in the airplane returning to the Air Force in 2006. At the time of its retirement, the NB-52B was the oldest operational (and lowest flight time) B-52 in the Air Force.*401


The final name used by the FRC for the follow-on research program was "test-bed experiments," although the Research Airplane Committee and other sources continued to call it the "follow-on program." The effort was formally announced in a news release on 13 April 1962: "The hypersonic X-15 will become a ‘service’ airplane to carry out new experiments in aeronautical and space sciences, in a program planned to make use of its capabilities for extremely high speeds and altitudes beyond Earth’s atmosphere. The new program adds at least 35 flights… and may take two years to complete." John Stack and Hubert M. Drake announced that an ultraviolet stellar photography experiment from the Washburn Observatory at the University of Wisconsin would be the first.1841

Experiment #1: Ultraviolet Stellar Photography

The NASA Office of Space Sciences sponsored experiment #1 to investigate the ultraviolet emissions of large, hot stars, and the properties of interstellar media. Researchers had already obtained limited data using sounding rockets, but desired additional data prior to the launch of the Orbiting Astronomical Observatory (OAO). The purpose of the experiment was to obtain measurements of the stellar brightness between 1,800 and 3,200 200 ngstroms ( ). The ozone layer blocks this spectrum from observation by ground-based instruments. Dr. Arthur D. Code and Dr. Theodore E. Houck from the Washburn Observatory at the University of Wisconsin designed the experiment.-1851

During December 1960, North American conducted a few runs in the fixed-base simulator to determine whether a pilot could fly the X-15 precisely enough to allow the experiment to collect useful data; the answer appeared to be yes. The simulations, however, pointed out the need for the reaction augmentation system, and were yet another driver to develop and install the system in the first two airplanes.-861

Before the experiment began, researchers at the University of Wisconsin wanted to gather information on the ultraviolet intensity of the sky background. To accomplish this, they installed a photomultiplier in one of the upper bug-eye camera bays of X-15-1 in April 1962. The photomultiplier required power and the use of one recording channel, but little else in the way of support. The first flight of the instrument was made on 19 April 1962 (flight 1-26-46). Originally, the university planned to install the complete experiment in the skylight compartment of X-15-2 in August 1962, but scheduling priorities delayed this until December 1962. Unfortunately, Jack McKay’s accident on flight 2-31-52 would postpone all future uses of X-15-2.871

Ultimately, the experiment consisted of an ultraviolet "star tracker" and horizon scanner installed on a stabilized platform in the skylight compartment on X-15A-2 after it was modified. The star tracker first flew on flight 2-33-56 and functioned properly in the caged mode. The next X-15A – 2 flight repeated the same tests. Flight 2-35-60 was intended to check out an uncaged (i. e., free to move) stabilized platform without opening the skylight doors, but a blown fuse prevented this. The experiment was successfully checked out on the next two X-15A-2 flights.881

The experiment was carried on five additional flights (2-38-66 through 2-41-73); however, little usable star-tracking data were obtained because of problems in maintaining the precise attitudes required for the experiment. Nevertheless, data from flight 2-39-70 confirmed speculation that the sky background was somewhat brighter than originally expected. The brightness gave less contrast between the star or constellation and the sky, making acquisition and observation more difficult. After the last flight in this series, researchers temporarily discontinued the experiment because of the position of the desired stars during the winter in southern California. The position of the stars supported three additional flights (2-46-83 through 2-48-85) the following summer. All of these flights were successful and obtained good data.88

Researchers determined that the atmosphere above 45 miles did not absorb the light from stars of moderate or larger magnitude. During flight 2-47-84, the experiment successfully photographed the stars Eta Aurigae, Alpha Aurigae, and Rho Aurigae from altitudes above 246,000 feet, which were some of the first stellar ultraviolet images. In late 1966, NASA removed the experiment from X-15A-2 in preparation for its Mach 8 envelope-expansion program.88

