Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

During the mid-1960s, a proposed delta-wing modification to X-15-3 might have kept the program flying until 1972 or 1973. Unlike many proposals, such as the "orbital X-15," the delta wing was a real project and was the subject of a great deal of research and engineering.

The delta-wing X-15 grew out of hypersonic cruise research vehicle studies conducted during the early 1960s. A "hypersonic cruise" vehicle would spend minutes or tens of minutes at hypersonic velocities, in contrast to the original X-15 that spent only a few tens of seconds at that velocity. The delta-wing X-15 configuration used the third airplane with the MH-96 and the basic modifications made to X-15A-2. Proponents of the concept, particularly John Becker at Langley, found the idea very attractive. Becker opined that "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."[287]

Researchers held a meeting at the FRC on 9-10 December 1964 to determine exactly what research could be undertaken with a delta-wing hypersonic-cruise X-15. Attendees included researchers from Ames and Langley, and of course groups from the AFFTC and FRC. Not surprisingly, Ames and Langley did not necessarily agree on the exact nature of the research, and NASA Headquarters asked each center to submit a position paper outlining its preferences.

The Langley paper included an evaluation of possible aerodynamic and heat-transfer experiments, in-flight engine-inlet tests, and various structural recommendations that might be applicable to the delta-wing X-15. The entire paper was remarkably short-only 11 pages, or about the same size as the original 1954 paper that had been the genesis of the X-15 program.*288

John Becker and David E. Fetterman, Jr. led the Langley group that defined the aerodynamic and heat-transfer experiments primarily concerned with evaluating the differences between data gathered during full-scale flight tests and the results of wind-tunnel and analytical data. They noted that "[s]ince air-breathing hypersonic cruise vehicles will fly at such large Reynolds numbers that turbulent conditions will occur over the entire wing, it is obviously of great importance to establish the turbulent heat-transfer characteristics of delta wings. The lack of any rigorous theory to turbulent heat transfer places great emphasis on experimental determinations.

It has proved impossible, however, to achieve natural turbulent boundary layers on delta wing models in hypersonic ground facilities except over the rearward regions." Becker and Fetterman used similar logic to justify investigations into areas such as turbulent skin friction and Reynolds analogy, turbulent boundary-layer profile surveys, interference heating, and pressure distributions.-128^

The discussion of engine-inlet testing led to a rather surprising conclusion: "an inlet flight test program on the X-15 is not recommended." J. R. Henry believed that ground facilities such as the 20-inch hypersonic tunnel at Langley and the 3.5-foot hypersonic tunnel at Ames were more than adequate for research into inlet configurations up to Mach 8. Henry’s case was convincing, and despite the ongoing interest in the HRE experiment, Langley dropped engine-inlet testing from further consideration for the delta wing.[290]

Structural considerations were not so much possible experiments, but rather recommendations prepared by J. C. Robinson representing the views of the Structures Research Division at Langley. The primary recommendation was to manufacture the main wing structure from one of the nickel – or cobalt-base "superalloys" using simple construction methods such as "corrugated webs welded to machined cap members with a machined waffle plate skin between the cap members and welded or riveted to them." It was also recommended that the leading-edge material "should be refurbishable, fabricated of thoria-dispersed nickel or a refractory metal, radiation cooled, and should have expansion joints to accommodate differences in thermal expansion between it and the main structure."[291]

The Ames position paper submitted by J. Lloyd Jones was more extensive, consisting of 18 pages using much smaller typeface. The paper also levied some criticism of the existing X-15 program.[292]

Ames agreed with Langley that the primary area of research should be turbulent boundary layers "because of the difficulty of providing turbulent boundary layer flow on models in wind tunnel tests at hypersonic speeds." Ames noted that the existing X-15 had already validated wind-tunnel results up to Mach 5, and the Mach 8 data would be a natural extension. Ames also pointed out that the delta-wing X-15 "is more representative of hypersonic cruise aircraft configurations" and would therefore yield more useful data. Ames also believed that "knowledge that will accrue of the handling qualities of a delta winged vehicle representative of current concepts of airbreathing cruise configuration will certainly be of value."[293]

The researchers at Ames seemed to want to take the lead on the delta-wing configuration, much as Langley had done on the original X-15, and their list of potential research areas was a good deal longer. Besides turbulent-boundary-layer research, Ames wanted to look at skin-friction surveys, pressure distributions and local flow fields, boundary-layer-transition definitions, ablative studies, panel-flutter studies, and low-speed and hypersonic-handling qualities. To accompany these experiments, Ames believed that "an increased program of ground based research relating to hypersonic cruise aircraft technology should be initiated."[294]

Although Ames acknowledged that "the X-15 program to date unquestionably has been very successful" and had turned into "a research facility with which to conduct studies never envisioned at its inception," it believed the program was open to some criticism. "For example, measurement of heat transfer was one of the major research experiments conducted on the airplane, but design inflexibility resulted in the acquisition of heat transfer data in an environment which compromised their value to the extent that they cannot be fully explained nor understood."[295]

To overcome this fault on the new airplane, Ames indicated, "the key to the potential benefits…

for a delta wing X-15 program lies in the design of the airplane to accomplish well defined experimental tasks as primary objectives." To this end, Ames made several recommendations for the vehicle configuration:12961

1. Provision for removable test panels in the areas selected for measurements… to provide for heat transfer, boundary layer and structural tests on both experimental and prototype panels. Design of the test panels and supporting structure should be guided by the requirement to avoid localized heat sinks that withdraw heat away from the boundary layer in a non-uniform manner.

2. Provide a smooth primary test area along the bottom centerline of the airplane free of all protuberances. This consideration may well lead to the requirement for a low-wing configuration to insure a test region with a definable uniform flow field.

3. A removable fuselage nose section ahead of the wheel well with an instrument compartment and a removable tip. This feature would provide for replacement nose sections having different geometry.

4. An instrumentation bullet located at the wing-fin juncture may be advantageous.

Ames went on to describe an elaborate research program that began with wind-tunnel studies, followed by flights using X-15A-2, and finally tests with the delta-wing aircraft. The program would concentrate most of the theoretical and wind-tunnel work at Ames. Based on the scope of the work described, it probably would have been expensive, although the report did not include a cost estimate.

After reviewing the papers submitted by Ames and Langley, with additional input from the Air Force and FRC, NASA defined two primary objectives for the delta-wing X-15 program:12921

1. Aerodynamic research-The flight tests would provide realistic aerodynamic data under fully developed turbulent flow conditions to supplement ground-based research where such conditions cannot be achieved. Answers would be obtained to key questions relating to hypersonic aerodynamics of delta wings, large-scale behaviors of flap-type controls, tip-fin interference effects, and handling qualities of a configuration typical of present thinking for a future hypersonic air-breathing vehicle. Aerodynamic research on this vehicle would be unclouded by propulsion effects, inasmuch as most of the data would be taken under gliding conditions.

2. Structural research-The delta-wing proposal would permit the evaluation in a practical flight application of a hot radiation-cooled structure designed for repeated flights at temperatures between 1,500 degrees and 2,200 degrees Fahrenheit. It would also focus technical effort on a refurbishable, hot-wing leading edge design.

Paul Bikle concluded that "[i]n general, a delta-wing X-15 program could establish a baseline of confidence and technology from which decisions regarding the feasibility and design of advanced air-breathing vehicles could be realistically made. The proposed time for the delta X-15 fits well with that for an overall hypersonic research vehicle program and the cost does not appear to be unreasonable."12981

Things seemed to be progressing rapidly. In January 1965 the FRC drafted a statement of work for North American to conduct a detailed study of the delta-wing concept. This document indicated that "NASA is considering a hypersonic cruise vehicle research program which involves a modification of the basic X-15 configuration for study of various aerodynamic, structural, and flight control problems. The program will also include limited investigations of flight to altitudes extending to about 180,000 feet." Extreme high-altitude research was not a requirement.12991

NASA suggested that an existing "X-15 airplane would be modified to incorporate a representative slender hypersonic wing substituted for the present wing and horizontal tail. The wing structure would be designed for sustained hypersonic flight at a Mach number of 7, but would also be capable of flight to Mach number 8 for limited time periods. The basic X-15 fuselage structure, rocket engine, flight control and other systems would be retained with minimum modification, and the present B-52 launch system and high range facility would be utilized."13001

The work statement went on to indicate that the airplane should have a load limit of 5 g and a

2,0- psf design dynamic pressure. Potential "improvements" included relocating the wing to be flush with the bottom of the fuselage, increasing the dynamic pressure to 2,500 psi and the load factor to 7.33 g, including external propellant tank(s), and relocating the nose landing gear. Other possibilities included the addition of a permanent thermal-protection system (in lieu of ablative coatings) over the fuselage to prevent contamination of the wing with ablative products.-13011

The delta wing seemed to languish at the FRC for the remainder of the year while engineers put together a project development plan. By the end of the year, the FRC had released the second draft of the plan for internal review, providing more detail on how things might progress. The opening paragraph provided the justification for the program:13021

Three of the most probable uses for hypersonic airbreathing aircraft are transport over long ranges, military reconnaissance, and as maneuverable reusable first-stage boosters. There are currently no military or civilian requirements of over-riding importance for any one of these. Their potential, however, constitutes a clear justification to proceed with comprehensive programs to develop the required hypersonic technology.

