Transiting from Air to Space
The North American X-15

SECTION VI
RESEARCH AT THE EDGE OF SPACE



The first of the three X-15s (serial 56-6670) arrived at the Air Force Flight Test Center at Edwards Air Force Base, California, in mid-October 1958, trucked over the hills from the plant in Los Angeles for testing at the NASA High-Speed Flight Station (subsequently redesignated the NASA Flight Research Center). It was joined by the second airplane (serial 56-6671) in April 1959. In contrast to the relative secrecy that had attended flight trials with the XS-1 (X-1) a decade before, the X-15 program offered the spectacle of pure theater.

The X-15's contractor program lasted two years, from mid-1959 through mid-1960. North American had to demonstrate the craft's general airworthiness during flights above Mach 2, and successful operation of its new XLR99 engine before delivering the craft to NASA. Anything beyond Mach 3 was considered a part of the government's research obligation. The task of flying the X-15 during the contractor program rested in the capable hands of Scott Crossfield, who had left NACA to join North American and help shepherd the craft through its long development. Crossfield completed the first captive flight on 10 March 1959 and first glide flight on 8 June. Just prior to landing, the plane began a series of increasingly wild pitching motions; thanks to Crossfield's instinctive corrective action, the plane landed safely; Crossfield feared for the plane's design, but fortunately, for naught. North American's engineers subsequently modified its boosted control system to increase the control rate response, and the X-15 never again experienced the porpoising motions that had threatened it on its first flight. On 17 September, the X-15 completed its first powered flight, when Crossfield flew the second airplane to Mach 2.11. 1

A series of ground and in-flight accidents marred the X-15's contractor program, fortunately without injuries or even greatly delaying the program. On 5 November 1959 an engine fire – always extremely hazardous in a volatile rocket airplane – forced an emergency landing on Rosamond Dry Lake; the X-15 landed with a heavy load of propellants and broke its back, grounding this particular X-15 for three months. During a ground engine test with the third X-15 (the first one equipped with the large Thiokol engine), a stuck pressure regulator caused the craft to explode, necessitating virtual rebuilding. The second X-15 was actually the first of the series to test-fly the large XLR99 engine, and after adding the engine to the other two craft, North American delivered the last of the X-15s to NASA in June 1961. By that time, NASA, Air Force and Navy test pilots had been operating the X-15 on government research flights for just over a year. 2 The research phase of the X-15's flight program involved four broad objectives: verification of predicted hypersonic aerodynamic behavior and hypersonic heating rates, study of the X-15's structural characteristics in an environment of high heating and high flight loads, investigation of hypersonic stability and control problems during atmospheric exit and reentry, and investigation of piloting tasks and pilot performance. By late 1961, these four areas had been generally examined, though detailed research continued to about 1964 using the first and third aircraft, and to 1967 with the second (the X-15A-2). Before the end of 1961, the X-15 had attained its Mach 6 design goal and had flown well above 200,000 feet; by the end of the next year the X-15 was routinely flying above 300,000 feet. Within a single year, the X-15 had extended the range of winged aircraft flight speeds from Mach 3.2 to Mach 6.04, the latter achieved by Air Force test pilot Bob White on 9 November 1961.

The intensive flight program on the X-15 revealed a number of interesting things. physiologists discovered the heart rates of X-15 pilots varied between 145 and 180 beats per minute in flight, as compared to a normal of 70 to 80 beats per minute for test missions in other aircraft. Researchers eventually concluded that prelaunch anticipatory stress, rather than actual post launch physical stress, influenced the heart rate. They believed, correctly, that these rates could be considered as probable baselines for predicting the physiological behavior of future pilot-astronauts. Aerodynamic researchers found remarkable agreement between the tunnel tests of exceedingly small X-15 models and actual results, with the exception of drag measurements. Drag produced by the blunt aft end of the aircraft proved 15% higher on the actual aircraft than wind-tunnel tests had predicted. At Mach 6, the X-15 absorbed eight times the heating load it experienced at Mach 3, with the highest heating rates occurring in the frontal and lower surfaces of the aircraft, which received the brunt of airflow impact. During the first Mach 5+ excursion, four expansion slots in the leading edge of the wing generated turbulent vortices that increased heating rates to the point that the external skin behind the joints buckled. As a solution, technicians added small Inconel alloy strips over the slots, and the X-15 flew without further evidence of buckling. It offered ”… a classical example of the interaction among aerodynamic flow, thermodynamic properties of air, and elastic characteristics of structure.” 3

