Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

One of the unique items included in the X-15 design was a side-stick controller. Actually, the airplane included two side sticks: one on the right console for the aerodynamic controls, and one on the left console for the ballistic controls. The right and center controllers were linked mechanically and hydraulically to provide simultaneous movement of both sticks; however, the side stick required only one-third as much movement to obtain a given stabilizer motion.[182]

NASA had installed a similar side stick in one of the North American YF-107A aircraft to gain experience with the new controller. A review of early X-15 landing data (using the side-stick) revealed a "striking similarity" with landings made in the YF-107. Despite large differences in speed and L/D ratios, the variations in angle of attack, normal acceleration, pitching velocity, and horizontal stabilizer position exhibited the same tendencies for the pilot to over-control the airplane using the side stick. During the YF-107 program, several flights were generally required before a pilot became proficient at using the controller and could perform relatively smooth landings; the same was true of the X-15.[183]

Regarding the side-stick controller, Bob White commented that "the side aerodynamic control stick designed for the X-15 has received the usual critical analysis associated with a departure from the conventional." As pilots reported their experiences using the side stick, North American began making minor modifications to correct undesirable characteristics. In the end, the company found that most of the initial design features were satisfactory. The most frequent complaint was the location of the stick in relation to the pilot’s arm, since the stick had been located based on Scott Crossfield’s input, and other pilots differed in size and proportions. However, Crossfield was a strong proponent of the side stick and North American soon devised a way to adjust the stick into one of five different fore-aft locations prior to flight based on individual pilot preference. After this, the side stick gained favor rather quickly.-11841

The all-moving horizontal stabilizers deflected symmetrically for longitudinal control (elevators) and differentially for lateral control (ailerons). The rolling tail that had caused so much controversy within the government early in the program proved to be quite satisfactory in operation.

According to Bob White, "the pilot is not aware of what specific type of lateral control is allowing the roll motion. His only concern is in being able to get the aircraft response he calls for when

deflecting the control stick___ From experience to date [after 45 flights], the rolling tail has

provided a good rolling control for the X-15, and there have been no undesirable aircraft motions coupled in any axis because of lateral-control deflection."11881

Conventional rudder pedals actuated the movable portions of the dorsal and ventral vertical stabilizers. Just prior to the landing flare, the pilot would jettison the lower portion of the dorsal stabilizer to provide sufficient ground clearance; otherwise, the dorsal rudder would contact the ground before the landing skids. Speed brakes were located on each side of the fixed portion of the dorsal and ventral stabilizers. Irreversible hydraulic actuators actuated all of the aerodynamic control surfaces.11861

The aerodynamic controls were effective up to about 150,000 feet. Nevertheless, many X-15 pilots manually used the ballistic control system in addition to the aerodynamic controls above 100,000 feet, and the MH-96 on X-15-3 automatically began blending in the ballistic control system thrusters above 90,000 feet. As Neil Armstrong, who was a principle engineer on the MH- 96, commented, "a rule of thumb is that when dynamic pressure on control surfaces reduces to 50 psf, there should be a switchover from aerodynamic to reaction control." Despite some early concerns about controlling a vehicle above the sensible atmosphere, in practice it quickly became routine.11871

The Westinghouse-manufactured stability augmentation system (SAS) dampened the aerodynamic

controls in all three axes. The system consisted of three rate gyros, two pitch-roll servocylinders, one yaw servocylinder, and various electronics, displays, and controls. Essentially, the system included a channel for each axis that sensed the aircraft rate of change in pitch, roll, and yaw, and automatically provided signals to the respective servocylinders to move the horizontal and vertical stabilizers to oppose the airplane angular inputs. An additional interconnect damper, called "yar," provided a crossfeed of the yaw-rate signal to the roll damper. This interconnection was necessary for stability at high angles of attack, primarily because of the high roll input of the lower rudder. The yar interconnect was disabled when the lower rudder was removed during later flights. The authority of the SAS was equal to the pilot’s authority in pitch and yaw, and to twice the pilot’s authority in roll. The pilot could turn dampening on or off for each individual axis, and select the damping gain for each axis. Originally, the SAS gyro package was located in the instrument compartment behind the pilot. However, a vibration at high gains reported by Scott Crossfield during the first X-15 captive flight resulted in North American moving the gyros to the center of gravity compartment under the wings, thus removing the gyro from a point influenced by fuselage bending.-1188-

North American incorporated two side-stick controllers in the X-15 cockpit. The controller on the right console operated the aerodynamic flight control systems while the controller on the left operated the ballistic control system thrusters. The aerodynamic controller was mechanically linked to the conventional center stick. In X-15-3, the MH-96 adaptive flight control system automatically blended the ballistic thrusters in when needed, eliminating the need for the pilot to use the left side-controller. (NASA)

The SAS caused numerous pilot comments. During early flights below Mach 3.5, the dampers used moderate gains and the pilots quickly expressed a desire for "a stiffer aircraft," particularly in pitch and roll. North American subsequently increased the gain, resulting in generally favorable pilot opinions. It is interesting to note that at angles of attack above 8 degrees with low damper gain or with the roll damper off, pilots had great difficulty in controlling the lateral and directional motions to prevent divergence. This was primarily because of an adverse dihedral effect that was present above Mach 2.3. Although this was of some concern to the pilots, and the subject of a great deal of investigation by the researchers, the airplane exhibited acceptable handling characteristics as long as the dampers were functioning. In general, the airplane exhibited about the same handling qualities expected based on extensive simulations at Ames, and the pilots thought the damper-off handling was slightly better than the simulator predicted, but still considered the natural stability to be marginal.-1189

The SAS was unique for the time because it provided 10 pilot-selectable gain rates for each axis. However, the system experienced some annoying problems during development and early operations. During the first studies using the fixed-base simulator, the dampers sustained unwanted limit cycles (or continuous oscillations) from linkage lags and rate limiting. Pilots later observed the phenomenon in flight. The frequency of the limit cycle was about 3.2 cycles per second, resulting in changes in bank angle of about 1 degree. This limit cycle was not constant,

changed due to control input, and had a tendency to "beat." North American was unable to identify a way to eliminate the limit cycles, but modified the electronic filter to reduce its lag. This greatly lowered the amplitude of the limit cycles, and the pilots found the results acceptable.-190

The X-15 made extensive use of a stability augmentation system to dampen the aerodynamic controls in all three axes. The SAS was unique for the time since it provided ten pilot-selectable gain rates for each axis via rotary switches in the cockpit. Flight simulations showed that it would be nearly impossible for a pilot to control the X-15 in some flight regimes without the SAS. (NASA)

Although the modified filter greatly improved the issue with the limit cycles in roll, a new problem soon arose. It became apparent during ground tests that it was possible to excite and sustain a SAS-airplane vibration at 13 cycles per second with the modified filter. A breadboard of the modified filter was flown (flight 2-12-23) at higher damper gains, but Scott Crossfield failed to excite the vibration. During the rollout after landing, however, Crossfield encountered a severe vibration that required disabling the SAS. This experience led to the mistaken belief that the vibration could only occur on the ground. To prevent a recurrence, North American installed a switch that automatically lowered the gain whenever the pilot extended the landing gear.

However, five flights later (2-14-28), Joe Walker encountered a 13-cps vibration during reentry from 169,600 feet. After the flight, Walker reported that the vibration was the most severe he had ever encountered (or ever wanted to). The shaking was triggered by pilot inputs at 130 psf dynamic pressure and continued until the damper gain was reduced and the dynamic pressure climbed above 1,000 psf. Fortunately, the amplitude of the shaking was constrained by the rate limits of the control surface actuators. North American and NASA began investigating the problem again.-11911

TOTAL HOURS

TOTAL FLIGHTS

Failures of the stability augmentation system contributed to the maintenance woes suffered by the X-15 early in the flight program, but oddly, most of the failures were on the ground; the system seldom failed in flight. Nevertheless, an auxiliary stability augmentation system was added to the first two airplanes as insurance against an SAS failure. The X-15-3 did not carry an SAS or ASAS since the mH-96 adaptive flight control system performed both functions. (NASA)

The problem was that the lightly damped horizontal stabilizers were excited at their natural frequency (13 cps) by pilot inputs to the control system. The gyro picked up this vibration and the dampers were able to sustain the vibration with input to the control surfaces. Engineers also found a second natural frequency for the stabilizers at 30 cps. North American subsequently installed notch filters in the SAS and pressure feedback valves in the control surface actuators, eliminating the vibrations.-192

The SAS proved to be unreliable in the beginning, but fortunately most failures occurred during ground testing. The program recorded only seven in-flight failures during the first 78 flights (defined as NB-52 takeoff to X-15 landing). Of these failures, one was an electronic module, three were malfunctioning cockpit gain switches, and three were broken wires in the X-15. Engineers ultimately traced all except the failed electronics module to human error.192

There was never any doubt that the X-15 flight program would take place at Edwards AFB, California. However, Edwards would play a key role as infrastructure was developed to support the X-15. The program was an involved undertaking, and the operational support required was extensive. Logistically, Edwards would become the linchpin of the entire effort.

MUROC TO EDWARDS

The Mojave Desert-called the "high desert" because of its altitude-is approximately 100 miles northeast of Los Angeles, just on the other side of the San Gabriel Mountains. First formed during the Pleistocene epoch, and featuring an extremely flat, smooth, and hard surface, Rogers Dry Lake is a playa, or pluvial lake, that spreads out over 44 square miles of the Mojave, making it the largest such geological formation in the world. Its parched clay and silt surface undergoes a cycle of renewal each year as desert winds sweep water from winter rains to smooth the lakebed out to an almost glass-like flatness.-Ш-

Lieutenant Colonel Henry H. "Hap" Arnold decided that Rogers Dry Lake would make a "natural aerodrome," and in September 1933 the Army Air Corps established the Muroc Bombing and Gunnery Range as a training site for squadrons based at March Field near Riverside, California. It continued to serve in that capacity until 23 July 1942, when it became the Muroc Army Air Field. During World War II the primary mission at Muroc was to provide final combat training for aircrews before their deployment overseas.-12-

Until the beginning of World War II, the Army Air Corps conducted the majority of its flight-testing at Wright Field, Ohio. However, the immense volume of testing created by the war was one of the factors that led to a search for a new location to test the first American jet fighter, the Bell XP- 59A Airacomet. The urgent need to complete the program immediately dictated a location with year-round flying weather. In addition, the risks inherent in the radical new technology used in the aircraft dictated an area with many contingency landing areas, and one that minimized the danger of crashing into a populated area. After examining a number of locations around the country, the Army Air Forces selected a site along the north shore of Rogers Dry Lake about six miles away from the training base at Muroc.-13-

When Bell test pilot Robert Stanley arrived at the base in August 1942, he found just three structures: an unfinished hangar, a wooden barrack, and a water tower. Things would begin to change quickly as more than 100 people arrived at the base to support the project. On 2 October 1942, Stanley made the first "official flight" of the XP-59A (it had actually lifted off for the first time on the previous day during high-speed taxi tests), introducing flight-testing to the high desert. Only five years later, on 14 October 1947, Captain Charles E. "Chuck" Yeager became the first man to exceed (barely) the speed of sound in level flight when he achieved Mach 1.06 (approximately 700 mph) at 42,000 feet in the Bell XS-1 research airplane. Muroc’s place in the history books was firmly established.[4]

However, with the arrival of the X-1, flight-testing at Muroc began to assume two distinct identities. The Air Force typically flew the research airplanes, such as the X-3, X-4, X-5, and XF – 92A, in conjunction with the NACA in a methodical fashion to answer largely theoretical questions. The bulk of the testing, however, focused on highly accelerated Air Force and contractor evaluations of prototype operational aircraft, and was often much less methodical as they tried to get new equipment to combat units as quickly as possible at the height of the Cold War.[5]

Not surprisingly, the rather informal approach to safety that prevailed during the late 1940s, and even into the 1950s, was one of the factors that contributed to a horrendous accident rate. There were, of course, a number of other factors. The corps of test pilots at Muroc remained small and commonly averaged more than 100 flying hours per month. They flew a wide variety of different types and models of aircraft, each with its own cockpit and instrument panel configuration. Chuck Yeager, for example, reportedly once flew 27 different types of airplanes in a single one-month period. The year 1948 was particularly tragic, with at least 13 fatalities recorded at or near the base. One such fatality was that of Captain Glen W. Edwards, who was killed in the crash of a Northrop YB-49 flying wing on 4 June 1948. In December 1949 the Air Force renamed the base in his honor, while other pilots have streets named after them.[6]

Edwards AFB, California, hosted the X-15 flight program. The "new" main base complex is located at the center left in this photo, with the NASA Flight Research Center being slightly above the main base on the edge of the lakebed. Rogers Dry Lake was the planned site for all X-15 landings, and 188 times, it worked out that way. Two would land at Cuddeback, one at Delamar, four at Mud, one at Rosamond, one at Silver, and one at Smith Ranch; the X-15-3 broke up in flight and did not land on its last flight. (U. S. Air Force)

On 25 June 1951, the government established the Air Force Flight Test Center (AFFTC) at Edwards, and a $120 million master plan was unveiled for construction at the base. Part of the appropriation paid to remove the Atcheson, Topeka, and Santa Fe railroad from the northern portion of Rogers Dry Lake and bought out the silt mines that had been located along the route. However, the major undertaking was to relocate the entire base two miles west of the original South Base location and construct a 15,000-foot concrete runway. With the increased number of flight test programs at the base, the natural surfaces of the Rogers and Rosamond dry lakebeds took on even greater importance as routine and emergency landing sites. The first AFFTC commander, Brigadier General Albert Boyd, later commented that the dry lakes were nothing less than "God’s gift to the U. S. Air Force." That same year, the USAF Test Pilot School moved from Wright Field to the high desert.[7]

