[SHB] X-15: Lessons for Reusable Winged Spaceflight

[Orionid]

X-15: Lessons for Reusable Winged Spaceflight (1966)
29 November 2017 David S. F. Portree


A dagger-shaped X-15 rocket plane separates from its B-52 carrier aircraft. During this 9 November 1961 flight, the 45th in the X-15 series, U.S. Air Force Major Robert White piloted X-15 No. 2 to a world-record speed of Mach 6.04 (4093 miles per hour). It was the first time a piloted vehicle exceeded Mach 6. Image credit: NASA

The X-15 a strong contender for the title of "Everyone's Favorite X-plane." Conceived in the 1952-1954 period, before Sputnik (4 October 1957) and the birth of NASA (1 October 1958), the North American Aviation-built rocket plane was intended to pioneer the technologies and techniques of piloted hypersonic flight - that is, of flight faster than Mach 5 (five times the speed of sound).

Between 1959 and 1968, three X-15 rocket planes, two modified B-52 bombers, and a dozen pilots took part in joint U.S. Air Force/NASA X-15 research missions. Before the start of each mission, an X-15 was mounted on a pylon attached to the underside of a wing of a B-52 carrier aircraft at Edwards Air Force Base, California. Wearing a silver pressure suit, a single pilot boarded the 50-foot-long X-15 as it hung from the pylon, then the B-52 taxied and took off from a runway.

Early X-15 missions were "captive" flights, meaning that the rocket plane stayed attached to the B-52, or gliding flights, meaning that it carried no propellants and relied on its wings, which spanned only 22 feet, to make a controlled - though fast and steep - descent to a landing. Early powered flights used stand-in rocket engines taken from earlier X-planes. By late 1960, however, the X-15's throttleable 600,000-horsepower XLR99 rocket engine was ready. The engine was designed to burn the nine tons of anhydrous ammonia fuel and liquid oxygen oxidizer in the X-15's tanks in about 90 seconds at full throttle.

Most missions followed two basic profiles. "Speed" missions saw the rocket plane level off at about 101,000 feet and push for ever-higher Mach numbers. The X-15 reached its top speed - Mach 6.72, or about 4520 miles per hour - during the 188th flight of the series on 3 October 1967 with Air Force Major William "Pete" Knight at the controls.

Knight flew X-15A-2, the former X-15 No. 2, which had rolled over during an abort landing on 9 November 1962, seriously injuring its pilot, John McKay. When NASA and the Air Force rebuilt X-15 No. 2, they modified its design to enable faster flights. McKay resumed X-15 flights after his recovery, though injuries he sustained plagued him until his death in 1975 at age 52.

For "altitude" missions, the X-15 climbed steeply until it exhausted its propellants, then arced upward, unpowered. X-15 reached its peak altitude - 354,200 feet (almost 67 miles) above the Earth's surface - on 22 August 1963, with NASA pilot Joseph Walker in the cockpit.

During altitude missions, the pilot experienced several minutes of weightlessness as the X-15 climbed toward the high point of its trajectory, above 99% of the atmosphere, then fell back toward Earth. Aerodynamic control surfaces (for example, ailerons) could not work while the X-15 soared in near-vacuum, so the space plane included hydrogen peroxide-fueled attitude-control thrusters so that the pilot could orient it for reentry.

It was during an altitude mission that the X-15 program suffered its only pilot fatality. On 15 November 1967, Major Michael McAdams piloted X-15 No. 3 to 266,000 feet despite an electrical problem that made control difficult. During descent, McAdams lost control of the space plane, which went into a flat spin at Mach 5, then an upside-down dive at Mach 4.7. McAdams might have recovered control at that point, but then an "adaptive" flight control system malfunctioned, thwarting maneuvers that might have damped out excessive pitch oscillations and compensated for increasing atmospheric density. The X-15 broke apart at about 65,000 feet.