Experiment #2: Ultraviolet Earth Background

The Air Force Geophysics Research Directorate sponsored experiment #2 to measure the total Earth background radiation (albedo) and horizon in support of designing missile-warning surveillance satellites. Researchers expected that the Earth’s atmosphere would absorb most of the ultraviolet rays and thus appear very black to an ultraviolet sensor. Any missile rising from the surface of the Earth would show as a bright point of light in the ultraviolet, and thus could be easily detected. As originally envisioned, the experiment would use an array of spectrometers installed in the lower bug-eye camera bays. Researchers wanted to obtain data during each of the four seasons and at altitudes above 132,000 feet to be above the ozone ultraviolet absorption level, but otherwise did not require special flight considerations. The experiments would obtain spectral background data in the middle ultraviolet spectrum, high-angular-resolution data relative to the solar-blind ultraviolet horizon gradient, high-angular-resolution data in the solar – blind gradient near 3,100 , and vacuum ultraviolet background data. The experiment was

scheduled to begin in late 1962, but was postponed almost a year because key Air Force personnel were busy with other projects.-91

Researchers planned to fly the experiment on X-15-2, but a meeting on 17 September 1962 between Captain Hugh D. Clark and Captain James H. Smith from the ASD, and James E. Love and Lannie D. Webb from the FRC resulted in a decision to use X-15-3 instead. This decision also affected the ultraviolet exhaust-plume characteristics (#3) and infrared-exhaust-signature (#10) experiments."

During the postponement, the Air Force briefly canceled the experiment due to a lack of funding, but ultimately reinstated it. The experiment required at least one flight in excess of 150,000 feet to calibrate the test package, and then six further flights to acquire data. The equipment consisted of a high-resolution Barnes ultraviolet scanning spectrometer and a solar-blind radiometer mounted on a stabilized platform in the tail-cone box on X-15-3. Mechanical problems with the experiment precluded any data collection through the end of 1963, and equipment and scheduling problems continued to conspire against the experiment until the Air Force finally canceled it in early 1965 without acquiring any useful data. Instead, researchers decided to concentrate their efforts on experiment #3, which used the same basic equipment aimed at a specific point behind the X-15 to measure its exhaust.-1931


Neil Armstrong flew the X-15 for 20 months from 30 November 1960 until 26 July 1962, making seven flights. These included two flights with the XLR11 and five with the XLR99. Armstrong reached Mach 5.74, a maximum speed of 3,989 mph, and an altitude of 207,500 feet. His accomplishments include making the first flight with the ball nose and the first flight with the MH-96 adaptive control system.

Neil Alden Armstrong was born on 5 August 1930 in Wapakoneta, Ohio. He attended Purdue University, earning his bachelor of science degree in aeronautical engineering in 1955. During Korea, which interrupted his engineering studies, Armstrong flew 78 combat missions in F9F-2 fighters, for which he earned the Air Medal and two Gold Stars. He later earned a master of science degree in aerospace engineering from the University of Southern California.

Armstrong joined the NACA Flight Propulsion Research Laboratory (now the Lewis Research Center) in 1955. Later that year he transferred to the High-Speed Flight Station (HSFS) as an aeronautical research scientist and then as a pilot. Armstrong served as the project pilot on the F – 100A, F-100C, F-101, and F-104A, and flew the X-1B, X-5, F-105, F-106, B-47, KC-135, and Paresev. He left with over 2,450 flying hours.

102A and F5D aircraft. In 1962, when he was flying the X-15, Armstrong was one of nine pilots selected for the second NASA astronaut class. In March 1966 he was the commander of Gemini 8, with David Scott as pilot (this mission accomplished the first successful docking of two vehicles in orbit). On 20 July 1969, during the Apollo 11 mission, Armstrong became the first human to land on the Moon. Armstrong has a total of 8 days and 14 hours in space, including 2 hours and 48 minutes walking on the Moon.

After his lunar flight, Armstrong became the deputy associate administrator for aeronautics at NASA Headquarters. He resigned from NASA in August 1971 to become professor of engineering at the University of Cincinnati, a post he held until 1979. Armstrong became chairman of the board of Cardwell International, Ltd., in 1980 and served in that capacity until 1982. During 1982-1992, he was chairman of Computing Technologies for Aviation, and from 1981 to 1999 he served on the board of directors for the Eaton Corporation. He was also vice chair of the Rogers Commission, which investigated the Space Shuttle Challenger accident in 1986.

Armstrong has been the recipient of numerous awards, including the Presidential Medal of Freedom and the Robert J. Collier Trophy in 1969, the Robert H. Goddard Memorial Trophy in 1970, and the Congressional Space Medal of Honor in 1978.[3]


Milt Thompson flew the X-15 for 22 months from 29 October 1963 until 25 August 1965, making 14 flights with the XLR99 engine. Thompson reached Mach 5.48, a maximum speed of 3,723 mph, and an altitude of 214,100 feet.