Bell would have seemed a logical choice to develop the new research airplane since the company had developed the X-1 series and X-2 high-speed research aircraft that had ushered in a new era of flight research. They were also doing studies on much faster vehicles in search of the BoMi boost-glide bomber. The company had direct experience with advanced heat-resistant metals and with the practical issues of powering manned aircraft using liquid-fueled rocket engines. In fact, Bell had an in-house group that built rocket engines, including one under consideration for the X-15. Lawrence Bell, Robert Woods, and Walter Dornberger were already legends. Somehow, all of this was lost in the proposal.[48]

Unsurprisingly, Bell engineers decided the Bell-manufactured XLR81 was the most promising engine, and it became the baseline; however, the XLR30 offered certain advantages and Bell proposed the alternative D-171B variant using this engine. The design had three XLR81s arranged in a triangular pattern with one engine mounted above the others, much like the later Space Shuttle Orbiter. Bell believed that the ability to operate a single XLR81 at its 8,000-lbf "half­thrust" setting was an advantage, based on a reported comment from the NACA that "a high percentage of the flight testing would be conducted in the lower speed and altitude ranges." Bell did not record who made the comment, but given that only 36 of the eventual 199 X-15 flights were below Mach 3, it was obviously incorrect. Unfortunately, it seemed to influence the Bell proposal throughout.-^

A throttle lever controlled engine thrust by actuating a series of switches arranged so that thrust increased as the pilot pushed the lever forward in the conventional manner. The initial switch fired the first engine at its 8,000-lbf half-power setting. The second switch caused this engine to go to 14,500-lbf full power. The next switch fired the second engine at its 14,500-lbf setting, resulting in a 29,000-lbf thrust. The last switch started the third engine, resulting in a full thrust of 43,500 lbf. The engineers did not consider the slightly asymmetrical thrust provided by the triangular engine to be a problem.[50]

The selection of a conventional aerodynamic configuration simplified the arrangement of the fuselage and equipment systems. The fuselage had six major sections. The forward section contained the pilot’s compartment, nose gear, and research instrumentation, followed by the forward oxidizer tank. A center section housed the wing carry-through, main landing skids, and pressurization systems, followed by the aft oxidizer tank and fuel tank. The aft section contained the engine and empennage. A pressurized area just behind the cockpit contained the hydraulic and electrical systems, environmental control equipment, and research instrumentation. The hydrogen peroxide supply, the main landing gear, and the structure for suspending the research airplane from the carrier aircraft were located in the center of the fuselage between the two oxidizer tanks. A flush-mounted canopy minimized drag and avoided discontinuities in the airflow that could result in thermal shocks on the glass.[51]

One of the unfortunate consequences of selecting the XLR81 was that the red, fuming nitric acid required a large storage volume, which caused the oxidizer to be stored in two tanks (one on either side of the wing carry-through). This was necessary to maintain the center of gravity within acceptable limits, but complicated the attachment of the wing to the fuselage. Bell investigated bolting the wing directly to the oxidizer tank or passing the structure through the tank. This, however, was not considered ideal "since it would present a hazard in the form of a possible fatigue failure as the result of the combination of localized wing loads and tank pressurization loads." The 61S-T aluminum propellant tanks were generally similar to those used on the Bell MX – 776 (GAM-63) RASCAL missile program.[52]

The wing had a leading-edge sweep of 37 degrees to moderate center-of-pressure shifts at subsonic and transonic speeds. Engineers had discovered that higher sweep angles resulted in pitch-up and damping-in-roll difficulties that Bell wanted to avoid. At the same time, researchers found that the aspect ratio was not particularly important, so it was set to provide decent subsonic and landing attitudes. The total wing area was 220 square feet, allowing a reasonable landing speed of 170 mph.1531

Approximately one-third of the vertical stabilizer area was located under the fuselage to maintain high-speed stability. This ventral stabilizer was added "to provide sufficient directional stability to M=7.0. This lower surface is very effective at high Mach numbers because of the compressive flow field below the wing." Bell attempted to provide as much area as possible while still maintaining sufficient clearance for the D-171 to be loaded into the carrier aircraft without resorting to a folding or retractable design. Before the airplane could land, the pilot would jettison the ventral stabilizer to provide sufficient clearance for the landing gear. A parachute lowered the ventral to a safe landing, although Bell noted that deleting the parachute would save a little weight, with the ventral becoming expendable.-1541

Landing skids were a logical choice to save weight but the exact nature of these skids was the subject of some study. A two-skid arrangement-one forward and one aft—was considered too unstable during landing, although a drag chute could be used to overcome this, as was done on the SM-62 Snark missile. Still, the arrangement was undesirable. A nose wheel with a single aft skid was statically stable, but model tests showed that it was dynamically unstable. A good pilot could land the aircraft with this arrangement, but Bell rejected the configuration because it placed too placed a great burden on the pilot. Two forward skids and a single aft skid offered neutral stability, but experience with the Sud-Est SE5003 Baroudeur showed that it still placed a high burden on the pilot. Bell finally selected a conventional tricycle arrangement with a nose wheel and two main skids located midway aft on the fuselage. Both the nose gear and skids were retractable and covered with doors, unlike the eventual X-15 where the rear skids did not retract inside the fuselage.1551

The fully loaded airplane weighed 34,140 pounds at launch, including 21,600 pounds of propellants. The estimated landing weight was 12,595 pounds. Based on a launch at Mach 0.6 and 40,000 from a B-36 carrier aircraft, Bell estimated that the D-171 could exceed the basic performance requirements. The projected maximum altitude during the "space leap" was 400,000 feet. At altitudes between 85,000 and 165,000 feet, the velocity was in excess of 6,600 fps, with a maximum of 6,850 fps at 118,000 feet.1561

A set of reaction controls used eight hydrogen peroxide thrusters: one pointed up and another down at each wing tip for roll control, one up and one down at the tail for pitch control, and one pointing left and one right at the tail for yaw control. A single control stick in the cockpit controlled the thrusters and aerodynamic control systems. Bell noted that "no criteria are available for the design of such controls," so the company arbitrarily assumed that aerodynamic controls would be ineffective at dynamic pressures below 10 psf. Bell expected the X-15 to operate in flight regimes that required reaction controls for about 115 seconds per high-altitude mission, and provided 550 pounds of hydrogen peroxide. Operating all of the thrusters for the entire 115- second flight (something that obviously would not happen) used only 49% of the available propellant.1571

ЕЕЧ^МДС WINS
ISE.26
25%HORTAlL
IE5(I3FT 91Ц }
OVER-*-ALL SPAN
Z5.33
0+
126.3
STft
STA
pilot’s eye
-AIRCRAFT
TREAD1
t FUSELД6Е ft HCflli REF LINE
Pi A A DEPTH
s N….______ Ip
STROKE 10 В
WHEEL BASE STROKE
JETTISON ABLE
VENTHAL FIN

536 (44 FT в IN) OVER-ALL LENGTH

ThrsE Vievr Ol Airplane, Model DLVi

The Bell entry in the X-15 competition bore a subtle resemblance to their X-2 research airplane that had such an unhappy career. Bell had considerable theoretical experience with thermal protection systems as part of its ongoing work on the Air Force BoMi and RoBo programs, and much practical experience with high-speed X-planes such as the X-1 and X-2. Ultimately, the Bell proposal finished third in the competition. (Bell Aircraft Company)

The researchers at Bell did not believe the hot-structure data provided by the NACA from the Becker studies. This may have reflected a bias on the part of Bell engineers who had been working on alternate high-speed structures for several years. The Bell proposal contained a detailed discussion on why conventional or semi-conventional structures would not work, and the hot – structure concept fell into the latter category.

A survey of available materials showed that Inconel X was the best available high-temperature alloy for a conventional structure—the same conclusion reached at Langley. Bell estimated that an Inconel X airframe would weigh approximately 180% as much as an equivalent structure made from aluminum 75S-T. Bell noted that the "usual expedient" of adding additional material would not relieve all of the thermal stresses unless sufficient material were added to absorb the entire expected heat load, leading to a structure that would be too heavy to accomplish its assigned mission. The Bell engineers also thought that "the stresses and deformations produced by temperature gradients cannot generally be reduced by the simple addition of more material."[58]

The second approach was to use what Bell called semi-conventional structures. In addition to adding sufficient material to absorb the heat load, the designers attempted to develop structures that would be free to warp and bend as they heated. Bell believed that all of the design approaches they tried would fail in operation. For instance, Bell designers decided it would be impossible to use integral propellant tanks in a hot-structure airframe because "no suitable

structural arrangement has been found for attaching propellant tank ends and baffles to the outer shell without introducing serious thermal stresses." When they investigated the use of separate tanks, they found the weight penalty to be severe.

Bell also briefly investigated actively cooled structures, such as the "water wall" concept developed early in the BoMi studies. The basic structure weighed little more than a conventional aluminum airframe, but including the weight of coolant and pumping equipment resulted in the concept being 200-300% heavier.-1591

In a fuzzy look at things to come for the Space Shuttle, Bell investigated a structure protected by external insulation and concluded that "[c]eramic materials would seem attractive for insulation, except that the present state of development for this application is not well enough advanced…."*69!

As it turned out, Bell had an alternative, developed during the ongoing BoMi studies. This unique double-wall structure used air as an insulator, permitting heat transfer by radiation in addition to conduction. The outer wall consisted of a 0.005-inch-thick Inconel X skin panel, approximately 4 inches long and 8 inches wide, welded to a corrugated sheet of Inconel X. The corrugations were

0. 3125 inch deep with 0.3125-inch spacing. An outside retaining strip of Inconel X (approximately 1.25 inches wide and 0.056 inch thick), running along each edge, held each panel in place. The edges of the corrugations, top and bottom, were joggled 0.056 inch so that the outer surface was flush. In the bottom, joggled portion of each of the corrugations, 0.015-inch – deep protruding dimples provided support for the outer wall panels to the inner structure. The combination of the dimple and joggle raised the outer wall panel to a height slightly over 0.375 inch from the inner structure, providing the necessary air space for insulation. The retaining strip was broken into 4-inch lengths to permit expansion relative to the inner structure, and two screws and two floating inverted-type anchor nuts held each retaining strip to the structure.

These provided the required air space between the inner and outer walls to minimize heat conduction into the inner structure. Narrow strips of fibrous insulation located beneath the retaining strips prevented boundary air from leaking between the outer panels and their retaining strips.1611

This arrangement allowed the outer wall panels to expand in the direction parallel to the corrugations simply by sliding further under the retaining strips. Separating the skin into elements only 4 inches wide accommodated the thermal expansion of the outer skin of the outer wall. In order to prevent the parallel, free edges of this very thin skin from lifting due to aerodynamic forces, "Pittsburgh" joints interconnected the edges of adjacent panels. This is a standard sheet-metal joint, but in this application "considerable clearance" was used so that the adjacent panels were free to move relative to one another to permit thermal expansions.*621

Two pins set in the basic structure restrained each of the 4-by-8-inch outer wall panels against lateral movement. One of these pins fit snugly into a hole in a small square plate welded to the bottom of two adjacent corrugations, thus preventing any translations. The other pin fit into a slotted hole, permitting expansion but preventing rotation. Thus the outer wall had complete freedom of expansion relative to the underlying aluminum alloy structure. Its shallow depth (0.3125 inch) and uniformity minimized thermal gradients through the wall. Although they cost considerably more to manufacture, Bell proposed using Haynes 188 or similar alloys in areas where temperatures exceeded the capability of Inconel X. Researchers expected that ceramic panels or various sandwich materials could eventually replace the Inconel outer wall.*631

The primary advantage of the double-wall system was that it weighed some 2,000 to 3,000 pounds less than an Inconel X hot structure. The double-wall construction also minimized development time, according to Bell, since the primary structure of the airframe was conventional in every way, including its use of aluminum alloys. This limited, in theory, any development problems for the outer wall. Interestingly, Bell believed that the double-wall construction provided an advantage when it came to research instrumentation. Since the outer panels were easily removable, it greatly simplified the installation of thermocouples, strain gages, pressure orifices, and other sensors.[64]

The wing and empennage used the same double-wall construction, but the leading edges were of unique construction. Bell noted that "it cannot be assumed that the optimum design has been selected since the evaluation…requires a greater time than afforded in this proposal period." Bell engineers did not believe they could accurately predict the heat transfer coefficients, but noted that the equilibrium temperature of the leading edges could approach 2,500°F. At this temperature, Bell was not sure that any metallic alloy would be sufficient, or whether a ceramic was necessary instead. Nevertheless, Bell proposed a metal heat sink. A 0.040-inch-thick Inconel X shell formed the desired leading-edge shape with a chord-wise dimension of approximately 6.5 inches (normal to the leading edge). Properly spaced, welded ribs provided attachment fittings, and intermediate ribs provided support to ensure that air pressure would not deform the shell. Lithium, beryllium, magnesium, or sodium (listed in descending order of preference) filled the leading edge shell as a heat sink.*651

All of the leading edges were easily removable, facilitating the substitution of various types of leading-edge designs for flight research and evaluation. The wing leading edges were single­piece structures on each side of the airplane. The inboard attachment was fixed, but the other attach points were designed to allow span-wise motion to accommodate differences in linear expansion between the wing structure and the leading edge.[66]

At first, Bell selected a Boeing B-50 Superfortress for its carrier aircraft, mainly because it had experience with this type of airplane from the X-1 and X-2 programs. It soon became apparent, however, that the B-50 did not have the capability to carry the D-171 and its support equipment to the altitudes required. Attention then turned to the Convair B-36. A comparison of the two aircraft showed that the B-36 had a much better rate of climb, and could launch the D-171 at Mach 0.6 and 40,000 feet compared to Mach 0.5 and 30,000 feet for the B-50.*67

