By any measure the North American X-15 was an amazing aircraft. By the end of the decade long, 199 flight programme the three aircraft had pushed airspeed and altitude records way beyond all previous marks. Many X-15 pilots qualified as astronauts on their high altitude flights and the wealth of operational knowledge that was gained continues to influence aerospace programmes to this day.
Yet for all this, the X-15 is often overshadowed by NASA’s other activities during the 1960s and its legacy overlooked. It could never go as high or as fast as the capsules launched from the Cape, but the fact remains, at the time it was designed the X-15 looked like it would provide America’s first forays in human spaceflight and as Tom Wolfe points out in The Right Stuff, they would FLY their vehicle there and back.
Mach 1 and beyond
The story of the X-15 really starts as an extension of the high speed research programs being carried out by the NACA, Air Force and Navy beginning at the end of World War 2. Following the advent of effective liquid-fuelled rocket propulsion, the development of the jet engine and advances in aerodynamics during that conflict, it became clear that aviation would be pushing into new flight regimes. Famously the Bell X-1 (originally the XS-1) marked the first in a long line of experimental aircraft constructed to explore these new regimes and gather data which could later be applied to the design of operational aircraft.
Although research aircraft of the time were not exclusively concerned with high speed flight, many of the early X-planes such as the X-1 series and their Navy equivalents the Douglas Skystreak (D-558-1) and Skyrocket (D-558-2) were designed to examine the transonic and supersonic region. These designs tended to be purely functional and highly conservative serving to access a flight regime and gather the data safely while offering high margins of structural strength to cope with any unknowns that may be encountered. They weren’t generally designed to act as prototypes for future operational types – their role was to gather data and extend knowledge. Very rapidly though, the contemporary fighter types of the late 1940s and early 1950s began to match and often exceed the performance of the early research aircraft calling their practicality into question, but while designs such as Kelly Johnson’s F-104 Starfighter could now reach speeds in excess of Mach 2 on a routine basis, it was recognised that it would take a rocket powered research craft to probe speeds above Mach 3 and altitudes in excess of 100,000ft.
The much delayed Bell X-2 was anticipated to provide valuable data in these areas, but this troubled aircraft didn’t manage to make its first powered flight until 1955. On September 7th 1956, Air Force pilot Iven Kincheloe took the X-2 to a new altitude record of 126,200ft and it looked like the X-2 might start to make good on its promise, but on the very next flight just 20 days later, Milburn Apt was killed after reaching a speed in excess of Mach 3. Apt had fallen victim to a high speed aerodynamic phenomenon known as inertia coupling, where the aircraft’s control surfaces lose their ability to counteract the inertia of the fuselage resulting in a loss of control and tumbling in all three axis. Chuck Yeager had managed to survive an encounter with inertia coupling after he exceeded Mach 2.5 in the X-1A, but although Apt was able to recover control and separate the X-2s escape capsule, he became incapacitated and was unable to parachute to safety.
Although the X-2 had provided some data on high altitude flight and aerodynamic heating it was clear that much work remained for the next planned research vehicle.
Genesis of the X-15
As early as 1952, discussions had begun on the need for a research aircraft capable of reaching hypersonic velocities (exceeding Mach 5) and altitudes in excess of 50 miles. Initial ideas included an adapted X-2, but it quickly became clear that a new design would be required. By 1954 the NACA Langley facility created a baseline design for a hypersonic research plane which was able to include revised control surfaces to help counter inertia coupling. Concurrently the Office of Naval Research had been working with Douglas aircraft on a concept for a vehicle of broadly similar capabilities as a successor to the D-558-2. Although both their concept, known as the Model 671, and the proposed Langley vehicle had many unknown in terms of materials and propulsion, there was sufficient agreement from the NACA, Air Force and Navy that such a vehicle was technically feasible. In October 1954 a memorandum of understanding was signed by all three parties and Project 1226, soon to be given the new designation X-15, began in earnest.
Before the end of 1954, an invitation to bid was circulated to 12 prospective airframe contractors for three vehicles. Each contractor was obliged to specify their preferred power plant from a separate list. On May 9th 1955 proposals from Bell, Douglas, North American Aviation (NAA) and Republic were received and between them they represented a variety of approaches to meeting the specification. On paper it could be imagined that both Bell and Douglas would have an advantage given their previous experience with high speed research vehicles. This was new ground for NAA and Republic although both companies had gained substantial experience on contemporary supersonic fighter and interceptor projects by this time.
