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.

All 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

Six 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.

Over 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.