The basic installation in the B-36 was straightforward, and Convair already had data on the B-36 carrying large aircraft in its bomb bays from Project Fighter Conveyer (FICON).*68 Loading the D – 171 was the same as loading the X-1 or X-2: a pair of hydraulic platforms under the B-36 main landing gear allowed the ground crew to tow the research airplane underneath the raised bomber. Alternately, the bomber straddled an open pit in the ground and crews raised the research airplane into the bomb bays. The D-171 took up the forward three of the four B-36 bomb bays in order to keep the mated center of gravity at an acceptable position. This also minimized B-36 control problems when the D-171 dropped away from the bomber.-*69*

As had been the case with previous research airplanes, the mated pair would take off with the research airplane pilot in the carrier aircraft—not in the D-171. As the carrier climbed through 15,000 feet, the pilot would climb into the research airplane and the canopy would close. Equipment checks of the research airplane

would begin as the carrier climbed through 35,000 feet. When the checks were completed, the carrier aircraft would drop the research airplane.*701

Along with the baseline D-171 design, Bell proposed two slight variations. The D-171A two-seat version was a required response to the government request for proposal. Bell noted that that since the equipment compartment had a differential pressure of 2.5 psi to support the instrumentation, a small increase in structural weight would allow the higher pressure differential necessary to carry a second crew member. The observer would be seated on an upward-firing ejection seat and have two small side windows in a separate canopy. The gross weight was unchanged at 34,140 pounds since the weight of the observer and the ejection seat exactly matched the research instrumentation load normally carried. Performance was also unaffected because the propellant load was identical.1711

The second variant was the D-171B powered by a Reaction Motors XLR30 "Super Viking" engine. Although Bell preferred to use three XLR81 engines, it realized that the XLR30 offered some advantages. The D-171B had an empty weight about 200 pounds more than the baseline configuration, but a launch weight of some 1,000 pounds less. Bell listed the fact that the XLR30 used liquid oxygen as its oxidizer as its greatest disadvantage since this would require a top-off system in the carrier aircraft, which Bell believed would add "considerable greater weight" to the B-36.172 Bell also thought that the minimum thrust capability of the XLR30 (13,500 lbf) was unsatisfactory compared to the Hustler engine (8,000 lbf). On the positive side, the internal propellant tank arrangement for the XLR30-powered airplane was superior because only a single oxidizer tank would be needed, greatly simplifying propellant management for center-of-gravity control. Bell agreed that the single XLR30 thrust chamber (versus three for the XLR81 installation) was also an advantage. Although no two-seat XLR30 aircraft was described in the proposal, it is easy to imagine a two-seat variant since the forward fuselage was identical to that of the D-

171.IZ3]

Bell expected to have the basic design established six months after the contract was signed, and to finalize the design after 18 months. The first airplane would be available for ground tests 34 months after the start of the contract. Bell indicated that they attempted to compress the schedule into the required 30 months, but were unable to do so. It would take 40 months to get to the first glide flight, and six additional months before the first powered flight. Bell expected the government to provide a complete test engine in the 27th month, and a final propulsion system had to be delivered to Bell simultaneously with the first aircraft entering ground tests.1741

The X-15 landing gear was somewhat unusual, both in its approach and in its simplicity. The system consisted of a dual nose wheel and a pair of aft skids. Initially the cast magnesium nose wheels were fitted with standard aircraft tires pressurized with 240-psi nitrogen. The skids consisted of a 4130-steel skid and an Inconel X strut that was attached to the fuselage by trunnion fittings and through bell crank arms that were attached to shock struts inside the aft fuselage. The skids were free in pitch and roll, but fixed in yaw for parallel alignment. Drag braces attached to the fuselage ahead of the trunnion fittings and to the skids at the strut attachment pin. Bungee springs kept the skid in a nose-up position just before landing. Instead of retracting inside the fuselage, the skids and struts folded forward against the outside of the fuselage when retracted. The pilot lowered the landing gear by pulling a handle in the cockpit that attached via
cable to the uplock hooks and released the gear. North American designed the landing gear for an 11,000-pound airplane with a sink rate of 9 fps, touching down between 190 and 230 mph at an angle of attack of approximately 6 degrees.-1124

Three major test series of the landing-gear system were conducted prior to the first glide flights: 1) a dynamic-model test of stability during the landing run, 2) nose-wheel shimmy tests using the actual nose gear, and 3) full-scale skid tests at the lake-bed landing site.-1125

North American used the model tests to investigate the stability of the tricycle arrangement. Engineers constructed a 1/10-scale model that accurately reflected the size, weight, and mass moments of inertia for yaw and roll, but did not simulate the aerodynamic characteristics of the X-15. Scale-size metal skids were manufactured so that they could be installed in either the original mid-fuselage location proposed by North American, or the aft fuselage location eventually built. North American catapulted the model along a concrete runway using a 100-foot length of 0.625-inch-diameter shock cord. High-speed movie cameras on overhead towers recorded each run. The tests revealed some minor nose-wheel instability, which the company subsequently corrected.-1124

Researchers at Langley then tested the revised full-scale nose gear using the landing-loads track facility at speeds up to 125 mph. These tests evaluated the nose gear on smooth concrete, uneven concrete, wet pavement, sandy pavement, uneven tire pressure, one flat tire, and unbalanced wheels. Given that the X-15 was to land only on dry lake beds, some of the tests seemed extreme. Throughout the tests the co-rotating wheel arrangement proved extremely stable, with no tendency to shimmy. Researchers, therefore, concluded the shimmy damper and torque links were unnecessary and North American subsequently removed them, saving 25 pounds.-1127

North American conducted the landing-gear-skid tests on Rogers Dry Lake during April 1958. For these tests, researchers mounted the complete main gear on a two-wheel trailer vehicle and towed it behind a truck at speeds up to 70 mph. After the truck reached full speed, an electric switch actuated a bomb-release solenoid that dropped a 6,000-pound load on the skid landing gear. Instruments on the gear recorded vertical and drag loads, and shock-strut position. High­speed cameras mounted in the truck and trailer recorded the motion of the gear and skids. Test runs included straight-line landing on smooth lake surfaces, "fishtail" runs on rutted and bumpy areas near the edges of the lake, and one landing on the concrete runway just to make sure. The results of all the tests were satisfactory. Skid wear on the lake beds was light, and engineers determined that the skids would last for three or four landings. The tests revealed that the X-15 should leave depressions approximately 0.03 inch deep in the lake bed. As expected, wear on the concrete runway was severe, but the tests showed the X-15 could land on concrete if necessary.-1124

Despite all the tests, the first four actual landings pointed out several deficiencies in the landing gear, mainly because the aircraft was heavier than anticipated and sink rates were slightly higher. North American replaced the shock struts with higher-capacity units, and strengthened some of the structure inside the fuselage. The fourth landing resulted in X-15-1 breaking in half. This was not strictly a design error; Scott Crossfield had been unable to fully jettison the propellants prior to an emergency landing, and the airplane was significantly overweight. However, the landing gear contributed because the gas and oil mixture in the shock strut foamed, keeping the rear skids from absorbing as much of the impact as they should have. This forced a higher than normal load on the nose gear, aggravating the structural problem caused by being overweight.729

The X-15 was unique, even among X-Planes, in using a landing gear consisting of rear skids and a nose wheel. The skids solved several problems for designers since they were relatively small and could be stowed mostly outside the airframe. Interestingly, the X-15 landing gear was lowered by the pilot pulling a mechanical handle that was connected to a cable that released the uplock hooks and allowed a bungee to extend the skids. A similar system would have been used on the X-20 Dyna-Soar if that program had not been cancelled. (NASA)

In addition, during some of the early landings, engineers found that the nose wheel tire marks left on the dry lake bed were not continuous. After initial contact, the tire marks became very faint or disappeared for short distance and then reappeared. This puzzled the engineers since all of the early drop tests of the landing gear had been satisfactory.-1^00

The engineers became concerned that the nose-gear extension mechanism was not working properly. Normally, technicians manually retracted the nose gear after attaching the X-15 to the NB-52, and then they pumped dry nitrogen gas into the shock strut to preload it to 1,404 psi. Charlie Feltz had suggested this method as a way to minimize the size and weight of the nose gear compartment. What the engineers discovered was that upon lowering the landing gear, an orifice in the strut trapped the nitrogen gas below it and most of the shock-absorption oil above it. The design of the metering valve was such that it prevented a rapid change in position of the oil and nitrogen in the 10 seconds between gear extension and wheel touchdown. To better understand the problem, engineers conducted additional dynamic tests using the original test apparatus. Initial tests operated the apparatus with the nose gear serviced in the extended position, as had been done in the original tests. The performance appeared normal. The engineers then modified the test rig to allow the gear to be serviced in the retracted position, as was done on the airplane. A delay of 10 seconds was introduced between the gear being lowered and touching down, and the abnormal behavior was reproduced almost exactly.-1201

At first, engineers modified the orifice in the shock, but this failed to resolve the problem. After additional tests, the engineers determined that they could not pressurize the strut in its retracted (compressed) position. Unfortunately, the nose wheel compartment was not large enough to allow the nose gear to be retracted in its extended position. The final solution was to mount redundant nitrogen bottles on the gear strut itself. When the gear reached its fully extended position, a valve actuated and released the nitrogen to pressurize the strut. This worked and the first modified nose gears were available in July 1960. However, the engineers kept evaluating the problem and, later in the program, changed the design again. This time they installed a floating piston inside the strut that kept the oil and gas separated. Technicians could now pressurize the strut in the compressed position before flight, allowing the removal of the nitrogen bottles.-1202

During 1961, engineers instrumented the skids to gather additional data on skid landing gear in support of the Dyna-Soar program and possible future vehicles, such as the space shuttle. Standard NASA instrumentation was used to provide airplane upper-mass response, shock-strut force and displacement, main – and nose-gear drag forces, nose-gear vertical force, horizontal – and vertical-stabilizer setting, horizontal stabilizer load, airplane angle of attack, and airplane pitch velocity during the impact and slideout portion of a landing. Tests were conducted at the end of normal research flights while the pilots landed normally and performed specific control movements during slideout. Phototheodolite cameras on the ground furnished data for landing coordinates, airplane altitude, flight-path velocity, and vertical velocity at touchdown. The instrumentation remained on all three airplanes for the remainder of the flight program to monitor the severity of each landing.-1203

Landing-gear loads continued to be high, despite the minor modifications made early in the flight program. An analytical study of the landing dynamics showed that several important parameters affecting the landing loads were actually aerodynamic factors. One of the primary culprits was a down-load from the horizontal stabilizer caused by both the pilot and SAS. Immediately prior to touchdown, the stabilizer trim position was set to between 4 and 5 degrees with the leading edge down. If the pilot pulled back on the stick and put the leading edge further down, the landing loads increased. If the pilot pushed the stick forward to get the leading edge up, the loads decreased. Another factor affecting the gear loads was lift from the wing. Unfortunately, the severe nose-down angle of the X-15 after nose-gear touchdown effectively pushed the airplane into the ground, further increasing the stress on the landing gear. Unfortunately, this was an unchangeable consequence of the airplane configuration, and a similar problem occurred on the space shuttle orbiters.[204]

The most severe problem, however, was weight. The design landing weight had been 11,000 pounds. The initial landing weight of the airplane was 13,230 pounds, and by 1965 this had crept up to 15,500 pounds on a routine basis. Emergency landings with a partial propellant load could be as high as 17,000 pounds. The only way to execute safely a landing at 17,000 pounds was for the pilot to perform an active push maneuver to obtain low horizontal stabilizer settings. This would still exceed the design load on the airplane, but would most probably be below the yield (destructive) limit.-1203

The nose gear was more conventional, consisting of a pair of wheels and tires. Note how short the nose gear strut is, resulting in severe loads during landing. The length of the nose strut was largely dictated by the amount of room available to stow it when retracted. Space shuttle orbiters suffer from a similarly short nose gear strut. (NASA)

By 1965 the problem was no longer one of understanding the nature of the loads, but rather one of how best to reduce them. North American introduced a near-constant series of minor modifications to the skids, their struts, and the surrounding structure in an effort to provide additional margin for the landing gear. Of all the factors that affected gear loads, the most

difficult to control—without restricting the research role of the airplane—was weight. Engineers determined they could reduce landing gear loads if they prevented the stabilizer angle from moving in the leading-edge-down direction during landings. Training the pilots to perform a push maneuver during landing accomplished this. In addition, North American installed a switch in the cockpit that disengaged the SAS at main gear touchdown to prevent the dampers from forcing the stabilizer leading edge down. Experience showed that under normal circumstances the pilots were efficient at pushing the stick at the right moment, even though the maneuver had to occur within 0.4 second after main gear touchdown to be effective in reducing gear loads. However, this maneuver was unnatural for the pilots, who tended to revert to habits formed through long hours of previous experience during emergencies and pull back on the stick. For this reason, the FRC began developing an automatic stick-kicker.-1206

In fact, this very condition occurred during the Jack McKay’s accident in X-15-2. The airplane was 1,000 pounds heavy with residual propellants, and as he landed, McKay pulled back on the stick, driving the stabilizer leading edge down to its maximum value. As it happened, the flaps failed on this flight and resulted in a down-load on the main wing, and therefore on the main landing gear. The combined resulted was a severely overstressed gear that, of course, failed.-207

Following the accident with X-15-2, engineers considered designing a new landing gear for the modified X-15A-2. The original location of the nose gear was approximately 23 feet ahead of the center of gravity, and moving the landing gear back could significantly reduce main-gear loads, with the forward bulkhead of the liquid-oxygen tank representing the rear-most location in the existing airframe. One of the ideas engineers investigated was moving the nose landing gear rearward to the instrumentation compartment behind the pilot. The nose gear would occupy the lower half of the compartment, with most of the instrumentation that normally resided there being moved forward to the old nose-gear compartment ahead of the pilot.208

However, fiscal and schedule constraints involved with repairing the aircraft precluded such major modifications, and the existing gear locations were reused on the modified airplane. Nevertheless, engineers made some basic changes, such as increasing the shock strut stroke from 3.66 inches to 5.03 inches, and modifying the relief valve setting from 17,000 pounds to 22,000 pounds.