Unfortunately, the justification also included a rationale for not supporting the program, given the budget crunch NASA was experiencing as it continued the Apollo program to the detriment of the aeronautics budget. Nevertheless, the FRC pressed on with detailed plans. The FRC considered the delta-wing project an extension of the X-15 program, and assumed that all existing agreements between the Air Force and NASA would continue. Researchers believed that the manpower requirements for the program could be satisfied "with the present complements of the Langley, Ames, and Flight Research Center."13031

Researchers at the FRC estimated the program would cost $29,750,000 spread between FY67 and FY73. Of this, the airframe contractor would receive $24,600,000 to build the flight vehicle, while the other $5,150,000 would be "in-house" expenses. The second year represented the largest annual expenditure: $14,500,000 to the contractor and a little over $1,000,000 in-house. The planners warned, however, that "if the military withdraws their operational support from the general X-15 program, this project would be responsible for additional expenses over the 4-1/2 year operational period. These expenses could amount to as much as 17 million dollars." The preliminary schedule showed a request for proposals in August 1966, a contract award in March 1967, modifications to X-15-3 between December 1967 and October 1968, and a first flight in January 1969. The 37-flight research program continued until December 19 72.13041

In the project plan, the FRC had expected the X-15 Project Office at Wright-Patterson AFB to handle the procurement of the modifications, although NASA would pay the bills. Although it would seem logical for North American to perform the modifications, several other contractors (Lockheed, Northrop, and Republic) had expressed interest in the program, so the FRC proposed to make it a competitive process.13051

The Air Force was not totally in favor of this since they saw their involvement with the X-15 winding down, and had little apparent interest in the delta-wing program. Despite negotiations between the Air Force and NASA at various levels, the X-15 Project Office declined to participate in the expected procurement of the delta-wing airplane; however, it did agree to transfer the X – 15-3 airframe to NASA for the modification at the end of its flight program.

After nearly two years of delay, on 13 May 1967 the delta-wing program had progressed far enough for the FRC to issue a request for proposals (PR-7-174) for a formal conceptual design study. The primary objectives of the study were to 1) develop a preliminary design for evaluating the modification of X-15-3 to a delta-wing configuration, and 2) formulate an accurate estimate of performance, weight, cost, and schedule for such modification. A secondary objective was to analyze alternate approaches, such as unsegmented leading edges; eliminate the use of ablatives; and incorporate a fly-by-wire control system, multiplane airfoils, symmetrical tip fins, and different propulsion systems.-1306

There appears have been only a single respondent to the request for proposals. North American submitted a two-volume, 500-page proposal containing detailed engineering concepts and cost data-and that was just the proposal to do the study! By this time, North American had already been testing the delta-wing X-15 in wind tunnels for over a year. The North American low-speed and hypersonic tunnels and the Langley 20-inch hypersonic tunnel had tested 1/15-scale and 1/50-scale models at Mach numbers between 0.2 and 6.9 and Reynolds numbers up to 10,000,000 (equivalent, based on model length).-307

The proposed North American delta-wing X-15-3 was not a simple conversion. The 603-square – foot delta-wing planform had a 76-degree leading-edge sweep, but the span was the same as that of the original X-15 to ensure that there would be no clearance issues with the NB-52 carrier aircraft. Elevons (30.8 square feet each) at the trailing edge provided longitudinal and roll control by deflecting up to 4.5 degrees up or 5.0 degrees down. The existing dorsal and ventral stabilizers provided directional stability with the addition of wing-tip fins, and the existing dorsal rudder provided directional control. Engineers could adjust the removable tip fins on the ground for cant and tow-in, which allowed them to change the relative levels of directional and lateral stability to investigate the handling qualities of the vehicle. The tip fins compensated for the blanking of the normal centerline vertical stabilizers by the fuselage at hypersonic speeds and large angles of attack.308

The position of the wing was the subject of a great deal of study since the HRE was expected to be carried on the ventral stabilizer (as the dummy was on X-15A-2), and the aircraft also had to be stable in flight without the 1,000-pound engine. It proved to be a difficult problem. The final answer before NASA terminated the program was to position the wing in the best location to compensate for the HRE. On flights without the engine, as much research equipment or ballast as possible would be located in a new aft experiment compartment. Likewise, the shape of the wing leading edge was of some concern, but North American noted that "the effect of leading edge radius on the low-speed aerodynamic characteristics of highly swept delta wings is not well understood." Engineers did not think the effect on the leading-edge shape at supersonic speeds would be significant, because positive pressure on the wing lower surface would produce most of the lift. Nevertheless, since the leading-edge shape significantly influenced the landing characteristics, North American investigated various configurations in its low-speed wind tunnel. The company had not found a satisfactory answer, and was awaiting further data from NASA wind-tunnel tests, when the program ended.309

During the mid-1960s, a proposed delta-wing modification to X-15-3 might have kept the program flying until 1972 or 1973. Unlike many proposals, such as the "orbital X-15," the delta wing was a real project and was the subject of a great deal of research and engineering. Despite endorsements from the Flight Research Center and John Becker at Langley, support remained lukewarm within the Air Force and NASA. The FRC was still evaluating the proposals for the delta­wing study on 15 November 1967 when the crash of the X-15-3 effectively ended all thought of such a modification. (NASA)

North American was also somewhat uncertain about the leading-edge material, mainly because of the expected 2,200°F temperatures encountered on the design mission. North American built a segmented leading edge made from columbium alloy that successfully passed tests at 2,400°F. This appeared satisfactory, at least for initial use. The company expected that no available material would prove satisfactory for the lower surface of the wing, and that some form of thermal protection system would have to be developed. Unsurprisingly, many of the North American ideas looked similar to concepts the company was investigating for the space shuttle. One of the most promising ideas was to use metallic heat shields supported by standoff clips with a layer of low – density insulation sandwiched between the shield and the wing skin. Only an area about 2 feet wide just behind the leading edge needed this type of protection since the airflow further aft smoothed out sufficiently to keep temperatures within the ability of alloys such as TD nickel to

Г3101

Originally, North American had envisioned using upswept wing tips to replace the directional stability lost by the removal of the ventral rudder from the delta-wing configuration. Although the X-15 program seldom used the ventral rudder, this was because most missions flew at high angles of attack, where the lower rudder was detrimental to stability. The delta-wing program, on the other hand, wanted to fly sustained high-speed cruise missions that would require little high – angle-of-attack work. The initial round of tests in the North American hypersonic tunnel revealed that the upswept wing tips were inadequate above Mach 6. Researchers tested various configurations in both the North American hypersonic tunnel and the 20-inch Langley hypersonic tunnel until they found a set of tip fins that extended both above and below the wing centerline to be adequate. Nevertheless, engineers decided to make it easy to replace the fins just in case the wind-tunnel tests proved to be inaccurate.13111

The large-angle-of-attack capability of the basic X-15s was no longer required since researchers did not intend the mission to go to high altitudes. Given that the maximum angle of attack envisioned for the new airplane was less than 15 degrees, engineers decided to use a fixed-flow direction sensor to sense the angle of attack and sideslip. A hemispherical nose with five pressure taps could provide an air-data computer with sufficient information to derive the necessary angles without the complexity and weight of the ball nose. Conceptually, this was identical to the fixed alpha nose flown during six of the last X-15-1 flights.13121

North American proposed to stretch the fuselage 10 feet to 62.43 feet overall, and to manufacture what was essentially a new fuselage from the cockpit rearward. The company would stretch the propellant tank section 91 inches and provide new mounting provisions for the delta wing. North American manufactured test specimens from Rene 41 and Inconel 718 to determine which would be the best material for this area; these tests were in progress when NASA canceled the program. The space between the liquid-oxygen and ammonia tanks accommodated the standard center-of – gravity instrumentation compartment, and North American added a new 29-inch-long compartment behind the fuel tank but ahead of the engine to hold fuel and gases for the HRE. Designers also wanted to replace the existing ogive forward fuselage (in front of the canopy) with a 20-degree, included-angle-cone section. This semi-monocoque structure would use Rene 41 outer skin and Inconel X (or Inconel 718) frames, and titanium would be used for the inner skin of the equipment compartment.13131

North American investigated several different powerplants for the airplane, with the leading challenger being an Aerojet YLR91-AJ-15 from the second stage of a Titan II ICBM. This engine used unsymmetrical dimethylhydrazine and nitrogen tetroxide as propellants, and had already been man-rated for the NASA Gemini program. When equipped with a 25:1 nozzle, this engine completed the reference mission without the use of external propellant tanks. In fact, at a launch weight of 52,485 pounds and a burnout weight of 18,985 pounds, the YLR91 would have allowed a maximum velocity of 8,745 fps, well in excess of the 7,600 fps required by NASA. The effect of carrying the 1,000-pound HRE would have reduced this by about 400 fps.13141

North American briefly investigated the idea of using a separate "sustainer" engine to provide thrust to overcome drag during hypersonic cruise. Although the integration issues involved with incorporating a second engine and its propellants into the airframe eventually convinced all concerned that it would be too difficult, the particular engines investigated show how a program could come full circle. One of the engines investigated was the Bell YLR81-BA-11, a variation of the one of the engines proposed to power the X-15 in 1954. North American also investigated several variants of the Reaction Motors LR11 family that had been used for the initial X-15 flights,

along with the Aerojet LR52 (AJ-10).-1315

The engine that was ultimately selected was a modified XLR99 that provided 83,000 Ibf at 100,000 feet and was throttleable down to 8,000 lbf for sustained cruise. This was the version of the XLR99 that used a single thrust chamber and nozzle, not the Reaction Motors concept that used a second, remotely located alternate chamber. The increased internal fuel would permit sustained flights at Mach 6.5 using the low-thrust "sustainer" capability of the modified XLR99 to overcome drag but not produce any acceleration. The addition of a single centerline external tank would allow Mach 8 flights.-316

The main landing gear would be a version of the gear developed for the X-15A-2, appropriately strengthened for the almost 19,000-pound normal landing weight of the delta-wing design. As on the X-15-A-2, North American proposed to use both short and long versions of the rear shock struts; the long ones would provide clearance for the HRE under the ventral, while flights that did not carry the HRE would use the short ones. The nose gear would be moved to the instrument compartment behind the pilot, and the recorders and other research instrumentation normally carried there would be moved to a new compartment in front of the pilot where the original nose gear well was.-1317!

