The X-15 as Research Aircraft

In a previous post, I offered a brief summary of the X-15 program in which I highlighted its features that enabled it to take on the designation of a ‘space plane’. I also mentioned that its nature is two-fold; it is at once a space plane and a research aircraft. (Left, an engineer runs wind tunnel tests on a scale model of an X-15.)

For many involved with the X-15 program, the aircraft was the first space plane – it’s record altitude was above the 50-mile limit of space. The aircraft was poised to be the first in a line of orbit-capable space planes. The proposed follow-up X-20 program built on the basic space plane design. But as the space race gathered steam, the X-15 took a backseat to, and was eventually eclipsed by, the Mercury ballistic capsule. Thus, in the wake of Mercury’s success, the X-15 took on a second nature – the last in a long line of research aircraft.

The official history of the Mercury program cites March of 1958 as the starting point. In response to the launch of Sputnik, the NACA and the Advanced Research Projects Agency (ARPA, which later became DARPA) agreed to pursue the ballistic method as the quickest means to manned spaceflight.

That same month, both organizations met with representatives from leading aeronautical research corporations to review and pick a design for the Mercury capsule. In part, the size and shape of the capsule was determined by the available booster rockets. The Redstone and Atlas ballistic missiles (both products of the Army Ballistic Missile Association) had a fixed circumference and a limited lift capacity. Of all proposals, Maxim Faget’s blunt body design was eventually selected. The program began in earnest in 1959 – the astronauts were selected and trained, and the capsules built and man rated. (Right: Three scale models of launch vehicles. The Redstone (right) and Atlas (centre) that launched the Mercury program stand next to the Saturn V, which launched Apollo to the moon, on the left.)

At the same time, the X-15 program was beginning test flights. First proposed in 1954, the X-15 was a joint venture undertaken by the United States Air Force, the National Advisory Committee for Aeronautics, the United States Navy, and North American Aviation, the contractor who built the physical aircraft.

The X-15’s flight path took it through two very different environments: the powered ascent and unpowered descent were in an atmospheric environment, while highest point of the ballistic flight was done at an altitude with little to no atmosphere. This unique flight path necessitated three separate flying styles. The X-15 flew like a jet during the powered ascent, it functioned as a spaceship during the top portion of the flight path, and finally it flew like a glider during its unpowered descent).

The X-15's flight plan. This schematic shows its release from the B-52 launch plane, powered ascent, arc at peak altitude, and gliding descent pattern.

To move seamlessly through these stages, the X-15 was equipped with two sets of controls. For the atmospheric portions, the pilot used traditional pitch, yaw, and roll controls, and during the non-atmospheric flight, reaction controls in the nose of the plane controlled the attitude of the aircraft.

After NASA’s inception in 1958, it absorbed its predecessor organization the NACA as well as its programs, including the X-15. So in its infancy, NASA was simultaneously pursuing two very different methods of spaceflight.

In 1961, the Mercury and X-15 programs were underway; the former had completed the initial stages and had progressed to record-breaking high altitude flights of over 200,000 feet, and the latter program had begun with Alan Shepard’s suborbital flight. Both programs were also reaching their inherent limitations. The X-15 was too small to achieve orbit and the Mercury spacecraft was too small to achieve long-duration flight. NASA needed to make a decision about the next generation of manned spaceflight.

The X-15 had proven the concept of a space plane and Mercury had proven the concept of a ballistic spacecraft, and both programs had the potential to grow into the next generation of manned space flight.

The follow-up program building on the X-15 success was the X-20 program – also knownas Dyna-Soar for Dynamic Soaring) – which was originally proposed in late 1957. Dyna-Soar was essentially a larger X-15, combining the latter’s controllability with the capacity for orbital flight. Dyna-Soar was based on the concept of an aerodynamic lifting body – a spacecraft whose inherent aerodynamic design gives it a certain amount of control during its descent. It would be launched into Earth orbit on the nose of a missile and returned to a landing on a runway like an aircraft. (Right, an artist concept of a Dyna-Soar launch.)

Similarly, the basic design of the Mercury capsule could be developed into a larger and more manoeuvrable spacecraft to accommodate a multi-manned crew on a long-duration flight. The planned development was originally known as the Mercury Mark II program, an extension of the existing capsule design that eventually became the Gemini program in 1962.

Many involved in the X-15 program, from the USAF as well as from NASA, were sceptical that the Mercury-style capsule would retain any centrality in the future of manned space flight.

