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The Development of the X-15 at NACA Langley Aeronautical Laboratory, 1952-1958
It used liquid-fueled rocket thrust to travel outside the atmosphere, on flights that earned astronaut wings for its pilots. It had wings, and after reentry it landed like a glider. Over the course of 199 flights, it carried scientific experiment packages that pushed back the frontiers of human knowledge in astronomy, geosciences, and life science. And its first flight was in 1959, 22 years before the Space Shuttle. The X-15 was built by North American Aviation as a joint research project by NASA, the Air Force, and the Navy. Built to explore the effects of hypersonic (greater than Mach 5) flight, the X-15 set numerous speed and altitude records over the high desert of Edwards Air Force Base, some of which still stand today. The birthplace of the X-15, however, was not in the Mojave Desert but rather on the shores of the Chesapeake Bay at Virginia’s NACA Langley Aeronautical Laboratory, known today as NASA Langley Research Center.
In 1952, the National Advisory Committee for Aeronautics (NACA) passed a resolution to “increase its program dealing with problems of unmanned and manned flight…at altitudes between 12 and 50 miles, and at Mach numbers between 4 and 10” and to “devote a modest effort” to flight above 50 miles and greater than Mach 10. In support of this resolution, Langley Aeronautical Laboratory was asked to “consider the research problems related to outer atmosphere and space flight.”  Langley Director Henry J. E. Reid responded by forming a study group led by Clinton E. Brown. After reviewing an informal study that had already been conducted by researchers at the Langley Pilotless Aircraft Research Division (PARD), the Brown group concluded that a modified X-2 with an internal rocket engine could reach speeds of Mach 3.7, but would face engineering challenges due to structural heating. 
[top] The Becker Study Group
The NACA Interlaboratory Research Airplane Panel took another look at the hypersonic flight issue in 1954 and suggested that instead of using a modified X-2, the NACA should “look into the possibility of obtaining a new research airplane having higher speed and altitude capabilities than present types.”  All four NACA research laboratories, including Langley, established ad-hoc committees in response.  The Langley committee was chaired by John V. Becker, who had launched a hypersonic research program at Langley in 1945. The Langley program included a piece of equipment that until 1950 had only been referred to only as “Project 506” and was so secret that the engineers and technicians who used it were not allowed to tell their spouses what kind of work they were doing. In 1950, “Project 506” was revealed to be the 11-Inch Hypersonic Tunnel, which was capable of aerodynamic testing at the unheard-of velocity of Mach 6.86. 
Based on their experience with the 11-Inch Hypersonic Tunnel, Becker’s team conducted an intense four-week feasibility study and generated a preliminary design for a “hypersonic research airplane.” The team determined that their design could achieve speeds of Mach 6.3 and peak altitudes “outside the sensible atmosphere.” In order to do so, the aircraft skin would reach temperatures of 1,200 degrees F, heating unevenly and causing the wingtips to flex upward as much as 12 to 14 inches. These problems would be solved by manufacturing the aircraft’s skin panels from a high-temperature alloy known as Inconel X and using an internal structure of shear webs and trusses to relieve the stresses caused by heat-created expansion.  The design also used thrusters on the wingtips and tail for control outside the atmosphere, where conventional aircraft control surfaces would not work because there was no air for them to deflect.  For flight within the atmosphere, the team faced a serious control problem. Conventional, thin stabilizers on supersonic research aircraft lost control effectiveness with increasing airspeed, which created a phenomenon called “inertia coupling” that rendered the aircraft completely uncontrollable (inertia coupling caused the crash of the X-2 and the death of pilot Milburn Apt in 1956.)  To solve this problem, the Becker group used a new, wedge-shaped stabilizer design that was pioneered by Langley engineer Charles McLellan. 
[top] A Partnership in Speed: Langley and North American Aviation
While McLellan’s team worked to refine the Becker group’s concept in the 11-Inch Hypersonic Tunnel, the results of the Langley feasibility study were presented at NACA headquarters to an audience composed of NACA, USAF, and Navy representatives.  All three organizations saw the merits in the hypersonic research design, and agreed to a Memorandum of Understanding (MoU) that assigned construction oversight to the USAF, split the construction costs between the USAF and Navy, and placed the NACA in charge of flight testing. Several major aircraft manufacturers were invited to submit bids, and North American Aviation won the contract to build the new aircraft. During the bidding competition, the USAF informed NACA, Navy, and industry representatives that the unclassified designator for the program was “Project 1226” and the aircraft would be designated “X-15.” 