Milton Orville Thompson was born on 4 May 1926 in Crookston, Minnesota. Thompson began flying with the Navy and served in China and Japan during World War II. Following six years of active duty, Thompson entered the University of Washington and graduated with a bachelor of science degree in engineering in 1953. After graduation Thompson became a flight-test engineer

for the Boeing Aircraft Company, testing, among other things, the B-52.

Thompson joined the HSFS on 19 March 1956 and became a research pilot in January 1958. At the time, there were only five pilots at the station: Joe Walker, Stan Butchart, Jack McKay, Neil Armstrong, and Thompson. In 1962, Thompson became the only civilian pilot on the X-20 Dyna – Soar, but Robert McNamara canceled that program just over a year later. On 16 August 1963, Thompson became the first person to fly a lifting body, the lightweight M2-F1. He flew it 47 times and made the first five flights of the all-metal M2-F2. Thompson concluded his active flying career in 1967 and became chief of research projects two years later. In 1975, he became chief engineer and retained the position until his death on 6 August 1993. Thompson also served on NASA’s Space Transportation System Technology Steering Committee during the 1970s. In this role he was successful in leading the effort to design the Space Shuttle orbiters for power-off landings rather than increase weight with air-breathing engines. His committee work earned him the NASA Distinguished Service Medal.

Thompson was a member of the Society of Experimental Test Pilots, and he received the organization’s Iven C. Kincheloe Trophy as the outstanding experimental test pilot of 1966 for his research flights in the M2 lifting bodies. He also received the 1967 Octave Chanute award from the AIAA for his lifting-body research. In 1990, the National Aeronautics Association selected Thompson as a recipient of its Elder Statesman of Aviation award (this award has been presented each year since 1955 to individuals who made contributions "of significant value over a period of years" in the field of aeronautics). Milt Thompson died on 6 August 1993.Г26

Thompson wrote about his experiences with the X-15 in At the Edge of Space: The X-15 Flight Program (Washington, D. C.: Smithsonian Institution Press, 1992). Anybody who is interested in an inside look at the program should pick up a copy; it is a fascinating read.

Project 7969

The Air Force initiated Project 7969, the manned ballistic rocket research system, in February 1956 with a stated goal of orbiting and recovering a manned space capsule. By the end of 1957, a joint Air Force-NACA team had evaluated at least 10 serious proposals during a conference held at Wright Field on 29-31 January 1958. Avco, Convair, Goodyear, Lockheed, Martin, and McDonnell proposed spherical reentry vehicles or blunt capsules, while Bell, North American, Republic, and Northrop all proposed winged vehicles.234

The North American proposal included a "stripped" X-15 with an empty weight of 9,900 pounds. Cape Canaveral would launch the vehicle on a two-stage booster that allowed a single orbit with an apogee of 400,000 feet and a perigee of 250,000 feet. The launch vehicle consisted of four Navaho boosters. Three were clustered together in the first stage and one acted as the second stage. The XLR99 in the X-15 was the third stage. The X-15 would be equipped with beryllium oxide leading edges and a Rene 41 alloy shingle heat shield, plus a thicker Inconel X hot structure. Due to the low perigee and aerodynamics of the X-15, no retrorocket was required for reentry. The pilot would eject and descend by parachute just before ditching the X-15 in the Gulf of Mexico, with the aircraft being lost. North American expected that it could conduct the first manned orbital flight 30 months after a go-ahead, at a cost of $120 million.-1135

Given the early state of development of the X-15, there was almost no real engineering associated with this proposal. Nevertheless, it was further along than many of the others since researchers already knew that the basic X-15 shape was stable in most flight regimes, and both the airframe and XLR99 were at least under active development.