Flights of early rocket-powered X planes, such as the first aircraft to break the sound barrier, the Bell X-1, took place over Edwards Air Force Base, but the X-15 needed more room for its speed and altitude flights. In both powered X-15 mission profiles, the B-52 released the X-15 about 45,000 feet above northern Nevada with its nose pointed southwest toward its landing site on Edwards dry lake bed. Two radio relay stations and six emergency landing sites on dry lake beds were established along the X-15 flight path. McAdams might have landed on Cuddeback dry lake bed, 37 miles northeast of Edwards, had he regained control of X-15 No. 3.


This NASA cutaway of the X-15 displays the aircraft's XLR99 engine, weight-saving aft skids, propellant tanks, wing, fin, and fuselage structure, cockpit, and forward landing gear. The lower tail fin was necessary for stability, but got in the way during landing, so was designed to drop away during approach.

During high-speed flight and Earth atmosphere reentry, the X-15 compressed the air in front of it, generating temperatures as high as 1300° Fahrenheit on its nose and wing leading edges. The rocket plane's designers opted for a "hot structure" approach to protecting it from aerodynamic heating. An outer skin made of Inconel X, a heat-resistant nickel-chromium alloy, covered an inner skin of aluminum and spun glass, which in turn covered a titanium structure with a few Inconel X parts. Heat caused the skin and structure to expand, warp, and flex, but they would return to their original shapes as they cooled. The X-15's cockpit temperature could reach 150° Fahrenheit, but the pilot usually remained cool in his pressure suit.

NASA's Project Mercury, which began officially on 6 October 1958, opted for a different approach to aerodynamic heat management: a blunt, bowl-shaped, ablative heat shield (that is, one that charred and broke away during atmosphere reentry, carrying away heat). As piloted Mercury capsule flights commenced (5 May 1961) and President John F. Kennedy put NASA on course for the moon (25 May 1961), public attention shifted away from the X-15 and Edwards Air Force Base and toward Mercury, Apollo, and Cape Canaveral, Florida. X-15 research planes continued to fly, however, pushing the hypersonic flight envelope well past their original design limits.

In the same period, some within NASA planned Earth-orbiting space stations. Before Kennedy's moon speech, a space station was seen as the necessary first step toward more advanced space activities. It would serve as a laboratory for exploring the effects of space conditions on astronauts and equipment and as a jumping-off place for lunar and interplanetary voyages. Station supporters often envisioned that it would reach orbit atop a two-stage Saturn V rocket, and that reusable spacecraft for logistics resupply and crew rotation would make operating it affordable. After the moon speech, station proponents hoped that, once Kennedy's politically motivated moon goal was reached, piloted spaceflight could resume its "proper" course by shifting back to space station development.

In November 1966, James Love and William Young, engineers at the NASA Flight Research Center at Edwards Air Force Base, completed a brief report in which they noted that the reusable suborbital booster for a reusable orbital spacecraft would undergo pressures, heating rates, and accelerations very similar to those the X-15 experienced. They acknowledged that the X-15, with a fully fueled mass of just 17 tons, might weigh just one-fiftieth as much as a typical reusable booster. They nevertheless maintained that X-15 experience contained lessons applicable to reusable booster planning.

Love and Young wrote that some space station planners expected that a reusable booster could be launched, recovered, refurbished, and launched again in from three to seven days. The X-15, they argued, had shown that such estimates were wildly optimistic. The average X-15 refurbishment time was 30 days, a period which had, they noted, hardly changed in four years. Even with identifiable procedural and technological improvements, they doubted that an X-15 could be refurbished in fewer than 20 days.

At the same time, Love and Young argued that the X-15 program had demonstrated the benefits of reusability. They estimated that refurbishing an X-15 in 1964 had cost about $270,000 per mission. NASA and the Air Force had accomplished 27 successful X-15 flights in 1964. The cost of refurbishing the three X-15s had thus totaled $7.3 million.