North American and Langley engineers worked closely together to develop the new aircraft. Before the first X-15 was rolled out of the North American assembly plant in California, North American engineers and technicians would build at least six different scale models, some with the direct assistance of Langley personnel. Additionally, NACA engineers and fabricators would build several scale models in-house, including a free-flight model used in the Full Scale Tunnel. Langley facilities would test X-15 models at speeds ranging from the subsonic through Mach 1.15, 1.43, 3.0, 4.0, up to a maximum, hypersonic velocity of Mach 6.86.  In addition to wind-tunnel testing, Langley research divisions made numerous other contributions to the development of the X-15. In order to understand low-speed stability and control characteristics of the new design, models were taken to a rural airport nearby and filmed as they dropped from a helicopter hovering overhead.  Other Langley research support included airframe evaluation by the Structures Research Division, proposed panel flutter tests by the Pilotless Aircraft Research Division (PARD), control-motion studies by the Stability Research Division, flight simulators built and operated by the Flight Research Division (and flown by X-15 pilots Joe Walker and Scott Crossfield), and testing of electrical components by the Instrumentation Research Division (IRD). 
[top] Operational History
The first flight of the X-15 took place on June 8, 1959. During the operational phase of the program, the X-15 exceeded nearly every expectation envisioned in the Becker group’s original concept. Three X-15 aircraft were flown by twelve pilots for a combined total of 199 missions over a ten year period. At the end of that period, eleven pilots and two aircraft survived. Flight milestones included a maximum altitude of 354,200 feet during a flight by NACA/NASA pilot Joseph A. Walker on August 22, 1963 (over 100,000 feet higher than the contract specifications established by the NACA), and a maximum airspeed of Mach 6.70 (4,520 mph) established by Major William J. “Pete” Knight, USAF, on October 3, 1967. 
Although the program did experience a handful of less serious mishaps, the only X-15 fatality took place on November 15, 1967, during the seventh X-15 flight of Major Michael J. Adams, USAF. As the aircraft approached a maximum altitude of 266,000 feet, it drifted slightly off of the correct heading, a condition that worsened when Adams attempted a correction using the RCS thrusters. As the aircraft traveled through a ballistic arc and began to reenter the atmosphere, neither Adams nor his ground support team at Edwards realized that an initial yaw angle of 15 degrees had increased to 90 degrees, and his aircraft was literally coming back into the atmosphere sideways. At an altitude of 230,000 feet and a speed of Mach 5, Adams and X-15-3 entered the only hypersonic spin in the history of human flight. Although the aircraft did recover from the spin, the resulting aircraft gyrations created severe positive and negative g-loads that incapacitated Adams and destroyed the aircraft at an altitude somewhere above 60,000 feet. 
In spite of the Mike Adams crash, the X-15 program provided operational procedures and experience that would serve as the foundation of America’s fledgling space program. The “High Range” tracking and telemetry network that stretched across California and Nevada to support X-15 missions served as the progenitor of similar networks that supported spaceflight operations in the 1960s and 1970s. The practice of using astronaut Capsule Communicators (CAPCOMs) to maintain voice communication with fellow astronauts during space operations can be directly traced to the use of X-15 pilots at the “NASA-1” console at Edwards to communicate with other X-15 pilots during flight operations. Walt Williams, Max Faget, and Gerald Truszynski played key roles in the development of the X-15 program and later made significant contributions as managers in the Mercury, Gemini, and Apollo programs.  Two X-15 pilots would blaze their own trails in the Astronaut Corps – Neil Armstrong, who left one of the most famous footprints in history, and Joe H. Engle, the only astronaut in the history of the Space Shuttle program to fly an orbiter manually through reentry all the way from Mach 25 to touchdown. 
[top] A Stepping Stone to the Shuttle
Not surprisingly, the X-15 program had an even more significant impact on the Space Shuttle, another winged vehicle that, like the X-15, was built in California, utilized “throttleable” rocket engines to leave the atmosphere, and transitioned from reentry at hypersonic speeds to land like a conventional airplane. Operational procedures for the Shuttle that came directly from the X-15 program include:
• Extensive use of flight simulators to build and maintain pilot proficiency before and between occasional (and expensive) operational missions 
• Practice approaches and landings flown in aircraft with similar flight characteristics to the operational vehicle (F-104 Starfighters for the X-15, Gulfstream Shuttle Training Aircraft for the Shuttle) 
• Use of multiple chase aircraft to monitor weather, survey landing conditions, and provide backup altitude information during approach and landing 
• Emergency landing sites – the Shuttle’s Transatlantic Abort (TAL) airfields in Europe and Africa, and the X-15’s pre-surveyed dry lake emergency landing sites in California and Nevada 
The greatest contribution made by X-15 operations to the Shuttle program, however, was the energy management procedures that were developed for unpowered X-15 approaches and landings at Edwards. X-15 pilots used the “Drinkwater Approach” that was established by NACA Ames test pilot Fred Drinkwater to make power-off approaches in an aircraft that had a poor lift-to-drag ratio (L/D) and steep glide angle compared to many conventional aircraft. The X-15’s ten-year operational record proved that the Drinkwater Approach could be used safely and reliably. Not long after the conclusion of the X-15 program, when designers of the Shuttle (another low L/D design) realized that their plans to deploy air-breathing engines for approach and landing would be prohibitive in terms of weight and complexity, the X-15’s use of the Drinkwater Approach provided a solution. As X-15 and Shuttle pilot Joe H. Engle noted at the close of the 30-year Shuttle program, "If it were not for the energy management procedures that were demonstrated during the X-15 program, it would have taken longer to proceed with the Space Shuttle...the X-15 allowed us to have confidence that it was not necessary to develop air breathing engines to make the approach and landing task more benign." 