North American manufactured two sets of strengthened struts—one set that was the same length as the original units, and another set that was lengthened from 53.6 inches to 59.0 inches. The longer units provided sufficient ground clearance to land with the functional ramjet sill attached to the ventral, but it appears that all flights of the X-15A-2 used the shorter units. Engineers also lengthened the skid 6.75 inches. In addition, engineers made some changes to the nose gear, primarily increasing the shock strut stroke to accommodate the increased length of the airplane. North American lowered the trunnion 9 inches to allow an attitude at nose-gear touchdown similar to that of the basic X-15. Despite these changes, the landing dynamics of the new gear were not appreciably changed, and X-15A-2 inherited most of the deficiencies of the basic system.209-

In addition, during the first part of 1965, North American investigated increasing the capability of the X-15A-2 gear. NASA wanted the maximum landing weight with the "short" main landing gear to increase to 16,374 pounds normal and 18,519 pounds emergency. The "long" gear used with the ramjet would increase to 17,855 pounds normal and 20,000 pounds emergency. A preliminary analysis indicated that incorporating the stick-kicker and changing the shock-strut relief valve setting would allow these increases. However, at 20,000 pounds there were concerns about whether the fuselage structure just behind the cockpit would be strong enough.219

Researchers installed a prototype stick-kicker in the FRC fixed-base simulator in May 1965 to determine the optimum stick forces. Subsequently, engineers installed the first stick-kicker in X – 15-3 during the weather down period at the beginning of 1966, and in X-15-1 by the end of that year. Apparently, NASA never installed the stick-kicker in X-15A-2. An emergency landing at 17,700 pounds, the highest landing weight yet encountered by the program, illustrated the effectiveness of the stick-kicker.-211

North American also conducted an investigation during early 1965 to determine the modifications needed to increase the landing weight of X-15-1 and X-15-3 to 16,000 pounds normal and 17,000 pounds emergency. The analysis included the use of the stick-kicker to rotate the horizontal stabilizer at landing to reduce main skid loads, although this would not eliminate the need to modify the skids for the higher weights. Preliminary studies showed that relocating the nose gear trunnion (as done on X-15A-2) would appreciably reduce landing loads on the other two airplanes, even without the addition of a stick-kicker.-212-

At the same time, engineers studied the feasibility of incorporating a third main skid attached to the fixed portion of the ventral stabilizer. This third skid could redistribute the landing loads and relieve the critically stressed gear components, particularly if either the stick-kicker of the landing flaps failed to operate. NASA installed the skid on X-15-3 in time for flight 3-52-78 on 18 June 1966, and by the end of 1966 it had used the third skid for four landings. These landings, however, were not at a sufficient weight to require the skid, and during the slideout the third skid contacted the lake surface with little or no load applied to it. Nevertheless, the third skid seemed like a good idea and NASA modified X-15-1 in time for flight 1-71-121 on 22 March 1967.

NASA did not install the third skid on X-15A-2 since it would have interfered with the ramjet installation.215-

The X-15A-2 experienced some of the more bizarre problems with landing gear. On the second flight (2-33-56) of the modified aircraft, after obtaining a maximum Mach number of 5.23, the nose gear unexpectedly extended as the airplane decelerated below Mach 4.2. William P. Albrecht, the X-15 project engineer for the flight, wrote that "[u]pon arrival in the Edwards area, chase aircraft confirmed that the nose gear was extended fully, and that the tires appeared badly burned, although still inflated. Major Rushworth elected to land the X-l5, and skillfully did so. The tires remained intact on touchdown but disintegrated after approximately 300 feet of rollout, the remainder of the 5,630 foot rollout being taken by the magnesium rims of the nose wheels." Considering the circumstance, it was a good landing.-214!

The subsequent investigation revealed that the nose-gear uplock hook was severely bent, the point of the hook having opened by approximately 0.25 inches. However, engineers determined the hook had not bent far enough to release the gear without the occurrence of some other deflection. The pilot lowered the X-15 landing gear via a simple cable arrangement that connected the landing gear extension handle in the cockpit to the uplock hook. Engineers measured the slack in the landing gear actuating cable (used to compensate for fuselage expansion due to heating effects) at 1.18 inches after the flight, within the specified limits. However, an analysis by North American indicated that the thermal growth of the fuselage was approximately 1.90 inches for this flight. This pointed out that the slack allowance was inadequate. Since the same mechanism operated all three landing gear components, it could not be ascertained in advance which of the three landing gears (left main, right main, or nose) would be first affected by partial actuation of the extension system, since that one with the least cable loading (due to friction, air loads, etc.) would tend to operate first. NASA duplicated the failure in the High-Temperature Loads Calibration Laboratory by simulating the fuselage expansion and applying heat to the nose-gear door. As Albrecht observed afterward, "Needless to say some modification to the landing gear mechanism seems to be in order."-215-

North American modified the cable to provide 2.25 inches of slack to compensate for thermal expansion. Although the engineers did not believe the problem affected the other two airplanes, they also received the modification. The only major drawback to this modification was that the pilot now had to pull the gear handle through almost 14 inches of travel to release the landing gear, which led to several complaints. Subsequently, engineers at the FRC designed a differential pulley that shortened the pull to 11 inches.-1216

These modifications, however, did not totally fix X-15A-2. During the next flight (2-34-57) on 29 September 1964, Bob Rushworth experienced a similar, but less intense, noise and aircraft trim change at Mach 4.5: the small nose-gear scoop door opened. This had already happened several times during the flight program on all three airplanes, fortunately without disastrous results.

There were two initial thoughts on how to fix the problem. The first was to eliminate the scoop door altogether; except for inspection and servicing, the door would be bolted shut prior to flight. Alternately, engineers could design a new uplock for the scoop door that featured a positive retention of the door roller on the uplock hook. In the end, NASA selected the second route and installed a new uplock hook, scoop door hook, and associated bell cranks.-217

NASA conducted two captive-carry flights of X-15A-2 to verify proper deployment of the redesigned nose scoop door and nose landing gear after cold soak. During flight 2-C-58 the nose gear required approximately 5.4 seconds to lock down—an unacceptably long time. Subsequent inspection showed that an incorrect orifice had been installed in the nose-gear snubber (which controlled the deployment rate). NASA installed the correct orifice, and the deployment time on flight 2-C-59 was an acceptable 2.7 seconds. Researchers collected data on both these captive flights data regarding the scoop door hook position and scoop door roller loads. Hook movement was negligible (less than 1/16 inch) and NASA subsequently modified the other two airplanes as well. Jack McKay took X-15A-2 on a perfect flight (2-35-60) on 30 November 1964.

Like the rear skids, the nose wheel was lowered by the pilot pulling a handle that was connected to a cable that released the uplocks. On two separate flights, the nose gear extended while Major Robert A. Rushworth was flying the X-15A-2 above Mach 4, resulting in some interesting flying characteristics and two sets of burned tires. Researchers finally deduced that the fuselage of the airplane was expanding due to heat, and that the landing gear release cable did not have enough slack to compensate. North American increased the slack in the cable, but the pilots now had to pull the release handle more than 14 inches to get the landing gear to deploy. (NASA)

However, it did not end there. Rushworth was in the cockpit again for the next fight (2-36-63) of X-15A-2 on 17 February 1965 when the right main skid extended at Mach 4.3 and 85,000 feet. The chase pilot was able to verify that the gear appeared structurally sound, and Rushworth managed to make a normal landing. Investigation of the right-hand main skid uplock revealed that thermally induced bowing of the main strut caused excessive loading of the main uplock hook. Ground heating tests of the main-gear struts during a "hot-flight" profile caused bending of the hook and release of the gear. Consequently, NASA modified the main-gear uplock to include a stronger hook, a Belleville washer mounting system to accommodate approximately 0.14 inch bowing of the strut, and a stronger support structure. In addition, it was necessary to reinforce the sheet-metal fuselage longeron structure around the main-gear drag-brace anchor fittings. While the repair itself was not complicated, access was extremely difficult since it

To test the hypothesis that the fuselage expanded more than the release cable, researchers at the Flight Research Center heated one of the X-15 forward fuselages using heat lamps. The test confirmed the theory. (NASA)

This ended the significant problem with the landing gear on the X-15A-2 (and the other airplanes), although the ever-increasing landing weight continued to be a concern and a set of small modifications (such as stronger struts) continued to be implemented until the end of the flight program.

On 30 September 1946, Walter C. Williams and four other engineers from NACA Langley arrived at the Muroc Army Air Field to assist in flight-testing the XS-1. It was supposed to be a temporary assignment, and the group did not even have an official name, although they called themselves the NACA Muroc Flight Test Unit. This marked the beginning of the joint USAF-NACA research airplane program that would culminate with the X-15. The NACA Muroc Flight Test Unit received permanent status from Hugh Dryden on 7 September 1947, with Walt Williams named as head of a group that now numbered 27 people.[8]

By the time the NACA redesignated the unit the High-Speed Flight Research Station (HSFRS) on 14 November 1949, roughly 100 people worked for Williams, who was named chief of the station. In February 1953, the Air Force formally leased 175 acres at the north end of the main taxiway to the NACA for a permanent installation.[9]

On 26 June 1954, the now-200-strong NACA contingent moved from its primitive quarters on South Base to a new headquarters, located in Building 4800 on the north side of the new Edwards flight line. This building still serves as the core of the Dryden Flight Research Center (DFRC). The new facility cost $3.8 million. In contrast to the small NACA station at Edwards, the Air Force contingent at the AFFTC numbered over 8,000. The NACA organization was renamed the High­Speed Flight Station (HSFS) on 1 July 1954, with Williams still in charge.-10

On 27 January 1953, a ground-breaking ceremony was held at the site of the future NACA High­Speed Flight Station on the new main base at Edwards AFB. Shown in the photo are (left to right) Gerald Truszynski, Head of Instrumentation Division; Joseph Vensel, Head of the Operations Branch; Walter Williams, Head of the Station, scooping the first shovel full of dirt; Marion Kent,

Head of Personnel; and California state official Arthur Samet. (NASA)

On 1 October 1958, the National Aeronautics and Space Administration (NASA) replaced the NACA, and on 27 September 1959, NASA Headquarters redesignated the HSFS the Flight Research Center (FRC). By the time the station became a center, Williams was gone. At the behest of Hugh Dryden, on 14 September 1959 he had joined Project Mercury (the first American manned space effort) as its operations director. In his place came Paul F. Bikle, a Pennsylvanian with long experience in flight-testing at the nearby AFFTC. Bikle replaced Williams on 15 September 1959, oversaw its transition to the FRC, and remained for the next 12 years. Bikle believed in doing things quietly and with a minimum of fuss and outside attention. "Under Paul Bikle," one engineer recalled, "we were well aware that headquarters was 3,000 miles away." Like Williams before him, Bikle impressed those who encountered him with his bluntness, drive, and engineering sense.-1111

The first challenge faced by Bikle was shifting from planning for the X-15 program to conducting it. He needed people, and asked Ira Abbott at Headquarters for authority to add 80 new positions to the rapidly growing X-15 team. The personnel and facilities at the FRC expanded throughout the 1960s, with the budget going from $3.28 million in 1959 to $20.85 million in 1963, and to $32.97 million in 1968. The staff went from 292 to a peak of 669 in 1965; by the end of the X – 15 program, the staff was down to 566.-112

The employees of the NACA High-Speed Flight Station are gathered for a 1954 photo shoot on the front steps of building 4800, the new NACA facility at Edwards AFB. This new building was considerably larger than the earlier NACA buildings on South Base to support a staff that had increased from 132 in 1950 to 250 in 1954. As the workload increased and more research flights were completed the complement of employees grew to 662 in 1966, largely the result of the X-15 flight program. (NASA)

The X-15 was an airplane of accelerations. When an X-15 pilot looks back on his X-15 flights. it is the accelerations he remembers. The first of these sensations was the acceleration due to B-52 lift. which held the X-15 at launch altitude and prevented it from falling to Earth. When the X-15 pilot hit the launch switch. the B-52 lift was no longer accessible to the X-15. The X-15 fell at the acceleration due to Earth’s gravity. which the pilot recognized as "free fall" or "zero g." Only when the pilot started the engine and put some "g" on the X-15 was this sensation of falling relieved.

The next impression encountered on the X-15 flight came as the engine lit. just a few seconds after launch. A 33.000-pound airplane was accelerated by a 57.000-lbf engine. resulting in a chest-to-back acceleration of almost 2 g. Then. as the propellant burned away and the atmosphere thinned with increasing altitude. the chest-to-back acceleration increased and the drag caused by the atmosphere lessened. For a standard altitude mission (250.000 feet). the weight and thrust were closer to 15.000 pounds and 60.000-lbf at shutdown. resulting in almost 4-g chest-to-back acceleration. The human body is not stressed for 4 g chest to back. and by shutdown the boost was starting to get a little painful. Milt Thompson once observed that the X – 15 was the only aircraft he had ever flown where he was glad when the engine quit.