After the launch of Sputnik 1, Project 7969 was reoriented into the Man In Space Soonest (MISS) project to ensure that a U. S. Air Force pilot would be the first human in outer space. On 27 February 1958, General Curtiss E. LeMay, the Air Force vice chief of staff, was briefed on three alternatives that included the X-15 derivative, speeding up the Dyna-Soar program, and building a simple non-lifting ballistic capsule that could be boosted into low orbit by an existing ICBM – derived booster. LeMay apparently expressed no preference, and although it was a long and complicated process, the result was that a ballistic capsule appeared to offer the best hope of immediate success. This idea formed the basis for Project Mercury after NASA was formed on 1 October 1958 and the first American manned space effort was transferred to the civilian agency.-135


Nevertheless, engineers at North American continued to refine their Project 7969 concept. A few days after the Soviet Union orbited Sputnik 1 on 4 October 1957, North American packaged everything into a neat report and Harrison Storms took the idea to Washington. This version used two Navaho boosters clustered together as the 830,000-lbf first stage, a single Navaho booster as the 415,000-lbf second stage, and an X-15B powered by a 75,000-lbf Rocketdyne XLR105 Atlas sustainer engine as the third stage. Unlike the 7969 proposal, this one had a great deal more engineering in it, although it was still very preliminary since North American had not conducted wind-tunnel tests or detailed calculations on heating or aero loads.-137

The X-15B was larger than the basic X-15 and was capable of carrying two pilots. The Inconel X skin was made thicker to withstand the increased reentry heating, and the vehicle had larger propellant tanks to feed the Atlas sustainer engine that replaced the XLR99. However, the shape and many of the internal systems were identical to those of the basic X-15 then under construction. Engineers had already demonstrated the supersonic and subsonic stability of the X – 15 during numerous wind-tunnel tests, and keeping the same shape eliminated the need to repeat many of them.

The flight plan was simple. Eighty seconds after launch from Cape Canaveral, the first stage would drop away and the second stage would fire. At an altitude of about 400,000 feet, the second stage would burn out and the X-15B would continue using its own power. The vehicle would eventually get up to 18,000 mph, enough for three orbits. The pilot would fire the XLR105 at a point that would allow the X-15B to land at Edwards using the reentry profiles already developed for the basic X-15. It was a grand plan, and years ahead of its time. Unfortunately, when Storms got back from Washington he reported that "there were exactly 421" other people who had competing proposals. Eventually the X-15B just quietly faded from sight.138

Project 7969

In the excitement caused by the Soviet launch of Sputnik, North American proposed a heavily modified X-15B as an early orbital vehicle. Although the aerodynamics of the X-15 were well – understood by this time, the X-15B did not have nearly the maneuverability of the Air Force Dyna-Soar while returning from orbit, and in fact, many X-15B proposals had the pilot ejecting over water instead of attempting to land. (North American Aviation)

The Third Industry Conference

November 1961 saw the first industry conference held in three years (NASA had held previous conferences in 1956 and 1958). The classified conference at the FRC featured 24 papers from 56 authors, including 4 X-15 pilots, and was attended by 442 people. Of the authors, 5 came from North American, 37 from various NASA centers, 13 from the Air Force, and 1 from the Navy. The attendees represented virtually every major aerospace contractor in the country, all of the NASA centers, several universities, the various military services, and the British Embassy.-154

At the time the papers for the conference were prepared, the program had made 45 flights during the 29 months since the initial X-15 flight. The first of these was a glide flight, and of the subsequent powered flights, 29 had used the XLR11 engines and 15 used the XLR99. A maximum altitude of 217,000 feet (flight 2-20-36) and a velocity of 6,005 feet per second (flight 2-21-37) had been achieved.155

Researchers had already accomplished quite a bit of analysis on aerodynamic heating, one of the primary research objectives of the X-15. Several theoretical models had been developed to predict heating rates, but little experimental data were available to validate them since it was uncertain whether wind tunnels were capable of realistically simulating the conditions. The X-15 provided the first real-world experience at high Mach numbers in a well-instrumented, recoverable vehicle. Data from the X-15 showed that none of the models were completely accurate, although all showed some correlation at different Mach numbers. The data showed that the wind tunnels were reasonably accurate.156

A particular area of interest to researchers was how the boundary layer transitioned at different Mach numbers and angles of attack. Researchers used two methods to detect laminar and turbulent areas on the airplane in flight. The first was to use thermocouple data reduced to heat – transfer coefficients, which showed a much higher level of heat transfer in a turbulent boundary layer than in a laminar one. The second method was to use temperature-sensitive "DetectoTemp" paint applied over large areas of the airplane. In general, NASA applied the paint to the left side of the airplane, and the thermocouples were on the right side.157

