Category Hypersonics Before. the Shuttle

Other Systems

In early 1958, at the very height of the furor over the problems with the XLR99, a note of warning sounded for the General Electric auxiliary power unit (APU). On 26 March 1958 and again on 11 April 1958, General Electric notified North American of its inability to meet the original specifications in the time available, and requested approval
of new specifications. North American, with the concurrence of the Air Force, agreed to modify the requirements. The major changes involved an increase in weight from 40 to 48 pounds, an increase in start time from five to seven seconds, and a revision of the specific fuel consumption curves.50

By the end of the summer 1958, the APU seemed to have reached a more satisfactory state of development, and production units were ready for shipment.51 The early captive flights beginning in 1959 would reveal some additional problems, but investigation showed that the in-flight failures had occurred partial­ly because captive testing subjected the units to an abnormal operational sequence that would not be encountered during glide and powered flight. Some components were redesigned, but the APU would continue to be relatively troublesome in actual service.

During the course of the X-l 5 program, many concerns were voiced over the development of a pressure suit and an escape system. Although full-pressure suits had been studied during World War II, attempts to fabricate a practical garment had met with failure. The

Other Systems

Soule to Storms: “You have a little airplane and a big engine with a large thrust margin."

And indeed they did. The XLR99 provided

57.0 pounds-thrust to propel an aircraft that only weighed

30.0 pounds. Consider that the con­temporary F-104 Starfighter, considered something of a hot rod, weighed 20,000 pounds and its J79 only produced 15,000 pounds-thrust in full afterburner. (NASA)











20 30 40 50 60


80 90


Other Systems

Air Force took renewed interest in pressure suits in 1954 when it had become obvious that the increasing performance of aircraft was going to necessitate such a garment. The first result of the renewed interest was the creation of a suit that was heavy, bulky, and unwieldy; the garment had only limited mobility and various joints created painful pressure points. However, in 1955 the David Clark Company succeeded in producing a garment using a distorted-angle fabric that held some promise of ultimate success.51

Despite the early state-of-development of full-pressure suits, Scott Crossfield was con­vinced they were the way to go for X-l5. North American’s detail specifications of 2 March 1956 called for just such a garment— to be furnished by North American through a subcontract with the David Clark Company.55 A positive step toward Air Force acceptance of the idea occurred during a conference held at the North American plant on 20-22 June 1956. A full-pressure suit developed by the Navy was demonstrated during an inspection of the preliminary cockpit mockup, and although the suit still had a number of defi­ciencies, it was concluded that “… the state – of-the-art on full pressure suits should permit the development of such a suit satisfactory for use in the X-15.”54

After an extremely difficult and prolonged development process, Scott Crossfield received the first new MC-2 full-pressure suit on 17 December 1958 and, two days later, the suit successfully passed nitrogen contamina­tion tests at the Air Force Aero Medical Laboratory. The X-15 project officer attrib­uted much of the credit for the successful and timely qualification of the full-pressure suit to the intensive efforts of Crossfield.55

Fortunately, development did not stop there. On 27 July 1959, the Aero Medical Laboratory brought the first of the new A/P22S-2 pressure suits to Edwards. The consensus amongst the pilots was that it rep­resented a large improvement over the earli­er MC-2. It was more comfortable and pro­vided greater mobility; and it took only 5 minutes to put on, compared to 30 minutes for the MC-2. However, it would take anoth­er year before fully-qualified versions of the suit were delivered to the X-15 program.56

While not directly related to the pressure suit difficulties, the type of escape system to be used in the X-15 had been the subject of debate at an early stage of the program; the decision to use the stable-seat, full-pressure – suit combination had been a compromise based largely on the fact that the ejection seat was lighter and offered fewer complications than the other alternatives.

As early as 8 February 1955, the Aero Medical Laboratory had recommended a cap­sular escape system, but the laboratory had also admitted that such a system would prob­ably require extensive development. The sec­ond choice was a stable seat that incorporated limb retention features and one that would produce a minimum of deceleration.51 During meetings held in October and November 1955, it was agreed that North American would design an ejection seat for the X-15 and would also prepare a report justifying the use of such a system in preference to a capsule. North American was to incorporate head and limb restraints in the proposed seat.58

Despite the report, the Air Force was not completely convinced. At a meeting held at Wright Field on 2-3 May 1956, the Air Force again pointed out the limitations of ejection seats. In the opinion of one NACA engineer who attended the meeting, the Air Force was still strongly in favor of a capsule—partly because of the additional safety a capsule system would offer, and partly because the use of such a system in the X-15 would pro­vide an opportunity for further developmen­tal research. Primarily due to the efforts of Scott Crossfield, the participants finally agreed that because of the “time factor, weight, ignorance about proper capsule design, and the safety features being built into the airplane structure itself, the X-15 was probably its own best capsule.” About

the only result of the reluctance of the Air Force to endorse an ejection seat was a request that North American yet again docu­ment the arguments for the seat.59

The death of Captain Milbum G. Apt in the crash of the Bell X-2, which had been equipped with an escape capsule, in September 1956 renewed apprehension as to the adequacy of the X-15’s escape system.® By this time, however, it was acknowledged that no substantive changes could be made to the X-15 design. Fortunately, North American’s seat development efforts were generally proceeding well.’’1

Sled tests of the ejection seat began early in 1958 at Edwards with the preliminary tests concluded on 22 April. Because of the high cost of sled runs, the X-15 project office advised North American to eliminate the planned incremental testing and to conduct the tests at just two pressure levels—125 pounds per square foot and 1,500 pounds per square foot. The X-15 office felt that suc­cessful tests at these two levels would fur­nish adequate proof of seat reliability at intermediate pressures.62

Between 4 June 1958 and 3 March 1959, the X-15 seat completed its series of sled tests. Various problems, with both the seat and the sled, had been encountered, but all had been worked through to the satisfaction of North American and the Air Force. The X-15 seat was cleared for flight use.62

Another item for which the Air Force retained direct responsibility was the all-attitude iner­tial flight data system. It was realized from the beginning of the X-15 program that the air­plane’s performance would necessitate a new means of determining altitude, speed, and air­craft attitude. This was because the traditional use of static pressure as a reference would be largely impossible at the speeds and altitudes the X-15 would achieve; moreover, the tem­peratures encountered would rule out the use of tradition pitot tube sensing devices. The NACA had proposed a “stable-platform iner­tial-integrating and attitude sensing unit” as the means of meeting these needs.64 A series of miscommunications resulted in the NACA assuming the Air Force had already developed a satisfactory unit and would provide it to the X-15 program.65 After it was discovered that a suitable unit did not exist, emergency efforts were undertaken to develop one without impacting the X-15 program. After a consid­erable amount of controversy, a sole-source contract was awarded to the Sperry Gyroscope Company on 5 June 1957 for the development and manufacture of the stable – platform.66 The cost-plus-fixed-fee contract, signed on 5 June 1957, was for $1,213,518.06 with a fixed fee of $85,000.67

In April 1958, the Air Force concluded that the scheduled delivery of the initial Sperry unit in December would not permit adequate testing to be performed prior to the first flights of the X-15. Consequently a less capa­ble interim gyroscopic system was installed in the first two aircraft and the final Sperry system was installed in the last X-15.68

By the end of 1958, the two major system components (the stabilizer and the computer) were completed and ready to be tested as a complete unit. The systems were shipped to Edwards in late January 1959, and during the spring of 1959 plans were made to use the NB-52 carrier aircraft as a test vehicle.69 In addition, arrangements were made to test the stable-platform in a KC-97 that was already in use as a test aircraft in connection with the B-58 program.™ The first test flights in the KC-97 were carried out in late April.71 By June, North American had successfully installed the Sperry system in the third X-15 22 In January 1961, wiring was installed in the NB-52B to allow the stable-platform to be installed in a pod carried on the pylon under the wing. The first complete stable – platform system installed in the B-52 pod was flown on 1 March 1961, Since the B-52 was capable of greater speeds and higher altitudes than the KC-97, it provided addi­tional data to assist Sperry in resolving prob­lems being encountered with the unit.7’

Resolution Adopted by NACA Committee on Aerodynamics, 5 October 1954 —

Подпись: This resolution was the official beginnings of the X-15 research airplane program.RESOLUTION ADOPTED HI NACA

WHEREAS, The necessity of maintaining supremacy In the «dr continues to place great urgency on solving the problems of flight with man-carrying aircraft at greater speeds and extreme altitudes, and

WHEREAS, Propulsion systems are now capable of propelling such aircraft to speeds and altitudes that impose entirely new and unexplored aircraft design problems, and

WHEREAS, It now appears feasible to construct a research airplane capable of initial exploration of these problems,

3E IT HEREBY RESOLVED, That the Committee on Aerodynamics endorses the proposal of the 1mediate initiation of a project to design and construct a research airplane capable of achieving speeds of the order of Mach Number 7 and altitudes of several hundred thousard feet for the exploration of the problems of stability and control of manned aircraft end aerodynamic heating in the severe form asaociated with flight at extreme speeds and altitudes.