X-15 ready for flight on the flight line. (NASA)

On a mission to high altitude (above 250,000 feet), the pilot did not regain any sensible air with which to execute a pullout until about 180,000 feet, and could not pull 1 g of lift until 130,000 feet. Flying a constant angle of attack on reentry, the pilot allowed g to build up to 5, and then maintained 5 g until the aircraft was level at about 80,000 feet. There was a deceleration from Mach 5 at 80,000 feet to about Mach 1 over the landing runway, and the pilot determined the magnitude of the deceleration by the use of speed brakes. This ended the high-g portion of the flight, except for one pilot who elected to start his traffic pattern at 50,000 feet and Mach 2, and flew a 360-degree overhead pattern from that starting point.

Flight to high altitude represented about two-thirds of the 199 X-15 flights. Flights to high speed or high dynamic pressure accounted for the other third, and those flights remained well within the atmosphere for the entire mission. The pilot of a high-speed flight got a small taste of chest-to – back acceleration during the boost (thrust was still greater than drag, but not by such a large margin as on the high-altitude flights). The deceleration after burnout was a new sensation. This condition was high drag and zero thrust, and it had the pilot hanging in his shoulder straps, with perspiration dripping off the tip of his nose onto the inside of his face plate.

Milt Thompson collected anecdotes about the X-15 that remain astonishing to this day. Milt noted that at Mach 5, a simple 20-degree heading change required 5 g of normal acceleration for 10 seconds. Milt also pointed out that on a speed flight, the (unmodified) X-15-1 accelerated from Mach 5 to Mach 6 in six seconds. These were eye-opening numbers at the time of the X-15 program.

Those of us in the program at flight 190 thought that the X-15 would continue indefinitely. Then, on flight 191, Major Michael J. Adams experienced electrical irregularities that made the inertial flight instruments unreliable and may have disoriented him. In any case, at peak altitude (266,000 feet), the X-15 began a yaw to the right. It reentered the atmosphere, yawed crosswise to the flight path, and went into a high-speed spin. It eventually came out of the spin but broke up

during the reentry, killing the pilot.

The loss of the airplane and pilot was the death knell for the entire program. Program management decided not to fly the X-15A-2 again, and to fly X-15-1 only for calendar year 1968. The X-15 flew its last flight on 24 October of that year, and then faded into aeronautical history.

William H. Dana

Test Pilot, Dryden Flight Research Center Pilot, last X-15 flight

PREFACE: ROCKETS OVER THE HIGH DESERT

years since the flight program ended, it is unlikely that many of the actual hardware lessons are still applicable. Having said that, the lessons learned from hypersonic modeling and pilot-in-the – loop simulation, and the insight gained by being able to evaluate actual X-15 flight test results against wind-tunnel and theoretical predictions greatly expanded the confidence of researchers during the 1970s and 1980s.m

It would not have surprised anybody involved that the actual X-15 technology did not find further application. Researchers such as John Becker and Norris Dow, and engineers like Harrison Storms and Charlie Feltz never intended the design to represent anything other than a convenient platform to acquire aero-thermo data. Becker once opined that proceeding with a general research configuration rather than a prototype of a vehicle designed to achieve a specific mission was critical to the ultimate success of the X-15. Had the prototype route been taken, Becker believed, "we would have picked the wrong mission, the wrong structure, the wrong aerodynamic shapes, and the wrong propulsion." They are good words of advice.-12

In fact, the decision to pursue a pure research shape was somewhat controversial at the beginning. Kelly Johnson, for one, believed the vehicle should be adaptable as a strategic reconnaissance aircraft. Indeed, several of the proposals for the X-15 sought to design a vehicle with some future application. Nevertheless, the original Langley concept of a vehicle optimized to collect the desired data as safely as possible ultimately won. As Harley Soule told Harrison Storms, "You have a little airplane and a big engine with a large thrust margin. We want to go to 250,000 feet altitude and Mach 6. We want to study aerodynamic heating. We do not want to worry about aerodynamic stability and control, or the airplane breaking up. So, if you make any errors, make them on the strong side. You should have enough thrust to do the job." North American succeeded brilliantly.-13

It had taken 44 years to go from Kitty Hawk to Chuck Yeager’s first supersonic flight in the X-1. Six more years were required before Scott Crossfield got to Mach 2 in the D-558-2 Skyrocket. A remarkably short three years had passed when Mel Apt coaxed the X-2 above Mach 3, before tumbling out of control to his death. There progress stalled, awaiting the arrival of the three small black airplanes that would more than double the speed and altitude milestones.

The X-15 flight program began slowly, mostly because the XLR99 was not ready. This undoubtedly worked in the program’s favor since it forced the engineers and pilots to gain experience with the airplane and its systems prior to pushing the envelope too far. The first 20 months took the X-15 from Crossfield’s glide flight to essentially duplicating the performance of the X-2: Mach 3.5 and 136,500 feet. Then the XLR99s arrived and things got serious. Six days after the last flight with the interim XLR11s, Bob White took X-15-2 past Mach 4, the first time a piloted aircraft had flown that fast. Mach 5 fell, also to Bob White, four months later. Mach 6, again to White, took six more months. Once the X-15 began flying with the ultimate engine, it took only 15 flights to double the maximum Mach number achieved by the X-2.

Altitude was a similar story. Iven Kincheloe was the first person to fly above 100,000 feet, in the X-2 on 7 September 1956. Thirteen flights with the big engine allowed Bob White to fly above 200,000 feet for the first time. Three months later, he broke 300,000 feet. Once it began flying with the ultimate engine, the X-15 took only 19 months to double the maximum altitude achieved by the X-2. These were stunning achievements.

1 hour, 25 minutes, and 33 seconds of hypersonic flight. At the other end of the spectrum, just two flights were not supersonic (one of these was the first glide flight), and only 14 others did not exceed Mach 2. It was a fast airplane. Similarly, there were only four flights above 300,000 feet (all by X-15-3), but only the initial glide flight was below 40,000 feet.[4]

Despite appearances, however, the program was not about setting records.-151 The actual speed and altitude achieved by the program was not the ultimate test, and the fact that the basic airplane never achieved its advertised 6,600 feet per second velocity was of little consequence. What interested the researchers was the environment in which the airplane flew. They wanted to study dynamic pressures, heating rates, and total temperatures. More specifically, the goals were to:

1. Verify existing (1954) theory and wind-tunnel techniques

2. Study aircraft structures and stability and control under high (2,000 psf) dynamic pressures

3. Study aircraft structures under high (1,200°F) heating

4. Investigate stability and control problems associated with high-altitude boost and reentry

5. Investigate the biomedical effects of both weightless and high-g flight

The X-15 achieved all of these design goals, although Project Mercury and other manned space efforts quickly eclipsed the airplane’s contribution to weightless research. The program ultimately achieved a velocity of 6,629 fps (with X-15A-2), 354,200 feet altitude, 1,350°F, and dynamic pressures over 2,200 psf.[6]

With 40 years of hindsight, it is apparent that the most important lessons to be learned from the X-15 concern not the hardware, but the culture. The world was different during the 1950s, certainly within the government-contracting environment. The military and NACA initiated and funded the X-15 program without congressional approval or oversight, although this was not an effort to hide the program or circumvent the appropriations process. The military services had contingency funds available to use as they saw fit. They ultimately needed to explain to Congress and the White House how they spent the funds, but there was little second-guessing from the politicians. This allowed the program to ramp up quickly and absorb the significant cost overruns that would come. Following its likely origin in February 1954, the Air Force awarded the X-15 development contract in September 1955 and North American rolled out the first airplane in October 1958. The maiden glide flight was in June 1959, just over five years from a gleam in John Becker’s eye to Scott Crossfield soaring over the high desert. It could not happen today.

There is a story in the main text about a meeting Harrison Storms attended at Edwards, and some important words of wisdom: "[TJhere is a very fine line between stopping progress and being reckless. That the necessary ingredient in this situation of solving a sticky problem is attitude and approach. The answer, in my opinion, is what I refer to as ‘thoughtful courage.’ If you don’t have that, you will very easily fall into the habit of ‘fearful safety’ and end up with a very long and tedious-type solution at the hands of some committee. This can very well end up giving a test program a disease commonly referred to as ‘cancelitis,’ which results in little or no progress."[7]

Storms must have had a crystal ball. In today’s environment, the system will not allow programs to have problems. If the Air Force and NASA were trying to develop the X-15 today, Congress would cancel it long before the first flight. A series of configuration changes and production problems added weight and lowered the expected performance before the airplane flew. The XLR99 engine was tremendously behind schedule, so much so that the program selected interim engines just to allow the airplane to begin flying. Ultimately, however, the airplane and the engine were hugely successful. Compare this to how the X-33 program reacted to issues with its composite propellant tanks.

When Crossfield finally released from the carrier aircraft on the initial glide flight in X-15-1, his landing was less than ideal. In today’s world, the program would have stood down to work out this issue and assess the risk. In 1959 North American made some adjustments and launched Crossfield again three months later. It was a short-lived reprieve. Less than 60 days later, Crossfield broke the back of X-15-2 during a hard landing that followed an in-flight abort. Instead of canceling the program, the X-15 went back to the factory for repair. Three months later Crossfield was flying again.

During the initial ground-testing of the ultimate XLR99 engine in X-15-3 at Edwards, an explosion destroyed the airplane. Nobody was seriously hurt and North American subsequently rebuilt the airplane with an advanced flight control system intended for the stillborn X-20 Dyna – Soar. The program was flying two months later using X-15-1 and the rebuilt X-15-3 went on to become the high-altitude workhorse.

It was the same across the board. When Jack McKay made his emergency landing at Mud Lake that essentially destroyed X-15-2, the Air Force did not cancel the program. Five weeks later Bob White made a Mach 5.65 flight in X-15-3; McKay was his NASA-1. North American rebuilt X-15- 2 and the airplane began flying again 18 months later. Jack McKay went on to fly 22 more X-15 flights, although the lingering effects of his injuries shortened his lifetime considerably.

In each case the program quickly analyzed the cause of the failure, instituted appropriate changes, and moved on. Always cautious, never reckless. No prolonged down times. No thought of cancellation. It would not happen that way today. One of the risks when extending any frontier is that you do not understand all the risks.

Paul Bikle, the director of the Flight Research Center, had long warned that the flight program should end when it achieved the design speed and altitude. However, the X-15s provided an ideal platform for follow-on experiments that had little or nothing to do with the design aero-thermo research mission. The temptation was too great, and NASA extended the flight program several years. Bikle knew that eventually the odds would catch up with the program. The day they did, Mike Adams was at the controls of X-15-3, and the consequences were as bad as anything Bikle could have imagined. The crash killed Mike Adams and destroyed X-15-3. Even so, the program made sure it learned from the accident and was flying again less than four months later. This time, however, it would not be for long. Eight more flights were conducted before the program ended when funding expired at the end of 1968.

John 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 made his comment, it is highly unlikely that a program such as the X-15 could gain approval.-18

Dill Hunley, a former DFRC historian, once opined that "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." It is certain that the development of the Space Shuttle would have carried a far greater risk if not for the lessons learned from the development and flight-testing of the X-15.

Fifty years later, the X-15 experience still provides the bulk of the available hypersonic data available to aircraft designers.-19

Perhaps we have not learned well enough.

Dennis R. Jenkins Cape Canaveral, Florida

The Model 684 was a conceptual follow-on to the successful D-558-1 and D-558-2 research airplanes that Douglas had built under Navy sponsorship beginning in 1944. It also benefited from the experience Douglas gained from investigating the Model 671, which is generally referred to as the D-558-3, during the "High Altitude and High Speed Study."1751 Douglas took a unique approach to designing the structure of the Model 684, somewhat following the hot-structure concept developed at NACA Langley, but adding several new twists. The most obvious was that instead of Inconel X, Douglas chose a magnesium alloy "of sufficient gage that the structure [sic] temperature will not exceed 600°F." The use of copper for the leading edges permitted temperatures approaching 1,000°F. All of the proposed structure could be manufactured using conventional methods.1761

The Model 684 weighed only 25,300 pounds fully loaded and had a landing weight of 10,450 pounds, making it the lightest of the competitors. The single Reaction Motors XLR30 allowed the airplane to exceed the performance specifications, with a maximum 6,655 fps velocity at 110,000 feet altitude expected. Douglas noted that it appeared "possible to explore altitudes up to

approximately 375,000 feet without exceeding the structural limits of the airplane or the physiological limits of the pilot."[77]

Oddly, Douglas did not just dust off the work it had accomplished for the Navy on the D-671 and submit it for the X-15 competition. The D-684 was a much different design that intrigued many of the evaluators during the competition, and Douglas ultimately lost largely because the Inconel X hot-structure on the North American entry better supported thermal research. The Douglas proposal finished second in the competition. (Douglas Aircraft Company)

The most controversial aspect of the Douglas proposal was the material selected for the hot structure. In advance, Douglas defended this action: "a careful study was made of all the various metals that have satisfactory strength properties at elevated temperatures." During this study Douglas eliminated everything except Inconel X and a thorium-zirconium alloy of magnesium called HK31.178

Douglas noted that the structural properties of Inconel X and HK31 fell off rapidly as the temperature approached 1,200°F and 600°F, respectively, and observed that "[s]ince we are concerned with heating of short duration, not with stabilized temperature, the specific heat[79] of the material becomes a very important factor." The study showed that HK31 had twice the specific heat of Inconel X. Since the strength-to-weight ratios of the two metals were roughly equal, Douglas believed the magnesium alloy was a better choice. "One must realize that less heat will be re-radiated by magnesium because of its lower temperature," allowing less internal insulation around critical components such as the instrumentation and pilot.[80]

attained." Douglas also found that less internal structure was required to support the magnesium skin. Combined with the ability to machine the metal more precisely, Douglas estimated that a magnesium airframe would weigh approximately 25% less than an equivalent Inconel X airframe. The weight of the Model 684 seemed to confirm this.181

The choice of magnesium was not a surprise, since Douglas had manufactured the fuselage for both the D-558-1 and D-558-2 from a similar alloy. Nevertheless, it was a departure from the C-110M titanium-alloy structure investigated for the earlier Model 671. Of course, that airplane would have required an ablative coating-something that was not desirable on the X-15 because of the desire to do research into high temperature structures.-1821

Douglas summarized the advantages of HK31 as follows:1831

1. There will be far fewer parts due to the greater skin thickness and all of the parts can be manufactured and assembled with existing manufacturing facilities. An Inconel airplane would require special tooling and techniques [further details omitted].