Heating and turbulent flow generated by the protruding cockpit enclosure posed other problems; on two occasions, the outer panels of the X-15's heavy glass cockpit windshields fractured because heating loads in the expanding frame overstressed the soda-lime glass. NASA solved the difficulty by changing the cockpit frame from Inconel to titanium, modifying its configuration, and replacing the outer glass panels with high-temperature alumina silica glass. Another problem concerned an old aerodynamics and structures bugaboo, panel flutter. Panels along the flanks of the X-15 fluttered at airspeeds above Mach 2.4, forcing engineers to add longitudinal metal stiffeners to the panels. a All this warned aerospace designers to proceed cautiously. John Becker, writing in 1968, noted of the X-15 experience that: 4

The really important lesson here is that what are minor and unimportant features of a subsonic or supersonic aircraft must be dealt with as prime design problems in a hypersonic airplane. This lesson was applied effectively in the precise design of a host of important details on the manned space vehicles.

A serious roll instability predicted for the airplane under certain reentry conditions posed a serious challenge to flight researchers. To simulate accurately the reentry profile of a returning winged spacecraft, the X-15 had to fly at angles of attack of at least 17°. Yet the cruciform ”wedge” tail, so necessary for stability and control in other portions of the plane's flight regime, actually prevented it from being flown safely at angles of attack greater than 20° because of potential rolling problems. By this time, FRC researchers had gained enough experience with the XLR99 engine to realize that fears of thrust misalignment – a major reason for the large vertical fin – were unwarranted. The obvious solution was simply to remove the lower half of the ventral fin, a portion of the fin that X-15 pilots had to jettison prior to landing anyway so that the craft could touch down on its landing skids. Removing the ventral produced an acceptable tradeoff. While it reduced stability by about 50%. at high angles of attack, it greatly improved the pilot's ability to control the airplane. With the ventral off, the X-15 could now fly into the previously ”uncontrollable” region above 20° angle of attack with complete safety. Eventually the X-15 went on to reentry trajectories of up to 26°, often with flight path angles of -38° at speeds up to Mach 6, a much more demanding piloting task than the shallow entries flown by manned vehicles returning from orbital or lunar missions. Its reentry characteristics were remarkably similar to those of the later NASA Space Shuttle orbiter. 5 b

When Project Mercury took to the air, it rapidly eclipsed the X-15 in glamour, but the two programs really were complementary in nature, though Mercury dominated some of the research areas that had first interested X-15 planners, such as ”zero g” weightlessness studies. The use of reaction controls to maintain a vehicle's attitude in space proved academic after Mercury flew, but the X-15 had already proved them and would also furnish valuable design information on the use of blending reaction controls with conventional aerodynamic controls during an exit and reentry, a matter of concern to subsequent Shuttle development. The X-15 experience clearly demonstrated the ability of pilots to fly rocket-propelled aircraft out of the atmosphere and back in to precision landings. Flight Research Center director Paul Bikle saw the X-15 and Mercury as a: 6

parallel, two-pronged approach to solving some of the problems of manned space flight. While Mercury was demonstrating man's capability to function effectively in space, the X-15 was demonstrating man's ability to control a high-performance vehicle in a near-space environment … considerable new knowledge was obtained on the techniques and problems associated with lifting reentry.

Operationally, the X-15 gave its team a number of headaches. Because of the complexity of its systems, the plane experienced a number of operational glitches that delayed flights, aborted them before launch, or forced abandonment of a mission after launch. Early in the program, the X-15's stability augmentation and inertial guidance systems were two major problem areas. NASA eventually replaced the Sperry inertial unit with a Honeywell unit first designed for the Dyna-Soar. The plane's propellant system had its own weaknesses. Pneumatic vent and relief valves and pressure regulators gave the greatest difficulties, followed by spring pressure switches in the auxiliary power units, the turbopump, and the gas generation system. NASA's mechanics routinely had to reject 24 to 30% of spare parts as unusable, a clear indication of the difficulties of devising industrial manufacturing and acceptance test procedures when building for use in an environment at the frontier of science. 7 Weather posed a critical factor. Many times Edwards enjoyed fine weather, the lakebed bone-dry, while upcountry the High Range was covered with clouds, alternate landing sites were flooded, or some other meteorological condition postponed a mission. In one case, weather and minor maintenance kept one X-15 grounded from mid-October 1961 to early January 1962. When it finally flew, the pilot had to make an emergency landing up range. Weather and maintenance then grounded the plane until mid-April. 8 On an average, the X-15 completed 1.77 flights per month – a figure comparing well within the shuttle's own subsequent experience (until the loss of Challenger.)