The High Range

Previous rocket aircraft, such as the X-l and X-2, had been able to conduct the majority of their flight research in the skies directly over the Edwards test areas. The capabilities of the X-l5, however, would use vastly more air­space. The proposed trajectories required an essentially straight flight corridor equipped with multiple tracking, telemetry, and com­munications sites, as well as the need for suit­able emergency landing areas. This led to con­struction of the X-l5 High Range extending from Wendover, Utah, to Edwards AFB. Radar and telemetry stations were installed at Ely and Beatty, Nevada, as well as Edwards. Telemetry from the X-l 5, as well as voice communications, were received, recorded, and forwarded to Edwards by the stations at Ely and Beatty. Each of these stations was also manned by a person to back up the prime “communicator” (NASA 1) at Edwards in
case the communication links went down. Each ground station overlapped the next, and they were interconnected via microwave and land-line so that timing signals, voice com­munication, and radar data would be available to all. Provisions were made for recording the acquired data on tape and film, although some of the data was directly displayed on strip and plotting charts. The design and construction of the range was accomplished by Electronic Engineering Company of Los Angeles under contract with the Air Force.74 North American and the NACA also conducted numerous evaluations of various dry lakes to determine which were suitable for emergency landings along the route (see the summary included as an appendix to this monograph).

Carrier Aircraft

The group at Langley had sized their X-l5 proposal around the potential of using a

The use of a B-36 car­rier aircraft would have allowed the pilot to exit the aircraft while in transit to the drop area, or in case of emergency. However, personnel at the FRC worried that the B-36 would not be supportable since it was being phased out of active service. In the end, the B-52 pro­vided much better per­formance and was ultimately selected.

The High Range(AFFTC History Office)



Convair B-36 as the carrier aircraft. This was a natural extension of previous X-planes that had used a Boeing B-29 or B-50 as a carrier. The B-36 would be modified to carry the X-15 partially enclosed in its bomb bays, much like the X-l and X-2 had been in earlier projects. This arrangement had some advantages; the pilot could freely move between the X-15 and B-36 during climb-out and the cruise to the launch location. This was extremely advanta­geous if problems developed that required jet­tisoning the X-15 prior to launch. At the time of the first industry conference in 1956, it was expected that a B-36 would be modified begin­ning in the middle of 1957 and be ready for flight tests in October 1958.75

As the weight of the X-15 and its subsystems grew, however, the Air Force and NASA began to look for ways to recover some of the lost performance. One way was to launch the X-15 at a higher altitude and greater speed. In addition, the personnel at Edwards believed that the ten-engine B-36 would be difficult to maintain7" since it was being phased out of the Air Force inventory. Investigations showed that the X-15, as designed, would fit under the wing of one of the new Boeing B-52 Stratofortresses; the configuration of the B-52 . precluded carrying the X-15 in the bomb bay. This was not the ideal solution—the X-15 pilot would have to be locked in the research airplane prior to takeoff, and the large weight transition when the X-15 was released would provide some interesting control problems for the B-52. Further analysis concluded that the potential problems were solvable, and that the increase in speed and altitude capabilities were desirable. Fortunately, two early B-52s were completing their test duties, and the Air Force made them available to the program.

On 29 November 1957, the B-52A (52-003) arrived at Air Force Plant 42 in Palmdale, California, after a flight from the Boeing plant in Seattle. The aircraft was placed in storage pending modifications. On 4 February 1958, the B-52A was moved into the North American hanger at Plant 42 and modified with a large pylon under the wing, the capa­
bility to monitor to the X-15, and a system to replenish the X-15 LOX supply. The aircraft, now designated77 NB-52A, was flown to Edwards AFB on 14 November 1958; it was later named “The High and the Mighty One.” The Air Force also supplied a B-52B (52-008) that arrived in Palmdale for similar modifica­tions on 5 January 1959, and was flown, as an NB-52B, to Edwards on 8 June 1959.

Roll Out

As the first X-15 was being completed, the NACA held the second X-15 industry con­ference in Los Angeles on 28-29 July 1958. North American began the conference with a paper detailing the developmental status of the aircraft. Twenty-seven other papers cov­ered subjects such as stability and control, simulator testing, pilot considerations, mis­sion instrumentation, thermodynamics, structures, materials and fabrication. There were approximately 550 attendees,78

On 1 October 1958, High-Speed Flight Station employees Doll Matay and John Hedgepeth put up a ladder in front of the sta­tion building at the foot of Lilly Avenue and took down the winged-shield NACA emblem from over the entrance door. NASA had arrived in the desert, bringing with it a new era of space-consciousness, soaring budgets, and publicity. The old NACA days of concentra­tion on aeronautics, and especially aerody­namics, were gone forever, as was the agency itself. On this day, the National Aeronautics and Space Administration was created.79

The X-15 construction process eventually consumed just over two years, and on 15 October 1958, the first aircraft (56-6670) was rolled out. Following conclusion of the official ceremonies, it was moved back inside and prepared for shipment to Edwards. On the night of 16 October, cov­ered completely in protective heavy-duty wrapping paper, it was shipped by truck to Edwards for initial ground test work.


The first of three let­ters attached to the Memorandum of Understanding that created the X-15 research program. Since it was nominally an Air Force program, the Air Force began the signature process.






NOV 9 1954


The early 1950s was an era where carbon paper and onion-skin copies were kept. Forty-five years later they are not repro­ducible, so the three letters have been recreated.



SUBJECT: Principles for the Conduct of a Joint Project for a Hew

High Speed Research Airplane

1. The Air Force concurs in the establishment of a joint NACA- Navy-Air Force project to design and construct a research airplane capable of achieving speeds of the order of Mach Number 7 and altitudes of several hundred thousand feet.

2. Attached is a Memorandum of Understanding. signed in tripli­cate by the Air Force, setting forth the principles fox the conduct by the NACA, the Navy, and the Air Force of this joint project. It 1b reguested that the Navy sign this Memorandum, in triplicate, and forward the signed copies to the Director of the NACA for signature and distribution bach to the signatory agencies.

3. The Air Force is designating Brigadier General В, В. Кеївеу, Deputy Director of Research and Development, as the Air Force representa­tive on the ‘Research Airplane Oonmittee’ referred to in paragraph В

of the Memorandum of understanding.


The letters remained SECRET until 3 July 1963 when they were downgraded to CONFIDENTIAL.

It was not until 9 November 1966 that they were finally declassified.



Trevor Gardner
Special Assistant (Rid))



Memo of understanding

w/1 incl fin trip)


The Flight Research Program

During the ten years of flight operations, five major aircraft were involved in the X-15 flight research program. The three X-15s were des­ignated X-15-1 (56-6670), X-15-2 (56-6671), and X-15-3 (56-6672). Early in the test pro­gram the first two X-15s were essentially iden­tical in configuration; the third aircraft was completed with different electronic and flight control systems. When the second aircraft was extensively modified after an accident mid­way through the test program, it became the X-15A-2. The two carrier aircraft were an NB-52A (52-003) and an NB-52B (52-008); they were essentially interchangeable.

The program used a three-part designation for each flight. The first number represented the specific X-15; 1 was for X-15-1, etc. No differentiation was made between the origi­nal X-15-2 and the modified X-15A-2. The second position was the flight number for that specific X-15. This included free-flights only, not captive-carries or aborts; the first flight was 1, the second 2, etc. If the flight was a scheduled captive-carry, the second position in the designation was replaced with a C; if it was an aborted free-flight attempt, it was replaced with an A. The third position was the total number of times any X-15 had been carried aloft by either NB-52. This
number incremented for each captive-carry, abort, and actual release. The 24 May 1960 letter from FRC Director Paul Bikle estab­lishing this system is included as an appen­dix to this monograph.

Initial Flight Tests

The X-15-1 arrived at the Air Force Flight Test Center at Edwards AFB, California, on 17 October 1958; trucked over the hills from the North American plant in Los Angeles for testing at the NASA High-Speed Flight Station. It was joined by the second airplane in April 1959; the third would arrive later. In contrast to the relative secrecy that had attended flight tests with the XS-1 (X-l) a decade before, the X-15 program offered the spectacle of pure theater.1

As part of the X-15’s contractor program, North American had to demonstrate each air­craft’s general airworthiness during flights above Mach 2 before delivering it to the Air Force, which would then tum it over to NASA. Anything beyond Mach 3 was con­sidered a part of the government’s research obligation. The contractor program would last approximately two years, from mid – 1959 through mid-1960.

Two different mission profiles were flown— one for maximum speed; and one for maximum altitude.