2. The reduction in the required amount of internal structure provides greater access to all control and instrumentation equipment, wiring, hydraulic actuators and piping, and allows better placing of this equipment.

3. The 600°F temperature limit for the magnesium greatly eases the temperature problem for the pilot and equipment in the airplane. This should result in less design time.

4. A psychological advantage in favor of magnesium might be that the pilot would prefer to fly in a gray airplane at 600°F rather than in one that is glowing red at 1200°F.

The last point was probably questionable, but the reduction in internal structure was striking. Photographs accompanying the proposal showed a typical wing panel constructed of each material. The HK31 panel used skin almost 0.5 inch thick and needed support only along the four edges of the panel. The Inconel X structure, on the other hand, used skin only 0.1 inch thick and needed support across its entire surface. Both samples could withstand the same aero and thermal loads.1841

The HK31 skin was thick throughout the vehicle. Skin gages on the upper half of the fuselage varied from 0.38 inch near the nose to 0.12 inch at the end of the ogive. On the lower surface, the gage varied from 0.92 inch near the nose to 0.25 inch at the end of the ogive on the bottom centerline. The skin on the upper surface of the wing was 0.35 inch thick over the entire exposed area, and 0.25 inch thick where the wing crossed inside the fuselage. The lower surface of the wing tapered from 0.64 inch near the leading edge to 0.43 inch 4 feet aft of the leading edge.1851

The wing used seven truss-type spars that ran continuously through the fuselage. The skin used thick, tapered sheets stiffened by the spars and truss-type chord-wise ribs. Increasing the skin thickness at the wing-fuselage intersection created heat sinks to absorb the heating load. All of the leading edges (wing, empennage, and canopy frame) were made of copper that extended far enough aft to conduct the extremely high temperatures in the stagnation areas away to cooler areas of the airframe.1861

The forward part of the fuselage consisted of the pressurized instrumentation compartment and the cockpit. If desired, the airplane could carry an observer in lieu of the normal research instrumentation, although the accommodations were cramped, and the observer had no visibility and sat in an awkward position. Another small, pressurized compartment (2.5-psi differential) was located in the aft fuselage to contain the gyros, accelerometers, and other subsystems.1871

In case of an emergency, the entire forward fuselage separated from the rest of the airplane via explosive bolts and a JATO bottle located near the center of gravity of the nose section. Afterwards, a 5-foot-diamter metal drogue chute would deploy in the reefed position. When the load reached a predetermined level, the reefing device would automatically release and the metal drogue chute would fully open. A 50-foot-diamter fabric main parachute deployed when the load on the open drogue chute dropped below a predetermined value or the altitude reached 15,000 feet.[88]

Douglas hedged its bets slightly: "It is too early to determine whether this escape system will be satisfactory in the event of an emergency at extremely high altitudes, but no other system will be as good…. The jettisonable nose will be the most satisfactory system for escape under the high Mach number, high Q, and high G conditions at which this airplane is most likely to get into trouble." As events with the Bell X-2 would later show, the capsule concept did not significantly alter the chance of survival. Of course, the Douglas system did have one advantage over the X-2: as proposed for the Model 684, the entire nose would descend to the ground, at which time the pilot would unbuckle and walk out of the capsule. In the X-2, the pilot had to unbuckle and jump out of the capsule after it separated but before it hit the ground. This assumed that the pilot had remained conscious during what was sure to be violent tumbling and accelerations during the escape. The pilot of the Model 684 had a small back-type parachute "in case he prefers to bail out in the conventional manner." [89]

A liquid air supply provided a maximum differential pressure of 5 psi for the cockpit and instrumentation compartment. The pressurized areas were insulated from the structural heating by a 0.25-inch layer of high-temperature fiberglass insulation located near the skin, followed by a light-gage stainless-steel radiant barrier that was covered by another 1.5 inches of batt insulation. The liquid air also cooled a heat exchanged that conditioned the recirculated cockpit air to a constant 80°F, and the instrumentation compartment to 150°F. There was sufficient liquid air for 30 minutes of full-load operation, and a warning system told the pilot to turn off the instrumentation if the liquid air supply ran low. The pilot’s pressure suit used air diverted from the cockpit supply, and a small electric heater warmed the air to maintain the pilot’s comfort.-90

The windshield consisted of a 0.75-inch panel of high-temperature glass insulated by a 0.25- inch air gap from a 0.25-inch safety glass panel on the inside of the cockpit. Douglas calculated that the outer panel would not exceed 500°F, which was well within the capabilities of the glass. The tinted inner panel resisted radiant heat and ultraviolet light. One of the items Douglas had trouble with was developing a canopy seal. The heat surrounding the cockpit structure made a normal inflated rubber seal impractical. Engineers discovered that the preferred Teflon seal gave off a "small quantity of fluorine" between 400°F and 600°F. This was considered toxic and corrosive, but might be tolerable given that the cabin pressure differential was in the right direction (i. e., fumes would be expelled overboard). If a Teflon seal was used, it would have to be replaced after every flight.-1911

Unlike the other competitors, Douglas proposed a conventional landing gear consisting of two main wheels, a nose wheel, and a tail wheel. The nose gear was located far back on the fuselage (behind the cockpit), while the main gear retracted into compartments under the wing. The ventral stabilizer housed the tail wheel, which was needed because of the relatively high approach attitude of the research airplane. Ground-clearance issues during takeoff dictated that the ventral and tail wheels be retracted on the ground prior to loading in the carrier aircraft. They automatically rotated into the proper position for flight when the pilot started the auxiliary power units prior to launch.-921

A single liquid-oxygen tank was located forward of the wing, but to maintain the correct center of gravity there were two ammonia tanks: one in the upper fuselage over the wing carry-through and another behind the wing. All of the main propellant tanks were integral parts of the structure. Three hydrogen peroxide tanks were located under the wing carry-through between the main gear wells. A single 62-gallon tank powered the XLR30 turbopump, and two smaller tanks supplied the reaction control system. The Douglas proposal noted that the compartment that contained these tanks "must be kept clean to prevent combustion in the event of fuel spillage and it is therefore sealed, vinyl coated and vented to an adjacent compartment through a filter that will prevent dirt contamination."1931

Two completely independent power systems each used a separate Walter Kidde ethylene-oxide auxiliary power unit with sufficient propellant for a 30-minute flight. Each auxiliary power unit drove a hydraulic pump and an AC/DC generator, and operated simultaneously, although either could provide all the required power.194

The flight controls were completely conventional, with the all-moving horizontal stabilizer, rudder, and ailerons all being power-boosted. Hydraulically operated two-position speed brakes located in the extreme aft end of the fuselage provided a constant deceleration of 1.5-g when opened. The speed brakes automatically closed at pressures above 1,000 psf.1951

The Douglas proposal acknowledged that "there are many formidable problems in the design of an airplane to operate over the wide Mach number and altitude ranges encountered by this airplane." Douglas embraced the wedge principle developed by Charles McLellan at Langley, and used the shape for the vertical and horizontal stabilizers. Douglas also flared the aft fuselage to provide additional stability at high Mach numbers.-1961

"Flight out of the atmosphere is another new problem" that caused Douglas to provide a reaction control system with 12 hydrogen peroxide thrusters, two in each direction about each axis. Two completely independent systems were provided (hence the two thrusters at each location), and either system was capable of maneuvering the airplane. The thrusters were powerful enough to rotate (and stop) the airplane through an angle of 90 degrees in 14 seconds when both systems were operational. The pitch and yaw thrusters were rated at 50 lbf each, while the roll thrusters were rated at 12.5 lbf each. Because of the large uncertainties involved, Douglas provided 640% of the amount of propellant estimated necessary for a single flight. In a note of caution, Douglas "recommended that a device be constructed for the purpose of training the pilot in this type of flight."1971

The Model 684 was light enough that a Boeing B-50 Superfortress was a satisfactory carrier aircraft. This seemingly ignored the maintenance problems and low in-service rate of the B-29 and B-50 carrier aircraft experienced at Edwards, and was a radical step backwards from the apparent use of a B-52 in the earlier D-558-3 study. Surprisingly, the existing X-2 carrier aircraft required very little modification to accommodate the Model 684—mainly the front and rear bomb bay openings had to be made a little larger.1981

Douglas conducted preliminary wind-tunnel tests on the Model 684 on 21-22 April 1955 in the company-owned facility in El Segundo. Normally, Douglas would have used the more elaborate tunnel at the Guggenheim Aeronautical Laboratory at the California Institute of Technology (GALCIT), but there was insufficient time to build the more sophisticated model required at GALCIT. The El Segundo tunnel had a test cell that measured 30 by 45 inches and could generate a dynamic pressure of 60 psf. The tests did not generate any truly useful data, but demonstrated that the 6.5% scale model was reasonable stable at low speeds.1991

North American seemed to be at a disadvantage, having never built an X-plane of any description. The company, however, did have a great deal of experience in building early missile prototypes. Their Missile Development Division conducted Project NATIV experiments during the late 1940s using captured German V-2 rockets, and then built major parts of similar vehicles itself. The company had almost completed the design of the Navaho, a large intercontinental cruise missile designed to fly at Mach 3. In addition, the company had developed what were arguably the three highest-performance fighters of their eras: the P-51 Mustang of World War II; the F-86 Sabre, which made its mark in Korea; and the F-100 Super Sabre, the first operational supersonic aircraft. North American was also involved in studies that would eventually lead to the fastest and most advanced bomber ever built: the XB-70A Valkyrie. They were on a roll, and the designers embraced the idea of building a hypersonic aircraft.-1100

Unlike the other competitors, who went in their own directions, Hugh Elkin and the North American Advanced Design Group stayed fairly true to the configuration that John Becker and the team at Langley had proposed; in fact, the resemblance was striking. Their goal was also similar: "the design objective must be to provide a minimum practical and reliable vehicle capable of exploring this regime of flight. Limiting factors are time, safety, state of the art, and cost.’,[101]

North American truly grasped what the government was trying to accomplish with the project. The other competitors—even Douglas, who otherwise came closest-worked at designing an airplane that met the performance requirements. North American, on the other hand, "determined that the specification performance can be obtained with very moderate structural temperatures; however, the airplane has been designed to tolerate much more severe heating in order to provide a practical temperature band within which exploration can be conducted." Put another way, "This performance is attained without recourse to untested or complicated solutions to design problems. This should allow the major effort to be expended on obtaining the desired research information." This was, after all, the point of the whole exercise.-102

North American engineers spent a great deal of time talking to the researchers and other personnel at Edwards, recognizing that "a secondary, but important, factor considered in preliminary design is the desirability of meshing with the present operational pattern for research aircraft. By following the established pattern of operations, a considerable saving in learning time should be achieved." Given the significant increase in performance promised by the X-15, this was not completely possible, but it showed that North American was attempting to eliminate as many variables as possible. Along the same lines, North American did not attempt to design an operational aircraft, recognizing that a "compromise in favor of extreme simplicity in order to assure a high degree of ruggedness and reliability" would go a long way toward improving the aircraft’s research utility.102

An interesting passage from the proposal, especially considering the current trend toward trying to eliminate all programmatic risk, is found in the summary: "Detailed definition and solution of all problems which will be encountered in this program are believed impossible for a proposal of this scope; indeed, if this were possible, there would be little need for a research airplane." Nevertheless, North American attempted to mitigate the inherent risk "by allowing for easy modification of critical areas if the need arises," again showing an understanding of the fundamental intent of the program. An example was that the forward nose section, the leading edges, and the wing tips were made easily replaceable "to allow panel structures and aerodynamic shapes to be tested economically." Unfortunately, some of these innovations would never make it

off the drawing board.-1104!

All of the bidders, as well as the NACA and Air Force, recognized that structural heating would be the major design problem. "At a Mach number of 7, the boundary layer recovery temperature will be on the order of 3,499°F and the skin equilibrium temperature, where heat input is balanced by radiation output, will exceed 1,200°F even at altitudes above 100,000 feet." North American noted that this approached the upper limits of Inconel X, but believed the conditions were survivable "if flight duration is low and the skins are thick enough to form a heat sink of sufficient capacity."!1051

North American noted that the wing leading edges might experience temperatures of 1,400°F during extreme conditions, well beyond the ability of Inconel. To allow this without causing permanent damage to the aircraft, the company proposed to use a laminated glass cloth that would "melt or burn locally during these extreme cases." The flight-test group could replace the leading-edge sections after each flight, and alter the shape and material as desired or necessary.!1061

The North American Aviation entry in the competition bore the greatest overall resemblance to the original NACA Langley study, but the company had refined the concept into a vehicle that would support all of the required research without compromising the safety of the pilot. The North American proposal placed first in the evaluation. (North American Aviation)

The North American design was structurally similar to the one developed at Langley. Fabricating the basic wing as a complete semi-span assembly ensured rigidity, and fuselage ring frames transferred the wing skin loads across the fuselage. The ring frames were made of titanium alloy with numerous web beads to minimize thermal stresses. The wing structural box extended from the 25% chord line to the 75% chord line, and a span-wise series of shear beams made from corrugated 24S-T aluminum and titanium-manganese alloy attach points provided the support for the taper-milled Inconel X skins. The spar corrugations resisted the normal crushing loads and served to relieve thermal stresses. The relatively low modulus of elasticity of the titanium – manganese attach angles reduced the thermal stresses induced from the hot Inconel X skins. The skin panels varied from 0.060 inch thick at the tips to 0.125 inch thick at the fuselage fairing intersection.-1071

TIME FROM LAUNCH – SECONDS

TIME FROM LAUNCH – SECONDS

mm

North American met the required performance requirements with an anticipated maximum altitude of 250,000 feet and a velocity of 7,000 fps. In reality, the eventual X-15 would greatly exceed the predicted altitude, while not quite meeting the velocity estimate. Still, the slight performance shortfall did not compromise the research data and the airplane met the expectations of the researchers. (North American Aviation)

One controversial aspect of the North American design was the use of large fuselage side fairings to carry propellant lines, control cables, and wiring around the integral propellant tanks. Oddly, a similar fairing located on top of the Douglas Model 684 received much less comment from the government. Insulation was required around the liquid-oxygen tank to keep the cold temperatures out of the tunnel, and all along the outer skin to protect against the hot temperatures. Segmenting the Inconel X fairings every 20 inches reduced the thermal deflections and stresses.-11081 Initially the government was concerned about possible aero – and thermodynamic effects of the tunnels, but early wind-tunnel studies helped North American

reshape them slightly and they actually ended up providing beneficial lift. It was later determined that the panels were susceptible to hypersonic panel flutter, and additional stiffeners were added during the flight program.