Although it was not part of the delta-wing baseline, North American was investigating the use of a fly-by-wire control system on the delta-wing X-15. Engineers believed that this would reduce the overall system size, weight, and volume, and provide better overall performance. This system would have used an analog flight-control system, not a digital one. The MH-96 adaptive control system was capable of accepting electrical inputs that were equivalent to flying in a fly-by-wire mode, and Honeywell designed the MH-96 to interface to the fly-by-wire flight-control system in the Dyna-Soar. In X-15-3 these were paralleled with mechanical linkages, and the delta wing could eliminate these mechanical linkages altogether. Given the facts that a fly-by-wire system had never flown, and the delta-wing airplane was flying in a new performance envelope anyway, NASA was not supportive of this effort.316

The final delta-wing design spent a considerable amount of time in this NASA Langley wind tunnel before the program was cancelled. (NASA)

Although the delta-wing airplane was generally described as a modification of X-15-3, about the only structure from the original airplane that would remain would be the cockpit and the aft thrust structure. Most of the electronics (i. e., the inertial system and MH-96) would also remain.

However, at least by weight, the majority of the aircraft would be new. Since the North American study had not been completed when X-15-3 was lost, the company had not estimated the final cost, but the amount would probably have been substantial.

Although no formal contracting arrangement existed, North American pressed on with a great deal of research into the delta-wing configuration. By March 1967, wind-tunnel models had accumulated over 300 hours of testing, with a 1/50-scale model used for high-speed tests and a 1/15-scale model used for low-speed tests. According to a North American news release, "the four year research program has also enabled North American to check out the integrity of components using new super alloys that will be required at hypersonic speeds. Tanks, wing sections, and other components have been fabricated of such materials and put through exhaustive thermal and structural tests."[319]

Despite endorsements from the FRC and John Becker at Langley, support remained lukewarm within the Air Force and NASA. The FRC was still evaluating the proposals for the delta-wing study on 15 November 1967 when the crash of the X-15-3 effectively ended all thought of such a modification. Since the general concept depended upon the use of the electronic systems that were unique to X-15-3, most researchers did not consider it readily possible to convert one of the other airframes; besides, the accident effectively sealed the fate of the entire X-15 program.-132^

Forrest "Pete" Petersen flew the X-15 for 15 months from 23 September 1960 until 10 January 1962, making five flights. These included two flights with the XLR11 and three flights with the XLR99. He reached Mach 5.30, a maximum speed of 3,600 mph, and an altitude of 101,800 feet.

Forrest Silas Petersen was born on 16 May 1922 in Holdrege, Nebraska. After he graduated from the Naval Academy in June 1944, he reported to the destroyer USS Caperton (DD 650) and participated in campaigns in the Philippines, Formosa, and Okinawa. Petersen switched from the "black shoe" Navy to "brown shoes" when he graduated from flight training in 1947 and was assigned to VF-20A.

Petersen completed two years of study at the Naval Post Graduate School and received a bachelor of science degree in aerospace engineering. He continued his studies at Princeton University and received a master’s degree in engineering in 1953. In 1956 he attended the Naval Test Pilot School and remained as an instructor following graduation. The Navy assigned him to the X-15 program in August 1958, and he served with NASA until January 1962. He was a joint recipient of the 1961 Robert J. Collier Trophy presented by President John F. Kennedy at the White House in July 1962, and the NASA Distinguished Service Medal presented by Vice President Lyndon B. Johnson.

Petersen served as commanding officer of VF-154 prior to being assigned to the office of director, Division of Naval Reactors, Atomic Energy Commission, for nuclear power training. He reported to USS Enterprise in January 1964 and served as executive officer until April 1966. Petersen received the Bronze Star during Enterprise’s first combat tour in Vietnam. Afterward, he became an assistant to the director of naval program planning in the office of the chief of naval operations. In November 1967 he assumed command of USS Bexar (APA-237) and received the Navy Commendation Medal with Combat V. He later served as deputy chief of naval operations for air warfare, and commander of the Naval Air Systems Command. Vice Admiral Petersen retired from active duty in May 1980 and died of cancer in Omaha, Nebraska, on 8 December 1990.

Finally, at 0838 hours on 8 June 1959, Scott Crossfield and X-15-1 dropped from the NB-52A at Mach 0.79 and 37,550 feet. Just prior to launch, the SAS pitch damper failed, but Crossfield elected to proceed with the flight and switched the pitch channel to standby. At launch, the X-15 separated cleanly and Crossfield rolled to the right with a bank angle of about 30 degrees. Usually the obedient test pilot, on this flight Crossfield allowed himself to deviate slightly from the flight plan and perform one unauthorized aileron roll. However, not all was well. On the final approach to landing, the X-15 began a series of increasingly wild pitching motions. Crossfield: "[T]he nose of the X-15 pitched up sharply. It was a maneuver that had not been predicted by the simulator… I was frankly caught off guard. Quickly I applied corrective elevator control. The nose came down sharply. But instead of leveling out, it tucked down. I applied reverse control. The nose came up but much too far. Now the nose was rising and falling like the bow of a skiff in a heavy sea… I could not subdue the motions." The X-15 was porpoising wildly, sinking toward the desert at 175 knots.*63*

The airplane touched down safely at 150 knots and slid 3,900 feet while turning slightly to the right. After he landed, Crossfield said he believed that the airplane exhibited a classic case of static instability. Harrison Storms, on the other hand, was sure that the cure was a simple adjustment. In the end, Storms was right. As he would on all of his flights, Crossfield had used the side-stick controller during the flare instead of the center stick, and this subsequently proved to be the contributing cause of the oscillations. The side-stick controller used small hydraulic boost actuators to assist the pilot since it would have been impossible (or at least impractical) to move the side stick through the same range of motion required for the center stick. However, the engineers had decided to restrict the authority of these hydraulic cylinders somewhat, based on a best guess of the range of movement required. The guess had been wrong, and because of this a cable in the control system was stretching and retracting unexpectedly. What appeared to be pilot-induced oscillations during landing actually reflected the mechanics of the control system. The fix was to provide more authority to the hydraulic cylinder by changing an orifice—a simple adjustment.*643

Although the impact at landing was not particularly hard, later inspection revealed that bell cranks in both main landing skids had bent. Unfortunately, North American had not instrumented the main skids on this flight, so no specific impact data were gathered. However, the engineers generally believed that the shock struts had bottomed and remained bottomed because of higher – than-predicted landing loads. Excessive rebound loads caused by a foaming of the oil in the nose gear strut compounded the issue, although it took several more landings to realize this. As a precaution against the main skid problem occurring again, the metering characteristics of the shock struts were changed, and engineers conducted additional lakebed drop tests at even higher loads with the landing-gear test trailer used to qualify the landing-gear design. The landing gear would continue to be a concern throughout the flight program. All other airplane systems operated satisfactorily on this flight, clearing the way for the first powered flight using X-15-2. The following day North American moved X-15-1 into the hangar to hook up its XLR11s and propellant system and make other changes in preparation for its first powered flight.*65*

Scott Crossfield climbs out of X-15-1 after the first captive-carry flight. The X-15 landing gear had been deployed during the flight to demonstrate it would work after being cold-soaked at altitude. A member of the ground crew installs protective covers on the nose-mounted air data boom. (AFFTC History Office)

While the NB-52 was carrying X-15-1 as expensive wing cargo, engineers were testing the XLR11s at the Rocket Engine Test Facility using X-15-2. Despite the successful 22 May test, things were not going particularly well. Perhaps the engines had been out of service for too long between programs, or maybe too much knowledge had been lost during the coming and goings of the various engineers and technicians over the years, but the initial runs were hardly trouble-free. Various valves and regulators in the propellant system also proved to be surprisingly troublesome.

Moreover, sometimes things just went to hell. After one engine run, the ground crew began purging the hydrogen-peroxide lines of all residual liquid by connecting a hose from a ground nitrogen supply to a fitting on X-15-2. On this day, it was a new hose. Despite the careful procedures and great caution used, the hose had a slight residue of oil. When the technician applied gas pressure to the hose, the film of oil ran into the hydrogen-peroxide lines. The only thing truly compatible with peroxide is more peroxide, not oil. The result was an immediate explosion and fire that raced through the X-15 engine compartment. As always, the Edwards fire crew was standing by and quickly extinguished the fire, but not before gutting the engine bay.

One X-15 crewman was badly burned; if he had been standing two feet closer, he likely would have been killed. It took weeks to repair the airplane.[66]

Forty-six days after the first glide flight, and after the damage from the explosion was repaired, the Nb-52A took X-15-2 for a captive-carry flight with full propellant tanks on 24 July 1959.