The Flight Research ProgramThe first X-15 (56-6670) immediately prior to the official roll­out ceremonies at North American’s Los Angeles plant on 15 October 1958. The small size of the trapezoid-shaped wings and the extreme wedge sec­tion of the vertical sta­bilizer are noteworthy. (North American Aviation)

The task of flying the X-15 during the con­tractor program rested in the capable hands of Scott Crossfield. After various ground checks, the X-15-1 was mated to the NB-52A, then more ground tests were con­ducted. On 10 March 1959, the pair made a scheduled captive-carry flight (program flight number 1-C-l). They had a gross take­off weight of 258,000 pounds, lifting off at 168 knots after a ground roll of 6,200 feet. During the 1 hour and 8 minute flight it was found that the NB-52 was an excellent carri­er for the X-15, as had been expected from numerous wind tunnel and simulator tests. During the captive flight the X-15 flight con­trols were exercised and airspeed data from the flight test boom on the nose was obtained in order to calibrate the instrumentation. The penalties imposed by the X-15 on the NB-52 flight characteristics was found to be minimal in the gear-up configuration. The mated pair was flown up to Mach 0.83 at 45,000 feet.2

The next step was to release the X-15 from the NB-52 in order to ascertain its gliding and landing characteristics. The first glide flight was scheduled for 1 April 1959, but was aborted when the X-15 radio failed. The
pair spent 1 hour and 45 minutes airborne conducting further tests in the mated config­uration. A second attempt was aborted on 10 April 1959 by a combination of radio failure and APU problems. Yet a third attempt was aborted on 21 May 1959 when the X-15’s stability augmentation system failed, and a bearing in the No. 1 APU overheated after approximately 29 minutes of operation.

The problems with the APU were the most disturbing. Various valve malfunctions, leaks, and several APU speed-control prob­lems were encountered during these three flights, all of which would have been unac­ceptable during research flights. Tests con­ducted on the APU revealed that extremely high surge pressures were occurring at the pressure relief valve (actually a blow-out plug) during initial peroxide tank pressuriza­tion. The installation of an orifice in the heli­um pressurization line immediately down­stream of the shut-off valve reduced the surges to acceptable levels. Other problems were found to be unique to the captive-carry flights and the long-run times being imposed on the APUs; they were deemed to be of lit­tle consequence to the flight test program

Long before the NB-52 first carried the X-15 into the air, engineers had tested the separation charac­teristics in the wind tunnels at Langley and Ames. Here an X-15 model drops – away from a model of the NB-52. Note that the X-15 is mounted on the wrong wing. This was necessary because the viewing area of the wind tun­nel was on the left side of the aircraft.

The Flight Research Program(NASA photo EL-1996-00114)

since the operating scenario would be differ­ent. The APUs underwent a constant set of minor improvements early in the flight test program, but nevertheless continued to be a source of irritation until the end.

On 22 May the first ground run of the inter­im XLR11 engine installation was accom­plished using the X-15-2. Scott Crossfield was in the cockpit, and the test was consid­ered successful, clearing the way for the eventual first powered flight; if the first X-15 could ever make its scheduled glide flight.

Another attempt at the glide flight was made on 5 June 1959 but was aborted even before the NB-52 left the ground3 when Crossfield reported smoke in the X-15-1 cockpit. Investigation showed that a cockpit ventila­tion fan motor had overheated.

Finally, at 08:38 on 8 June 1959, Scott Crossfield separated the X-15-1 from the NB-52A at Mach 0.79 and 37,500 feet. Just prior to launch the pitch damper failed, but Crossfield elected to proceed with the flight, and switched the SAS 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. The X-15 touched down on the dry lake at Edwards 4 minutes and 56 seconds later. Just prior to landing, the X-15 began a series of increasingly wild pitching motions; mostly as a result of Crossfteld’s instinctive corrective action, the airplane landed safely. Landing speed was 150 knots, and the X-15 rolled-out 3,900 feet while turning very slightly to the right. North American subsequently modified the control system boost to increase the control rate response, effectively solving the problem.

Although the impact at landing was not con­sidered to be particularly hard, later inspec­tion revealed that bell cranks in both main landing skids had bent slightly. The main skids were not instrumented on this flight, so no specific impact data could be ascertained, but it was generally believed that the shock struts had bottomed and remained bottomed as a result of higher than predicted landing loads. As a precaution against the main skid problem occurring again, the metering char­acteristics of the shock struts were improved, and lakebed drop tests at higher than previ­ous loads were made with the landing gear test trailer that had been used to qualify the landing gear design. All other airplane sys-

The Flight Research ProgramNorth American test pilot A. Scott Crossfield was responsible for demonstrating that the X-15 was airworthy.

His decision to leave NACA and join North American effectively locked him out of the high-speed and high – altitude test flights later in the program. (NASA photo EC-570-1

terns operated satisfactorily, clearing the way for the first powered flight.4

In preparation for the first powered flight, the X-15-2 was taken for a captive-carry flight with full propellant tanks on 24 July 1959. During August and early September, several attempts to make the first powered flight were cancelled before leaving the ground due to leaks in the APU peroxide system and hydraulic leaks. There were also several failures of propellant tank pressure regulators. Engineers and technicians worked to eliminate these problems, all of which were irritating, but none of which was considered critical.

The first powered flight was made by X-15-2 on 17 September 1959. The aircraft was released from the NB-52A at 08:08 in the morning while flying at Mach 0.80 and 37,600 feet. Crossfield piloted the X-15-2 to Mach 2.11 and 52,341 feet during 224.3 sec­onds of powered flight using the two XLR11 engines. He landed on the dry lakebed at Edwards 9 minutes and 11 seconds after launch. Following the landing, a fire was noticed in the area around the ventral stabiliz­er, and was quickly extinguished by ground
crews. A subsequent investigation revealed that the upper XLR11 fuel pump diffuser case had cracked after engine shutdown and had sprayed fuel throughout the engine compart­ment. Fuel collected in the ventral stabilizer and was ignited by unknown causes during landing. No appreciable damage was done, and the aircraft was quickly repaired.5

The third flight of X-15-2 took place on 5 November 1959 when the X-15 was dropped from the NB-52A at Mach 0.82 and 44,000 feet. During the engine start sequence, one chamber in the lower engine exploded. There was external damage around the engine and base plate, plus quite a bit of damage internal to the engine compartment. The resulting fire convinced Crossfield to make an emergency landing at Rosamond Dry Lake; he quickly shut off the engines, dumped the remaining fuel, and jettisoned the ventral6 rudder. Even so, within the 13.9 seconds of powered flight, the X-15 managed to accelerate to Mach 1. The aircraft touched down near the center of the lake at approximately 150 knots and an 11 degrees angle of attack. When the nose gear bottomed out, the fuselage literal­ly broke in half at station’ 226.8, with about 70 percent of the bolts at the manufacturing

Any landing you can walk away from…

The Flight Research ProgramThe X-15-2 made a hard landing on 5 November 1959, breaking its back as the nose settled on the lakebed. The dam­age looked worse than it was, and the aircraft was back in the air three months later. (NASA photo E-9543)

joint being sheared out. The fuselage contact­ed the ground and was dragged for approxi­mately 1,500 feet. Crossfield later stated that the damage was the result of a defect that should have broken on the first flight.8 The aircraft was sent to the North American plant for repairs, and was subsequently returned to Edwards in time for its fourth flight on 11 February I960.9

The X-15-1 made its first powered flight, using two XLRlls, on 23 January 1960. This was also the first flight using the stable platform, and the performance of the system was considered encouraging. Under the terms of the contract, the X-15 had still “belonged” to North American until they had demonstrated its basic airworthiness and operation. Following this flight, a pre-deliv­ery inspection was accomplished, and on 3 February 1960 the airplane was formally accepted by the Air Force and subsequently turned over to NASA.

The first government X-15 flight (1-3-8) was on 25 March 1960 with NASA test pilot Joseph A. Walker at the controls. The X-15-1 was launched at Mach 0.82 and 45,500 feet, although the stable platform had malfunc­tioned just prior to release. Two restarts were required on the top engine before all eight chambers were firing, and the flight lasted just over 9 minutes, reaching Mach 2.0 and 48,630 feet. For the next six months, Walker and Major Robert M. White alternated flying the X-15-1.10

It is interesting to note that the predictions regarding flutter made by Lawrence P. Greene at the first industry conference in 1956 did materialize, although fortunately they were not major and relatively easy to correct. During the early test flights, vibrations at 110 cycles had been noted and were the cause of some concern. Engineers at FRC added instrumentation to the X-15s from flight to flight in an attempt to isolate the vibrations and understand their origins, while wind tun­nel tests were conducted at Langley. It was finally determined that the vibrations were being caused by a flutter of the fuselage side tunnel panels. These had been constructed in removable sections with an unsupported length of over 6 feet in some cases." North American added longitudinal stiffeners along the underside of each panel, and this largely cured the problem.12

The X-15-1 flew three times in the two weeks between 4 August and 19 August 1960, with five aborted launches due to various problems (including persistent APU failures). Two of these flights were made by Joe Walker, and one by Bob White. The flight on 12 August was to an altitude of 136,500 feet, marking the highest flight of an XLR11 – powered X-15.