Contributor: Robert C Moyer, University of Maryland University College, 2011.
John Becker and the 11-Inch Hypersonic Tunnel in Bldg 1229
X-15 model in 4 X 4-Foot Supersonic Pressure Tunnel, Bldg 1236
Reaction Control System simulator in Bldg 1220
Flutter Tests of the X-15 Tail Surfaces in the Langley 9- by 18-Inch Supersonic Flutter Tunnel. 1957. 9 x 18-Inch Supersonic Flutter Tunnel.
Low Speed Stability and Control. 1957. Full Scale Tunnel.
Stability and Control Characteristics of the North American X-15 Airplane. 1957. Full Scale Tunnel. Boisseau, Peter C.
X-15 Force Tests. 1958. 11-Inch Hypersonic Tunnel.
Surface Flow on a Model of the X-15 at M=6.8. 11-Inch Hypersonic Tunnel.
Flight Tests of the X-15 Model Airplane. 1958. Full Scale Tunnel.
X-15 Nose Gear Shimmy Test. 1958. Landing Loads Track.
Aerodynamic Characteristics of the X-15/B-52 Combination. 1959. 7 X 10 Low Speed Tunnel. Alford, William J. Jr., and Robert T. Taylor.
Horizontal Tail Flutter of a X-15. 1959. Transonic Blowdown Tunnel.
Spin Recovery Characteristics. 1959. 20-Foot Vertical Spin Tunnel.
X-15 Lower Movable Vertical Stabilizer. 1959. 9 X 6-Foot Thermal Structures Tunnel.
Test of Full Scale X-15 Horizontal Stabilizer. 1950s. 9 X 6-Foot Thermal Structures Tunnel.
Subsonic Flight Tests of an X-15. 1960. Helicopter drop-tests of a radio-controlled model at West Point, VA. Hewes, Donald E., and James L. Hassell, Jr.
Remotely Controlled X-15 Model Tests. 20-Foot Vertical Spin Tunnel.
[top] Further Reading
Becker, John V. 1983. The Development of Winged Reentry Vehicles: 1952-1962.
Jenkins, Dennis R. 2000. Hypersonics Before the Shuttle: A Concise History of the X-15 Research Airplane.
Jenkins, Dennis R. 2007. X-15: Extending the Frontiers of Flight.
Moyer, Robert C. and Mary E. Gainer. 2012. Chasing Theory to the Edge of Space. Quest: The History of Spaceflight Quarterly. Vol 19, No 2. p4-18.
Stillwell, Wendell H. 1965. X-15 Research Results.
- ↑ Milton B. Ames, Jr., Acting Assistant Director for Research, Headquarters, National Advisory Committee for Aerodynamics [henceforth: NACA], to Henry J. E. Reid, Director, Langley Aeronautical Laboratory [henceforth: LAL], July 10, 1952, Box 176, X-15 Project Correspondence, May 1955 – October 1954, Entry 1 – Project Correspondence Files, 1918-1978, Records of the National Aeronautics and Space Administration, Record Group 255 (RG 255), National Archives and Records Administration – Mid-Atlantic Region (Philadelphia) [henceforth: Box 176, Entry 1, RG 255, NARA – Mid-Atlantic (PHL)].
- ↑ Dennis R. Jenkins, X-15: Extending the Frontiers of Flight (Washington, DC: U. S. Government Printing Office, 2007), 20 [henceforth: Jenkins, Extending].
- ↑ J. W. Crowley, Associate Director for Research, Headquarters, NACA, to Director, LAL, March 9, 1954, Box 176, Entry 1, RG 255, NARA – Mid-Atlantic (PHL)].
- ↑ Jenkins, Extending, 24.