One of the purposes of this flight was to evaluate the liquid-oxygen top-off system between the NB-52 and X-15. It proved to be erratic. Another test was to measure the time it took to jettison the propellants at altitude. While still safely attached to the wing of the NB-52, Crossfield jettisoned the hydrogen peroxide, which took 140 seconds. He then jettisoned the liquid oxygen and alcohol simultaneously, which took 110 seconds. The times matched predictions. The APUs and pressure suit performed flawlessly. Despite the failure of the top-off system, researchers considered the flight a success. The original contract had specified that North American would turn the first airplane over to the government in August 1959. For a while it looked like the company might deliver the first X-15 on schedule, but it was not to be.[67]

During August and early September, engineers canceled several attempts to make the first powered flight before the aircraft left the ground, due to leaks in the APU propellant system and hydraulic problems. There were also several failures of propellant tank pressure regulators, and on at least one occasion, liquid oxygen streamed out of the safety vent while the NB-52 carried the X-15. No flight occurred on that day. Charlie Feltz, Bud Benner, and John Gibb, along with a variety of other North American engineers and technicians, worked to eliminate these problems, all of which were irritating but not critical-other than to the morale.[68]

At the 30th anniversary celebration, Storms described the mood at the time:*69

A typical launch attempt would start the night before, and the crews would work all night preparing the X-15 and fueling it. About 8 a. m., Scott Crossfield would be in his flight gear and, after walking around the operation, get into the cockpit and start his checkout. Scott would stay in the ready condition as the countdown continued. This, unfortunately, might be as late as 3 or 4 in the afternoon before the B-52 would be allowed to take off. By the time it had reached launch altitude and attempted to hold for the required length of time with all systems in operation, sometime during this period a regulator would fail, a valve would fail, or the bearings on one or both APUs would go out. Then back to Edwards. When Scott returned, we would be scheduled to go to a press conference and meet many tired, and by that time somewhat edgy, reporters that always wanted answers that were just not available. These were not happy meetings for any of the participants.

Shortly after about the fourth such encounter, I was gathered up by General John McCoy of Wright Field and taken over to Mr. [James Howard ‘Dutch’] Kindelberger’s office, the then chairman of the board. The general explained that the country was in a bad spot with the Sputnik success and that our false starts were not very much of a positive boost to the national position. In short, "when were we going to launch that X-15?" This one time in my life all eyes were on me. Not the most desirable position. The answer I gave was to go over the conditions that we and the NASA had set up for a launch. Also, I gave my support to this approach and pointed out that we were attempting to put a new type of flying machine in the air without the loss of either millions of dollars worth of equipment or the pilot. However, if they wanted to, I would take them to the task force that set up the launch ground rules and they could either convince them of a different approach or overrule them, if possible. The whole meeting ended up with the Air Force’s plea for increased effort on out part and hope for early success. Fortunately for all concerned, the next attempt turned out to be a winner.

At last, Scott Crossfield made the first powered flight using X-15-2 on 17 September 1959. The NB-52A released the research airplane at 0808 in the morning while flying at Mach 0.80 and 37,600 feet. X-15-2 reached Mach 2.11 and 52,341 feet during 224.3 seconds of powered flight using the two XLR11 engines. Crossfield surprised everybody, including most probably himself, by performing another aileron roll, this time all the way around. As Crossfield remembers, "Storms was tickled." On a more serious note, he observed, "With the rolling tail one would expect very clean ‘aileron’ rolls without the classical adverse yaw from ailerons, and that is the way it rolled.

No big deal at all." The government’s concerns about the rolling tail were for naught.-701

Crossfield landed on Rogers Dry Lake 9 minutes and 11 seconds after launch, despite some concerns about a crosswind on the lake. Following the landing, ground crews noticed a fire in the area around the ventral stabilizer, but quickly extinguished it. A subsequent investigation revealed that the upper XLR11 fuel pump diffuser case had cracked after engine shutdown and sprayed fuel throughout the engine compartment. Alcohol collected in the ventral stabilizer and some unknown cause ignited it during landing. Crossfield noted that "the fire had burned through a large area, melting aluminum tubing, fuel lines, valves, and other machinery." For the second time in less than six months, X-15-2 went back to Inglewood. It took about three weeks to repair the damage.-1711

Edwards was not the only place where the X-15 created interest, although it was certainly the most visible. Back at Langley Research Center, just a month after the first powered flight, approximately 20,000 visitors attended the first anniversary inspection, held on Saturday, 24 October 1959. The crowds had come at NASA’s invitation, and local newspapers had spread the word that for the first time in its 42-year history Langley would be open to the public. NASA scientists, engineers, and technicians showed the public just what the new agency was doing to launch their country into space. The attractions included full-scale mockups of the X-15, XLR99, and a dummy in an MC-2 full-pressure suit. A small group of Langley secretaries acted as the hostesses for the exhibit, while both John Becker and Paul Bikle were nearby to answer questions. The event was a success with both the public and the media.-721

Back in the high desert, the third flight (2-3-9) of X-15-2 took place on 5 November 1959 when the NB-52A dropped the X-15 at Mach 0.82 and 44,000 feet. The flight got off to a bad start; during the engine start sequence, one chamber in the lower engine exploded. Chase planes reported external damage around the engine and base plate, and the resulting fire convinced Crossfield to land on Rosamond Dry Lake. Crossfield shut down both engines, but the 13.9 seconds of powered flight had been sufficient to accelerate the X-15 to Mach 1. Unfortunately, the flight attitude necessary to descend to the lakebed made it impossible to dump most of the remaining propellants. Crossfield initiated the landing flare at about 950 feet altitude and 253 knots. The aircraft touched down near the center of the lake at approximately 161 knots and a 10.8-degree angle of attack with a descent rate of 9.5 feet per second. Crossfield noted: "The skids dug in gently. The nose slammed down hard and the airplane plowed across the desert floor, slowing much faster than usual. Then she came to a complete stop within 1,500 feet instead of the usual 5,000 feet." When the nose gear had bottomed out, the fuselage literally broke in half at station 226.8, shearing out about 70% of the bolts at the manufacturing splice. The broken fuselage dug into the lakebed, creating a very effective brake.-173

A minor explosion during Flight 2-3-9 on 5 November 1959 resulted in an emergency landing on Rosamond Dry Lake that broke the back of X-15-2. As built, the X-15 was heavier than originally intended, and it did not help that Scott Cross field was unable to jettison all of the unused propellants before the emergency landing. The airplane was repaired in time for its fourth flight on 11 February 1960. (AFFTC History Office)

A contributing factor to the hard landing was the 15,138-pound touchdown weight. During development, engineers had established a limiting rate of sink of 9 fps based on design weight of 11,500 pounds. However, the as-built airplane had increased to 13,230 pounds. In addition, Crossfield had been unable to jettison some of the propellants because of the steep descending attitude necessary to reach the landing site, which further increased the landing weight. Crossfield later stated that the damage was the result of a structural defect that probably should have broken on the first flight.-74

Yet again, X-15-2 went to the Inglewood plant for repairs, and returned to Edwards in time for its fourth flight on 11 February 1960. North American repaired the damaged fuselage and strengthened the manufacturing splice by doubling the number of fasteners and adding a doubler plate, top and bottom, at the fuselage joint. The company also modified the other two airplanes to prevent similar problems.75

All was ready on the morning of 3 October 1967 as Colonel Joseph P. Cotton and Lieutenant Colonel William G. Reschke, Jr., started the engines on the NB-52B. Pete Knight had already been in the cockpit of X-15A-2 for over an hour performing the preflight checklist with the ground crew led by Charlie Baker and Larry Barnett and a host of support personnel in the NASA control room. At 1331 hours the mated pair took off from Edwards and headed to Mud Lake. An hour later, Knight "reached up and hit the launch switch and immediately took my hand off to [go] back to the throttle and found that I had not gone anywhere. It did not launch." This was not a good start, but a second attempt 2 minutes later resulted in the smooth launch of flight 2-53-97.[341

The flight plan showed that X-15A-2 would weigh 52,117 pounds at separation, more than 50% heavier than originally conceived in 1954. As the X-15 fell away, Knight lit the engine and set up a 12-degree angle of attack resulting in about 1.5 g in longitudinal acceleration. As normal acceleration built to 2 g, Knight had to hold considerable right deflection on the side stick to keep X-15A-2 from rolling left due to the heavier liquid-oxygen tank. When the aircraft reached the 35-degree planned pitch angle, Knight began to fly a precise climb angle. The simulator had predicted a maximum dynamic pressure of 540 psf, remarkably close to the 560 psf measured during the rotation. Knight maintained the planned pitch angle within 1 degree.[342]

Knight jettisoned the external tanks 67.4 seconds after launch at Mach 2.4 and 72,300 feet. Tank separation was satisfactory, but Knight described it as "harder" than it had been on flight 2-50­89. The parachute system performed satisfactorily and the Air Force recovered the tanks in repairable condition. Free of the extra weight and drag of the external tanks, the airplane began to accelerate quickly, and Knight came level at 102,100 feet. As Knight later recalled, "We shut down at 6,500 [fps] and I took careful note to see what the final got to. It went to 6,600 maximum on the indicator."[343]

Seventy-one seconds after engine shutdown, Knight performed the first of a series of planned rudder pulses with the yaw damper off. The sideslip indicator did not rotate as expected, but post-flight analysis revealed that the aircraft achieved a satisfactory yaw rate and lateral acceleration. Since the maneuver occurred at approximately the same time the unprotected ball nose reached its maximum temperature, researchers theorized that differential expansion in the nose may have resulted in a false instrument reading. Almost amusingly, despite the significant heating experienced by the rest of the airplane, the aft viewing Millikan 16-mm camera installed in the center-of-gravity compartment froze because of a malfunction of the thermal switch that activated the camera heater.[344]

This is how the ventral stabilizer and ramjet installation looked on the morning of 3 October 1967 prior to Flight 2-53-97. The skid landing gear is extended in this photograph. (NASA)

As Knight decelerated through Mach 5.5, the HOT PEROXIDE light came on; unknown to anybody, the intense heat from shock waves impinging on the dummy ramjet were severely damaging the airplane. Unfortunately, the peroxide light distracted Knight from his planned maneuvers and his energy management. As worries mounted, NASA-1 directed Knight to jettison his peroxide and began vectoring him toward high key. The X-15A-2 came across the north edge of Rogers at 55,000 feet and Mach 2.2. When Knight went to jettison the remaining propellants so that the chase plane could find him, nothing came out. There would be no help from the chase. Knight was high on energy, unable to jettison his propellants, and unsure about the condition of his airplane. He turned through high key at 40,000 feet but was still supersonic. While on final approach, Knight tried to jettison the ramjet, but later indicated that "I did not feel it go at all."