The Million Horsepower Engine13

The X-15-3 had arrived at Edwards on 29 June 1959 but had not yet flown when the first XLR99 flight engine (s/n 105) was installed in it during early 1960. It should be noted that the third X-15 was never equipped for the XLR11 engines. At the same time, the second X-15 was removed from flight status after its ninth flight (2-9-18) on 26 April 1960, in anticipation of replacing the XLR11 engines with the new XLR99. This left only the X-15-1 on active flight status.

The first ground run with the XLR99 in the X-15-3 was made on 2 June 1960. Inspection of the aircraft afterward revealed damage to the liquid oxygen inlet line brackets, the result of a water-hammer effect. After repairs were completed, another ground run was conducted on 8 June. A normal engine start and a short run at minimal power was made, followed by a normal shutdown, A restart was attempted, but was shutdown automati­cally by a malfunction indication. Almost immediately, a second restart was attempted, resulting in an explosion that effectively destroyed the aircraft aft of the wing. Crossfield was in the cockpit, which was thrown 30 feet forward, but he was not injured. Subsequent investigation revealed that the ammonia tank pressure regulator had failed open. Because of some ground han-

The top and bottom of the fuselage were usually covered in frost because the LOX tank was integral with the fuselage. Oxygen is liquid at -297 degrees Fahrenheit.

The Million Horsepower Engine13All three X-15s nor­mally carried a yellow NASA banner on their vertical stabilizers. (U. S. Air Force)

dling hoses attached to the fuel vent line, the fuel pressure-relief valve did not operate properly, thus allowing the fuel tank to over­pressurize and rupture. Tn the process, the peroxide tank was damaged by debris, and the mixing of the peroxide and ammonia caused an explosion.

Post-accident analysis indicated that there were no serious design flaws with either the XLR99 or the X-15. The accident had been caused by a simple failure of the pressure reg­ulator, exasperated by the unique configura­tion required for the ground test. Modification of the X-15-2 to accept the XLR99 continued, and several other modifications were incorpo­rated 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 for the installation of the ball-nose instead of the flight test boom that had been used so far in the program. The remains of the X-15-3 were returned to North American, which received authorization to rebuild the aircraft in early August.14

The installation of the ball-nose presented its own challenges since it had no capability to determine 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 been considered alternate locations. 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. This indicated that the data from all three locations were acceptable, so the original location was retained. After the ball-nose was installed, angle-of-attack data was compared to that 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.

The first flight attempt of X-15-2 with the XLR99 was made on 13 October 1960, but was terminated prior to launch because of a peroxide leak in the No. 2 APU, Just to show haw many things could go wrong on a single flight, there was also propellant impingement on the aft fuselage during the prime cycle, manifold pressure fluctuations during engine turbopump operation, and fuel tank pressure fluctuations during the jettison cycle. Nevertheless, two weeks later, Crossfield again entered the cockpit with the goal of making the first XLR99 flight. Again, prob­lems with the No. 2 APU forced an abort.

On 15 November 1960, everything went right, and Crossfield made the first flight of X-15-2 powered by the XLR99. The primary flight objective was to demonstrate engine operation at 50 percent thrust. The launch was at 46,000 feet and Mach 0.83, and even with only half the available power, the X-15 managed to climb to 81,200 feet and Mach 2.97. The sec­ond XLR99 flight tested the engine’s restart and throttling capability. Crossfield made the flight on 22 November, again using the sec­ond X-15. The third and final XLR99 demonstration flight was accomplished using X-15-2 on 6 December 1960. The objectives of engine throttling, shutdown, and restart were successfully accomplished. This marked North American Aviation’s, and Scott Crossfield’s, last X-15 flight. The job of fly­ing the X-15 was now totally in the hands of the government test pilots.15

After this flight, a work schedule was estab­lished which would permit an early flight with a government pilot using North American maintenance personnel. The flight was tentatively scheduled for 21 December i960 with Bob White as the pilot. However, a considerable amount of work had to be accomplished before the flight, including the
removal and replacement of the engine (s/n 103) which had suffered excessive chamber coating loss, installation of redesigned canopy hooks, installation of an unrestricted upper vertical stabilizer, rearrangement of the alternate airspeed system, and the reloca­tion of the ammonia tank helium pressure regulator into the fixed portion of the upper vertical. During a preflight ground run, a pinhole leak was found in the chamber throat of the engine. Although the leak was found to be acceptable for an engine run, it became increasingly worse during the test until it was such that the engine could not be run again. Since there was no spare engine avail­able, the flight was cancelled and a schedule established to deliver the aircraft to the gov­ernment prior to another flight. The X-15-2 was formally delivered to the Air Force and turned over to NASA on 8 February 1961. On the same day, X-15-1 was returned to the North American plant for conversion to the XLR99, having completed the last XLR11 flight of the program the day before with White at the controls.16

From the beginning of the X-15 flight test program in 1959 until the end of 1960, a total of 31 flights had been made with the first two

The Million Horsepower Engine13Six of the X-15 pilots {from left to right): Lieutenant Colonel Robert A. Rushworth (USAF), John B.

“Jack” McKay (NASA), Lieutenant Commander Forrest S. Petersen (USN), Joseph A. Walker (NASA), Neil A. Armstrong (NASA), Major Robert M. White (USAF). (NASA via the San Diego Aerospace Museum Collection)

X-15s by seven pilots. But the X-15-1 was experiencing an odd problem. When the APU was started, hydraulic pressure was either slow in coming up, or dropped off out of limits when the control surfaces were moved. The solution to the problem came after additional instrumentation was placed on the hydraulic system. The boot-strap line which pressurized the hydraulic reservoir was freezing, causing a flow restriction or stoppage. Under these conditions, the hydraulic pump would cavitate, resulting in little or no pressure rise. The apparent cause of this problem was the addition of a liquid nitrogen line to cool the stable platform. Since the nitrogen line was installed adjacent to the hydraulic lines, it caused the Orinite hydraulic oil to freeze. The solution to the problem was to add electric heaters to the affected hydraulic lines.

Joe Walker’s flight on 30 March 1961 marked the first use of the new A/P-22S full-pressure suit instead of the earlier MC-2. Walker reported the suit was much more comfortable and afforded better vision. But the flight pointed out a potential problem with the stability augmentation system (SAS). As Walker descended through

100,0 feet, a heavy vibration occurred and continued for about 45 seconds until recovery was affected at 55,000 feet. Incremental acceleration of approximately 1-g was noted in the vertical and transverse axes at a frequency of 13 cycles. This cor­responded to the first bending mode of the horizontal stabilator. The center of gravity of the horizontal surfaces was located behind the hinge line; consequently rapid surface movement produced both rolling and pitching inertial moments. Subsequent analysis showed the vibration was sustained by the SAS at the natural frequency of the horizontal surfaces. Essentially, the oscilla­tions began because of the increased activi­ty of the controls on reentry which excited the oscillation and stopped after the pilot reduced the pitch-damper gain.’7

Two solutions to the problem were discussed between the FRC, North American, the Air Force, and the manufacturer of the SAS, Westinghouse; a notch filter for the SAS and a pressure-derivative feedback valve for the main stabilator hydraulic actuator. The notch filter eliminated SAS control surface input at 13 cycles, and the feedback valve damped the stabilator bending mode. In essence, the valve corrected the source of the problem, while the notch filter avoided the problem. Although it was felt that either solution would likely cure the problem, the final deci­sion was to use both.