In the late 1950s, NASA Administrator T. Keith Glennan chose the X-15 as the preferred method of spaceflight over capsule designs. He did, however, concede that the Mercury spacecraft had distinct advantage of gaining experience with men in space much faster, explaining that NASA accepted the lack of control in the interest of time. (Right, a schematic of a typical mission profile for an orbital Mercury flight.)

As could be expected, the USAF stood strongly behind the X-15 as the most practical space vehicle. The design was particularly appealing when considering the long-term future of not only trained astronauts but also scientists and even tourists in space. The space plane’s shallower re-entry (compared with a capsule-style spacecraft) would be much gentler on less physically fit men.

Opinions from engineers from both NASA as well as NAA were mixed. From both camps, engineers agreed that an X-15 type vehicle would dominate the future of spaceflight. NASA landing gear engineer Jim McKay anticipated the influence of the X-15 on future space vehicles, particularly in terms of its landing gear. The rear skids (or ski gear as he described it) is the optimum way to slow a vehicle quickly upon landing.

In 1960 and 1961, when NASA began planning its follow-up program to Mercury, the organization had two equally viable methods for spacecraft under development, and a decision had to be made as to which method would persist. Another event in 1961 effectively set the mode of spaceflight for the ensuing decade and a half.

NASA’s decision regarding which method to pursue in space was made against the backdrop of mounting pressure for the US to prove its dominance in space. On 12 April 1961, Yuri Gagarin became the first man to orbit the Earth. NASA followed two weeks later with its slightly less impressive first manned space flight. On 5 May, Alan Shepard became the first American in space by travelling in a ballistic arc. The mission lasted less than 15 minutes.

Three days after his flight on 8 May, Alan Shepard received a NASA Distinguished Service Medal from President Kennedy at a ceremony at the White House. Shepard, who recounts the evening in his memoirs, found himself with the other Mercury astronauts President Kennedy, and a handful of his advisers and NASA officials discussion NASA’s next move. (Left, Kennedy presents Shepard with the Distinguished Service Medal, May 8, 1961.)

Kennedy suspected that the US could never best the Soviets if confined to Earth orbit, so concluded that the country’s hope for dominance in space lay in setting a goal both countries were equally unprepared to reach. This distant goal not only enabled the US to bide time while NASA developed its next generation program, it had the added benefit of successful efforts serving as a testament to American technology and innovation.

Twenty days later, Kennedy pledge the US would be the first nation to land a man on the moon and return him to the earth, effectively setting the mode of spaceflight for NASA’s immediate future.

This proclamation gave NASA a strict time frame with only nine years in which to accomplish a lunar landing. The organization had already begun a move away from space planes with the decision not to pursue the Dyna-soar program with the USAF. Reflective of that decision, the follow-up program to Mercury that was already in the planning stages (Mercury Mark II) was building on the successes of the capsule approach.

NASA abilities of the moment favoured the capsule-designed spacecraft, effectively killing the space plane’s chances for a lunar program. The capsule method had been proven, and with launch and recovery methods known, and could simply be expanded with sufficient speed to support the lunar goal.

The X-15, while a significant step forward in aviation, became was little more than a research aircraft for NASA after Kennedy’s lunar proclamation.  The project yielded vital data on the effects of reentry on manned vehicles applicable to future programs, including Gemini and Apollo. More directly, the X-15 influenced the Space Shuttle, which acts exactly like the X-15 during its reentry and descent. NASA’s focus at the time, however, was on achieving its goals as fast as possible, and so the capsule-style spacecraft promised the greatest results.


Suggested Reading/Selected Sources

1. Grimwood, James. Project Mercury: A Chronology. Washington: NASA. 1963.

2. Godwin, R. X-15: the NASA mission reports, incorporating files from the USAF: Collector’s Guide Publishing. 2000.

3. Shepard, Alan and Slayton, Deke. Moonshot; Inside America’s Race to the Moon. Turner Publishing, Kansas City. 1994.

4. Thompson, Milton O. At the edge of space: the X-15 flight program: Smithsonian Inst Pr. 1992.

5. Thompson, Milton O. Flying Without Wings: NASA lifting bodies and the birth of the space shuttle. Washington: Smithsonian Institution Press. 1999.

6. Tregaskis, Richard. X-15 Diary: The Story of America’s First Space Ship. Bison Books. 2004.

7. “Dynasoar”. [Accessed August 13, 2010].

8. “X-Planes”. [Accessed August 13, 2010].



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