Love and Young cited North American Aviation estimates when they placed the cost of a new X-15 at about $9 million. They then calculated that 27 missions using expendable X-15s would have cost a total of $243 million. This meant, they wrote, that the cost of the reusable X-15 program in 1964 had amounted to just three percent of the cost of building 27 X-15s and throwing each one away after a single flight.


NASA test pilot Neil Armstrong flew the X-15 seven times in 1960-1962. Armstrong became a member of NASA Astronaut Group 2 ("The New Nine") in September 1962. He orbited the Earth as commander of Gemini 8 (March 1966) and became the first man to set foot on the moon during Apollo 11 (July 1969). Another X-15 pilot, Joseph Engle, became a member of NASA Astronaut Group 5 in April 1966. Engle flew the Orbiter Enterprise during Space Shuttle Approach and Landing Test (ALT) flights in 1977, commanded Columbia for mission STS-2 in November 1981, and commanded Discovery for mission STS 51-I in August-September 1985. Image credit: NASA

The last X-15 flight, the 199th in the series, took place on 24 October 1968. Flight experience gained and hypersonic flight data collected during the nine-year program contributed to the development of the U.S. Space Shuttle, though not exactly as Love and Young had envisioned.

When, in 1968, NASA Headquarters management first floated Space Station/Space Shuttle as the space agency's main post-Apollo piloted program, the Shuttle was conceived as a reusable piloted orbiter vehicle with a reusable piloted suborbital booster - that is, the design that Love and Young had assumed. By late 1971, however, funding limitations forced NASA to opt instead for a semi-reusable booster stack comprising an expendable External Tank and twin reusable solid-propellant Solid Rocket Boosters.

The space agency was also obliged to postpone its Space Station plans at least until after the Space Shuttle became operational. Saturn V was on the chopping block, so the semi-reusable Shuttle would be used to launch the Station as well as to resupply it and rotate its crews.

Shuttle Orbiter Columbia first reached Earth orbit on 12 April 1981, but no Orbiter visited a space station until Discovery rendezvoused with the Russian Mir station on 6 February 1995 during mission STS-63. The first Shuttle Orbiter to dock with a station - again, Russia's Mir - was Atlantis during mission STS-71 (27 June-7 July 1995).

Sources

Survey of Operation and Cost Experience of the X-15 Airplane as a Reusable Space Vehicle, NASA Technical Note D-3732, James Love and William Young, November 1966

"I Fly the X-15," Joseph Walker and Dean Conger, National Geographic, Volume 122, Number 3, September 1962, pp. 428-450

Hypersonics Before the Shuttle: A Concise History of the X-15 Research Airplane, Monographs in Aerospace History No. 18, Dennis R. Jenkins, NASA, June 2000

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Source: Lessons for Reusable Winged Spaceflight

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NASA Armstrong Fact Sheet: X-15 Hypersonic Research Program
Feb. 28, 2014

X-15 Hypersonic Research Aircraft


One of the X-15s landing on Rogers Dry Lake in Edwards, California. The nose wheel did not steer and the skids were the only braking it had: friction.

In the joint X-15 hypersonic research program that NASA conducted with the Air Force, the Navy, and North American Aviation, Inc., the aircraft flew over a period of nearly 10 years and set the world's unofficial speed and altitude records of 4,520 mph (Mach 6.7) and 354,200 feet in a program to investigate all aspects of piloted hypersonic flight. Information gained from the highly successful X-15 program contributed to the development of the Mercury, Gemini, and Apollo piloted spaceflight programs as well as the Space Shuttle program.

Manufactured by North American Aviation, Inc., three rocket-powered X-15s flew a total of 199 times, with North American (and former National Advisory Committee for Aeronautics or NACA) pilot Scott Crossfield making the first, unpowered glide flight on June 8, 1959. NASA's William H. Dana was the pilot for the final flight in the program on Oct. 24, 1968. All of these flights took place within what was called the "High Range" surrounding but mostly to the east of Edwards Air Force Base, Calif., and NASA's Flight Research Center (later called the NASA Dryden Flight Research Center).