NASA research pilot William Dana made a check flight in a specially-modified JF-100C (53-1709) at Ames on 1 November 1960, delivering the aircraft to the FRC the follow­ing day. The aircraft had been modified as a variable-stability trainer that could simulate the X-15’s flight profile. This made it possi­ble to investigate new piloting techniques and control-law modifications without using an X-15. Another 104 flights were made for pilot checkout, variable stability research, and X-15 support before the aircraft was returned to Ames on 11 March 1964.’®

The first government flight with the XLR99 engine took place on 7 March 1961 with Bob White at the controls. The X-15-2 reached Mach 4.43 and 77,450 feet, and the flight was generally satisfactory. The objectives of the flight were to obtain additional aerodynamic and structural heating data, as well as informa­tion on stability and control of the aircraft at high speeds. Post-flight examination showed a limited amount of buckling to the side-fuse­lage tunnels, attributed to thermal expansion. The temperature difference between the tunnel panels and the primary fuselage structure was close to 500 degrees Fahrenheit. The damage was not considered significant since the panels were not primary structure, but were only nec­essary to carry air loads. However, the buck­ling condnued to become more severe as Mach numbers increased in later flights, and eventually NASA elected to install additional expansion joints in the tunnel skin to minimize the buckling.141

By June 1961, government test pilots had been operating the X-15 on research flights for just over a year.20 The research phase of the X-15’s flight program involved four broad objectives: verification of predicted hyperson­ic aerodynamic behavior and heating rates, study of the X-15’s structural characteristics in an environment of high heating and high flight loads, investigation of hypersonic sta­bility and control problems during atmospher­ic exit and reentry, and investigation of pilot­ing tasks and pilot performance. By late 1961, these four areas had been generally examined, although detailed research continued to about 1964 using the first and third aircraft, and to 1967 with the second (as the X-15A-2). Before the end of 1961, the X-15 had attained its Mach 6 design goal and had flown well above 200,000 feet; by the end of 1962 the X – 15 was routinely flying above 300,000 feet. The X-15 had already extended the range of winged aircraft flight speeds from Mach 3.2’1 to Mach 6.04, the latter achieved by Bob White on 9 November 1961.

The X-15 flight research program revealed a number of interesting things. Physiologists discovered the heart rates of X-15 pilots var­ied between 145 and 185 beats per minute in flight, as compared to a normal of 70 to 80 beats per minute for test missions in other aircraft. Researchers eventually concluded that pre-launch anticipatory stress, rather than actual post launch physical stress, influ­enced the heart rate. They believed, correct­ly, that these rates could be considered as probable baselines for predicting the physio­logical behavior of future astronauts. Aerodynamic researchers found remarkable agreement between the wind tunnel tests of exceedingly small X-15 models and actual results, with the exception of drag measure­ments. Drag produced by the blunt aft end of the actual aircraft proved 15 percent higher than wind tunnel tests had predicted.

At Mach 6, the X-15 absorbed eight times the heating load it experienced at Mach 3, with the highest heating rates occurring in the frontal and lower surfaces of the aircraft, which received the brunt of airflow impact. During the first Mach 5+ flight, four expan­sion slots in the leading edge of the wing generated turbulent vortices that increased heating rates to the point that the external skin behind the joints buckled. It offered “… a classical example of the interaction among aerodynamic flow, thermodynamic proper­ties of air, and elastic characteristics of struc­ture.” As a solution, small Inconel X alloy strips were added over the slots and addi­tional fasteners on the skin.22

Heating and turbulent flow generated by the protruding cockpit enclosure posed other problems; on two occasions, the outer panels of the X-15’s glass windshields fractured because heating loads in the expanding frame overstressed the soda-lime glass. The difficulty was overcome by changing the cockpit frame from Inconel X to titanium, eliminating the rear support (allowing the windscreen to expand slightly), and replac­ing the outer glass panels with high temper­ature alumina silica glass. All this warned aerospace designers to proceed cautiously. During 1968 John Becker22 wrote: “The real­ly important lesson here is that what are minor and unimportant features of a subson­ic or supersonic aircraft must be dealt with as prime design problems in a hypersonic air­plane. This lesson was applied effectively in the precise design of a host of important details on the manned space vehicles.”

A serious roll instability predicted for the airplane under certain reentry conditions posed a dilemma to flight researchers. To accurately simulate the reentry profile of a returning winged spacecraft, the X-15 had to fly at angles of attack of at least 17 degrees. Yet the wedge-shaped vertical and ventral stabilizers, so necessary for stability and control in other portions of the flight regime, actually prevented the airplane from being flown safely at angles of attack greater than 20 degrees because of potential rolling prob­lems. By this time, FRC researchers had gained enough experience with the XLR99 engine to realize that fears of thrust mis-

A common sight dur­ing the 1960s over Edwards—an NB-52 carrying an X-15.This was a boy’s dream at the time; and the sub­ject of many fantasies.

The Million Horsepower Engine13Over the course of the program, the markings on the NB-52s changed significantly. Early on, they were natural metal with bright orange verti­cals; later they were overall gray. (NASA)

alignment—a major reason for the large sur­faces—were unwarranted. The obvious solu­tion was simply to remove the lower portion of the ventral, something that X-15 pilots had to jettison prior to landing anyway so that the aircraft could touch down on its landing skids. Removing part of the ventral produced an acceptable tradeoff; while it reduced stability by about 50 percent at high angles of attack, it greatly improved the pilot’s ability to control the airplane. With the ventral off, the X-15 could fly into the previously “uncontrollable” region above 20 degrees angle of attack with complete safety. Eventually the X-15 went on to reentry tra­jectories of up to 26 degrees, often with flight path angles of -38 degrees at speeds up to Mach 6.1J Its reentry characteristics were remarkably similar to those of the later Space Shuttle orbiter.

When Project Mercury began, it rapidly eclipsed the X-15 in the public’s imagina­tion. It also dominated some of the research areas that had first interested X-15 planners, such as “zero-g” weightlessness studies. The use of reaction controls to maintain attitude in space proved academic after Mercury flew, but the X-15 would furnish valuable information on the blending of reaction con­trols with conventional aerodynamic con­
trols during exit and reentry, a matter of con­cern to subsequent Shuttle development. The X-15 experience clearly demonstrated the ability of pilots to fly rocket-propelled air­craft out of the atmosphere and back in to precision landings. Paul Bikle saw the X-15 and Mercury as a “… parallel, two-pronged approach to solving some of the problems of manned space flight. While Mercury was demonstrating man’s capability to function effectively in space, the X-15 was demon­strating man’s ability to control a high per­formance vehicle in a near-space environ­ment… considerable new knowledge was obtained on the techniques and problems associated with lifting reentry.”25

Nearly all of the early XLR99 flights experi­enced malfunction shutdowns of the engine immediately after launch, and sometimes after normal engine shutdown or burnout. Since the only active engine system after shutdown was the lube-oil system, investiga­tions centered on it. Analyses of this condi­tion revealed very wide acceleration excur­sions during the engine-start phase. A rea­sonable simulation of this acceleration was accomplished by placing an engine on a work stand with the ability to rotate the engine about the Y-axis. Under certain con­ditions, the lube-oil pump could be made to

cavitate for about 2 seconds, tripping an automatic malfunction shutdown. To elimi­nate this problem, a delay timer was installed in the lube-oil malfunction circuit which allowed the pump to cavitate up to 6 seconds without actuating the malfunction shutdown system. After this delay timer was installed in early 1962, no further engine shutdowns of this type were experienced.26

But a potentially more serious XLR99 prob­lem was the unexpected loss of the Rokide coating from the combustion chamber during firing. A meeting was held at Wright Field on 13 June 1961 to discuss possible solutions. It was decided that the Wright Field Materials Laboratory would develop a new ceramic coating for the chambers, and that FRC would develop the technique and fixtures required to recoat chambers at Edwards. Originally, the Materials Laboratory award­ed a contract to Plasmakote Corp. to perform the coating of several chambers, but the results were unsatisfactory. By March 1962, the techniques and fixtures developed by the FRC allowed chambers to be successfully recoated at Edwards.

Early in the program, the X-15’s stability
augmentation and inertial guidance systems were two major problem areas. NASA even­tually replaced the Sperry inertial unit with a Honeywell system designed for the stillborn Dyna-Soar. The propellant system had its own weaknesses; pneumatic vent and relief valves and pressure regulators gave the greatest difficulties, followed by spring pres­sure switches in the APUs, the turbopump, and the gas generation system. NASA’s mechanics routinely had to reject 24-30 per­cent of spare parts as unusable, a clear indi­cation of the difficulties that would be expe­rienced later in the space programs in getting parts manufactured to exacting specifica­tions.27 Weather posed a critical factor. Many times Edwards enjoyed good weather while other locations on the High Range were cov­ered with clouds, alternate landing sites were flooded, or some other meteorological con­dition postponed a mission.

Follow-on Experiments

During the summer of 1961, a new research initiative was proposed by the Air Force’s Aeronautical Systems Division at Wright – Patterson AFB and NASA Headquarters; using the X-15 to carry a wide range of sci-

Подпись: tFollow-on ExperimentsOn 4 November 1960, the program attempt­ed to launch two X-15 flights in a single day. Here X-15-1 is mount­ed on the NB-52B and X-15-2 is on the NB-52A. Rushworth was making his first flight in X-15-1, a low (48,900 feet) and slow (Mach 1.95) familiar­ization. The X-15-2, with Crossfield as pilot, aborted due to a failure in the No. 2 APU. (NASA photo E-6186)

entific experiments unforeseen when the air­craft was conceived in 1954.