- ↑ Jim A. Penland, interview by Laura T. Freeze, June 28, 2007, from “An Oral History of the X-15 and Langley 11” Hypersonic Wind Tunnel: Recorded Interviews with Mr. Jim A. Penland”, LARSS intern program, NASA Langley Research Center [henceforth: Penland Interview, June 28, 2007].
- ↑ John V. Becker et al., “Research Airplane Study” (working paper, LAL, April 22, 1954), 8, 15, 18-19.
- ↑ Becker et al., “Research Airplane Study,” 11.
- ↑ Staff Members of the Langley Research Center, “Conception and Research Background of the X-15 Project” [ca. 1961], Jim A. Penland collection, NASA Langley Research Center, 11-12; Jay Miller, The X-Planes (Arlington, TX: Aerofax, 1988), 40-41, 43.
- ↑ Langley Staff, “Conception,” 12-12a; Becker et al., “Research Airplane Study,” 12-13; Penland Interview, June 8, 2007; Charles H. McLellan, “A Method for Increasing the Effectiveness of Stabilizing Surfaces at High Mach Numbers” (NACA RM L54F21, LAL, August 3, 1954), 11.
- ↑ Penland Interview, June 8, 2007; Jenkins, Extending, 36.
- ↑ Dennis R. Jenkins, Hypersonics Before the Shuttle: A Concise History of the X-15 Research Airplane, no. 18 of Monographs in Aerospace History (Washington, DC: NASA History Office, 2000), 86-90 (photocopies of original MoU and associated correspondence).
- ↑ Jenkins, Extending, 97-98.
- ↑ Jenkins, Extending, 62.
- ↑ D. H. Mason, Staff Engineer, Engineering Data Section, North American Aviation, Inc., to Commander, Air Materiel Command, Wright-Patterson AFB, OH [henceforth: Mason to Wright-Patterson AFB], “X-15 Research Aircraft Sixth Monthly Progress Letter,” June 8, 1956, Box 175, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Mason to Wright-Patterson AFB, “X-15 Research Aircraft Seventh Monthly Progress Letter,” July 12, 1956, Box 175, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Mason to Wright-Patterson AFB, “X-15 Research Aircraft Twelfth Monthly Progress Letter,” December 7, 1956, Box 174, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Hartley A. Soulé, Research Airplane Projects Leader, Langley Field, VA, to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for month of May 1956,” June 7, 1956, Box 175, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report from June 1, 1956 to July 9, 1956,” July 18, 1956, Box 175, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for months of January and February 1957,” March 19, 1957, Box 174, Entry 1, RG 255, NARA – Mid-Atlantic (PHL).
- ↑ Donald E. Hewes and James L. Hassell Jr., “Subsonic Flight Tests of a 1/7-Scale Radio-Controlled Model of the North American X-15 Airplane With Particular Reference to High Angle-of-Attack Conditions” (NASA TM X-283, NASA Langley Research Center, June 1960), 1.
- ↑ Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for months of September and October 1956,” November 15, 1956, Box 174, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for month of March 1956,” April 10, 1956, Box 175, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for months of November and December 1956,” January 17, 1957, Box 174, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for months of January and February 1957,” March 19, 1957, Box 174, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for March 1956,” April 10, 1956, Box 175, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for month of April 1956,” May 17, 1956, Box 175, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for month of May 1956,” June 7, 1956, Box 175, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for month of August 1956,” September 18, 1956, Box 174, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report for months of September and October 1956,” November 15, 1956, Box 174, Entry 1, RG 255, NARA – Mid-Atlantic (PHL); Soulé to members of NACA Research Airplane Projects Panel, “Project 1226 – Progress report from June 1, 1956 to July 9, 1956,” July 18, 1956, Box 175, Entry 1, RG 255, NARA – Mid-Atlantic (PHL).
- ↑ Jenkins, Extending, 603-658.
- ↑ Donald R. Bellman et al., “Investigation of the Crash of the X-15-3 Aircraft on November 15, 1967” (NASA/USAF report, Edwards Air Force Base, California, January 1968), 1.
- ↑ Jenkins, Hypersonics, 73-74.
- ↑ NASA Johnson Space Center, “Astronaut Bio: Joe Henry Engle (06/2009)” http://www.jsc.nasa.gov/Bios/htmlbios/engle-jh.html (accessed September 29, 2011).
- ↑ Jenkins, Hypersonics, 72-73.
- ↑ Jenkins, Hypersonics, 73.
- ↑ Jenkins, Hypersonics, 74.
- ↑ Jenkins, Hypersonics, 72.
- ↑ Jenkins, Hypersonics, 74; NASA History Office, “SP-3300 Flight Research at Ames, 1940-1997” http://history.nasa.gov/SP-3300/ch7.htm (accessed September 1, 2011); Joe H. Engle, email conversation with author, September 1, 2011.