There were 10 other pilots in the program for a total of 12: five from NASA, five from the Air Force, one from the Navy, and one, Crossfield, from North American. Generally, pilots used one of two types of flight profiles — a speed profile that called for the pilot to maintain a level altitude until time for descent to a landing, and a high-altitude flight plan that required maintaining a steep rate of climb until reaching altitude and then descending.


The X-15 #2 (56-6671) launches away from the B-52 mothership with its rocket engine ignited. Credits: NASA Photo

Because of the large fuel consumption of its rocket engine, the X-15 was air launched from a B-52 aircraft at about 45,000 feet and speeds upward of 500 mph. Depending on the mission, the rocket engine provided thrust for the first 80 to 120 seconds of flight. The remainder of the normal 8- to 12-minute flight was without power and ended in a 200-mph glide landing. Because the nose landing wheel lacked steering and the main landing gear employed skids, the X-15 had to land on a dry lakebed. The Rogers Dry Lake adjacent to Edwards and Dryden was the intended landing location for all flights, but there were numerous emergency lakebeds selected in advance for emergency landings.

Design 


Multiple views of X-15 hypersonic aircraft. Credits: NASA Illustrations
 
The X-15 was a follow-on research aircraft to the early X-planes, which had explored the flight regime from just below the speed of sound (Mach 1) to Mach 3.2. In 1952 the NACA had begun preliminary research into space flight and associated problems. Two years later, NACA's Research Airplane Projects Panel discussed the need for a new research airplane to study hypersonic and space flight. The NACA established the characteristics of what became the X-15 and presented them to the Air Force and Navy in July 1954. The two services and NACA signed a memorandum of understanding for the joint project in December 1954, and the Air Force selected North American to develop three X-15 research aircraft in September 1955.

A North American team headed by Chief Project Engineer Charles Feltz designed the aircraft, with technical guidance from the NACA's Langley Aeronautical Laboratory (later NASA's Langley Research Center), Hampton, VA, and High-Speed Flight Station (as Dryden was then called).

Although the number two aircraft was later modified, the basic X-15 was a single-seat, mid-wing monoplane designed to explore the areas of high aerodynamic heating rates, stability and control, physiological phenomena, and other problems relating to hypersonic flight (above Mach 5). Because the Reaction Motors Division of Thiokol Chemical Corp. did not have the throttleable XLR-99 engine ready for the early flights of the aircraft, the X-15 initially flew with two XLR-11 engines, producing a thrust of 16,380 lb. Once the XLR-99 was installed, the thrust became 57,000 lb.

The X-15 used conventional aerodynamic controls for flight in the dense air of the usable atmosphere. The controls consisted of rudder surfaces on the vertical stabilizers to control yaw (movement of the nose left or right) and canted horizontal surfaces on the tail to control pitch (nose up and down) when moving in synchronization or roll when moved differentially.

For flight in the thin air outside the Earth's atmosphere, the X-15 used a reaction control system. Hydrogen peroxide thrust rockets on the nose of the aircraft provided pitch and yaw control. Those on the wings furnished roll control.

The outer skin of the X-15 consisted of a nickel-chrome alloy called Inconel X, employed in a heat sink structure to withstand the results of aerodynamic heating when the aircraft was flying within the atmosphere. The cabin was made of aluminum and was isolated from the outer structure to keep it cool.

Program History

The first X-15 arrived at the NASA High-Speed Flight Station in the early months of 1959, and Scott Crossfield, who had helped with the design of the aircraft, soon began the contractor demonstration flights. During its research program, the aircraft set unofficial world speed and altitude records of 4,520 mph (Mach 6.7—on Oct. 3, 1967, with Air Force pilot Pete Knight at the controls) and 354,200 feet (on Aug. 22, 1963, with NASA pilot Joseph Walker in the cockpit).


After an ablative coating to protect the craft from the high temperatures flight X-15 was then covered with a white sealant coat and mounted with additional external fuel tanks.