The X-15 had its share of accidents, one of which killed an Air Force test pilot; another seriously injured a NASA research pilot. As previously mentioned, Scott Crossfield once made an emergency landing on Rosamond Lake with an X-15 damaged by an engine fire; the plane broke its back on landing, necessitating lengthy repairs. The third X-15 blew up during ground testing of its XLR99 engine, but it, too, was rebuilt. In November 1962, an engine failure forced Jack McKay to make an emergency landing at Mud Lake, Nevada, in the second X-15; its landing gear collapsed and the X-15 flipped over on its back. McKay was promptly rescued by an Air Force medical team standing by near the launch site, and eventually recovered to fly the X-15 again. But his injuries, more serious than at first thought, eventually forced his retirement from NASA. In November 1967, Mike Adams was killed in a strange accident in the third X-15 that will be discussed later in great detail. One of the most remarkable close calls in the X-15 program involved Air Force. test pilot Major William J. ”Pete” Knight. in June 1967 he experienced a complete electrical failure while climbing through 100,000 feet at Mach 4+. With no computed information and guidance, Knight continued to climb, suddenly reduced to ”seat of the pants” flying technique. During reentry he managed to restart one of the auxiliary power units, restoring some instruments, and made an emergency landing at Mud Lake, for which he received the Distinguished Flying Cross. c Within NACA and later NASA, developing the X-15 had been left largely in the hands of Langley, the center most closely involved in determining its mission and configuration, with important inputs from the other centers, especially the High-Speed Flight Station. The flight research program was the province of the Flight Research Center with liaison and support from the Air Force Flight Test Center at Edwards. In the summer of 1961, as the X-15 approached its maximum performance during test flights, a new initiative began, one that sprang jointly from the Air Force's Aeronautical Systems Division at Wright-Patterson AFB and from NASA Headquarters: using the X-15 as a ”testbed” or carrier aircraft for a wide range of scientific experiments unforeseen in its original conception.

Pressures had existed even before the X-15 first flew to extend the scope of the program beyond aerodynamics and structural research. Researchers at the Flight Research Center had proposed using the airplane to carry to high altitude some experiments related to. the proposed Orbiting Astronomical Observatory; others suggested modifying one of the planes to carry a Mach 5+ ramjet for advanced air-breathing propulsion studies. Over 40 experiments were suggested by the scientific community as suitable candidates for the X-15 to carry. In August 1961, after consulting with Bikle at FRC, NASA headquarters, and the Air Force Aeronautical Systems Division, NASA and the Air Force formed an X-15 Joint Program Coordinating Committee to prepare a plan for a follow-on experiments program. Most of the suggested experiments were in space science, such as ultraviolet stellar photography. Others supported the Apollo program and hypersonic ramjet studies. A series of meetings held at NASA headquarters over the fall of 1961 between the joint committee. Hartley Soule, and John Stack, then NASA's director of aeronautical research, culminated in approval of the proposed follow-on research program and the classification of two groups of experiments. Category A experiments consisted of well-advanced and funded experiments having great importance; category B included worthwhile projects of less urgency or importance. 9

Figure 1 - X-15/Blue Scout Launch System
Figure 1
X-15/BLUE SCOUT LAUNCH SYSTEM

In March 1962 the X-15 committee approved the ”X-15 Follow-on Program,” which NASA announced 13 April in a Headquarters news conference presided over by Stack and FRC planner Hubert Drake. Drake announced that the first task would be to fly an ultraviolet stellar photography experiment from the University of Wisconsin's Washburn Observatory. NASA had investigated the possibility of the X-15 carrying a Scout booster that could fire small satellites into orbit, the entire B-52/X-15/Scout becoming in effect a multistage satellite booster, but that the agency finally rejected the idea for reasons of safety, utility, and economy. The X-15's space science program eventually included twenty-eight experiments running from astronomy to micrometeorite collection, using wingtop pods that opened at 150,000 feet, and high-altitude mapping. Two of the follow-on programs, a horizon definition experiment from the Massachusetts Institute of Technology and tests of proposed insulation for the Saturn launch vehicle, directly benefitted navigation equipment and the thermal protection used on Apollo-Saturn launch vehicle.

