During the early 1960s, when the nation was agog over the Mercury astronauts, the X-15 pointed to a future in which piloted spaceplanes might fly routinely to orbit. The men of Mercury went water-skiing with Jackie Kennedy, but within their orbiting capsules, they did relatively little. Their flights were under automatic control, which left them as passengers along for the ride. Even a monkey could do it. Indeed, a chimpanzee named Ham rode a Redstone rocket on a suborbital flight in January 1961, three months before Alan Shepard repeated it before the gaze of an astonished world. Later that year another chimp, Enos, orbited the Earth and returned safely. The much-lionized John Glenn did this only later.82
In the X-15, by contrast, only people entered the cockpit. A pilot fired the rocket, controlled its thrust, and set the angle of climb. He left the atmosphere, soared high over the top of the trajectory, and then used reaction controls to set up his re-entry. All the while, if anything went wrong, he had to cope with it on the spot and work to save himself and the plane. He maneuvered through re-entry, pulled out of his dive, and began to glide. Then, while Mercury capsules were using parachutes to splash clumsily near an aircraft carrier, the X-15 pilot goosed his craft onto Rogers Dry Lake like a fighter.
All aircraft depend on propulsion for their performance, and the X-15’s engine installations allow the analyst to divide its career into three eras. It had been designed from the start to use the so-called Big Engine, with 57,000 pounds of thrust, but delays in its development brought a decision to equip it with two XLR11 rocket engines, which had served earlier in the X-l series and the Douglas Skyrocket. Together they gave 16,000 pounds of thrust.
Flights with the XLR1 Is ran from June 1959 to February 1961. The best speed and altitude marks were Mach 3-50 in February 1961 and 136,500 feet in August 1961. These closely matched the corresponding numbers for the X-2 during 1956: Mach 3-196, 126,200 feet.83 The X-2 program had been ill-starred—it had had two operational aircraft, both of which were destroyed in accidents. Indeed, these research aircraft made only 20 flights before the program ended, prematurely, with the loss of the second flight vehicle. The X-15 with XLR1 Is thus amounted to X – 2s that had been brought back from the dead, and that belatedly completed their intended flight program.
The Big Engine, the Reaction Motors XLR99, went into service in November
1960. It launched a program of carefully measured steps that brought the fall of one Mach number after another. A month after the last flight with XLR1 Is, in March
1961, the pilot Robert White took the X-15 past Mach 4. This was the first time a piloted aircraft had flown that fast, as White raised the speed mark by nearly a full Mach. Mach 5 fell, also to Robert White, four months later. In November 1961 White did it again, as he reached Mach 6.04. Once flights began with the Big Engine, it took only 15 of them to reach this mark and to double the maximum Mach that had been reached with the X-2.
Altitude flights were also on the agenda. The X-15 climbed to 246,700 feet in April 1962, matched this mark two months later, and then soared to 314,750 feet in July 1962. Again White was in the cockpit, and the Federation Aeronautique Internationale, which keeps the world’s aviation records, certified this one as the absolute altitude record for its class. A year later, without benefit of the FAI, the pilot Joseph Walker reached 354,200 feet. He thus topped 100 kilometers, a nice round number that put him into space without question or cavil.84
The third era in the X-15 s history took shape as an extension of the second one. In November 1962, with this airplanes capabilities largely demonstrated, a serious landing accident caused major damage and led to an extensive rebuild. The new aircraft, designated X-15A-2, retained the Big Engine but sported external tankage for a longer duration of engine burn. It also took on an ablative coating for enhanced thermal protection.
It showed anew the need for care in flight test. In mid-1962, and for that matter in 1966, the X-2s best speed stood at 4,104 miles per hour, or Mach 5-92. (Mach number depends on both vehicle speed and air temperature. The flight to Mach 6.04 reached 4,093 miles per hour.) Late in 1966, flying the X-l 5A-2 without the ablator, Pete Knight raised this to Mach 6.33. Engineers then applied the ablator and mounted a dummy engine to the lower fin, with Knight taking this craft to Mach 4.94 in August 1967. Then in October he tried for more.
But the X-15A-2, with both ablator and dummy engine, now was truly a new configuration. Further, it had only been certified with these additions in the flight to Mach 4.94 and could not be trusted at higher Mach. Knight took the craft to Mach 6.72, a jump of nearly two Mach numbers, and this proved to be too much. The ablator, when it came back, was charred and pitted so severely that it could not be restored for another flight. Worse, shock-impingement heating burned the engine off its pylon and seared a hole in the lower fin, disabling the propellant ejec-
X-15 with dummy Hypersonic Research Engine mounted to the lower fin. (NASA)
tion system and threatening the craft’s vital hydraulics. No one ever tried to fly faster in the X-15.85
It soon retired with honor, for in close to 200 powered flights, it had operated as a true instrument of hypersonic research. Its flight log showed nearly nine hours above Mach 3, close to six hours above Mach 4, and 87 minutes above Mach 5-86 It served as a flying wind tunnel and made an important contribution by yielding data that made it possible to critique the findings of experiments performed in ground-based tunnels. Tunnel test sections were small, which led to concern that their results might not be reliable when applied to full-size hypersonic aircraft. Such discrepancies appeared particularly plausible because wind tunnels could not reproduce the extreme temperatures of hypersonic flight.