More important than records, however, were the X-15's probing of hypersonic aerodynamic performance and heating rates, research into structural behavior during high heating and high flight loads, study of hypersonic stability and control during exit from and reentry of the atmosphere, and examination of pilot performance and physiology.

In the course of its flight research, the X-15's pilots and instrumentation yielded data for more than 765 research reports. As Dryden Chief Scientist Ken Iliff and his wife, aerospace research engineer Mary Shafer, have written, "The aircraft returned benchmark hypersonic data for aircraft performance, stability and control, materials, shock interaction, hypersonic turbulent boundary layer, skin friction, reaction control jets, aerodynamic heating, and heat transfer." (The boundary layer is the thin layer of air next to the body of the aircraft that has distinctive flow characteristics because of friction between the air and the surface of the aircraft; control of the flow in the boundary layer is critical to improving aircraft performance.)

The distinguished Langley aeronautical researcher John Becker, who had been an early advocate of the X-15 program, identified 25 specific accomplishments of the effort. These included:

-First application of hypersonic theory and wind tunnel work to an actual flight vehicle.

-First use of reaction controls for attitude control in space.

-First reusable superalloy structure capable of withstanding the temperatures and thermal gradients of hypersonic reentry.

-Development of (a servo-actuated ball) nose flow direction sensor for operation over an extreme range of dynamic pressure and a stagnation air temperature of 1,900° F (for accurate measurement of air speed and flow angle at supersonic and hypersonic speeds).

-Development of the first practical full pressure suit for pilot protection in space.

-Development of inertial flight data systems capable of functioning in a high dynamic pressure and space environment.

-Discovery that hypersonic boundary layer flow is turbulent and not laminar.

-Discovery that turbulent heating rates are significantly lower than had been predicted by theory.

-First direct measurement of hypersonic aircraft skin friction and discovery that skin friction is lower than had been predicted.

-Discovery of hot spots generated by surface irregularities. (These last four discoveries including the Space Shuttle.)

-Discovery of methods to correlate base drag measurements with tunnel test results so as to correct wind tunnel data (and thereby improve design criteria for future air- and spacecraft).

-Demonstration of a pilot's ability to control a rocket boosted aerospace vehicle through atmospheric exit.

-Successful transition from aerodynamic controls to reaction controls and back again.

-First application of energy-management techniques (for the positioning of the vehicle for all future reusable launch vehicles following their reentry from space.)

-Use of the three X-15 aircraft as testbeds to carry a wide variety of experimental packages.


Typical flight research paths flown by the X-15 diagram. Credits: NASA Illustration

These experiments - 28 of them - ranged from astronomy to micrometeorite collection. They included tests of horizon definition and proposed insulation that bore fruit in the navigation equipment and thermal protection used on the Saturn launch vehicles in the Apollo program, which dispatched 12 astronauts to the moon and back. Among the 12 was Neil Armstrong, the first human to step on the moon's surface and a former X-15 pilot who also flew many other research aircraft at the Flight Research Center.

In the area of physiology, researchers learned that the heart rates of X-15 pilots ranged from 145 to 185 beats per minute during flight. This greatly exceeded the normal 70 to 80 beats per minute experienced on test missions for other aircraft. The cause of the difference proved to be the stress X-15 pilots encountered during pre-launch in anticipation of each mission. As it turned out, the higher rates proved typical for the future physiological behavior of pilot- astronauts.

More intangibly but no less importantly, in the words of John Becker, the X-15 project led to "the acquisition of new piloted aerospace flight 'know how' by many teams in government and industry. They had to learn to work together, face up to unprecedented problems, develop solutions, and make this first manned [today, we would say piloted] aerospace project work. These teams were an important national asset in the ensuing space programs."

As the partial list of accomplishments suggests, the X-15 brilliantly achieved its basic purpose of supporting piloted hypersonic flight within and outside the Earth's atmosphere. In addition, it carried out the "explorations to separate the real from the imagined problems and to make known the overlooked and the unexpected problems" that Hugh Dryden had called for in 1956 when the X-15 was still in the design and development phase.