Figure 2 - map of Pacific missile range launch
Figure 2
PROFILE OF PACIFIC MISSILE RANGE LAUNCH

FRC quickly implemented the follow-on program. In 1964, fully 65% of all data returned from the three X-15 aircraft involved follow-on projects; this percentage increased yearly through conclusion of the program. 10

Figure 3 - map of Atlantic missile range launch
Figure 3
PROFILE OF ATLANTIC MISSILE RANGE LAUNCH

NASA's major X-15 follow-on project involved a Langley developed Hypersonic Ramjet Experiment (HRE). d FRC advanced planners had long wanted to extend the X-15's speed capabilities, perhaps even to Mach 8, by adding extra fuel in jettisonable drop tanks and some sort of thermal protection system. Langley researchers had developed a design configuration for a proposed hypersonic ramjet engine. The two groups now came together to advocate modifying one of the X-15s as a Mach 8 research craft that could be tested with a ramjet fueled by liquid hydrogen. The proposal became more attractive when the landing accident to the second X-15 in November 1962 forced the rebuilding of the aircraft. The opportunity to make the modifications was too good to pass up. In March 1963 the Air Force and NASA authorized North American to rebuild the airplane with a longer fuselage. Changes were to be made in the propellant system; two huge drop tanks and a small tank for liquid hydrogen within the plane were to be added; the drop tanks could be recovered via parachute and refurbished, as with the Space Shuttle's solid-fuel boosters nearly two decades later.

Forty weeks and $9 million later, North American delivered the modified plane, designated the X-15A-2, in February 1964. 11 The X-15A-2 (Figure 4) first flew in June 1964, piloted by Air Force test pilot Major Bob Rushworth. Early proving flights demonstrated that the plane retained satisfactory flying qualities at Mach 5+ speeds, though on three flights, thermal stresses caused portions of the landing gear to extend at Mach 4.3, generating "an awful bang and a yaw," but Rushworth landed safely despite (in one case) blow-out of the heat-weakened tires upon touchdown. In November 1966, Air Force pilot Pete Knight set an unofficial world's airspeed record of Mach 6.33 in the plane. NASA then grounded it for application of an ablative coating to enable it to exceed Mach 7. 12

Figure 4 - North American X-15A-2 research aircraft
Figure 4
NORTH AMERICAN X-15A-2 RESEARCH AIRCRAFT


Flight Research Center's technical staff had evaluated several possible coatings that could be applied over the X-15'.s Inconel structure to enable it to withstand the added thermal loads experienced above Mach 6. NASA hoped that such coatings might point the way toward materials that could be readily and cheaply applied to reusable spacecraft, minimizing refurbishment costs and turn-around time between flights. Such a coating would have to be relatively light; have good insulating properties; be easy to apply, cure, and then remove; and be easy to reapply before another flight. On FRC's advice, a joint NASA-Air Force committee selected an ablator developed by the Martin Company, MA-25S, in connection with some corporate studies on reusable spacecraft concepts. Consisting of a resin base, a catalyst, and a glass bead powder, it would protect the X-15's structure from the expected 2000°F heating as the craft sped through the upper atmosphere. Martin estimated that the coating, ranging from .59 inches thick on the canopy, wings, vertical, and horizontal tail down to 0.015 inches on the trailing edges of the wings and tail, would keep the skin temperature down to a comfortable 600°F. The first unpleasant surprise came, however, with the application of the coating to the X-15A-2: it took six weeks. Because the ablator would char and emit a residue in flight, North American had installed an ”eyelid” over the left cockpit window. It would remain closed until just before approach and landing. During launch and climbout, the pilot would use the right window, but residue from the ablator would render it opaque above Mach 6. 13