The X-15 set many of these questions to rest. In Becker’s words, “virtually all of the flight pressures and forces were found to be in excellent agreement with the low-temperature wind-tunnel predictions.”87 In addition to lift and drag, this good agreement extended as well to wind-tunnel values of “stability derivatives,” which governed the aircraft’s handling qualities and its response to the aerodynamic controls. Errors due to temperature became important only beyond Mach 10 and were negligible below such speeds.
B-52 mother ship with X-15A-2. The latter mounted a dummy scramjet and carried external tanks as well as ablative thermal protection. (NASA)
But the X-15 brought surprises in boundary-layer flow and aerodynamic heating. There was reason to believe that this flow would remain laminar, being stabilized in this condition by heat flow out of the boundary layer. This offered hope, for laminar flow, as compared to turbulent, meant less skin-friction drag and less heating. Instead, the X-15 showed mostly turbulent boundary layers. These resulted from small roughnesses and irregularities in the aircraft skin surface, which tripped the boundary layers into turbulence. Such skin roughness commonly produced turbulent boundary layers on conventional aircraft. The same proved to be true at Mach 6.
The X-15 had a conservative thermal design, giving large safety margins to cope with the prevailing lack of knowledge. The turbulent boundary layers might have brought large increases in the heat-transfer rates, limiting the X-15’s peak speed. But in another surprise, these rates proved to be markedly lower than expected. As a consequence, the measured skin temperatures often were substantially less than had been anticipated (based on existing theory as well as on wind-tunnel tests). These flight results, confirmed by repeated measurements, were also validated with further wind-tunnel work. They resisted explanation by theory, but a new empirical model used these findings to give a more accurate description of hypersonic heating. Because this model predicted less heating and lower temperatures, it permitted design of vehicles that were lighter in weight.88
An important research topic involved observation of how the X-15 itself would stand up to thermal stresses. The pilot Joseph Walker stated that when his craft was accelerating and heating rapidly, “the airplane crackled like a hot stove.” This resulted from buckling of the skin. The consequences at times could be serious, as when hot air leaked into the nose wheel well and melted aluminum tubing while in flight. On other occasions, such leaks destroyed the nose tire.89
Fortunately, such problems proved manageable. For example, the skin behind the wing leading edge showed local buckling during the first flight to Mach 5-3- The leading edge was a solid bar of Inconel X that served as a heat sink, with thin slots or expansion joints along its length. The slots tripped the local airflow into turbulence, with an accompanying steep rise in heat transfer. This created hot spots, which led to the buckling. The cure lay in cutting additional expansion slots, covering them with thin Inconel tabs, and fastening the skin with additional rivets. The wing leading edge faced particularly severe heating, but these modifications prevented buckling as the X-15 went beyond Mach 6 in subsequent flights.
Buckling indeed was an ongoing problem, and an important way to deal with it lay in the cautious step-by-step program of advance toward higher speeds. This allowed problems of buckling to appear initially in mild form, whereas a sudden leap toward record-breaking performance might have brought such problems in forms so severe as to destroy the airplane. This caution showed its value anew as buckling problems proved to lie behind an ongoing difficulty in which the cockpit canopy windows repeatedly cracked.
An initial choice of soda-lime glass for these windows gave way to alumino-sili – cate glass, which had better heat resistance. The wisdom of this decision became clear in 1961, when a soda-lime panel cracked in the course of a flight to 217,000 feet. However, a subsequent flight to Mach 6.04 brought cracking of an aluminosilicate panel that was far more severe. The cause again was buckling, this time in the retainer or window frame. It was made of Inconel X; its buckle again produced a local hot spot, which gave rise to thermal stresses that even this heat-resistant glass could not withstand. The original retainers were replaced with new ones made of titanium, which had a significantly lower coefficient of thermal expansion. Again the problem disappeared.90
The step-by-step test program also showed its merits in dealing with panel flutter, wherein skin panels oscillated somewhat like a flag waving in the breeze. This brought a risk of cracking due to fatigue. Some surface areas showed flutter at conditions no worse than Mach 2.4 and dynamic pressure of 650 pounds per square foot, a rather low value. Wind-tunnel tests verified the flight results. Engineers reinforced the panels with skin doublers and longitudinal stiffeners to solve the problem. Flutter did not reappear, even at the much higher dynamic pressure of 2,000 pounds per square foot.91
Caution in flight test also proved beneficial in dealing with the auxiliary power units (APUs). The APU, built by General Electric, was a small steam turbine driven by hydrogen peroxide and rotating at 51,200 revolutions per minute. Each X-15 airplane mounted two of them for redundancy, with each unit using gears to drive an electric alternator and a pump for the hydraulic system. Either APU could carry the full electrical and hydraulic load, but failure of both was catastrophic. Lacking hydraulic power, a pilot would have been unable to operate his aerodynamic controls.