Each proposal had it’s relative strengths and weaknesses and independent evaluations were carried out by the NACA, Air Force and Navy. As well as specifying their engine choice and how they proposed to cope with the thermal and aerodynamic challenges, all four proposals also had to offer alternative two-person configurations for their airframe to allow for an observer or primate to fly – a feature never used (probably fortunately given some of the solutions proposed).
By August, a decision had been reached to give the contract to NAA although further discussions were required as the contractors cost estimates significantly exceeded the existing project budget (perhaps not an unforeseen situation as both the X-1 and X-2 had experienced significant cost overruns), but at this point events took an unexpected turn. Already busy with what would become the XB-70 Valkyrie bomber and F-108 Rapier interceptor projects NAA became concerned about their ability to meet the stated 30 month timeframe, and as a result officially withdraw from the contest on the 30th August – prior to being informed that they were the successful bidder. Further discussions took place on both sides and an extension to the contract was finally agreed. On 11th June 1956 the contract was signed and North American Aviation were officially building the X-15.
The winning design
NAA’s winning design, internally designated ESO-7487, was just under 50ft long with a wingspan of 22.36ft and a large vertical fin and smaller ventral stabiliser below the engine. Rather than try and use a more complex multi-walled insulated structure, NAA had elected to go with a hot structure constructed from Inconel X and interestingly had elected not to provide an escape capsule (at the time escape capsules were becoming an Air Force policy for high performance bombers and interceptors), preferring the combination of an ejector seat and full pressure suit for the pilot. For exoatmospheric flight a set of reaction control jets were included on the nose and wingtips. The landing gear would be gravity deployed and feature a conventional wheeled nose gear and skids, located beneath the wings, for the main gear. NAA chose a single Reaction Motors XLR30 for propulsion with a proposed maximum thrust of 57,000lbf. Finally, the huge Convair B-36 bomber was selected as the launch aircraft.
NAA now entered the detailed design and construction phase which lasted 3 years with some 2 million man-hours dedicated to engineering and over 4,000 hours of wind tunnel testing. One key asset the contractor had on their side through this process was test pilot Scott Crossfield. Having become the first man to exceed Mach 2.0 in the Douglas Skyrocket, Crossfield had chosen to resign from the NACA to help NAA push through the development of the X-15 figuring his experience of piloting high speed rocket planes could prove invaluable.
The design changed in many respects over the development period, with the position of the main gear moving rearward to share the same structure as the engine mount and the fuselage side tunnels being shortened. Another key area to change were the control surfaces with a smaller wedge-shaped vertical fin being introduced to prevent inertial coupling. This would be countered by a similar ventral arrangement with the lower portion being jettisoned during final approach to allow sufficient clearance for landing.
The other major change to the aircraft during this period was the switch from the XLR30 engine to the more powerful and controllable XLR99. While this engine offered significant performance improvements, it was dogged by developmental problems and consequently massive cost overruns. Initially priced at just under $10 million, the XLR99 eventually came in at a colossal $68 million – to put this in context this total was more than 5 times the originally projected cost of the entire X-15 programme!
While the costs were worrying enough, the delays in engine production meant that the X-15 airframe would be complete far in advance of its powerplant. Fortunately a makeshift solution was at hand to allow the initial testing to take place. The first X-15 would be fitted with two of the proven XLR11 four-chamber motors (a single XLR11 had been used in the various X-1 variants). In its final XLR99 powered form NAA expected the X-15 to be capable of reaching a maximum speed of Mach 6.5 and a maximum altitude of 250,000ft.
With regards to air-launching the X-15, the decision was made to change from the B-36 based on its lack of performance in terms of speed and altitude and concerns at Edwards over its practicality from a maintenance perspective (the B-36 was in the process of being phased out by SAC by this time meaning the competency of service crews and availability of parts would only diminish during the expected programme lifetime). The SAC’s new supersonic Convair B-58 Hustler was considered, but was found to be ill-suited as the X-15’s relative size meant the bomber’s nose wheel could not have deployed. A better fit was the Boeing B-52 although this meant the X-15 would need to be mated to a pylon inboard of the first set of engines. While tests proved this solution was feasible it did mean that unlike on earlier air-launched research planes, the pilot would need to enter the X-15 prior to takeoff and, should an emergency occur, be jettisoned along with the rocket plane to fend for himself.
Preparations for flight
While NAA toiled to complete the aircraft and modify the B-52 parent, much else remained to be done before the X-15 flight programme could begin. The prime pilots for the programme would be Joe Walker (NACA/NASA), Iven Kincheloe (Air Force) and Forest Petersen (Navy), but tragically Kincheloe was killed in July 1958 when the engine of his F-104 flamed out and he was unable to eject safely. Air Force backup pilot, Bob White, took Kincheloe’s place. Scott Crossfield would fly the initial contractor demonstration phase for both the X-15 with XLR11 and XLR99 engines.