The ground crew reported that they did not see anything drop. Something was obviously wrong, but things were happening too quickly to worry about it.[345]

Fortunately, things mellowed out after that and Knight made an uneventful landing. Once on the ground, Knight realized that something was not right when a majority of the ground crew rushed to the back of the airplane. After he finally egressed and walked toward the rear of the X-15, he understood: there were large holes in the side of the ventral with evidence of melting and skin

rollback.-1346!

Post-flight analysis showed that the airplane had managed to attain Mach 6.70, equivalent to 4,520 mph (6,629 fps), at 102,700 feet, an unofficial speed mark for winged-vehicles that would stand until the return of the Space Shuttle Columbia from its first orbital mission in April 1981. This was the only X-15 flight to exceed the original 6,600-fps design goal.!3471

Later analysis showed that the shock wave from the spike nose on the ramjet had intersected the ventral and caused severe heating. Flight planner Johnny Armstrong observed, "So now maybe we knew why the ramjet was not there." The telemetry indicated that the ramjet instrumentation ceased to function 25 seconds after the XLR99 shut down. Later that afternoon several people, including Armstrong, were reviewing the telemetry when they noted an abnormal decrease in the longitudinal acceleration trace that indicated a sudden decrease in drag. The conclusion was that this was when the ramjet had separated. When the flight profile was computed, it was determined that this happened at about the 180-degree point during the turn over the south area of Rogers Dry Lake at about Mach 1 and 32,000 feet. Armstrong began correlating the telemetry with recorded radar data: "I could say that I did a detailed calculation of the drag coefficient for a tumbling ramjet, then a 5th order curve fit of the potential trajectory, corrected for winds-but actually, I just made an engineering estimate." In other words, he guessed.!3481

Not everybody believed Armstrong, but Bill Albrecht, the NASA operations engineer for X-15A-2, and Joe Rief, the AFFTC airfield manager, thought the theory had merit. Albrecht and Armstrong checked out a radio-equipped carryall van, cleared it with the tower, and headed out onto the Edwards impact range. Armstrong had previously marked up a map with some landmarks near where the telemetry and radar indicated the ramjet had separated. As they drove, Armstrong indicated a place to stop. They got out and walked about 200 yards directly to the ramjet, which was lying in two major pieces. The pair gathered up the nose cone and pressure probes and then headed back to the van (the main body of the ramjet was too heavy for only two men to lift). The next day Albrecht and Armstrong directed a helicopter to retrieve the ramjet. Subsequent inspection showed that three of the four explosive bolts that held the ramjet on had fired, probably due to the excessive temperatures that had melted large portions of the ramjet and ventral.!349!

The next experiment came from an unlikely source. On 3 August 1961, the Air Force Special Weapons Center at Kirtland AFB, New Mexico, delivered an ionizing radiation-detection device for use on the X-15. NASA installed the 10-pound package in the left side console of the cockpit outboard of the ejection seat. Actually, the first attempt to install the experiment failed because the space allocated in the cockpit was insufficient, but Kirtland soon repackaged it to fit.731

The experiment activated automatically when the pilot turned on the main instrumentation switch during flight. The package contained an ion chamber, two scintillators, a Geiger tube, and a self- contained multi-channel tape recorder. Different thicknesses of human-tissue-equivalent plastic encased the ion chamber and scintillators. With the Geiger tube acting as a count rate monitor, the detectors recorded radiation dose rates on the surface and at depths of 0.25 inch and 1.0 inch in the plastic between 1 millirad per hour and 100 millirads per hour.741

The first flight attempt was made on 29 September 1961, but this flight (1-A-38) aborted prior to launch due to a flight-control anomaly in the X-15. The package successfully flew on 4 October 1961 with Bob Rushworth at the controls of flight 1-23-39. The experiment subsequently flew several more times during late 1961. After each flight the taped data went to Kirtland for analysis, and the results ultimately showed that the pilots received essentially a normal background dosage of radiation (0.5 millirads per hour) during the flights. Since there seemed to be no cause for concern, the Air Force deleted the radiation detector from flights beginning in 1962.75

The program flew a different radiation experiment on X-15-2 from early 1961 until September 1963. The "Earth cosmic-ray albedo" experiment investigated the cosmic-ray environment at altitudes from 50,000 feet upward to determine the cosmic-ray environment in which future manned space vehicles would operate. The experiment consisted of placing small photographic

emulsion stacks in upper and lower structural (i. e., bug-eye) camera bays to obtain information on the cosmic-ray albedo flux and spectrum, as well as the flux and spectra of electrons and protons leaking out of the Van Allen belts. The X-15-2 carried the stacks to high altitudes on as many flights as practical and placed no restrictions on the flight path or trajectory. The NB-52 carried similar stacks during the same flights to provide lower-altitude references. Researchers at the University of Miami and the University of California at Los Angeles and Berkeley analyzed the film from the stacks.[76]

In a very similar experiment, X-15-1 and X-15-2 carried two nuclear-emulsion cosmic-radiation measurement packages from the Goddard Space Flight Center on the aft ends of their side fairings to investigate the cosmic-radiation environment at extreme altitudes. These emulsion stacks were considerably larger than the Earth cosmic-ray albedo stacks and were located external to the airplane. Several flights carried the packages to altitudes above 150,000 feet. The packages required no special maneuvers and no servicing other than installation just prior to flight, and removal after landing.-177

Perhaps the most unusual concept involving the use of X-15s was also the one that should have made the program most thankful it was never implemented. During the late 1950s and early 1960s, the Air Force investigated a single-stage-to-orbit concept called Aerospaceplane (not to be confused with the later National Aero-Space Plane (NASP)). The vehicles explored during this program included some very exotic propulsion concepts, such as LACES and ACES, that extracted oxygen from the atmosphere during ascent and used it once the vehicle left the sensible atmosphere.-13213

Most of the contractors involved in the program performed parametric evaluations of conventional concepts that carried all of the propellants from the ground-termed "propellants onboard at takeoff" (POBATO)-in addition to the air-collection schemes. However, an even more bizarre concept was called the "hypersonic in-flight refueling system" (HIRES), and designers at Convair, Douglas, and North American each considered trying to refuel the Aerospaceplane in flight at Mach 6. This concept actually advanced far enough that the Air Force and NASA had preliminary discussions about using two X-15s flying in formation to validate the idea. The logistics of getting two X-15s in formation would have been formidable, and the piloting task daunting. On two separate occasions the X-15 program attempted to fly two flights in a single day (but not at the same time, since the High Range could not support the concept), and each time one of the X – 15s had a system problem that led to the flight being scrubbed. Fortunately for the X-15 program, the refueling demonstration was never attempted.13221

Bob Rushworth flew the X-15 for 68 months from 4 November 1960 until 1 July 1966, making 34 flights. These included two flights with the XLR11 and 32 flights with the XLR99. Rushworth reached Mach 6.06, a maximum speed of 4,018 mph, and an altitude of 285,000 feet. His accomplishments include the first ventral-off flight, the maximum dynamic-pressure flight, the maximum temperature flight, the maximum Mach number (6.06) in the basic X-15, the first flight of X-15A-2, and the first flight with external tanks.

Robert Aitken Rushworth was born on 9 October 1924 in Madison, Maine. He joined the Army Air Forces, flying C-46 and C-47 transports in World War II and later combat missions in Korea. In 1943 he graduated from Hebron Academy, Maine. He received bachelor of science degrees in mechanical engineering from the University of Maine in 1951 and in aeronautical engineering from the Air Force Institute of Technology in 1954. He graduated from the National War College at Fort Lesley J. McNair, Washington, D. C., in 1967.[22]

Rushworth began his flight-test career at Wright Field and transferred to Edwards in 1956. Following graduation from the Experimental Test Pilot School, Rushworth reported to the fighter operations branch at Edwards and later became operations officer in the manned spacecraft section while flying the X-15. Prior to flying the X-15, Rushworth flew the F-101, TF-102, F-104, F-105, and F-106. He received the Distinguished Flying Cross for an emergency recovery of the X-15 after premature extension of the nose gear at near Mach 5 speeds, and the Legion of Merit for overall accomplishments in the national interest of initial space flights.-1231

He graduated from the National War College in August 1967 and attended F-4 Phantom II combat crew training at George AFB. In March 1968, Rushworth went to Cam Ranh Bay Air Base in Vietnam as the assistant deputy commander for operations with the 12th TFW and flew 189 combat missions. From April 1969 to January 1971, he was program director for the AGM-65 Maverick, and in February 1971 he became commander of the 4950th Test Wing at Wright-Patterson AFB. General Rushworth served as the inspector general for the Air Force systems command from May 1973 to February 1974 and returned to the AFFTC as commander until November 1975, when he became commander of the Air Force Test and Evaluation Center at Kirtland AFB, New Mexico.