The first use of the paint was on 4 August 1960 for flight 1-9-17, which was the XLR11 maximum speed attempt. The results were promising inasmuch as the paint established a semipermanent pattern of contrasting colors at different temperature levels. The pattern retained on the wing and vertical stabilizer after the flight clearly indicated all of the heat-sink locations and areas of high heating. For instance, the internal spars and ribs stood out as heat sinks, while areas such as the expansion joints on the wing leading edge stood out in the color pattern as concentrated heating areas. Researchers decided that they could use the paint to collect qualitative temperature data, particularly in small areas that were not equipped with thermocouples.-1581

One of the notable discoveries made using the paint was that patterns indicated high – temperature, wedge-shaped areas originating at the wing leading-edge expansion joints and extending for a considerable distance rearward. The 0.080-inch-wide expansion joints appeared to result in a turbulent flow during the entire flight, producing 1,000°F temperatures in an 8-inch wedged-shaped area behind them. The measured heat-transfer data on the other wing supported this view, offering "a classic example of the interaction among aerodynamic flow, thermodynamic properties of air, and elastic characteristics of structure." Although the rates were well within the limits of the airframe, engineers installed small 0.008-inch-thick Inconel X shields over the expansion joints in an attempt to minimize the interference. Flights with these covers showed that the turbulent wedges still existed, although they were smaller, and researchers theorized that they would be present for shorter periods on each flight.159

The conclusion drawn from this was that the "boundary layer transition, which may be produced by such discontinuities in the surface of a high-speed vehicle, would be extremely difficult to predict. As yet, for the X-15, there has not been established parametric correlation which would allow the prediction of the transition location on the wing a priori. Under these circumstances, it would seem that conservative estimates of transition should still be required."169

To show how the preflight estimates and flight data correlated, the authors presented data for one thermocouple on the lower surface of the right wing about 1.4 feet from the leading edge at mid-semispan. For the high-speed flight profile, the measured data indicated an all-turbulent flow with a high skin-heating rate and high maximum temperature. The calculated skin temperature agreed quite well during the high heating period, but slightly overestimated the measured value near its peak and during a period of cooling just afterwards. A close look at the trajectory during this period of disagreement showed a high angle of attack, and researchers believed the differences were due to their inability to properly predict the local flow conditions.

For a high-altitude mission, however, this point of the wing appeared to experience laminar flow, at least at times. An all-turbulent flow prediction resulted in a higher temperature than was actually measured during the exit phase of the trajectory, greater cooling during the ballistic portion, and an overestimate of the maximum temperature during reentry. The assumption of laminar flow during the latter part of the exit phase resulted in better agreement between the measured and calculated data. Researchers noted, however, that one of the turbulent wedges originating on a wing leading-edge expansion joint might affect the thermocouple in question. Researchers did not understand exactly what might cause the location to go laminar, but theorized that either the turbulent wedge vanished or its lateral spread was delayed.161

The wing leading-edge expansion slots produced problems in addition to the wedge-shaped boundary layer issue. On one flight the area directly behind the expansion slots buckled. One reason for this was that the fastener spacing directly behind the slot was wider than on other sections of the leading edge, providing less support for the area. It was also determined that the original segmentation of the leading-edge heat sink did not adequately relieve the thermal compression loads. The skins at the expansion slots acted as a splice plate for the solid heat-sink bar, and as a result buckled in compression. Engineers made several changes to solve this problem. The shield installed over each expansion slot to help the boundary layer problem minimized the local hot spot, but engineers also added a fastener near each slot and three additional expansion slots (with shields) in the outboard segments of the leading edge. This presented some concern since North American had designed the original expansion slots with shear ties to prevent relative displacement of the leading edge, and it was not cost-effective to provide shear ties for the new slots because the entire wing structure would have required modification. A structural analysis showed that sufficient shear stiffness was present in the leading edge to meet the design requirements without shear ties, but engineers expected some relative displacement at the three new slots. Actual flight tests showed that this displacement averaged about 0.125 inch. Overall, the modifications prevented any serious leading-edge buckling, although minor distortions continued throughout the flight program.-162

The Third Industry Conference

The X-15 program was one of the first to employ temperature-sensitive paint that established a semipermanent pattern of contrasting colors at different temperature levels. The paint clearly showed the different heating loads absorbed by the hot-structure airframe. In general, NASA applied the paint to the left side of the airplane; the thermocouples were on the right side. (NASA)