Late in the summer of 1967, the X-15A-2 was ready for flight with the ablative coating. It had already flown with a dummy ramjet affixed to its stub ventral fin; the ramjet, while providing a pronounced nose-down trim change, actually added to the plane's directional stability. The weight of the ablative coating – 125 pounds higher than planned – -together with expected increased drag reduced the theoretical maximum performance of the airplane to Mach 7.4, still a significant advance over the Mach 6.3 previously attained with the plane. The appearance of the X-15A-2 was striking, an overall flat off-white finish, the huge external tanks a mix of silver and orange-red with broad striping. NASA hoped that early Mach 7+ trials would lead to tests with an actual ”hot” ramjet rather than the dummy now attached to the plane. On 21 August 1967 Knight completed the first flight in the ablative coated plane, reaching Mach 4.94 and familiarizing himself with its handling qualities. His next flight, on 3 October 1967, was destined to be the X-15's fastest flight and the most surprising as well. 14

That day, high over Nevada, Knight dropped away from the B-52, the heavy X-15A-2 brimming with fuel. The following is an extract from the official AFFTC summary of the X-15A-2's envelope expansion program: 15

The launch transients were very mild with a bank angle excursion of 14 degrees. During the rotation the pilot had good control of the aircraft and increased the angle of attack to 15 degrees and felt the onset of buffet. The remainder of the rotation to the planned pitch angle was made at 12 to 13 degrees angle of attack. During this period the roll control was excellent and the bank angle did not deviate more than 8 degrees. The maximum dynamic pressure experienced during the rotation was 560 psf, close to the 540 psf observed on the simulator. The planned pitch angle of 35 degrees was reached in 38 seconds and was maintained within plus/minus one degree.

The external tanks were ejected 67.4 seconds after launch. Tank separation was satisfactory, however, the pilot felt the ejection was "harder" than the last one he had experienced (Flight No. 2-50-89). The longitudinal trim change to the aircraft was from 4.2 to -2 degrees angle of attack. The external tank recovery system performed satisfactorily and the tanks were recovered in repairable condition.

After tank ejection the planned 2 degree angle of attack was maintained within +1 degree. As the aircraft came level at an indicated altitude of 99,000 feet, the pilot increased the angle of attack to 6 degrees to maintain zero rate of climb. During this task the pilot reported that the pitch control was very sensitive and it was difficult to hold a constant angle of attack.

The pilot reported shutting down the engine at 6500 fps; however, the final radar data analysis revealed the maximum velocity to be 6630 fps. The total engine burn time was 141.4 seconds, which compared favorably with the 141 seconds planned. However, the aircraft had achieved a velocity which was 130 fps faster than that of the simulator during this time.

During the deceleration the pilot was concentrating on performing stability and control maneuvers and as a result the profile was not exactly as planned. After shutdown the aircraft did not descend at the rate planned, resulting in a lower dynamic pressure between 5500 and 4000 fps. This anomaly, along with the higher maximum velocity, presented the pilot with the task of managing higher energy in approaching the high key position. The region of largest dispersion from the planned ranging occurred at the time when the dynamic pressure was lower than planned. To regain the desired high key energy conditions, the pilot delayed the retraction of the speed brakes and flew the remainder of the deceleration at a higher dynamic pressure (a maneuver commonly used on X-15 flights).

The ability of the ablative material to protect the aircraft structure from the high aerodynamic heating was considered good except in the area of the dummy ramjet where the heating rates were significantly higher than predicted. Considerable heat damage occurred on the dummy ramjet and the ramjet pylon. The ramjet instrumentation ceased approximately 25 seconds after engine shutdown indicating that a burn through of the ramjet/pylon structure had occurred. Shortly thereafter the heat propagated upward into the lower aft fuselage area causing the engine hydrogen peroxide hot light to illuminate in the cockpit. Ground control, assuming a genuine overheat condition, requested the pilot to jettison the remaining engine peroxide. The high heat in the aft fuselage area also caused a failure of a helium control gas line allowing not only the normal helium source gas to escape, but also the emergency jettison control gas supply as well (because of the failure of a check valve). Thus, the remaining residual propellants could not be jettisoned. The aircraft was an estimated 1500 pounds heavier than normal at landing, but the landing was accomplished without incident.

The pilot performed a rudder pulse with the yaw damper off 71 seconds after engine shutdown and noted that the sideslip indicator did not oscillate as expected. Post-flight analysis of the maneuver revealed that the aircraft did in fact experience a reasonable yaw rate and lateral acceleration. The maneuver was performed at approximately the time of maximum temperature for the unprotected Ball Nose. It was concluded that the sphere of the Ball Nose experienced binding, possibly due to differential expansion.