Rushworth retired from the Air Force in 1981 as vice commander of the Aeronautical Systems Division at Wright-Patterson AFB. Bob Rushworth died of a heart attack on 18 March 1993 in Camarillo, California.-1241

The first ground-test XLR99 (s/n 101) arrived at Edwards on 7 June 1959, and the first hot test was accomplished without an actual X-15 at the Rocket Engine Test Facility on 26 August 1959. X-15-3 arrived at Edwards on 29 June 1959. It was essentially identical to the other two airplanes in that it was equipped with a standard Westinghouse stability augmentation system, a stable platform, and a normal cockpit instrument panel. What made it different, at this point, was that it had the XLR99 engine. X-15-3 was never equipped with the XLR11 engines. At the same time, North American removed the second X-15 from flight status after its ninth flight (2-9-18) on 26 April 1960 in anticipation of replacing the XLR11 engines with the XLR99. This left only X-15-1 on active flight status, although the XLR99-powered X-15-3 would soon be joining it.[90]

North American made the first ground run with the XLR99 in X-15-3 on 2 June 1960 at the PSTS. Subsequent inspection revealed damage to the liquid-oxygen inlet line brackets, the result of an unexpected, but easily corrected, water-hammer effect. After repairs were completed, the company conducted another ground run with satisfactory results. For all of the ground runs during the program, a pilot had to be in the cockpit since the nearby blockhouse could not operate the engine by remote control. For the early tests, the pilot was Scott Crossfield, although all of the pilots would participate in ground runs prior to their first flights. The MC-2 full – pressure suit was an order of magnitude more comfortable than earlier pressure suits, but Crossfield still had little desire to wear it more than necessary. Since there was no need for altitude protection during the engine runs on the ground, Crossfield generally wore street clothes in the cockpit. All other personnel required for the tests were in the blockhouse, with the exception of Air Force fire crews a relatively safe distance away.-191

The third ground run began on 8 June at approximately 1930 hours. The objectives were to demonstrate the restart capability and throttling characteristics of the XLR99. All pre-test operations, servicing, and APU starts were successful and all systems were operating normally.

The engine was primed, set to idle, and then ignited at 50% thrust. After the chamber pressure stabilized for 7 seconds, Crossfield advanced the throttle to 100% for 5 seconds and then moved the throttle to idle for 5 seconds before shutting down the engine. Nobody noted anything abnormal during these events. After 15 seconds, Crossfield moved the throttle to the 50% position. The turbopump started normally, first-and second-stage ignition occurred, and the main chamber start appeared normal. After the main chamber pressure stabilized, it rapidly fell off and the engine shut down automatically. At this time, a valve malfunction light came on in the cockpit, so Crossfield moved the throttle to the off position and the light went out. In order to

restart the XLR99 after a malfunction shutdown, the pilot had to push a switch that reset the automatic safety devices. As Crossfield wrote in his accident statement, "the reset button was depressed at which time the airplane blew up." It was approximately 1945 hours.-192

Crossfield later observed, "During this entire sequence except for the malfunction shut down, there was no evidence in the cockpit of difficulty." The explosion appeared to be centered forward of the engine compartment, and caused the aircraft to separate around fuselage station 483.5, just forward of the liquid-oxygen tank. Don Richter, who was in the main blockhouse, indicated that he observed the explosion originating 5 feet forward from the aft end of the airplane, with the fireball quickly expanding to about 30 feet in diameter.-192

The explosion threw the entire forward fuselage about 30 feet forward. Crossfield said, "In the explosion, which is not describable, the cockpit translated abruptly forward and to the right with an acceleration beyond the experience of this pilot." The basic X-15 airframe had been designed – largely at Crossfield’s urging-to protect the pilot in case of an emergency; it appeared to work well. Ever the competent test pilot, Crossfield turned on his Scott airpack, turned off the engine switches, and pulled all the circuit breakers. He attempted to contact personnel inside the blockhouse, but the explosion had severed communications with the ground.-194

The fire truck that had been standing by was on the scene within 30 seconds, water pouring from its overhead nozzle, and a second fire truck arrived a minute or two later to help extinguish the fires. Art Simone and a suited fireman rushed to the cockpit and Crossfield was rescued uninjured. Simone had inhaled ammonia fumes and received minor burns to his hands, but suffered no lasting effects. The fires were largely out within a few minutes of the explosion, and Crossfield was safe. It was time to figure out what had happened.-192

Representatives from the Edwards provost marshal’s office, the North American industrial security office, and the Edwards air police arrived on the scene and roped off the area pending an investigation. Around 2110 hours, North American photographer Stan Brusto arrived to photograph the wreckage; after this was complete, the Air Force removed the data recorders from the aircraft for analysis. The air police withdrew after putting into place procedures to limit access to the area, leaving one fire truck on standby just in case. Personnel spent the next 24 hours finding all the bits and pieces blown from the aircraft, tagging them, and preparing to move the remains of the aircraft back to Inglewood. Major Arthur Murray from the X-15 project office authorized the move on 10 June.-196

Engineers removed the XLR99 from the wreckage on 13 June and took it to hangar 1870 at Edwards for inspection. North American transported the remains of X-15-3 by truck from Edwards on 15 June, parking overnight at the intersection of Sepulveda and San Bernardo Road before continuing on to Inglewood on 16 June.-192 By 4 August the company had assessed the damage and determined that the airframe would have to be replaced from fuselage station 331.9 aft. The dorsal and ventral stabilizers, all four speed brakes, both horizontal stabilizers, both main landing skids, and both propellant tanks would be replaced. The company considered the wings repairable, as were the APUs and stable platform. All of the miscellaneous equipment in the rear and center fuselage, along with most of the research instrumentation in the aft fuselage, also required replacement. Reaction Motors did not consider the XLR99 repairable, although the company salvaged some parts for future use. The Rocket Engine Test Facility required major repairs, but was back on line by the end of June.-192

Force had incorporated a vapor-disposal system into the PSTS to allow the ammonia fumes to be vented from the airplane during engine testing without endangering people. Essentially the disposal system consisted of a 90-foot pipe that connected the airplane ammonia vent to a water pond where the ammonia was diluted. At the time of the explosion, the ammonia tank pressurizing gas regulator froze or stuck in the open position while the vent valve was operating erratically or modulating only partially open. North American had considered this condition a potential failure on the airplane itself, and had addressed the problem during development. However, when combined with the back pressure created by the vapor-disposal system attached to the ammonia vent, the tank pressure surged high enough to rupture the tank. In the process, debris damaged the hydrogen-peroxide tank, and the mixing of the peroxide and ammonia caused an explosion.-199

Post-accident analysis indicated that there were no serious design flaws in either the XLR99 or the X-15. The Air Force determined that the cause of the accident was a failure of the pressure regulator, exacerbated by the unique configuration required for the ground test. Nevertheless, North American devised several modifications to preclude similar failures in the future. These included redesigning the pressurizing gas regulator to reduce maximum flow through an inoperable regulator, providing the regulator with additional closing forces in the event of freezing, relocating the regulator to minimize the chances of moisture accumulation and subsequent freezing, and redesigning the relief valve and its surrounding plumbing.-11"

Rebuilding the aircraft was not as straightforward as it sounded. Besides the estimated $4.75 million cost, there would be a considerable delay in obtaining suitable replacement parts. The X – 15s were not mass-produced items, and structural spares were nonexistent. The time required to repair the airplane meant it would miss most of the envelope expansion program and was, therefore, somewhat redundant.

There had been considerable interest in testing a new Minneapolis-Honeywell MH-96 adaptive flight control system in a high-speed vehicle prior to its use on the X-20 Dyna-Soar. Given the unfortunate event, the Air Force took this opportunity to modify X-15-3 to include the system. Complicating this was the fact the X-15 program was operating under a "reduced budget"-$8.6 million for FY61 instead of the $10.5 million that had been requested. However, the X-15 program still enjoyed considerable support within the Pentagon, and in early August, Air Force Headquarters authorized the ARDC to release $1 million from existing funds (i. e., the $8.6 million) to cover the procurement of long lead items needed for the repairs. The remaining $3.75 million, along with the restoration of the $1.9 million removed from the program earlier, was to follow "at a later date." In the interim, the Pentagon directed the X-15 program to "operate on a fiscal 1961 schedule compatible with… funds of $10.5 million plus an additional $4.75 million to cover the repair of the damaged aircraft." Although money never came easily for the X-15 program, it always came.-1"

Scott Crossfield was at the controls of X-15-3 when it suffered a catastrophic explosion during a ground run of the XLR99. Fortunately, Cross field was not injured. Subsequent investigation showed there was nothing wrong with either the engine or airframe design and that the explosion had been caused by the failure of a minor component and the unique configuration required for ground testing. The X-15-3 was subsequently rebuilt to include the advanced MH-96 adaptive flight control system. (AFFTC History Office)

On 10 August, the Air Materiel Command requested that North American submit an estimate for the repair of X-15-3. Twelve days later the Air Materiel Command ordered the repair using the $1 million authorized by the Pentagon, and North American proceeded with the work. The company estimated that the aircraft could be completed in August 1961 and available for flight in October. The Pentagon came through at the end of March 1961, funding the X-15 program at $15.25 million-the original $10.5 million request plus the cost of rebuilding the damaged airplane.-1102!

