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 is 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.

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 through most of its career. 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.

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. One modification was the addition of a replaceable ablative heat shield so that it could withstand the higher temperatures that came with faster speeds. Ablative heat shields are designed to char and break away, carrying away heat.

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 Adams piloted X-15 No. 3 to 266,000 feet despite an electrical problem that made control difficult. During descent, Adams lost control of the space plane, which went into a flat spin at Mach 5, then an upside-down dive at Mach 4.7. Adams 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. Adams 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.

NASA's Project Mercury, which began officially on 6 October 1958, opted for a different approach to aerodynamic heat management: a bowl-shaped single-use ablative heat shield. 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 3% 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.

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)

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