Researchers at the FRC wanted to use the X-15 to carry high-altitude experiments related to the proposed Orbiting Astronomical Observatory; others suggested modifying one of the airplanes to carry a Mach 5-f – ram­jet for advanced air-breathing propulsion studies. Over 40 experiments were suggested by the scientific community as suitable can­didates for the X-15 to carry. In August 1961 NASA and the Air Force formed the “X-15 Joint Program Coordinating Committee” to prepare a plan for a follow-on experiments program. The committee held its first meet­ing on 23-25 August 1961 at the FRC.26

Many experiments suggested to the commit­tee related to space science, such as ultravio­let stellar photography. Others supported the Apollo program and hypersonic ramjet stud­ies. Hartley Soule and John Stack, then NASA’s director of aeronautical research, proposed the classification of experiments into two groups: category A experiments consisted of well-advanced and funded experiments having great importance; cate­gory В included worthwhile projects of less urgency or importance.2’

In March 1962 the committee approved the “X-15 Follow-on Program,” and NASA announced that an ultraviolet stellar photog­raphy experiment from the University of Wisconsin’s Washburn Observatory would be first. The X-15’s space science program eventually included twenty-eight experi­ments including astronomy, micrometeorite collection (using wing-top pods on the X-15- 1 and X-15-3 that opened at 150,000 feet), and high-altitude mapping. The micromete­orite experiment was unsuccessful, and was ultimately cancelled. Two of the follow-on programs, a horizon definition experiment from the Massachusetts Institute of Technology, and test of insulation material for the Saturn launch vehicle, directly bene­fited the Apollo program. The Saturn insula­tion was applied to the X-15’s speed brakes, which were then deployed at the desired speed and dynamic pressure to test both the insulating properties and the bonding materi­al. By the end of 1964, over 65 percent of data being returned from the three X-15 air­craft involved follow-on projects; this per­centage increased yearly through conclusion of the program. ™

As early as May 1962, North American had proposed modifying one of the X-15s as a flying test bed for hypersonic engines. Since the X-I5s were being fully utilized at the time, neither the Air Force nor NASA expressed much interest in pursuing the idea. However, when the X-15-2 was damaged during a landing accident on 9 November 1962 (seriously injuring Jack McKay, who would later return from his injuries to fly the X-15 again), North American proposed mod­ifying the aircraft in conjunction with its repairs. General support for the plan was found within the Air Force, which was will­ing to pay the estimated $6 million.31

On the other hand, NASA was less enthusias­tic, and felt the aircraft should simply be repaired to its original configuration.32 Researchers at NASA believed that the Mach 8 X-l 5 would prove to be of limited value for propulsion research. However, NASA did not press its views, and in March 1963 the Air Force authorized North American to rebuild the aircraft as the X-15A-2. Twenty-nine inches were added to the fuselage between the existing propellant tanks. The extra vol­ume was to be used by a liquid hydrogen tank to power the ramjet, but the LH2 tank could be replaced by other equipment as needed. In fact, the compartment was frequently used to house cameras to test reconnaissance con­cepts, or to observe the dummy ramjet during flight tests, through three heat-resistant win­dows in the lower fuselage. The capability to carry two external propellant tanks was added to provide additional powered flight with the XLR99. The right wingtip was also modified to allow various wingtip shapes to be carried interchangeably, although it appears that this capability was never used.33

Forty weeks and $9 million later, North American delivered the X-15A-2,’4 The air­craft made its first flight on 25 June 1964 piloted by Bob Rushworth. Early flights demonstrated that the aircraft retained satis­factory flying qualities at Mach 5, although on three flights thermal stresses caused por­tions of the landing gear to extend at Mach 4.3, generating “an awful bang and a yaw.’"5 In each case Rushworth landed safely, despite the blow-out of the heat-weakened tires in one instance. On 18 November 1966, Pete Knight set an unofficial world’s speed record of Mach 6.33 in the aircraft. The drop tanks had been jettisoned at Mach 2.27 and 69,700 feet. A nonfunctional dummy ramjet was constructed in order to gather aerodynamic data on the basic shape in preparation for possible flight tests in the early 1970s. The first flight with the dummy ramjet attached to the ventral was on 8 May 1967. Although providing a pronounced nose-down trim change, the ramjet actually restored some of the directional stability lost when the lower ventral rudder had been removed.

NASA had evaluated several possible coat­ings that could be applied over the X-15’s Inconel X hot-structure to enable it to with­stand the thermal loads experienced above Mach 6. The use of such coatings could be beneficial since various ablators were being investigated by the major aerospace contrac­tors during the early pre-concept phases-16 of the Space Shuttle development." Such a coat­ing would have to be relatively light, have good insulating properties, and be easy to apply, remove, and reapply before another flight. The selected coating was MA-25S, an ablator developed by the Martin Company in connection with some early reusable space­craft studies. Consisting of a resin base, a cat­alyst, and a glass bead powder, it would pro­tect the hot-structure from the expected 2,000 degrees Fahrenheit heating at Mach 8. Martin esumated that the coating, ranging from 0.59 inches thick on the canopy, wings, vertical, and horizontal stabilizers, down to 0.015 inch­es on the trailing edges of the wings and tail, would keep the skin temperature below 600 degrees Fahrenheit. The first unpleasant sur­prise came, however, with the application of the coating to the X-15A-2: it took six weeks. Getting the correct thickness over the entire surface proved harder than expected. Also, every time a panel had to be opened to service the X-15, the coating had to be removed and reapplied around the affected area.

Because the ablator would char and emit a residue in flight, North American had installed an “eyelid” over the left cockpit window; it would remain closed until just before landing. During launch and climbout, the pilot would use the right window, but residue from the ablator would render it opaque above Mach 6. The eyelid had already been tested on several flights.™

Late in the summer of 1967, the X-15A-2 was ready for flight with the ablative coat­ing. The weight of the ablator—125 pounds higher than planned—together with expected increased drag reduced the theoretical maxi­mum performance of the airplane to Mach 7.4, still a significant advance over the Mach

6.3 previously attained. The appearance of the X-15A-2 was striking, an overall flat off – white finish, the external tanks a mix of sil­ver and orange-red with broad striping. On 21 August 1967, Knight completed the first flight in the ablative coated X-15A-2, reach­ing Mach 4.94 and familiarizing himself with its handling qualities. His next flight was destined to be the program’s fastest flight, and the last flight of the X-15A-2.’9

On 3 October 1967, 43,750 feet over Mud Lake, Knight dropped away from the NB-52B. The flight plan showed the X-15A-2 would weigh 52,117 pounds at separation, more than 50 percent heavier than originally conceived in 1954* The external tanks were jettisoned 67.4 seconds after launch at Mach

2.4 and 72,300 feet; tank separation was satis­factory, however, Knight felt the ejection was “harder” than the last one he had experienced (2-50-89). The recovery system performed satisfactorily and the tanks were recovered in repairable condition. The XLR99 burned for

140.7 seconds before Knight shut it down. Radar data showed the X-15A-2 attained Mach 6.70 (4,520 mph) at 102,700 feet, a winged-vehicle speed record that would stand until the return of the Space Shuttle Columbia from its first orbital flight in 1981.41

The post-landing inspection revealed many things. The ability of the ablative material to protect the aircraft structure from the high aerodynamic heating was considered good except in the area around the dummy ramjet where the heating rates were significantly higher than predicted. The instrumentation on the dummy ramjet had ceased working approximately 25 seconds after engine shut­down, indicating that a bum through of the ramjet/pylon structure had occurred. Shortly thereafter the heat propagated upward into the lower aft fuselage causing the hydrogen – peroxide hot light to illuminate in the cock­pit. Assuming a genuine overheat condition, William Dana in the NASA 1 control room had requested Knight to jettison the remain­ing peroxide. The high heat in the aft fuse­lage area also caused a failure of a helium check valve allowing not only the normal helium source gas to escape, but also the emergency jettison control gas supply as well. Thus, the remaining residual propel­lants could not be jettisoned. The aircraft was an estimated 1,500 pounds heavier than normal at landing, but the landing occurred without incident.

wave impinged on the ramjet and its sup­porting structure. The heat in the ramjet pylon area was later estimated to be ten times normal, and became high enough at some time during the flight to ignite 3 of the 4 explosive bolts holding the ramjet to the pylon. As Knight was turning downwind in the landing pattern, the one remaining bolt failed structurally and the ramjet separated from the aircraft. Knight did not feel the ramjet separate, and since the chase aircraft had not yet joined up, was unaware that the ramjet had separated.

The position of the X-15 at the time of sepa­ration was later established by radar data and the most likely trajectory estimated. A ground search party discovered the ramjet on the Edwards bombing range. Although it had been damaged by impact, it was returned for study of the heat damage.