The Aircraft

Except for the number two X-15 when modified as the X-15A-2, the X-15s were roughly 50 ft long, with a 22-ft wing span. The wedge-shaped vertical tail was 13 ft high. Because the lower vertical tail extended below the landing skids when they were deployed, a part of the lower vertical tail was jettisoned just before landing and recovered by a parachute. The aircraft was powered by a Thiokol (Reaction Motors Division) XLR-99 throttleable rocket engine powered by anhydrous ammonia and liquid oxygen. It provided a maximum thrust of 57,000 lb and a minimum thrust of 28,000 lb. Launch weight of the aircraft was 31,275 lb, decreasing to 12,295 lb at burnout.

The X-15A-2, modified from the number two aircraft and delivered to NASA in February 1964, included among other new features, a 28-in. fuselage extension to carry liquid hydrogen for a supersonic combustion ramjet that was flown (as a dummy) but never tested. It also had external tanks for liquid ammonia and liquid oxygen. These tanks provided roughly 60 seconds of additional engine burn and were used on the aircraft's Mach 6.7 flight. While adding to the speed the X-15 did achieve, the tanks also increased the aircraft's weight to almost 57,000 lb and added significantly to the drag experienced by the aircraft in flight.

 X-15 pilots in order by dates of first flights # of Flights

A. Scott Crossfield, North American Aviation, 14
Joseph A. Walker, NASA, 25
Robert M. White, United States Air Force (USAF), 16
Forrest S. Petersen, United States Navy, 05
John B. McKay, NASA, 29
Robert A. Rushworth, USAF, 34
Neil A. Armstrong, NASA, 07
Joe H. Engle, USAF, 16
Milton O. Thompson, NASA, 14
William J. Knight, USAF, 16
William H. Dana, NASA, 16
Michael J. Adams, USAF, 07
Total Number of Flights: 199


Cutaway drawing of the North American X-15 showing the various components of the aircraft.Credits: NASA Illustration

The X-15 had its share of emergency landings and accidents, but only two produced serious injuries or death. On Nov. 9, 1962, Jack McKay experienced an engine failure and landed at Mud Lake, NV. The landing gear collapsed, flipping him and the aircraft on its back. Although he recovered from his injuries sufficiently to fly again, he eventually had to retire because of them.

On Nov. 15, 1967, on Michael Adams seventh flight, he entered a spin from which he was able to recover but could not bring it out of an inverted dive because of a technical problem with the adaptive flight control system. He died in the resultant crash of the X-15 number three.

Sources

-Milton O. Thompson, At the Edge of Space: The X-15 Flight Program (Washington, DC, and London: Smithsonian Institution Press, 1992).
-Richard P. Hallion, On the Frontier: Flight Research at Dryden, 1946-1981 (Washington, DC: NASA SP-4303, 1984).
Wendell H. Stillwell, X-15 Research Results (Washington, DC: NASA SP-60, 1965).
-John V. Becker, "The X-15 Program in Retrospect," 3rd Eugen Sänger Memorial Lecture, Bonn, Germany, Dec. 4-5, 1968, copy in the NASA Dryden Historical Reference Collection.
-Kenneth W. Iliff and Mary F. Shafer, Space Shuttle Hypersonic Aerodynamic and Aerothermodynamic Flight Research and the Comparison to Ground Test Results (Washington, DC: NASA Technical Memorandum 4499, 1993), p. 2 for quotation and see also their "A Comparison of Hypersonic Flight and Prediction Results," AIAA-93-0311, paper delivered at the 31st Aerospace Sciences Meeting & Exhibit, Jan. 11-14, 1993, in Reno, NV.
-R. L. Schleicher, "Structural Design of the X-15," Journal of the Royal Aeronautical Society (Oct. 1963): 618-636.
Proceedings of the X-15 First Flight 30th Anniversary Celebration (Washington, DC: NASA Conference Publication 3105, 1991).