The heat in the ramjet pylon area became high enough to ignite 3 of the 4 explosive bolts retaining the ramjet to the pylon at some time during the flight. As the pilot was performing a turn to downwind in the landing pattern, the one remaining bolt failed structurally and the ramjet separated from the aircraft. The pilot did not feel the ramjet separate. Since the landing chase aircraft had not yet joined up, the pilot was not aware that the unit had separated.

The position of the aircraft at the time of separation was established by radar data and the most likely trajectory estimated. A ground search party discovered the ramjet impact point on the Edwards AFB bombing range. Although it had been damaged by impact, it was returned for study of the heat damage that had occurred.




 RAMJET SEPARATION CONDITIONS



 FLIGHT NO. 2-53-97



 Velocity           980 fps          Angle of attack   8°

 Altitude           35,500 feet      Roll angle        57° left

 Mach Number        0.98             Normal accel.     1.6 g

 Dynamic Pressure   340 psf


The unprotected right-hand windshield was, as anticipated, partially covered with ablation products. With the pilot's visibility being restricted (the left window was still covered by the eyelid) his guidance to the high key position was based on radar vectors from ground control. The eyelid was opened at approximately 1.6 Mach number as the aircraft was over Rogers Lake and the visibility out this window was good.

Knight landed at Edwards, the plane resembling burnt firewood. It had been an eventful flight; now the engineers sat down and took a long look at what it all meant.

What it really meant was the end of the refurbishable spray-on ablator concept. It was the closest any X-15 came to structural failure induced by heating. The plane was charred on its leading edges and nosecap. The ablator had actually prevented cooling of some hot spots by keeping the heat away from the craft's metal heat-sink structure. On earlier flights without the ablator, some of those areas remained relatively cool because of heat transfer through the heavy Inconel structure. Some heating effects, such as at the tail and body juncture and where shockwaves intersected the structure, had been the subject of theoretical studies, but had never before been seen on an actual aircraft in flight. To John Becker at Langley, the flight underscored ”… the need for maximum attention to aerothermodynamic detail in design and preflight testing.” 16 To Jack Kolf, an X-15 project engineer at the FRC, the X-15A-2's condition ”… was a surprise to all of us. If there had been any question that the airplane was going to come back in that shape, we never would have flown it.” 17 The ablator had done its job, but refurbishing for another flight near Mach 7 would have taken five weeks. Technicians would have had great difficulty in ensuring adequate depth of the ablator over the structure. Obviously, a much larger orbital vehicle would have had even greater problems. The sprayed-on refurbishable ablator concept thus died a natural death. The unexpected airflow problems with the ramjet ended any idea of using that configuration on the X-15, as did the ramjet's own shortcomings as a design (as is discussed subsequently). After the flight, NASA sent the X-15A-2 to its manufacturer for general maintenance and repair. Though the plane returned to Edwards in June 1968, it never flew again. It is now on exhibit--in natural black finish – at the Air Force Museum, Wright-Patterson AFB, Ohio. The third X-15 (serial 56-6672) featured specialized flight instrumentation and displays that rendered it particularly suitable for high-altitude flight research. A key element of its control system was a so-called ”adaptive” flight control system developed by Honeywell; it automatically compensated for the airplane's behavior in various flight regimes, combining the aerodynamic control surfaces and the reaction controls into a single control ”package.” This offered much potential for future high-performance aircraft such as the anticipated Dyna-Soar and supersonic transports, should the latter be built.

By the end of 1963, this X-15 had flown above 50 miles, the altitude that the Air Force recognized as the minimum boundary of spaceflight. FRC pilot Joe Walker set an X-15 record for winged spaceflight by reaching 354,200 feet, a record that stood until the orbital flight of Columbia nearly two decades later. These flights, and others later, acquired reentry data considered applicable to the design of future ”lifting reentry” spacecraft. By mid-1967, the X-15-3 had completed sixty-four research flights, twenty-one at altitudes above 200,000 feet. It became the prime testbed for carrying experiments to high altitude, especially micrometeorite collection and solar-spectrum analysis experiments.