The unprotected right-hand windshield was, as anticipated, partially covered with ablation products. Since the left eyelid remained closed until well into the recovery maneuver, Knight flew the X-15 using on-board instru­ments and directions from William Dana in the NASA 1 control room. The eyelid was opened at approximately Mach 1.6 as the air­craft was over Rogers Dry Lake, and the visi­bility was considered satisfactory. Knight landed at Edwards 8 minutes and 12 seconds after launch.

Подпись: An internal general arrangement of the modified X-15A-2. (NASA)
Follow-on Experiments

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

The ablator obviously was not totally success­ful; in fact this was the closest any X-15 came to structural failure induced by heating. Post­flight inspection revealed that the aircraft was

charred on its leading edges and nose. The ablator had actually prevented cooling of some hot spots by keeping the heat away from the hot-structure. Some heating effects, such as where shock waves impinged on the ramjet had not been thoroughly studied. To John Becker the flight underscored.. the need for maximum attention to aerothermodynamic detail in design and preflight testing.”42 To Jack Kolf, an X-15 project engineer at the FRC, the post-flight condition of the airplane “… was a surprise to all of us. If there had been any question that the airplane was going to come back in that shape, we never would have flown it.”1-1

Some of the problems encountered with the ablator were nonrepresentative of possible future uses. The X-15 had been designed as an uninsulated hot structure. Any future vehicle would probably be designed with a more conventional airframe, eliminating some of the problems encountered on this flight. But some of the problems were very
real. The amount of time it took to apply the ablator was unacceptable. Even considering that the learning curve was steep, and that after some experience the time could be cut in half or even further, the six weeks it took to coat the relatively small X-15 bode ill for larger vehicles. Nevertheless, ablators would continue to be proposed on various Space Shuttle concepts, in decreasing quantity, until 1970 when several forms of ceramic tiles and metal “shingles” would become the preferred concepts.44

Follow-on Experiments

Подпись: The X-15A-2 drops away from the NB-52 on its last flight. Note the dummy ramjet attached to the ventral and the overall white finish applied to the ablator. The drop tanks would be jettisoned 67.4 seconds after engine ignition, at a speed of Mach 2.4 and 72,300 feet altitude. Pete Knight would attain Mach 6.70 on this flight. (NASA)

It was estimated that repairing the X-15A-2 and refurbishing the ablator for another flight near Mach 7 would have taken five weeks. The unexpected airflow problems around the ramjet ended any idea of flying it again. NASA sent the X-15A-2 to North American for general maintenance and repair, and although the aircraft returned to Edwards in June 196S, it never flew again. It is now on exhibit—in natural black finish—at the Air Force Museum, Wright-Patterson AFB, Ohio.

Ultimately, Garrett did deliver a functioning model of the ramjet, and it was successfully tested in a wind tunnel in late 1969. In this case successful meant that supersonic com­bustion was achieved, although for a very short duration and under very controlled and controversial conditions.45

Adaptive Controls

The X-15-3 featured specialized flight instrumentation and displays that rendered it particularly suitable for high-altitude flight research. A key element was the Minneapolis Honeywell MH-96 “adaptive” flight control system originally developed for the X-20 Dyna-Soar. This system automatically com­pensated for the airplane’s behavior in vari­ous flight regimes, combining the aerody­namic control surfaces and the reaction con­trols into a single control package. This was obviously the way future high-speed aircraft and spacecraft would be controlled, but the technology of the 1960s were severely taxed by the requirements for such a system.

By the end of 1963, the X-15-3 had flown above 50 miles altitude. This was the altitude that the Air Force recognized as the mini­mum boundary of space flight, and five Air Force pilots were awarded Astronaut Wings for their flights in the X-15.4* All but one of these flights was with X-15-3 (Astronaut Joe Engle’s first space flight was in Х-15-t). NASA did not recognize the 50 mile criteria, using the international 62 mile standard instead. Only a single NASA pilot went this high; Joe Walker set a record for winged space flight by reaching 354,200 feet (67 miles), a record that stood until the orbital flight of Columbia nearly two decades later. By mid-1967, the X-15-3 had completed sixty-four research flights, twenty-one at altitudes above 200,000 feet. It became the primary aircraft for carrying experiments to high altitude.

The X-15-3 would also make the most tragic flight of the program. At 10:30 in the morning on 15 November 1967, the X-15-3 dropped away from the NB-52B at 45,000 feet over Delamar Dry Lake. At the controls was Major Michael J. Adams, making his seventh X-15 flight. Starting his climb under full power, he was soon passing through 85,000 feet. Then an electrical disturbance distracted him and slightly degraded the control of the aircraft; having adequate backup controls, Adams con­tinued on. At 10:33 he reached a peak altitude of 266,000 feet. In the NASA 1 control room, mission controller Pete Knight monitored the mission with a team of engineers. As the X-15 climbed, Adams started a planned wing-rock­ing maneuver so an on-board camera could scan the horizon. The wing rocking quickly became excessive, by a factor of two or three. At the conclusion of the wing-rocking portion of the climb, the X-15 began a slow drift in heading; 40 seconds later, when the aircraft reached its maximum altitude, it was off head­ing by 15 degrees. As Adams came over the top, the drift briefly halted, with the airplane yawed 15 degrees to the right. Then the drift began again; within 30 seconds, Adams was descending at right angles to the flight path. At 230,000 feet, encountering rapidly increas­ing dynamic pressures, the X-15 entered a Mach 5 spin.47

In the NASA 1 control room there was no way to monitor heading, so nobody suspect­ed the true situation that Adams now faced. The controllers did not know that the air­plane was yawing, eventually turning com­pletely around. In fact, Knight advised Adams that he was “a little bit high,” but in “real good shape.” Just 15 seconds later, Adams radioed that the aircraft “seems squirrely.” At 10:34 came a shattering call: “I’m in a spin, Pete.” Plagued by lack of heading information, the control room staff saw only large and very slow pitching and rolling motions. One reaction was “disbelief; the feeling that possibly he was overstating the case.” But Adams again called out, “I’m in a spin.” As best they could, the ground controllers sought to get the X-15 straight­ened out. There was no recommended spin recovery technique for the X-15, and engi­neers knew nothing about the aircraft’s

realizing that the X-15 would never make Rogers Dry Lake, went into afterburner and raced for the emergency lakes; Ballarat and Cuddeback. Adams held the X-15’s controls against the spin, using both the aerodynamic control surfaces and the reaction controls. Through some combination of pilot tech­nique and basic aerodynamic stability, the airplane recovered from the spin at 118,000 feet and went into an inverted Mach 4.7 dive at an angle between 40 and 45 degrees.48

Adams was in a relatively high altitude dive and had a good chance of rolling upright, pulling out, and setting up a landing. But now came a technical problem; the MH-96 began a limit-cycle oscillation just as the airplane came out of the spin, preventing the gain changer from reducing pitch as dynamic pressure increased. The X-15 began a rapid pitching motion of increasing severity, still in a dive at 160,000 feet per minute, dynamic pressure increasing intolerably. As the X-15 neared 65,000 feet, it was diving at Mach 3.93 and experiencing over 15-g vertically, both positive and negative, and 8-g laterally.

The aircraft broke up northeast of the town of Johannesburg 10 minutes and 35 seconds after launch. A chase pilot spotted dust on Cuddeback, but it was not the X-15. Then an Air Force pilot, who had been up on a delayed chase mission and had tagged along on the X-15 flight to see if he could fill in for an errant chase plane, spotted the main wreckage northwest of Cuddeback. Mike Adams was dead; the X-15-3 destroyed.49

NASA and the Air Force convened an acci­dent board. Chaired by NASA’s Donald R. Bellman, the board took two months to pre­pare its report. Ground parties scoured the countryside looking for wreckage; critical to the investigation was the film from the cock­pit camera. The weekend after the accident, an unofficial FRC search party found the camera; disappointingly, the film cartridge was nowhere in sight. Engineers theorized that the film cassette, being lighter than the camera, might be further away, blown north by winds at altitude. FRC engineer Victor Horton organized a search and on 29 November, during the first pass over the area, Willard E. Dives found the cassette.

Most puzzling was Adams’ complete lack of awareness of major heading deviations in spite of accurately functioning cockpit instru­mentation. The accident board concluded that he had allowed the aircraft to deviate as the result of a combination of distraction, misin­terpretation of his instrumentation display, and possible vertigo. The electrical distur­bance early in the flight degraded the overall effectiveness of the aircraft’s control system and further added to pilot workload. The MH-96 adaptive control system then caused the airplane to break up during reentry. The board made two major recommendations: install a telemetered heading indicator in the control room, visible to the flight controller; and medically screen X-15 pilot candidates for labyrinth (vertigo) sensitivity.5" As a result of the X-15 ’s crash, the FRC added a ground – based “8 ball” attitude indicator in the control room to furnish mission controllers with real time pitch, roll, heading, angle of attack, and sideslip information.