Last Updated: Aug. 7, 2017
Editor: Yvonne Gibbs

Source: NASA Armstrong Fact Sheet: X-15 Hypersonic Research Program
https://www.nasa.gov/centers-and-facilities/armstrong/x-15/

[Orionid]

X-15 pilot and future NASA astronaut Neil Armstrong stands beside one of the planes.
Image Credit: NASA

Editor’s note: This is the first in a series of features that will culminate with NASA’s 60th anniversary on Oct. 1, 2018. Rather than a purely chronological view, the series will have a broader emphasis, highlighting historical moments and programs that brought NASA to where it is today.

Though the X-planes became a formative part of the space program, their development was rooted in engineers’ efforts to simply make airplanes go faster. Researchers had learned by the 1930s that planes driven by piston engines and propellers ultimately ran into performance problems at about 350 mph, and data was already showing there would be additional problems at even higher speeds.

The X-1 program resolved enough of those problems to allow Chuck Yeager to fly faster than sound in 1947, and researchers turned to possibilities of “hypersonic” flight, Mach 5 or higher. In the mid-1950s, those efforts became the X-15 program, led by the National Advisory Committee on Aeronautics (NACA), NASA’s predecessor agency, in partnership with the U.S. Air Force and Navy.

As it did so many other things, the Soviet Union’s launch of Sputnik changed how the X-15 was viewed. With no U.S. satellite launch on the horizon, Americans focused on the new vehicle. It made its public debut on Oct. 15, 1958. (Ironically without its managing organization; two weeks earlier, NACA had become NASA.)


X-15 #2 in flight in 1961. Image Credit: NASA

As Harrison “Stormy” Storms of North American Aviation, the X-15’s builder, said, “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 of the elements and most of the problems of a true space vehicle.”

Ultimately, the X-15 flew 199 times, and on Oct. 3, 1967, U.S. Air Force pilot Pete Knight set a speed record of Mach 6.7. But as important as the records was everything that NACA, and later NASA, researchers learned about operating craft at high speeds and altitudes. Roger Bilstein summarized the achievements this way in “Testing Aircraft, Exploring Space:”

"The fallout was far-reaching in numerous crucial areas . . . The X-15’s survival encouraged extensive use of comparatively exotic alloys, such as titanium and Inconel-X, which led to machining and production techniques that became standard in the aerospace industry. . . . (T)he chance of accidental loss of pressurization . . . prompted development of the first practical full-pressure suit for pilot protection in space. The X-15 was the first to use reaction controls for attitude control in space; re-entry techniques and related technology also contributed to the space program, and even earth science experiments were carried out by the X-15 in some of its flights."


The X-15 was air launched from a B-52 aircraft at 45,000 feet and at a speed of about 500 mph. After dropping from the B-52, the rocket engine provided thrust for the first 80 to 120 seconds of flight. The remainder of the normal 10 to 11 min. flight was powerless and ended with a 200-mph glide landing. Image Credit: NASA

Taking over the X-15 program produced more than technical knowledge, though. “Although NACA in essence bootstrapped air force and navy funds for the X-15,” Bilstein wrote, “it was very much a NACA idea and design from start to finish. In many ways, the X-15 program signaled a shift to the research, development and management functions that characterized the NASA organization soon to come.”

The X-15 and other X-planes are more than a historical legacy for NASA. The program is the core of NASA’s New Aviation Horizons, an array of new experimental aircraft that will carry on the legacy of demonstrating advanced technologies to push back the frontiers of aviation. Goals include showcasing how airliners can burn half the fuel and generate 75 percent less pollution during each flight as compared to now, while also being much quieter than today’s jets – perhaps even when flying supersonic.


The Quiet Supersonic Technology, or QueSST, concept is in the preliminary design phase and on its way to being one of NASA’s first X-planes. Image Credit: NASA

Source: X 15