The conclusion drawn from the available data was that "when the boundary layer is known to be either laminar or turbulent, the skin temperatures can be predicted with reasonable accuracy." The problem was to figure out what the boundary layer would do under different flight conditions.163

The effect of temperature is not linear, and at Mach 6 the heating load on the X-15 was eight times that experienced at Mach 3. Unsurprisingly, the front and lower surfaces of the aircraft experienced the highest heating rates. During the conference, researchers discussed several intriguing aspects of the temperature problems. One was surprising, given that the program had always worried about high temperatures: "The first temperature problem occurred on the side­fairing panels along the LOX tank before the X-15 was first flown. Pronounced elastic buckles appeared in the panels as a result of contraction when the tank was filled for the first time."

Adding a 0.125-inch expansion joint to the tunnel fairing near the wing leading edge relieved the buckling.1164!

However, after a Mach 4.43 flight (2-13-26) on 7 March 1961, several permanent 0.25-inch buckles formed in the outer sheet of the fairing between the corrugations near the edge of a panel. Since the panel only carried air loads (not structural loads), the buckles did not seriously affect structural integrity. During the flight, the panels that buckled had experienced temperatures between 490°F (near the wing leading edge) to 590°F (near the front of the fairing). On this particular flight, the pilot shut down the engine prior to propellant depletion, leaving about 20% of the liquid oxygen in its tank. The maximum temperatures occurred after shutdown, and it was theorized that the cold tank (-260°F), together with the high outer-skin temperatures, resulted in large thermal gradients that caused the buckles. These gradients were higher than had been calculated for the original design, since the estimates had assumed propellant depletion on all flights. Based on this experience, engineers added four expansion joints in the fairing ahead of the wing that allowed a total expansion of slightly over 1 inch. This modification appeared to prevent any further buckling.!1661

Researchers expected the surface irregularities produced by the buckles to cause local hot spots during high-speed flights. To investigate this, NASA covered the buckled areas with temperature – sensitive paint for flight 2-15-29. The results from the Mach 4.62 flight showed that the maximum temperature in the buckle area was essentially the same as in the surrounding areas with no evidence of local hot spots. The researchers went back to their slide rules to come up with revised theories.-11661

Other heating problems experienced during the early flight program included hot airflow into the interior of the airplane, which caused unexpected high temperatures around the speed brake actuators, and loss of instrumentation wires in the wing roots and tail surfaces. In a separate incident, cabin pressure forced the front edge of the canopy upward, allowing hot air to flow against and damage the seal. NASA resolved the canopy problem by attaching a shingle-type strip to the fuselage just ahead of the canopy joint to prevent airflow under the edge of the canopy. A similar problem developed in the nose landing-gear compartment: a small gap at the aft end of the nose-gear door was large enough to allow the airstream to enter the compartment and strike the bulkhead between the nose-gear compartment and the cockpit. This stream caused a local hot spot that melted some aluminum tubing used by the pressure-measuring system on flight 2­17-33. During the Mach 5.27 flight, the bulkhead heated to 550°F, high enough to scorch the paint and generate some smoke inside the cabin. It was a potentially catastrophic problem, but fortunately no significant damage resulted. In response, engineers added an Inconel compression seal to the aft end of the nose-gear door and installed a baffle plate across the bulkhead.11671


On 23 August 2005, 40 years of aerospace controversy ended. For years, many aviation historians and enthusiasts had questioned why the Air Force pilots who flew the X-15 to altitudes above 50 miles received astronaut ratings, while the NASA pilots who accomplished the same feat in the same airplanes did not. The answer came on a small stage at the DFRC when Navy Captain Kent V. Romminger, chief of the NASA Astronaut Office at the Johnson Space Center, presented certificates proclaiming three NASA test pilots as astronauts. NASA administrator Shawn O’Keefe authorized the recognition, and Romminger, associate administrator for the Space Operations Mission Directorate William F. Readdy, and DFRC director Kevin L. Peterson signed the certificates. The purposefully small ceremony was a private moment for a very special group of men and their families.-*402*


In August 2005, NASA finally recognized three NASA pilots who had flown over 50 miles altitude but had not received astronaut wings like their military counterparts. Three former X-15 pilots joined in the ceremony. From left, Robert M White, William H. Dana (proudly wearing the new wings on his flight jacket), Neil A. Armstrong, and Joe H. Engle. The families of Jack McKay and Joe Walker were present to accept their astronaut wings from Kent Romminger, the Chief of the Astronaut Office. (NASA)

