Transiting from Air to Space
The North American X-15


The first call for an X-15-class research vehicle came from Robert J. Woods, a colleague of Walter Dornberger at Bell, during a meeting of the prestigious NACA Committee on Aerodynamics on October 4, 1951. He reiterated his support for such a vehicle during subsequent meetings and, as a result, the NACA committee passed a motion on June 24, 1952 that charged the agency to expand its research aircraft program to include studying the problems of manned and unmanned flight at altitudes between 12 and 50 miles, and velocities of Mach 4 to Mach 10, as well as devoting "a modest effort" to study exoatmospheric flight from Mach 10 to escape velocity. The major NACA field centers exchanged various paper plane proposals. NACA engineers L. Robert Carman and Hubert Drake of the High-Speed Flight Station drew up configurations for Mach 3+ launch aircraft carrying small hypersonic research aircraft including, in August 1953, a five-phase proposal culminating in the design of an orbital air-launched hypersonic boost-glide winged vehicle. The NACA shelved this bold proposal as too futuristic, which it was; its advocacy of a "two-stage to orbit" research vehicle was one of the earliest of the "piggyback" concepts predating the current Space Shuttle. The NACA, like other federal and private organizations, favored a more modest approach. In October 1953, the Air Force's Scientific Advisory Board recommended development of a Mach 5-7 research aircraft, and at the same time, the Office of Naval Research had funded the Douglas Aircraft Corporation to study the feasibility of a Mach 7+ rocket-propelled research airplane, informally referred to as the D-558-3 1.

Figure 1 - an early American aircraft/orbiter design
Figure 1

During 1954, the NACA, in partnership with the Air Force and Navy, further explored the hypersonic aircraft concept. The agency's Langley laboratory (later NASA's Langley Research Center) had formed a hypersonic study team comprised of chairman John V. Becker, Maxime Faget, Thomas Toll, N. F. Dow, and J. B. Whitten, and this group subsequently evolved a baseline design that closely resembled the ultimate X-15 configuration. Their conception incorporated Inconel alloy heat-sink construction, had a cruciform tail configuration, a wedge vertical fin for increased directional stability, and similar weights and specifications as the final aircraft. In December 1954, the NACA, Air Force, and Navy agreed to undertake joint development of the proposed hypersonic research aircraft, and in January 1955 it received the designation X-15. That same month, the Air Force (which administered the design and construction phases of the project) held the first briefings for potential contractors. This culminated in a competition between North American, Bell, Douglas, and Republic, which North American won on September 30, 1955. The Bell entry, which featured a novel form of "double-wall" construction, reflected the firm's obsession with Sänger-like boost-gliders (indeed, in April 1952, Bell's Dornberger had journeyed to France in a vain attempt to convince Sänger and his wife to join the company), and had no real hope of winning. The subsequent technical development of the North American X-15 went smoothly, with the exception of its rocket powerplant, which generated great concern before it, too, reached fruition 2.

The X-15, "Round Two" in the parlance of the NACA, had many features that separated it from the previous rocket research aircraft and placed it at an intermediate level between the purely supersonic aircraft (such as the X-1) and the purely winged reentry vehicles (like the proposed "Round Three" Dyna-Soar and the eventual Space Shuttle). For example, it incorporated a reaction control system of hydrogen peroxide rocket thrusters for keeping the aircraft under control at high altitudes; the pilot wore a full pressure pilot protection suit (the Clark MC-2) having provisions for physiological monitoring. It was the first flight vehicle to blend the application of hypersonic aerodynamic theory to an actual aircraft. It incorporated high temperature seals and lubricants, and had a "Q-ball" flow direction sensor capable of operating with stagnation air temperatures of 3500° F. The pilot relied on inertial flight data systems developed especially for operation under space-like conditions. The X-15's Inconel structure was the first reusable super-alloy structure capable of withstanding the temperatures and thermal gradients of hypersonic reentry. Subsequently, during its flight program, the X-15 spawned development and application of a refurbishable ablative heat protection system (the Martin MA-25S) 3.