Crossfield had also been working hard on the Human Factors side of the project including the development of the MC-2 pressure suit by the David Clark Company and setting initial flight profile parameters via simulator runs in the centrifuge at the Navy’s Johnsville facility. The extensive use of simulators was a key development during the X-15 programme, initially for pilot training but also as an integral part of mission planning. Before flight testing began, a variety of simulators were constructed NAA’s Inglewood plant, the Flight Research Centre at Edwards and Johnsville.
The X-15 would present a number of new challenges for its pilots ranging from flying in a full pressure suit in the pressurised nitrogen cockpit environment to using both aerodynamic and reaction controls during different phases of the flight. As well as a regular centre stick, the cockpit featured sidestick controllers to help the pilots under high positive (during the boost phase) or negative (during reentry/pull-up) G conditions.
The X-15 also required very careful energy management – once the fuel was exhausted it was just a very fast but poor glider and the pilot needed to ensure the right amount of energy was retained to make it back to the runway at Edwards. All of these new challenges could be addressed and at least partially mastered in the simulators (some surprises would only reveal themselves during actual flight test).
The High Range and Control Room
Unlike the earlier rocket planes tested at Edwards, the X-15 couldn’t be operated within the immediate vicinity of Rogers Dry Lake – it would need a far longer test range to reach the speeds and altitudes expected. The solution to this was the establishment of the High Range, essentially a corridor of airspace stretching from Wendover, Utah all the way back to Edwards in the Mojave. The plentiful dry lakes along this route provided a range of options for emergency landing sites, with the X-15 always needing to be launched within gliding range of one of these in case the engine failed to light. Missions would be planned with a number of interim landing sites between launch and landing to ensure the pilot would always be within range of a suitable site. Three radar stations were established to provide coverage for the High Range, the first at the Flight Research Centre itself and additional stations at Beatty and Ely in Nevada.
The radar stations would transmit data back to the Control Room at the Flight Research Centre from where the flight would be monitored and all related activities co-ordinated. The X-15 was one of the first flight test programmes to involve transmission of telemetry in realtime, allowing the vehicle to be monitored throughout the flight – the X-15’s position was also recorded on large map plotting boards. A fellow test pilot would perform the role of NASA-1 – the key communications link between the Control Room and the X-15 Pilot.
By October 1958 the first X-15 was completed and ready for rollout, but it entered a very different world to the one in which it had originally been conceived. In the years since the programme had been initiated, ideas of human spaceflight had gone from being somewhat remote to a matter of some national urgency.
The launch of Sputnik by the Soviet Union a year earlier had ushered in the ‘space age’ and it seemed only a matter of time before Soviets would use their superior boosters to launch the first person beyond the atmosphere. Although he had initially resisted any hasty response, President Eisenhower had decided that any American space programmes should be conducted under the auspices of a civilian agency. Somewhat to the disappointment of the Air Force and Army their plans for human spaceflight were absorbed into a new effort to be conducted by the newly formed National Aeronautics and Space Administration (NASA) built around the existing framework of the NACA.
Early in October 1958, NASA had announced Project Mercury with the aim of placing a manned capsule in space atop a missile. While for many within the industry, especially those who had been involved with high speed flight and the X-Planes, Mercury seemed somewhat inferior to a piloted spaceplane. Some of the X-15’s thunder had been stolen by the time it rolled out from NAA’s Inglewood plant on October 15th, but the X-15 had never been primarily about placing a person in space. It had a job to do gathering data on high speed high altitude flight and now it was almost ready to get started. Referring to the trip into the unknown that lay ahead of the three X-15s Walter Cronkite commented “They might be appropriately called the Nina, Pinta and the Santa Maria”. Then Vice President Richard Nixon simply stated that, in his career “…there will be no day more exciting than this”
At the Edge of Space – Milton O Thompson
Illustrated History of Space Shuttle: US Winged Spacecraft X-15 to Orbiter – Melvyn Smith
The X-Planes: X-1 to X-45 – Jay Miller
Hypersonic: The story of the North American X-15 – Dennis R. Jenkins & Tony R. Landis
The X-15 Rocket Plane: Flying the First Wings into Space – Michelle Evans
The Right Stuff – Tom Wolfe
X-15: Extending the Frontiers of Flight – Dennis R. Jenkins
Hypersonic Before the Shuttle: A Concise History of the X-15 Research Airplane – Dennis R. Jenkins