Midway through 1962 a sudden series of failures in a main gear began to show up. On two occasions, a pilot experienced complete gear failure and loss of one APU, forcing him to rely on the second unit as a backup. Following the second such flight, the other APU gear also proved to be badly worn. The X-15 aircraft then were grounded while investigators sought the source of the problem.
They traced it to a lubricating oil, one type of which had a tendency to foam when under reduced pressure. The gear failures coincided with an expansion of the altitude program, with most of the flights above 100,000 feet having taken place during 1962 and later. When the oil turned to foam, it lost its lubricating properties. A different type had much less tendency to foam; it now became standard. Designers also enclosed the APU gearbox within a pressurized enclosure. Subsequent flights again showed reliable APU operation, as the gear failures ceased.92
Within the X-15 flight-test program, the contributions of its research pilots were decisive. A review of the first 44 flights, through November 1961, showed that 13 of them would have brought loss of the aircraft in the absence of a pilot and of redundancies in onboard systems. The actual record showed that all but one of these missions had been successfully flown, with the lone exception ending in an emergency landing that also went well.93
Still there were risks. The dividing line between a proficient flight and a disastrous one, between life and death for the pilot, could be narrow indeed, and the man who fell afoul of this was Major Mike Adams. His career in the cockpit dated to the Korean War. He graduated from the Experimental Test Pilot School, ranking first in his class, and then was accepted for the Aerospace Research Pilot School. Yeager himself was its director; his faculty included Frank Borman, Tom Stafford, and Jim McDivitt, all of whom went on to win renown as astronauts. Yeager and his selection board picked only the top one percent of this school s applicants.94
Adams made his first X-15 flight in October 1966. The engine shut down prematurely, but although he had previously flown this craft only in a simulator, he successfully guided his plane to a safe landing on an emergency dry lakebed. A year later, in the fall of 1967, he trained for his seventh mission by spending 23 hours in the simulator. The flight itself took place on 15 November.
As he went over the top at 266,400 feet, his airplane made a slow turn to the right that left it yawing to one side by 15 degrees.95 Soon after, Adams made his mistake. His instrument panel included an attitude indicator with a vertical bar. He could select between two modes of display, whereby this bar could indicate either sideslip angle or roll angle. He was accustomed to reading it as a yaw or sideslip angle—but he had set it to display roll.
“It is most probable that the pilot misinterpreted the vertical bar and flew it as a sideslip indicator,” the accident report later declared. Radio transmissions from the ground might have warned him of his faulty attitude, but the ground controllers had no data on yaw. Adams might have learned more by looking out the window, but he had been carefully trained to focus on his instruments. Three other cockpit indicators displayed the correct values of heading and sideslip angle, but he apparently kept his eyes on the vertical bar. He seems to have felt vertigo, which he had trained to overcome by concentrating on that single vertical needle.96
Mistaking roll for sideslip, he used his reaction controls to set up a re-entry with his airplane yawed at ninety degrees. This was very wrong; it should have been pointing straight ahead with its nose up. At Mach 5 and 230,000 feet, he went into a spin. He fought his way out of it, recovering from the spin at Mach 4.7 and 120,000 feet. However, some of his instruments had been knocked badly awry. His inertial reference unit was displaying an altitude that was more than 100,000 feet higher than his true altitude. In addition, the MH-96 flight-control system made a fatal error.
It set up a severe pitch oscillation by operating at full gain, as it moved the horizontal stabilizers up and down to full deflection, rapidly and repeatedly. This system should have reduced its gain as the aircraft entered increasingly dense atmosphere, but instead it kept the gain at its highest value. The wild pitching produced extreme nose-up and nose-down attitudes that brought very high drag, along with decelerations as great as 15 g – Adams found himself immobilized, pinned in his seat by forces far beyond what his plane could withstand. It broke up at 62,000 feet, still traveling at Mach 3-9. The wings and tail came off; the fuselage fractured into three pieces. Adams failed to eject and died when he struck the ground.97
“We set sail on this new sea,” John Kennedy declared in 1962, “because there is new knowledge to be gained, and new rights to be won.” Yet these achievements came at a price, which Adams paid in full.98