The new money allowed the AFFTC to increase the propellants it had ordered because of 1) the high consumption of propellants required for component testing at the PSTS, 2) the high level of development testing of the APU and ballistic control system, and 3) the increased development testing of the XLR99 at the Rocket Engine Test Facility. A quick review shows the quantities and costs involved in this usually overlooked matter;!103!

At Inglewood, North American was installing the XLR99 in X-15-2 and incorporating several other changes at the same time. These included a revised vent system in the fuel tanks as an additional precaution against another explosion, revised ballistic control system components, and provisions to install the ball nose instead of the flight-test boom used so far in the program. The company had been looking to conduct the first flight in early September, but discovered corrosion in the engine hydrogen-peroxide tank. While North American was taking care of the corrosion, Reaction Motors tore down one of the ground-test engines to determine the condition of the individual components after 2 hours of engine operation. The inspection revealed no outstanding deficiencies or indications of excessive wear, clearing the way for the first X-15 flight with the million-horsepower engine.-1104

The installation of the ball nose presented its own challenges since it had no capability to determine airspeed. The possibility of a failure in the ball-nose steering mechanism also made it unsuitable as a total-pressure port to derive airspeed. The X-15 was designed with an alternate airspeed probe just forward of the cockpit, although two other locations-one well forward on the bottom centerline of the aircraft, and one somewhat aft near the centerline-had also been considered. Several early flights compared the data available from each location, while relying on the data provided by the airspeed sensors on the flight-test boom protruding from the extreme nose. The primary location exhibited some velocity-indication sensitivity at speeds over 345 mph and angles of attack over 4 degrees. At 8 degrees alpha the indicated airspeeds were generally about 25 mph low. The tests indicated that the data from all three locations were about the same, so the engineers decided to retain the original location. An interesting discovery was that the error was substantially less after the ball nose was installed, which led to a theory that the extended nose boom was contributing to the errors. Fortunately, the airspeed indications were consistent at the speeds and angles of attack encountered in the landing pattern, so researchers simply adjusted the instruments to compensate. After NASA installed the ball nose, engineers compared angle-of-attack data (based on the horizontal stabilizer position) with those from previous flights using the flight-test boom. The data were generally in good agreement, clearing the way for operational use of the ball nose.-105

From the Martin Marietta post-flight report: "The actual flight environment in the area of the modified ventral fin proved to be much more severe than anticipated. The condition was directly attributable to interaction effects of the shock waves generated by the dummy ramjet, the ventral, and the pressure probes. The ablator applications in this area were inadequate to protect the structure under these flow conditions, and the vehicle suffered localized damage in the area."!330!

The flight had completely eroded the ablator application, including both the molded leading-edge detail and the sprayed MA-25S layer, from the forward portion of the ventral. The vehicle skin sustained major damage due to the excessively high heating in the shock impingement, which burned through at the leading edge and on the sides of the ventral at the torque box assembly. This also damaged the torque box and destroyed the wiring and pressure lines in the forward compartment.-!3331

A study of the thermocouple responses in the area of the ventral indicated that the ablator had provided at least some protection for the first 140 seconds of flight. Continual erosion of the ablator surfaces was occurring during this period, and by approximately 160 seconds the degradation was such that all protection broke down. The ablator materials should have had zero surface recession, but instead eroded away. The particles from the forward sections of ablator, in turn, caused severe impact erosion of the downstream ablator layer. The lower speed brakes were bare of ablator, and the material on the inboard edges of the main landing skids and the undersides of the side fairings experienced considerable abrasion.[352]

Otherwise, the ablator had performed well enough. The flight had uniformly charred the details over the leading edges of the wings, horizontal stabilizers, canopy, and dorsal stabilizer along their lengths. All of the parts had retained their shape, and the char layer attachment was firm. There were some signs of localized surface melt in areas of shock impingement during peak heating, but because of a continually varying velocity during the flight, shock presence in any one area was limited and the degradation was "insignificant." The nose-up trim attitude degraded the lower surface of the wing details more heavily than the upper surfaces; the reverse was true for the horizontal stabilizers.-1353

The lower, fixed portion of the dorsal stabilizer leading edge charred more heavily than the upper, movable rudder, and some evidence of unsymmetrical heating of the rudder was present, with the left side sustaining a higher heat load. The ablator details for both vane antennas were heavily charred and experienced local erosion or spallation of the char from their surface. They looked worse than they were; measurements showed that more than half an inch of ablator remained on the antennas, which were undamaged in any case.[354]

The sprayed MA-25S layer over the fuselage and side fairings showed varying degrees of effects. Thermal degradation, with the resultant reticulation of the ablator surface, occurred only on the forward areas of the nose. Ablator fissuring extended along the fuselage belly to approximately the forward vane antenna. The ablator on the crown of the fuselage and the belly aft of the vane antenna showed no evidence of thermal exposure.[355]

Engineers could easily correlate the varying amounts of charring experienced over the fuselage with their location or proximity to the various design features of the airplane. For instance, heavily charred areas were directly behind the pressure orifices in the ball nose. These openings were apparently sufficient to "trip" the flow, causing a rapid transition to turbulent boundary-layer conditions. The holes for the ballistic control-system thrusters greatly increased heating effects in their vicinity. Localized stagnation within the recesses apparently permitted burning of the ablator, evidenced by a surface discoloration. The thickness of the material behind the nose-gear door was seriously degraded.-1356

The various stacks and vents protruding from the airplane caused localized heating problems. Stagnation shock and trailing-wake damage were evident downstream from an external tank disconnect door that failed to close after the tanks were jettisoned. The ablator surface on the lower wing experienced varying degrees of charring over the whole area. This was heaviest adjacent to the molded leading edges, and some blistering was evident near the wing tips. However, the upper wing surfaces thermally degraded only near the leading edge details; the remainder of the surface was unaffected. Again, the ablator on the upper surface of the horizontal stabilizers degraded more heavily than the lower surfaces. Along the inboard edge of the stabilizers, next to the side fairings, sections of ablator were missing from both the top and bottom surfaces, forward of the torque tube. The open cavity of the stabilizer’s inboard closing rib and the adjacent fairing formed a channel to trap the airflow during flight. This resulted in severe heating within the cavity and caused degradation of the ablator from the back face.[357]

In addition to the thermal degradation, the stabilizer upper surfaces sustained a significant amount of impact damage. Some of the abrasions obviously occurred during landing since the exposed ablator was virgin material, while others had occurred early in flight and the exposed ablator had become charred. Engineers thought the likely cause was spallation of small pieces of upstream ablator of fluid droplets from the various vents and drains. As expected, ablation residue partially covered the unprotected right-hand windshield.13581

Pull tests were conducted at random locations on the surface of the ablator to determine whether it was still well bonded to the airplane. The results were generally acceptable. In the end, Martin Marietta believed that the ablator "performed satisfactorily except in the area of the modified ventral fin." Nevertheless, Martin went on to suggest a series of minor modifications that would solve some of the problems experienced on these two flights.-13591

The ablator obviously was not completely successful. Unexpectedly, the ablator actually prevented cooling of the airframe by preventing heat from absorbing into the underlying hot structure. The post-flight condition of the airplane was a surprise to Jack Kolf, an X0-15 project engineer at the FRC, who noted, "If there had been any question that the airplane was going to come back in that shape, we never would have flown it."13601

Engineers had not fully considered possible shock interaction with the ramjet shape at hypersonic speeds. As it turned out, the flow patterns were such that a tremendous shock wave impinged on the ramjet and its supporting structure. Researchers later estimated that the heat in the ventral stabilizer was 10 times higher than normal. The warning signs had been there in various wind – tunnel tests and previous flights, but researchers had not recognized them.13611

It is interesting to note that post-flight photographs of the X-15A-2 damage normally highlight two areas. The first is the ventral stabilizer and ramjet. Heating effects unquestionably damaged this area, although there had been indications on the previous flight that something was not right. The second area shown is the large fissures around the nose. When NASA replaced the ball nose before this flight, it used an outdated batch of MA-25S because it was all that was available. Although its application characteristics, cure rate, and appearance were the same as those of the "fresh" ablator used elsewhere, thermal exposure resulted in a greater shrink rate than the newer material. This produced much more pronounced fissuring, but it appeared that the ablator provided sufficient protection.13621

The original contract with Martin Marietta indicated the company was responsible for "touching up" the ablator twice to allow three flights with the initial application. The damage sustained by the ventral stabilizer precluded the aircraft from flying again in the near future. Consequently, the Air Force directed Martin to remove the ablator so that it could return the aircraft to North American for inspection and repair. NASA technicians under the direction of a Martin engineer, however, performed the actual removal. The technicians removed the MA-25S-1 strips from the service panel peripheries, cleaned the panel edges, and then applied polyethylene tape to protect the aircraft interior from contamination. They stripped the ablator layer using plexiglass scrapers and scrubbed the surface to remove all residual ablator material. The final cleaning was performed with aluminum wool and nylon pads with powdered cleanser, and wooden toothpicks proved useful for dislodging the ablator material from skin gaps and the heads of permanent fasteners.13631

This is the ventral stabilizer after Flight 2-53-97; the ramjet had fallen off during landing. The X – 15A-2 skin sustained major damage due to the excessively high heating in the shock impingement, which burned through at the leading edge and on the sides of the ventral at the torque box assembly. This also damaged the torque box and destroyed the wiring and pressure lines in the forward compartment. (NASA)

NASA sent X-15A-2 to North American for repair and general maintenance. The airplane returned to Edwards on 27 June 1968, and a series of nondestructive load and thermal tests on the instrumented right wing began on 15 July in the FRC High Temperature Loads Calibration Laboratory. As it turned out, the airplane would never fly again.-1364-