As had happened in some other research aircraft programs, a fatal accident signaled the end of the X-15 program. On 15 November 1967 at 10:30 a.m., the X-15-3 dropped away from its B-52 mothership at 45,000 feet near Delamar Dry Lake. At the controls was veteran Air Force test pilot, Maj. Michael J. Adams. Starting his climb under full power, he was soon passing through 85,000 feet. Then an electrical disturbance distracted him and slightly degraded the control of the aircraft. Having adequate backup controls, Adams continued on. At 10:33 he reached a peak altitude of 266,000 feet. In the FRC flight control room, fellow pilot and mission controller Pete Knight monitored the mission with a team of engineers. Something was amiss. As the X-15 climbed, Adams started a planned wing-rocking maneuver so an on-board camera could scan the horizon. The wing rocking quickly became excessive, by a factor of two or three. When he concluded the wing-rocking portion of the climb, the X-15 began a slow, gradual drift in heading; 40 seconds later, when the craft reached its maximum altitude, it was off heading by 15°. As the plane came over the top, the drift briefly halted, with the plane yawed 15° to the right. Then the drift began again; within 30 seconds, the plane was descending at right angles to the flight path. At 230,000 feet, encountering rapidly increasing dynamic pressures, the X-15 entered a Mach 5 spin. 18

In the flight control room there was no way to monitor heading, so nobody suspected the true situation that Adams now faced. The controllers did not know that the plane was yawing, eventually turning completely around. In fact, control advised the pilot that he was ”a little bit high,” but in ”real good shape.” Just 15 seconds later, Adams radioed that the plane ”seems squirrelly.” At 10:34 came a shattering call: ”I'm in a spin, Pete.” A mission monitor called out that Adams had, indeed, lost control of the plane. A NASA test pilot said quietly, ”That boy's in trouble.” Plagued by lack of heading information, the control room staff saw only large and very slow pitching and rolling motions. One reaction was ”disbelief; the feeling that possibly he was overstating the case.” But Adams again called out, ”I'm in a spin.” As best they could, the ground controllers sought to get the X-15 straightened out. They knew they had only seconds left. There was no recommended spin recovery technique for the plane, and engineers knew nothing about the X-15's supersonic spin tendencies. The chase pilots, realizing that the X-15 would never make Rogers Lake, went into afterburner and raced for the emergency lakes, for Ballarat, for Cuddeback. Adams held the X-15's controls against the spin, using both the aerodynamic control surfaces and the reaction controls. Through some combination of pilot technique and basic aerodynamic stability, the plane recovered from the spin at 118,000 feet and went into a Mach 4.7 dive, inverted, at a dive angle between 40 and 45 degrees. 19

Adams was in a relatively high altitude dive and had a good chance of rolling upright, pulling out, and setting up a landing. But now came a technical problem that spelled the end. The Honeywell adaptive flight control system began a limit-cycle oscillation just as the plane came out of the spin, preventing the system's gain changer from reducing pitch as dynamic pressure increased. The X-15 began a rapid pitching motion of increasing severity. All the while, the plane shot downward at 160,000 feet per minute, dynamic pressure increasing intolerably. High over the desert, it passed abeam of Cuddeback Lake, over the Searles Valley, over the Pinnacles, arrowing on toward Johannesburg. As the X-15 neared 65,000 feet, it was speeding downward at Mach 3.93 and experiencing over 15 g vertically, both positive and negative, and 8 g laterally. It broke up into many pieces amid loud sonic rumblings, striking northeast of Johannesburg. Two hunters heard the noise and saw the forward fuselage, the largest section, tumbling over a hill. On the ground, NASA control lost all telemetry at the moment of breakup, but still called to Adams. A chase pilot spotted dust on Cuddeback, but it was not the X-15. Then an Air Force pilot, who had been up on a delayed chase mission and had tagged along on the X-15 flight to see if he could fill in for an errant chase plane, spotted the main wreckage northwest of Cuddeback. Mike Adams was dead, the X-15 destroyed. NASA and the Air Force convened an accident board. 20