Unlike Bell, which did not believe that a hot structure was compatible with integral propellant tanks, North American proposed such an arrangement from the beginning. The liquid-oxygen and anhydrous-ammonia tanks each consisted of four sections (top, bottom, and beaded sides) welded together with intermediate Inconel X bulkheads and end-dome bulkheads. Beading the sides of the liquid-oxygen tank reduced stress in areas shielded from the temperature of the air stream by the fuselage side tunnels. One bulkhead in each tank had a manhole that allowed access to the tank for maintenance.-110^

In the sections of the fuselage that were not part of the propellant tanks, North American decided to use a series of bulkheads spaced 25 inches apart as the primary support for a semi – monocoque structure. The bulkheads used a series of radial beads to stiffen them and reduce thermal stresses. Engineers worried that using conventional longerons and stiffeners would lead to unwanted temperature gradients that would cause the structure to warp or fail, so they avoided this technique. Instead, thick Inconel skins covered a simple Inconel X structure.-11^

The pressurized areas used an aluminum-alloy inner shell to retain compartment pressurization. The canopy seal was isolated from the hot skins, permitting the use of a conventional "blow-up" seal operated by nitrogen. This was in contrast to the problems Douglas expected with their Teflon canopy seal. The windshield consisted of heavy fused silica or Pyrex outer panes and stretched acrylic inner panes. The inner low-temperature panels provided the normal pressure seal. All of the panes were flat to simplify fabrication and eliminate distortion.–111^

Although it was a landmark preliminary design, the Langley study intentionally ignored many of the details necessary to build an airplane. One such detail was keeping the internal temperatures at an acceptable level for the pilot and instrumentation. North American noted that "the lack of any convenient source of large quantities of either compressed air or ram air, such as is associated with conventional jet aircraft, requires that a new and different approach be taken to the solution of pressurization and cooling." The company’s approach-using compressed gas (in this case nitrogen)-was hardly unique, being similar to that taken by the other competitors. The cryogenic nitrogen, plus the available heat absorption inherent in its vaporization, formed the necessary heat sink for refrigeration. The resulting gaseous nitrogen served as the atmosphere and pressurizing agent for the cockpit and equipment compartments.-1121

This led directly to one North American proposal that occupied quite a bit of discussion after contract award. The company also wanted to pressurize the pilot’s full-pressure suit with nitrogen, providing breathing oxygen to the pilot through a separate inner breathing mask.-1131 Done partly for simplicity, engineers believed that keeping oxygen exposure to the minimum was the simplest method to guard against fire in the cockpit or suit. Many within the NACA and the Air Force disagreed with this approach, and discussions surrounding the full-pressure suits (and the use of a neck seal or a face seal) would come up many times during the first year of development, with Scott Crossfield leading the charge for North American.

Like the choice of a face-mask oxygen system, North American’s decision to provide a simple ejection seat and a full-pressure suit for the pilot would later prove controversial. This combination resulted in "minimum weight and complexity" and exceeded the survival probabilities of "any capsule of acceptable weight which could be developed within the allowable time period." North American went on:-1141

In the event the pilot is required to bail out, the normal procedure will be to use the ejection seat. The design dynamic pressures encountered are not higher than those assumed for present-day high performance aircraft, so the pilot in his seat should be able to clear the aircraft satisfactorily at any altitude. The protection afforded by the pressure suit will probably conserve body heat and provide sufficient oxygen for a free fall from very high altitudes. However, the two relatively unknown effects of high stagnation temperatures attained on the exterior of the suit upon entering the atmosphere after falling through space, and the possible high rates of angular rotation of the pilot’s body during free fall will have to be studied in detail to determine the maximum altitudes at which it is feasible to bail out. Current developments at NAA [North American Aviation] indicate that with the protection against the air stream afforded by a full pressure suit, a suitably stabilized ejection seat may be designed which will assure escape under extreme conditions.

The wedge principle developed at Langley was evident in the vertical stabilizer proposed by North American. The dorsal stabilizer had a 10% wedge section; the ventral used a 15-degree wedge. Like the Douglas entry, the vertical was nominally a double-edge shape with the thickest part at 50% chord. A split trailing edge could open to form a "relatively obtuse blunt wedge" that greatly increased the lift curve slope at high Mach numbers and provided "sufficient directional stability without actual increase of tail area."115

Another innovative feature that was the subject of some debate after the contract was awarded was the use of all-moving "rolling" horizontal stabilizers instead of conventional ailerons and elevators.-1116! These operated symmetrically for pitch control and differentially for roll control. "Available aerodynamic data indicates that the configuration presented is reasonable when the complete speed range is considered. The all-movable surfaces for pitch, roll, and directional control are known to be satisfactory at the higher Mach numbers. Negative dihedral is incorporated on the horizontal tail to lessen abrupt trim changes due to shock impingement or wake immersion." There was an all-moving dorsal stabilizer that provided directional control, and a smaller fixed smaller ventral stabilizer. Split speed brakes were located on the sides of both the dorsal and ventral stabilizers.-117!

A separate "space control system" for use outside the atmosphere used Reaction Motors XLR32- RM-2 thrusters (four 90-lbf units in a cruciform arrangement at the nose, and one 17-lbf thruster at each wing tip). Unlike several of the other competitors that used the same control stick for the aerodynamic and reaction systems, North American used a separate lever on the right console.

The amount of propellant for the reaction controls seemed low by comparison with the other competitors: whereas Bell provided 47 gallons of hydrogen peroxide and Douglas provided nearly the same amount, North American provided only 3.15 gallons (36.2 pounds). The company expected this to be sufficient for "five gross attitude changes about each axis at approximately 6 degrees per second."118! This shows the amount of uncertainty that existed regarding the amount of use the reaction controls would receive—the first manned space flight was still six years away.

Like Douglas (and the alternate, Bell), North American chose the Reaction Motors XLR30 engine, but stated that "it appears feasible to use any engine or engines in the same performance category." Propellants would be stored in seam-welded Inconel X tanks, with the liquid-oxygen and main ammonia tanks being integral parts of the fuselage. A smaller, nonstructural ammonia tank slightly increased the fuel supply. Helium for propellant system pressurization was stored at 3,000 psi and -300°F in an Inconel X tank located on the centerline inside the liquid-oxygen tank. Surprisingly, there were only sufficient pressurizing gas and igniter propellants for three starts.-1191

Electrical and hydraulic power came from a pair of Reaction Motors X50AP-1 monopropellant gas turbine auxiliary power units in the aft fuselage. The systems were redundant, and either could provide sufficient power to operate the airplane. North American used two bladder-type tanks for both the APU and reaction control propellant, with 68.5% allocated to the APUs.[120]

North American believed it had a handle on the problem of acquiring air data in the hypersonic flight regime, and that "development time for this system will be minimized." The multipurpose air data system used existing components to measure pitot-static pressures, differential dynamic pressures due to angle of attack and angle of sideslip, and air-stream temperatures. North American never stated exactly where the pressure data would be sensed, although two devices originally designed for the Navaho missile program were the basis for the system.-1121

DESIGN MISSIONS

… femperafure vs. time

The temperature-versus-time estimates generated by North American essentially agreed with those made earlier at NACA Langley. The North American proposal used the same non-insulated Inconel X hot-structure airframe conceived at Langley, and this was one of the primary criteria that resulted in North American winning the competition. (North American Aviation)

The landing gear consisted of two strut-mounted skids that retracted against the outside of the fuselage beneath the wing leading edge and a two-wheel nose gear located far forward. The pilot deployed the landing gear via a manual cable release of the uplocks, with gravity and a bungee spring taking care of the rest. A small "tail bumper" skid in the aft edge of the ventral stabilizer protected the aft fuselage during landing. North American solved the problem of developing a landing system that was compatible with the large ventral stabilizer "by simply allowing the airplane to touch down and ‘rotate in’ about the tail bumper and providing adequate energy absorption in the main and nose gears." No retraction mechanisms existed, and the ground crew manually retracted the landing gear after each flight.-1122

North American chose the skids as much because they saved space inside the relatively small airframe as for any other reason: "the stowage of a wheel would not adapt itself to the configuration of the airplane without increasing the cross section area and wetted area." The friction between the skid and the ground accomplished braking, and the estimated landing rollout was 8,000 feet, well within the limits of the dry lakes at Edwards.[123]

In order to accommodate ease of maintenance, North American attempted to "incorporate the absolute minimum of systems and components which require servicing." Access to most wiring, cables, and hydraulic lines was gained through the easily removable side fairing panels. The research instrumentation was concentrated in a single equipment compartment equipped with large doors on each side. The fuselage panels around the engine were removable for service and inspection. All hydraulic components were concentrated in the aft fuselage.-11241

As required by the government, North American performed an engineering study on a two-seat X-15 to meet the Navy’s desire to "provide an observer." A second cockpit and ejection seat took the place of the research instrumentation, and an entirely new one-piece clamshell canopy covered both cockpits and faired into the upper fuselage further back than the normal canopy.

The observer had large flat-pane side windows, an intercom, and "an abbreviated presentation of flight and research data." The engineers estimated that "inasmuch as the launch and burn-out weights and airplane drag are identical to those of the single-place version, no change in performance will result."11251

The proposal and its included reports contained an extensive discussion on carrier aircraft. Of course, North American was the only company without some directly related experience with carrier aircraft. Bell and Douglas had both built research airplanes that were air launched, while Republic was manufacturing the RF-84Ks that were carried in the bomb bay of Convair GRB-36Ds as part of the FICON project.

North American chose a B-36 mostly because the only other available aircraft—the Boeing B-50 Superfortress—could not lift the X-15 above 25,000 feet, and North American wanted a higher launch altitude. From a modification perspective, the B-36 appeared to be excellent; only one bulkhead needed to be replaced, and the FICON project had already accomplished the basic engineering. The flight profiles developed by North American assumed a launch at Mach 0.6 and 30,000 feet, but the proposal suggested that the B-36 could actually achieve 38,000 feet with no difficulty. North American expected the separation characteristics to be excellent.11261

As North American was completing assembly of the first X-15, the Research Airplane Committee held the second X-15 industry conference at the IAS Building in Los Angeles on 28-29 July 1958. Forty-three authors (15 from North American, 14 from Langley, 6 from the High Speed Flight Station, 3 from the WADC, 2 from Ames, and 1 each from the AFFTC, Reaction Motors, and the Naval Aviation Medical Acceleration Laboratory at NADC Johnsville) presented 28 papers. There were 443 registered participants representing all of the military services and most of the major

(and many minor) aerospace contractors. Interestingly, there was no university participation this time. Notable attendees included Dr. David Myron Clark from the David Clark Company, Dr. Charles Stark Draper, and all of the original X-15 pilots. It is interesting to note how at least one of the participants registered; for instance, Harrison Storms listed his affiliation as "NACA Committee on Aircraft, Missile, and Spacecraft Aerodynamics" instead of "North American Aviation."[219]

The 1958 conference began, appropriately, where the 1956 conference had ended. Lawrence P. Greene from North American, who had presented the closing paper at the first conference, gave the technical introduction. One of his first statements summed up the progress: "It can be positively said that through the efforts of all concerned, the development of the X-15 research system has been successfully completed."1220

The airplane North American was building was the "Configuration 3" that had been inspected by the Air Force in mockup form. Configuration 1 was the initial North American proposal, while Configuration 2 was the one presented during the 1956 industry conference. Greene highlighted the important changes:[221

1. The side fairings were shortened ahead of the wing to improve longitudinal stability.

2. The horizontal stabilizer was moved 5.4 inches rearward, although the original fuselage location of the hinge line was retained. This modification moved the hinge line from the 37% to the 25% mean aerodynamic chord of the exposed horizontal stabilizer. Although flutter requirements dictated the change, this, combined with a 3.6-inch forward wing movement and the side-fairing changes, provided adequate longitudinal stability near zero lift at the maximum Mach number.

3. The vertical stabilizer area was increased to provide adequate directional stability with the speed brakes retracted and a 10-degree full wedge section was found to be optimum. The planform was then made nearly symmetrical (dorsal and ventral) for dynamic-stability considerations in the exit phase of the mission, since thrust asymmetry considerations in the zero to moderate angle-of-attack range necessitated a reduction in roll due to yaw.

4. Asymmetrical thrust effects also indicated the need for a low value of roll-due-to-yaw control in the low angle-of-attack region. For this purpose, an all-movable directional control was incorporated on the outer span of both the upper and lower vertical stabilizers. Incorporating the control in the lower vertical stabilizer was equally necessary for providing directional control at high angles of attack at high speed because of the ineffectiveness of the upper surface at these conditions. This, in turn, dictated some added complexity in the damper system.