Some of the problems encountered with the ablator were non-representative of possible future uses. North American had designed the X-15 with an uninsulated hot structure, but researchers expected to design any future vehicle with a more conventional airframe that would eliminate some of the problems encountered on this flight. However, other problems were very real. The amount of time it took to apply the ablator was unacceptable. Even considering that after they gained some experience the technicians could cut the application time in half or even more, the six weeks it took to coat the relatively small X-15 bode ill for larger vehicles.-1365-

This is the nose of X-15A-2 after Flight 2-53-97. NASA had replaced the ball nose before this flight because of a maintenance issue, and had used an outdated batch of MA-25S to patch the area because it was all that was available. Although its application characteristics, cure rate, and appearance were the same as the "fresh" ablator used elsewhere, thermal exposure resulted in a greater shrink rate than the newer material. This produced much more pronounced fissuring, but analysis indicated that the ablator provided sufficient protection, despite appearances. (NASA)

The use of an ablative coating on X-15A-2 came at an interesting time. The development of what became the space shuttle was just beginning, with various study efforts being initiated under the auspices of NASA and the Air Force. It was obvious that some sort of reusable thermal protection system was going to be required on a space shuttle, and a great deal of attention initially turned to ablatives because they were the most mature technology available at the time. The experience with the X-15 provided very meaningful insights into the problems that the space shuttle undoubtedly would have encountered using this technology. Nevertheless, various contractors continued to propose the use of ablators on their space shuttle concepts, in decreasing quantity, until 1970 when several forms of ceramic tiles and metal "shingles" became the preferred concepts. Based at least partially on the results of the X-15 tests, the space shuttle program decided to go down a different road; whether that road was truly superior is open to debate. At least it represented a different set of problems.

At 10:30:07.4 on 15 November 1967, X-15-3 dropped away from the NB-52B 45,000 feet over Delamar Dry Lake. Major Michael J. Adams was at the controls, making his seventh X-15 flight. Adams had spent slightly over 23 hours in the fixed-base simulator practicing this particular mission (3-67-95), which was intended to evaluate the Ames boost guidance display and conduct several experiments, including measuring the ultraviolet plume of rocket exhausts at high altitude. About 1 minute after launch, as X-15-3 passed through 85,000 feet, an electrical disturbance caused the MH-96 dampers to trip out. It was later determined the disturbance most probably had emanated from electrical arcing in the experiment in the nose of the right wing-tip pod that was being flown for the first time. Adams reset the dampers and continued.-366

As planned, Adams switched the cockpit sideslip attitude indicator to an alternate display mode. One of the more controversial aspects of the attitude indicator was a second use for the cross­pointers, which were developed late in the program to allow precise pointing of several experiments. In this mode the cross-pointers displayed vernier attitude errors (pitch error on the alpha needle, and bank error on the beta needle). A switch allowed the pilot to control the display mode. During the climb, the pilot switched the display to the vernier-attitude-error mode, and would normally have switched back to the sideslip mode prior to reentry.367-

Unlike the other two airplanes, X-15-3 automatically blended the ballistic control-system thrusters with the aerodynamic controls as needed using the right side stick, allowing the pilot to largely ignore the dedicated ballistic controller on the left. The electrical disturbances fooled the flight-control system into believing that the dynamic pressure was higher than it actually was, resulting in the system failing to engage the ballistic control system as would normally occur at high altitude. Adams felt the lack of response as the airplane approached maximum altitude and began using the left side stick to operate the thrusters. Unfortunately, Adams reverted to flying the vertical needle on the attitude indicator as if it were still showing sideslip instead of its actual vernier-attitude-error display.366

Pete Knight was NASA-1 on the ground. As the X-15 climbed after engine shutdown, Adams initiated a wing-rocking maneuver to sweep the ultraviolet plume experiment up and down across the horizon. Because Adams was apparently interpreting the attitude indicator incorrectly, he began rocking the wings excessively. After Adams stopped the wing rocking, the X-15 began to drift toward its peak altitude, flying with a 15-degree sideslip to the right. As Adams descended, the drift began again and X-15-3 yawed at a right angle to the flight path. The airplane entered a hypersonic spin as it encountered rapidly increasing dynamic pressure at 130,000 feet.369

The designers of the NASA control room had not thought to provide a heading indication, so the controllers were unaware of the attitude of the airplane. Everybody knew the ball nose did not accurately align with the relative wind at altitudes above 250,000 feet, so there was little concern when the angle of attack and angle of sideslip began drifting off nominal values near peak altitude. In reality, the airplane was yawing wildly, eventually turning completely around. Fifteen seconds later Adams reported that the airplane "seems squirrelly" and at 1034 hours he advised, "I’m in a spin, Pete." Adams radioed again, "I’m in a spin," followed by groans as the pilot was subjected to heavy accelerations. Engineers knew very little about the hypersonic spin characteristics of the X-15, and there was no recommended spin recovery technique.-1370-

At 10:30:07.4 on 15 November 1967, X-15-3 dropped away from the NB-52B 45,000 feet over Deiamar Dry Lake with Major Michael J. Adams at the controls. Technical problems combined with possible piloting issues caused the X-15-3 to break-up at approximately 62,000 feet with a velocity of about 3,800 fps and a dynamic pressure of 1,300 psf. The wreckage fell near Johannesburg, California. (NASA)

Realizing that X-15-3 would never make it back to Rogers Dry Lake, the chase pilots-Hugh M. Jackson and Bill Dana-shoved their F-104s into afterburner and raced for Ballarat and Cuddeback, the most likely emergency landing lakes. In the X-15, Adams used the combined power of the aerodynamic and ballistic controls against the spin. Eventually, largely through a weathervane effect, the airplane recovered at 120,000 feet and 140 psf. It then entered an inverted Mach 4.7 dive at an angle of nearly 45 degrees. At this point, it looked like Adams might pull out of the dive. However, a technical problem emerged as the MH-96 entered a limit-cycle oscillation when the airplane emerged from the spin. This prevented the system from reducing the pitch gain in response to the increasing dynamic pressure. While descending at over 2,700 fps, the X-15 began to exhibit an increasingly severe rapid pitching motion. The severe oscillations in the flight – control system effectively blocked pilot inputs. As it passed through 65,000 feet, X-15-3 was descending in an inverted dive at Mach 3.93 and approaching both the side-load and normal-load limits. At 1034:57.5, the airplane broke up at approximately 62,000 feet with a velocity of about 3,800 fps and a dynamic pressure of 1,300 psf. An Air Force pilot spotted the wreckage near the town of Johannesburg. Mike Adams was dead, and X-15-3 was destroyed.-1371!

accident, an unofficial search party from the FRC found the cockpit camera but not the film cartridge. Since the film cassette was lighter than the camera, engineers theorized that the cassette must have been blown north by winds at altitude. A search party organized by Victor Horton converged on the area on 29 November, and Willard E. Dives found the cassette. The film was flown to the EG&G laboratory in Boston for processing.

Johnny Armstrong and Jack Kolf began analyzing the cockpit film when it returned. Armstrong later recalled, "We had the time history from the flight recorded in the control room. We could see the vertical needle on the attitude indicator in the film and correlated the time of the film and the recorded time history. It became clear to us that the pilot was making manual ballistic inputs as if the vertical needle was sideslip rather than roll angle. His inputs were in the correct direction to make sideslip zero if it had been sideslip. However since it was roll angle his inputs drove the nose further from away from the flight path and eventually into… a spin."372

Mike Adams flew the X-15 for 13 months from 6 October 1966 until 15 November 1967, making seven flights. All of these were with the XLR99 engine and he reached Mach 5.59, a maximum speed of 3,822 mph, and an altitude of 266,000 feet. Adams died on Flight 3-65-97. The Air Force posthumously awarded Mike Adams an astronaut rating for his last flight in X-15-3, which had attained an altitude of 266,000 feet (50.38 miles). This was the only fatality during the program’s 199 flights. (NASA)

The accident board concluded that Adams misinterpreted his instruments, and combined with distraction and possible vertigo, this led him to allow the heading of the X-15-3 to deviate unexpectedly. The overall effectiveness of the MH-96 had been degraded by the electrical disturbance early in the flight, further adding to the pilot’s workload. The MH-96 then caused the airplane to break up. 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. Because of the crash, NASA added an attitude indicator in the control room to display real-time heading, pitch, roll, sideslip, and angle-of – attack information. Although it was not specifically called out in the accident report, many engineers came away with a more important lesson: do not use the same instrument to display multiple different indications in a high-workload or high-stress environment.-1373

The Air Force posthumously awarded Mike Adams an astronaut rating for his last flight in X-15- 3, which had attained an altitude of 266,000 feet (50.38 miles). This was the only fatality that occurred during the program’s 199 flights.-1374!

Researchers considered X-ray photographs important for understanding the problems of the solar atmosphere, which led to the "X-ray mapping of the sun" experiment. Instruments on sounding rockets had obtained similar photographs of the sun, but the excessive motion of the vehicle had greatly complicated measurements. NASA installed a small pinhole camera in one of the upper bug-eye camera bays on the X-15 in January 1962. This experiment flew above 150,000 feet several times between March 1962 and September 1963.[78]

The "electron-distribution determination" experiment measured electron distribution in the upper atmosphere using radiofrequency techniques. These measurements of the ionosphere D-layer (often as low as 50 miles) were important for investigators seeking to gain a basic understanding of the ionosphere. Since the temporal variation of the electron distribution was important, a series of flights was desirable; however, there appears to be no record indicating that the experiment actually flew or acquired any useful data.[79]