Chaired by NASA's Donald R. Bellman, the board took two months to prepare and write its report. Ground parties scoured the countryside looking for wreckage, any bits that might furnish clues. Critical to the investigation was the cockpit camera and its film. The weekend after the accident, a voluntary and unofficial FRC search party found the camera; disappointingly, the film cartridge was nowhere in sight. Engineers theorized that the film cassette, being lighter than the camera, might be further away, to the north, blown there by winds at altitude. FRC engineer Victor Horton organized a search and on 29 November, during the first pass over the area, W. E. Dives found the cassette, in good condition. Investigators meanwhile concentrated on analyzing all telemetered data, interviewing participants and witnesses, and studying the aircraft systems. Most puzzling was Adams' complete lack of awareness Of major heading deviations in spite of accurately functioning cockpit instrumentation. The accident board concluded that he had allowed the aircraft to deviate as the result of a combination of distraction, misinterpreting his instrumentation display – and possible vertigo. The electrical disturbance early in the flight degraded the overall effectiveness of the aircraft's control system and further added to pilot workload. The X-15's adaptive control system then broke up the airplane on reentry. The board made two major recommendations: install a telemetered heading indicator in the control room, visible to the flight controller, and medically screen X-15 pilot candidates for labyrinth (vertigo) sensitivity. As a result of the X-15's crash, FRC added a ground-based ”8 ball” attitude indicator, displayed on a TV monitor in the control room, which furnished mission controllers with ”"real time” pitch, roll, heading, angle of attack, and sideslip information available to the pilot, using this for the remainder of the X-15 program. 21

Figure 5 - proposed delta wing X-15 airplane design
Figure 5
THE PROPOSED DELTA-WING X-15, NOVEMBER 1964

The X-15 program itself did not long survive the loss of the X-15 #3. The X-15A-2, grounded for repairs, soon remained grounded forever. The first X-15 continued flying, with sharp differences of opinion about whether the research results returned were worth the effort and expense. The ramjet program had offered hope to zealots that the program might continue, but the X-15A-2's experience really ended all that. A proposed delta wing X-15 modification had offered supporters the hope that the program might continue to 1972 or 1973, but the loss of the third X-15 ended this hope as well, inasmuch as it would have been the third aircraft that would have been modified as a delta hypersonic testbed. The proposed delta wing X-15 (Figure 5) had grown out of studies in the early 1960s on using the X-15 as a hypersonic cruise research vehicle. Essentially, the delta X-15 would have made use of the third airframe with the adaptive flight control system, but also incorporated the modifications made to the X-15A-2 – lengthening the fuselage, revising the landing gear, adding external tankage, and provisions for a small-scale experimental ramjet. NASA proponents, particularly John Becker (chief of Langley's Aero-Physics Division) found the idea very attractive since, as Becker wrote in one internal memo: 22

The highly swept delta wing has emerged from studies of the past decade as the form most likely to be utilized on future hypersonic flight vehicles in which high lift/drag ratio is a prime requirement i.e., hypersonic transports and military hypersonic cruise vehicles, and certain recoverable boost vehicles as well.

Despite such endorsement, support remained lukewarm at best both within NASA and the Air Force (indeed, only within the flight testing and hypersonic communities of both organizations was there ever much support for the X-15 program at all); the loss of Mike Adams and the third X-15 sealed the fate of the delta proposal, though the idea did influence in a roundabout way the subsequent attempts to build hypersonic sustained cruise technology demonstrators in the 1970s such as the National Hypersonic Flight Research Facility (NHFRF).

Perhaps because of the generalized feeling that the X-15 had long passed the point of productive and timely research – a feeling that program participants would have contested – support for the X-15 dropped dramatically after 1963. As early as March 1964, in consultation with NASA Headquarters, Brig. Gen. James T. Stewart, director of science and technology for the Air Force, had determined to end the program in December 1968. 23 The first X-15, the only one of the three still flying after the Knight and Adams' flights, had just about exhausted its research ability, and it cost roughly $600,000 per flight. Other NASA programs could benefit from this funding, and thus NASA did not request a continuation of X-15 funding after December 1968. 24 During 1968 Bill Dana of NASA and Pete Knight of the AFFTC took turns flying the X-15, though a variety of weather, maintenance, and operational problems caused rescheduling and cancellation of a number of flights. On 24 October 1968, Dana completed the first X-15's 81st flight, the 199th flight of the series. The plane attained Mach 5.38 at 255,000 feet, carrying a variety of follow-on experiments. Though researchers tried to get a 200th flight before the end of the year, weather, maintenance and operational problems dictated otherwise. The X-15 program, after nearly a decade of flight operations, came to an end.




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