In the late 1960s, these three men-William H. Dana, John B. McKay, and Joseph A. Walker-had piloted the X-15 to altitudes in excess of 50 miles. Although five of their colleagues had received Air Force astronaut ratings for similar accomplishments, NASA had never recognized the three civilian pilots. Now, 40 years after the fact, the agency did. Only Bill Dana was still alive to receive his certificate, and to have his wife, Judy, place the blue and gold name tag with the astronaut wings emblem on his flight jacket. However, almost the entire McKay and Walker families were on hand to receive the tribute. Joe Walker, who always had a smile on his face, was the first human being to fly into space twice; now his son has a set of astronaut wings to proudly display. On hand to honor their colleagues were three former X-15 pilots: Joe H. Engle, who after his X-15 flights became the only person to fly the space shuttle back from orbit under manual control; Robert M. White, perhaps the least known of the test pilots (odd considering he was the first person to fly to Mach 4, Mach 5, and Mach 6, as well as the first person to fly to 200,000 feet and then to 300,000 feet; Bob still holds the world absolute-altitude record at 314,750 feet); and Neil A. Armstrong, who needed little introduction. Unfortunately, Scott Crossfield could not attend due to previous commitments. There was not a dry eye in the house.J403

Experiment #3: Ultraviolet Exhaust-Plume Characteristics

The ASD sponsored experiment #3 to measure the exhaust characteristics from a liquid-oxygen – ammonia rocket engine (the XLR99). It used the same basic equipment as experiment #2, without the stabilized platform. The first flight (3-41-64) was made on 23 April 1965 with Joe Engle at the controls. By the end of 1965, the high-resolution Barnes ultraviolet scanning spectrometer and solar-blind radiometer that had proved so troublesome on experiment #2 had successfully obtained good data. As a follow-up, researchers installed a Millikan dual-channel radiometer in X-15-3 during the weather down period at the beginning of 1966, and installed a vacuum ultraviolet spectrometer later in the year. The Millikan radiometer flew on flight 3-55-82 but froze due to a failed heater, and there is no record of it flying again. Similarly, no record exists of the spectrometer ever being flown.-194!

Experiment #4: Langley Horizon Definition a 12,000-foot mountain using a simple photometer and several interference filters. The data indicated that the "stable phenomena" hypothesis appeared to be correct, but emphasized the need for observations made from outside the Earth’s atmosphere using equipment that was more sophisticated. Researchers flew variations of the experiment on sounding rockets and the X-


Researchers installed a radiometer in the tail-cone box of X-15-3 along with a 16-mm motion – picture camera pointing out the rear. The camera provided wide-angle coverage to check for clouds or haze during the data-gathering period. The radiometer included a motor-driven scan mirror that provided a 30-degree field of view, and reflected energy into a parabolic mirror that focused the energy on the detector. The radiation passed through an optical bandpass filter to select the appropriate spectral band. The angle of the scan mirror and the output of the detector were recorded on an FM-FM magnetic tape recorder.1961

The experiment first flew on 2 May 1963 (flight 3-16-26) and made five additional flights during 1963. Three of these six flights provided meaningful data for the MIT-Apollo horizon photometer experiment (#17). Another successful flight (3-30-50) on 8 July 1964 investigated the near infrared in the 0.8-2.8-micron region. After the flight, the experiment returned to Langley for modifications, and was intended to fly at least three more times. In the end, only two additional flights were flown during 1965 (3-42-65 and 3-44-67), since the more sophisticated MIT experiment had already begun flying aboard X-15-1.

Langley was generally happy with the X-15 as an experiment platform: "Not only is the design of the experiment simplified because there are few restrictions due to size and weight limitations, but also the availability of standard X-15 attitude and position data are an important advantage… the radiometer is reusable… and good weather data is available." This was in contrast to sounding rockets that provided comparatively short flights, had minimal onboard instrumentation, and, of course, were not generally recoverable.-1971

This experiment provided the first infrared data gathered on the Earth’s limb from above 30 miles. From these data, researchers modeled the horizon profile to an accuracy of 4 kilometers for use in attitude-referencing systems carried aboard early orbiting spacecraft.1981