The X-15 spanned 22 ft. 4 in., and had a length of 50 ft. 9 in. It utilized a Thiokol (Reaction Motors Division) XLR-99 throttleable rocket engine, burning a mixture of anhydrous ammonia and liquid oxygen. (Delays in the development of this engine forced North American to install two XLR-11 engines in the X-15s during 1959, before beginning the research program, for purposes of checking out the aircraft and its systems; the first XLR-99 flight did not come until November 15, 1960). The three X-15 aircraft quickly established a number of speed and altitude marks, which often obscured the less glamorous but occasionally more important work they accomplished in mapping out the frontiers of' hypersonic flight. By the end of 1961, the X-15 had achieved its Mach 6 design speed, and had reached altitudes in excess of 200,000 feet. On August 22, 1963, NASA research pilot Joseph Walker reached 354,200 feet in the third X-15 aircraft, still a record for winged vehicles. X-15 testing revealed a number of interesting conditions about hypersonic flight, including the discovery that hypersonic boundary layer flow is turbulent and not laminar, that turbulent heating rates were lower than predicted by theory, that supersonic skin friction was likewise lower than predicted, that local surface irregularities generated hot spots (in one notable case, aerodynamic heating caused buckling of the wing skin behirid leading edge heat expansion slots), and that the cruciform tail configuration created a serious adverse roll problem at high angles of attack during atmospheric reentry (NASA cured this by removing the jettisonable lower half of the craft's ventral fin). The flights demonstrated that a pilot could successfully transition from aerodynamic to reaction controls and back again, function in a weightless environment (which became an academic question after Vostok and Mercury), control a rocket-boosted vehicle during atmospheric exit, and use energy management techniques to make a hypersonic/supersonic reentry and glide approach to a precision landing. The X-15 eventually made reentries at angles of attack up to 26 deg. and at flightpath angles as low as -38 degrees at Mach 6 flight speeds 4.

Figure 2 - X-15 rocket research airplane
Figure 2

As with the previous "Round one" rocket research airplanes, the X-15 was airlaunched, being dropped from a modified Boeing B-52 jet bomber. The flights were made over a specially instrumented 485-mile-long 50-mile-wide flight test corridor stretching from Nevada to Edwards Air Force Base in California. Following a landing accident with the second X-15, the Air Force and NASA authorized the manufacturer to modify it as a special testbed for NASA's planned Hypersonic Ramjet Experiment. North American lengthened the aircraft, making numerous modifications to it, and added provisions for two large jettisonable external tanks. Thus equipped, the aircraft, designated the X-15A-2, was capable of Mach 7 flight speeds, if equipped with a pr3per thermal protection system. NASA finally selected Martin to develop a suitable ablator, and that company derived the MA-25S, an ablator mix consisting of a resin base, a catalyst, and a glass bead powder. Hopes that such ablators could enable designers to build refurbishable spacecraft that could be stripped and recoated after each flight proved ill-founded, however. On October 3, 1967, the X-15A-2 attained Mach 6.72 (over 4,520 mph), while piloted by Air Force Maj. William J. Knight. Unfortunately, the plane landed in extremely worn condition -- a dummy ramjet had separated off the craft, in fact -- and the ablator would have required massive cleanup efforts prior to reapplication. North American repaired the craft and returned it to NASA, but it never flew again. The third X-15 made a number of notable high-altitude flights above 50 miles. Unfortunately, this aircraft was lost, together with pilot Michael J. Adams, on November 15, 1967. The first X-15 completed its last flight, the 199th flight for the type, on October 24, 1968 5.

Following awarding of the X-15 development contract, North American had considered a so-called "X-15B" orbital spacecraft (even before Sputnik), to be launched by two Navaho boosters and possibly carry a two-astronaut crew. After Sputnik, it went through a cycle of shelving and revival until finally overcome by the ballistic blunt-body spacecraft approach as taken by the McDonnell Mercury vehicle. The X-15 series itself, however, did perform a number of "Shuttle" like missions, for after 1962, the X-15 program switched concentration from hypersonic aerodynamics to using the vehicle as a testbed carrying a wide range of applications and experiments, such as insulation intended for the Saturn booster, and navigation instruments under development for Apollo. By 1964, fully 65 percent of all data returned from the X-15 related to follow-on programs, and this figure continued rising until the conclusion of the program in December 1968. NASA even briefly considered using the X-15 as a launcher for Scout rockets carrying small satellite payloads, the B-52/X-15/Scout becoming, in effect, one large booster, but after examining the idea, NASA rejected it on grounds of safety, cost, and practicality. Fittingly, in December 1968, the Deutsche Gesellschaft fUr Raketentechnik und Raumfahrt awarded John Becker and the X-15 team with the Eugen Sänger Medal, created to honor individuals and groups who have-made special contributions to the field of recoverable spacecraft 6.

The following case study of the X-15 was prepared by the late Robert S. Houston of the then-Historical Branch, Office of Information Services, Wright Air Development Center, Wright-Patterson AFB, Ohio, in 1959. It has been expanded and updated by the editor to treat the X-15's flight test program and research legacy as well, with much of this supplementary material drawing upon the editor's On the Frontier: Flight Research at Dryden, 1946-1981 (Washington, D.C.: NASA, 1984), and then-Captain Ronald G. Boston's "Outline of the X-15's Contributions to Aerospace Technology", prepared in support of the National Hypersonic Flight Research Facility effort in 1977. At the time, Captain Boston was an instructor in the Department of History, Air Force Academy, Colorado Springs, Colorado.

Figure 3 - modified X-15A-2 rocket research aircraft
Figure 3

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