29 November 2017

X-15: Lessons for Reusable Winged Spaceflight (1966)

An 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 aircraft 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 flight 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).


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

More Information

Space Station Resupply: The 1963 Plan to Turn the Apollo Spacecraft Into a Space Freighter

McDonnell Douglas Phase B Space Station (1970)

From Monolithic to Modular: NASA Establishes a Baseline Configuration for the Shuttle-Launched Space Station (1970)

An Alternate Station/Shuttle Evolution: The Spirit of '76 (1970)

Where to Launch and Land the Space Shuttle? (1971-1972)


  1. One of the X-15 pilots, Milton O Thompson wrote an excellent book about the program: "At The Edge Of Space: The X-15 Flight Program". One of the stories in it is the one about a Neil Armstrong flight where it went a little too long. Neil was flying past the Rose Bowl when he finally could make the turn back towards Edwards. He made it back but it was a close thing.

    1. I'll have to watch out for that book. That must've been pretty exciting - Armstrong seemed to court close calls. I'm thinking of Gemini 8's early return and Apollo 11 excitement after pitchover.


    2. Don't forget Armstrong's near-crash in the LEM trainer (Lunar Landing Training Vehicle).

    3. Phuzz:

      You know, I remembered that after I posted my comment. It's pretty uncanny, his very public set of close calls. Of course, other astronauts had them, too - perhaps we don't hear as much about them because they weren't the first man on the moon.


  2. NAA went even a Step further and proposed 1958 a Orbital X-15B
    That a strip down X-15 with mass of 4500 kg
    launched by 4 x G-26 Navaho booster stages or 4 x Titan I clusters.
    (even Saturn I derivate) from Cap Canaveral
    The X-15B would made a single orbit with an apogee of 120 km and a perigee of 75 km
    Means it reenter earth Atmosphere at end of curve.
    The Pilot glide with X-15B to the Gulf of Mexico were exit with ejection seat
    and wait in dinghy for rescue, while the X-15B is lost
    USAF and NASA were not happy about "almost a orbit and eject in final phase"
    So they reject the concept, irony Yuri Gagarin just made that in April 1961...

    1. Michel:

      Delicious irony. I hadn't heard of this before. I mean, I knew about the orbital X-15, but I didn't know about this scheme.


  3. Interesting what might have happened if things went differently. I have read a bit on the idea to make the C version a delta wing and a true space plane. Do you think the X-20 Dyna-Soar may have also muddied the waters going orbital with variant of the X-15 almost as much as the Mercury program did?

    1. After Dyna Soar was canceled came NAA in 1966 with the X-15C proposal
      For a Delta Wing version launch from B-52 or a modified XB-70 at Mach 3 !
      The goal of project was to gain information on Supersonic flight beyond Mach 5-6
      (up to Mach 8 in some Source)

      Also had NASA in 1962 the idea of X-15/Blue Scout as Satellite launcher !
      combo of B-52, X-15 with blue Scout rocket take of once the X-15 launch at high point
      it launch the blue Scout https://history.nasa.gov/hyperrev-x15/ch-6.html

    2. Michel:

      You know, that sounds an awful lot like the F-15/ASAT scheme tested back in the 1980s, only on a larger scale. I don't remember much about those tests, except they destroyed an operational science satellite, which annoyed some folks.


    3. That's infamous ASM-135 ASAT program
      irony the ASM-135 was build from stages of Scout rocket (like Blue Scout)
      but the target P78-1 or Solwind was on end of his lifespan
      The batteries were degrading and P78-1 suffers more and more of frequent "under-voltage cutoffs"
      Making Solwind unusable for it Mission and it owner DoD released it for target practice

      although the test that hit Solwind had unanticipated side effects: Space Debris

    4. Michel:

      I remember some scientists complaining about Solwind's destruction - apparently they were taking data from it even as it was destroyed. But the orbital debris issue was more consequential, for certain. At least at the time. I might be wrong, but I think all the detectable Solwind debris has reentered by now.

      How many ASM-135 tests took place?


    5. They made 5 test with ASM-135, one was failure (probably sensors in warhead)
      Program was canceled because of technical problems, testing delays, and far over budget.
      According some source was $500 million budget, finally it had to become $5.3 billion for deployment...

      Last debris of Solwing reenter around 2008


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