Mike Adams was posthumously awarded Astronaut Wings for his last flight in the X-15-3, which had attained an altitude of

266,0 feet—50.38 miles. In 1991 Adams’ name was added to the Astronaut Memorial at the Kennedy Space Center in Florida.

The X-15 program would only fly another eight missions. The X-15A-2, grounded for repairs, soon remained grounded forever. The X-15-1 continued flying, with sharp dif­ferences of opinion about whether the research results returned were worth the risk and expense.

A proposed delta wing modification to the X-15-3 had offered supporters the hope that the program might continue to 1972 or 1973. The delta wing X-15 had grown out of stud­ies in the early 1960s on using the X-15 as a hypersonic cruise research vehicle.

Essentially, the delta wing X-15 would have made use of the third airframe with the adap­tive flight control system, but also incorporat­ed the modifications made to the X-15A-2— lengthening the fuselage, revising the land­ing gear, adding external propellant tanks, and provisions for a small-scale experimen­tal ramjet. NASA proponents, particularly John Becker at Langley, found the idea very attractive since: “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.”51

Despite such endorsement, support remained lukewarm at best both within NASA and the Air Force; the loss of Mike Adams and the X-15-3 effectively ended all thought of such a modification.

As early as March 1964, in consultation with NASA Headquarters, Brigadier General

James T. Stewart, director of science and tech­nology for the Air Force, had determined to end the X-15 program by 1968.52 At a meeting of the Aeronautics/Astronautics Coordinating Board on 5 July 1966, it was decided that NASA should assume total responsibility for all X-15 costs (other than incidental AFFTC support) on 1 January 1968.51 This was later postponed one year. As it turned out, by December 1968 only the X-15-1 was still fly­ing, and it cost roughly $600,000 per flight. Other NASA programs could benefit from this funding, and thus NASA did not request a continuation of X-15 funding after December 1968.54 During 1968 William Dana and Pete Knight took turns flying the X-15-1. On 24 October 1968, Dana completed the X-15’s 199th, and as it turned out the last, flight reaching Mach 5.38 at 255,000 feet. A total of ten attempts were made to launch the 200th flight, but a variety of maintenance and weather problems forced cancellation every time. On 20 December 1968, the X-15-1 was demated from the NB-52A for the last time. After nearly a decade of flight operations, the X-15 program came to an end.

The instrument panel of the X-15-3 with the MH-96 adaptive con­trol system installed. The dark panel imme­diately ahead of the center control stick allowed the pilot to control how the MH-96 reacted.

Adaptive Controls(NASA photo E63-9834)

The Legacy of the X-15

The year 1999 marked the 40th anniversary of the first flight of the X-15; this anniversary occurred more than 30 years after the program ended. The X-15 was the last high-speed research aircraft to fly as part of the research airplane program. The stillborn X-30 of the 1980s never took flight, and the verdict is still out on the fate of the Lockheed Martin X-33 demonstrator. Neil Armstrong, among others, once called the X-15 “the most successful research airplane in history.”1

Twelve men flew X-15. Scott Crossfield was first; William Dana was last. Pete Knight went 4,520 mph (Mach 6.70); Joe Walker went 67 miles (354,200 feet) high. Five of the pilots were awarded Astronaut Wings. Mike Adams died. What was learned? What should have been learned?

In October 1968 John V. Becker enumerated 22 accomplishments from the research and development work that produced the X-15, 28 accomplishments from its actual flight research, and 16 from experiments carried by the X-15. Becker’s comments have been well documented elsewhere, but are quoted here as appropriate.2

Nearly ten years after Becker’s assessment, Captain Ronald G. Boston of the U. S. Air Force Academy’s history department reviewed the X-15 program for “lessons learned” that might benefit the development of the X-24C National Hypersonic Flight Research Facility Program, an effort that was cancelled shortly afterwards. Boston’s paper offered an interesting perspective on the X-15 from the vantage point of the mid-1970s?

In 1999, the historian at the Dryden Flight Research Center, J. D. “Dill” Hunley, wrote a lessons-leamed paper on the X-15. Drawing heavily but not uncritically upon Becker’s and Boston’s insights, it too pro­vides an interesting perspective, and is quot­ed several times in the pages that follow.4

Lessons Learned (or not)

The X-15 was designed to achieve a speed of Mach 6 and an altitude of 250,000 feet to explore the hypersonic and near-space envi­ronments. More specifically, its goals were:

(1) to verify existing (1954) theory and wind tunnel techniques;

(2) to study aircraft structures under high (1,200 degrees Fahrenheit) heating;

(3) to investigate stability and control problems associated with high-altitude boost and reentry; and

(4) to investigate the biomedical effects of both weightless and high-g flight.

All of these design goals were met, and most were surpassed. The X-15 actually achieved Mach 6.70, 354,200 feet, 1,350 degrees Fahrenheit, and dynamic pressures over 2,200 pounds per square foot.5 In addition, once the original research goals were achieved, the X-15 became a high-altitude hypersonic testbed for which 46 follow-on experiments were designed.

Unfortunately due to the absence of a subse­quent hypersonic mission, aircraft applica­tions of X-15 technology have been few. Given the major advances in materials and computer technology in the 30 years since the end of the flight research program, it is

unlikely that many of the actual hardware lessons are still applicable. That being said, the lessons learned from hypersonic model­ing, simulation, and the insight gained by being able to evaluate actual X-15 flight test results against wind tunnel and predicted results, greatly expanded the confidence of researchers during the 1960s and 1970s.

In space, however, the X-15 contributed sig­nificantly to both the Apollo and Space Shuttle programs. Perhaps the major contribu­tion was the final elimination of a spray-on ablator as a possible thermal protection sys­tem for the Space Shuttle. This would likely have happened in any case as the ceramic tiles and metal shingles were further developed, but the operational problems encountered with the (admittedly brief) experience on X-15A-2 hastened the departure of the abla­tors. Although largely intangible, proving the value of man-in-the-loop simulations and pre­cision “dead-stick” landings have also been invaluable to the Space Shuttle program.

The full value of X-15’s experience to designing advanced aircraft and spacecraft can only be guessed at. Many of the engi­neers (including Harrison Storms) from the X-15 project worked on the Apollo space­craft and the Space Shuttle. In fact, the X-15 experience may have been part of the reason that North American was selected to build later spacecraft. Yet X-15’s experience is overshadowed by more recent projects and becomes difficult to trace as systems evolve through successive programs. Nonetheless, many of those engineers are confident that they owe much to the X-15, even if many are at a loss to give any concrete examples.

Political Considerations

John V. Becker, arguably the father of the X-15, once stated that the project came along at “ … the most propitious of all possible times for its promotion and approval.” At the time it was not considered necessary to have a defined operational program in order to conduct basic research. There were no “glamorous and expensive” manned space projects to compete for funding, and the gen­eral feeling within the nation was one of try­ing to go faster, higher, or further. In today’s environment, as in 1968 when Becker was commenting, it is highly unlikely that a pro­gram such as the X-15 could gain approval.6

This situation should give pause to those who fund aerospace projects solely on the basis of their presumably predictable outcomes and their expected cost effectiveness. Without the X-15’s pioneering work, it is quite possible that the manned space program would have been slowed, conceivably with disastrous consequences for national prestige.7

According to Becker, proceeding with a gen­eral research configuration rather than with a prototype of a vehicle designed to achieve a specific mission as envisioned in 1954 was critical to the ultimate success the X-15 enjoyed. Had the prototype route been taken, Becker believed that “… we would have picked the wrong mission, the wrong struc­ture, the wrong aerodynamic shapes, and the wrong propulsion.” He also believed that a second vital aspect to the success of the X-15 was its ability to conduct research, albeit for very short periods of time, outside the sensi­ble atmosphere.®

The latter proved to be the most important aspect of X-15 research, given the contribu­tions it made to the space program. But in 1954 this could not have been foreseen. Few people then believed that flight into space was imminent, and most thought that flying humans into space was improbable before the next century. Fortunately, the hypersonic aspects of the proposed X-15 enjoyed “virtu­ally unanimous approval,” although ironical­ly the space-oriented results of the X-15 have been of greater value than its contributions to aeronautics.9

A final lesson from the X-15 program is that success comes at a cost. It is highly likely that researchers can never accurately predict the costs of exploring the unknown. If you under-

stand the problems well enough to accurately predict the cost, the research is not necessary. The original cost estimate for the X-15 pro­gram was $10.7 million. Actual costs were still a bargain in comparison with those for Apollo, Space Shuttle, and the International Space Station, but at $300 million, they were over almost 30 times the original estimate.10 Because the X-15’s costs were not subjected to the same scrutiny from the Administration and Congress that today’s aerospace projects undergo, the program continued. One of the risks when exploring the unknown is that you do not understand all the risks. Perhaps politi­cians and administrators should learn this par­ticular lesson from this early and highly suc­cessful program.