5. In order to avoid compound flutter problems, the speed brakes were reduced in size and relocated on the inboard or fixed parts of the vertical stabilizers.

Although initially it had been decided not to increase the load factor of the airplane from 5 g to 7.33 g, sometime in the intervening two years the change had been made, much to the relief of the pilots and researchers at the HSFS. In mid-1957 the NACA had asked the Air Force to double the amount of research instrumentation carried by the X-15. This became a major design driver.

In order to keep the airplane weight (and hence performance) from being too seriously degraded, numerous details were redesigned to save weight. The two areas that received the most rework were the propellant system plumbing and the nose gear. This is when Charlie Feltz came up with the idea of keeping the nose-gear strut compressed when it was stored, allowing a much more compact and lightweight installation.-12221

Changes in configuration also brought changes in weight. To support the additional loads, North

American strengthened the structure of the wing, fuselage, and empennage. This resulted in a revised specification that showed an airplane that was 765 pounds heavier than originally expected (184 pounds in empty weight and 581 pounds in useful load; this included the pilot, propellants, and gasses, but not research instrumentation). However, by the time North American began building the airplanes, even this had changed. The empty weight had increased by only 61 pounds (instead of 184), but the useful load had decreased by 196 pounds. The research instrumentation, on the other hand, had increased by 522 pounds. The empty weight increases were the result of the following changes:[223]

1. The wing was changed from 7 to 15 intermediate spars, the skin gage was reduced, and the heat-sink material was changed from titanium carbide with a nickel binder to Inconel X, resulting in a net decrease of 131 pounds.

2. A 17-pound net increase in the empennage resulted from a 58-pound increase to meet thermal requirements and a reduction of 41 pounds for changing the leading-edge heat­sink material from titanium carbide with a nickel binder to Inconel X.

3. Chem-milling pockets in the skin and reducing the skin gage by adding Z-stiffeners and substituting aluminum for Inconel X in a portion of the intermediate fuel – and oxidizer-tank bulkheads saved 102 pounds in the body ground, but a 15-pound increase was caused by the additional structure to accommodate the engine weight increase. The net fuselage change was a decrease of 87 pounds.

4. The landing gear group was reduced by 73 pounds by eliminating the shimmy damper on the nose wheel and reducing the gage of the main-landing gear skids.

5. A reduction of 12 pounds in surface controls was realized by changing from four direct – acting speed-brake actuators to two actuators with a linkage arrangement.

6. The engine dry weight increased 296 pounds.

7. The addition of an engine purge system increased the propulsion group by 67 pounds. However, this was partially offset by a reduction in the internal liquid oxygen system plumbing of 29 pounds, giving a net propulsion system increase of 38 pounds.

8. The 4-pound increase in the auxiliary powerplant group was due to an increase in the weight of the APUs.

9. Changes in the fixed equipment resulted in a net increase of 9 pounds, consisting of a 76- pound increase in the ejection seat, an 11-pound increase in instruments, a 34-pound decrease in the nitrogen system, and a 44-pound decrease in the air-conditioning system.

ANHYIMJUS AWMON A

ІДКК (FULL:

QIC OXYGEN

.■ TANK (OXCIZER)

lIQJO NTKOGFN

і AUXILIARY

А ТІ I ULli

HVCROGEW

PEHQXIDF

EJECTION

SLAI

This is the configuration of the X-15 presented at the 1958 Industry Conference, and largely
represents the airplane as built. The major components are annotated. The large area immediately behind the cockpit was the primary location for the research instrumentation recorders and other equipment that required a controlled environment. (NASA)

Changes made in the useful load included the following:

1. The turbopump monopropellant was reduced by 196 pounds.

2. Trapped propellants in the engine increased 70 pounds.

3. The helium required to pressurize the propellant tanks increased 13 pounds.

4. The nitrogen required to pressurize the cockpit was reduced by 82 pounds.

All of this resulted in an airplane that had an empty weight of 10,635 pounds, versus an original specification weight of 10,390 pounds and a revised specification of 10,574 pounds. The total gross weight was 31,662 pounds, versus the original target of 30,510 pounds and a revised specification of 31,275 pounds. For high-speed missions, NASA could remove 370 pounds of altitude-related instrumentation, resulting in a gross weight of 31,292 pounds—only 17 pounds over the revised specification.-12244

Perhaps the most notable (though hardly unexpected) item to come out of the second industry conference was that the XLR99 was significantly behind schedule, and initial flight-testing of the airplane would be undertaken using two interim XLR11-RM-5 engines.-12254

Previous rocket planes, such as the X-1 and X-2, had been able to conduct the majority of their flight research directly over Edwards and the lakebeds immediately surrounding the base. The capabilities of the X-15, however, would need vastly more airspace. The proposed trajectories required an essentially straight flight corridor almost 500 miles long, and the need to acquire real-time data necessitated the installation of radar, telemetry, and communications sites along the entire path. There was also a need for suitable emergency landing areas all along the flight corridor. Fortunately, the high desert was an ideal location for such requirements since many of the ancient lakes had long since vanished, leaving behind dry and hard-packed contingency landing areas.-113

As early as 7 April 1955 Brigadier General Benjamin S. Kelsey wrote to Hugh Dryden (both were members of the Research Airplane Committee) suggesting a cooperative agreement on the construction and operation of a new range to support the X-15 program. A range had been included in the initial Air Force cost estimates, with $1,500,000 budgeted for its construction. At a meeting of the Research Airplane Committee on 17 May 1955, the NACA agreed to cooperate with the WADC and AFFTC in planning the range: the Air Force would build and equip it, and the NACA would operate it after its completion. It was much the same agreement that governed the X-15 itself.114

However, this decision was not favorably received by AFFTC personnel, who felt they were "being relegated to the position of procurement agent" for the XACA. On 15 June, Walt Williams met with the AFFTC commander, Brigadier General J. Stanley Holtoner, to discuss the concept for the new X-15 range. Williams began by updating Holtoner on the status of the X-15 program since the general had not heard any details since the previous October. During this discussion, Holtoner indicated his willingness to cooperate in developing the range and agreed with Williams that the AFFTC should not become actively involved until the XACA was able to discuss "detailed items of hardware" and support. Nevertheless, he felt the AFFTC "should have a somewhat stronger position in the project."113

Despite the apparent lack of enthusiasm for the arrangements within the AFFTC, on 28 July 1955 an amendment to the original X-15 development directive was issued that clearly established the AFFTC’s responsibilities for building the range. However, since neither document discussed which organization would operate the range, the AFFTC renewed its efforts to acquire this responsibility.

A conference at ARDC Headquarters in Baltimore on 15 September 1955 set in place the basic architecture of the range. Technical personnel reviewed the availability of various types of radar and decided that all of the range stations should be similar and include telemetry receivers as well as radar equipment. Although no decision was made regarding the specific radar equipment, the choices were narrowed to the AFMTC Model II used on the Atlantic Missile Range, and the Canoga Mod 3 used by North American at White Sands. On 13 October the HSFS proposed expanding the use of telemetry beyond that used on earlier X-planes. In addition to the normal engine-related information that was traditionally monitored, the HSFS wanted to obtain aircraft information (structural, flight path, temperature, etc.), research data (cosmic ray concentrations, etc.), and pilot physiological effects. This was a stretch for the available technology.-1161

Developing the final specifications for the new range was the subject of a meeting on 16 November 1955. This is when the AFFTC made its move for control, stating that the Air Force would like to operate and maintain the range on the condition that the NACA could also use it for the X-15. The NACA reminded the Air Force that the verbal agreement between Hugh Dryden and General Kelsey had already settled the issue. The NACA representatives also pointed out that the safe operation of the X-15 would depend heavily upon data acquired by the ground stations, and that a division of responsibility would not be desirable. The issue, however, would not go away, and on 2 December 1955 the AFFTC deputy chief of staff for operations at the AFFTC, Lieutenant Colonel Bentley H. Harris, Jr., wrote to the commander of ARDC formally requesting that his center "be assigned the responsibility for operating, as well as developing, the test range." The ARDC reiterated that the NACA would operate the range, but the AFFTC could use it on a non­interference basis.-171

Despite this contentious beginning, in the end the NACA and AFFTC cooperated in planning and using the range. The HSFS instrumentation staff under Gerald M. Truszynski largely determined the requirements based on experience gained during prior research programs. In November 1955, Truszynski informed the Research Airplane Committee that the range should be at least 400 miles long, with three radar stations able to furnish precise data on aircraft position, reentry prediction, geometric altitude, and ground speed. The X-15 required a launch site located near an emergency landing area, intermediate landing sites, intermediate launch sites (for less than full-power/full- duration flights), airfields near the radar sites that could be used for support, and a "reasonably straight course" for the high-speed flight profile.-1181

Besides the technical issues, many other factors determined where the range and its associated ground facilities would be located. Because of the sonic booms, it was not desirable to have the X-15 fly over major metropolitan areas, at least not routinely. Avoiding commercial airline corridors would make flight planning easier, and avoiding mountains would make the pilots happier. Ground stations needed proper "look angles" so that at least one of them could "see" the X-15 at all times. Emergency landing sites had to be spaced so that the X-15 would always be within gliding distance of one of them. The parameters seemed endless.

Truszynski and his staff concluded that the best course lay on a straight line from Wendover,

Utah, to Edwards, with tracking stations near Ely and Beatty, Nevada, and at Edwards. The range would take the X-15 over some of the most beautiful, rugged, and desolate terrain in the Western hemisphere, flying high over Death Valley before swooping down over the Searles basin to a landing on Rogers Dry Lake.-181

All of this led to construction of the High Altitude Continuous Tracking Range, which is generally known simply as the High Range. Officially, the effort was known as Project 1876. The Electronic Engineering Company (EECo) of Los Angeles accomplished the design and construction of the range under an Air Force contract awarded on 9 March 1956. The requirements noted that the "range will consist of a ground area approximately 50 miles wide and 400 miles long wherein a vehicle flying at altitudes up to 500,000 feet can be tracked continuously."-201

Despite the hopelessly optimistic original budget of $1,500,000, the three tracking stations did not come cheap-the more-sophisticated Edwards station cost $4,244,000, and the costs of the other two together were about the same. The Air Force spent another $3.3 million on initial High Range construction, and the NACA would spend a similar amount for improvements over the first few years of operations. An office at Patrick AFB, Florida, managed the procurement of the radar equipment under a modification to an existing contract for the Atlantic Missile Range (later the Eastern Range).-211

The agreement between the NASA and the AFFTC stated that the Air Force would "retain title to the land, buildings, and equipment, except those physically located within NASA facilities." In addition, "control, operation and support of High Range will revert to USAF upon the conclusion of X-15 Flight Research or earlier if the Research Airplane Committee judges that the National Situation so dictates."-221

Although Truszynski and his staff at the HSFS had developed the basic configuration of the High Range, it was up to the EECo-with the advice and consent of the government-to select the actual sites for the tracking stations. Since the HSFS staff had already made rough site selections, the next step was developing a radar coverage map. This map showed considerations such as obstructions on the horizon, the curvature of the Earth, and the range in which a target could be "seen" by radar at specified altitudes. This map narrowed down the area that the EECo needed to investigate in detail. Next came a lot of field work.[23]

Preliminary investigations by AFFTC, NACA, and EECo personnel indicated a possible site called VABM 8002 located 1.5 miles northwest of Ely, Nevada (the number referred to the site’s elevation: 8,002 feet above sea level). However, measurements and photographs from this site taken by EECo personnel indicated that it would not provide the required radar sight lines because of an extremely wide and high blockage angle almost directly downrange from the site. In addition, constructing an access road would have required a "considerable amount" of rock blasting. EECo ruled out using the site.-124

An alternate site in Ely was on Rib Hill. This 8,062-foot-high location was a considerable improvement over VABM 8002 in terms of radar sight lines and the ability to build a road and construct the site itself. The downside was that it was adjacent to the Ruth Copper Pit, and the Kennecott Copper Corporation was already planning to extend the operation into the side of Rib Hill. Even if the hill went untouched, the mining operation would have created too much earth movement for a precision radar installation, so again the EECo ruled out the site.[25]

Fortunately, while investigating the Rib Hill site, EECo personnel ventured to the south ridge of the Rib Hill range. This site was promising because the radar sight lines were excellent. The civil engineering firm of F. W. Millard and Son conducted a detailed land survey, mapping out the best location of the buildings and the access road. The EECo estimated that a 5.65-mile-long, 12- foot-wide road from U. S. Highway 50 to the site would cost approximately $72,400, which included installing culverts and drainage ditches, cutting and filling slopes, clearing and compacting the base, and finishing the gravel road.[26] The road would take advantage of southerly exposures to gain maximum natural snow removal, and arrangements with the White Pine County Road Department and the Nevada Highway Department provided additional mechanical snow removal. It was 10 miles southeast to the town of Ely from the junction of the site access road and Highway 50. The Ely Airport, which was a scheduled stop for several commercial airlines, was five miles east of the town. There were some drawbacks, however. The Kennecott Copper Company offered to supply electricity for a nominal cost, but an evaluation of the mining company’s generators showed that the current could fluctuate 10%, which was unacceptable for the sensitive electronic equipment at the site. EECo estimated that voltage regulators and power lines would cost more than procuring primary and backup generators and generating the required power on-site. In addition, there was no water available at the site, so tank trailers would have to haul water from Ely and store it in a tank at the site.-127

The site at Beatty was somewhat easier to locate. Preliminary investigations by the AFFTC and NACA resulted in the selection of a location approximately six miles northwest of Springdale, Nevada. Further investigation by EECo personnel substantiated this selection. The site was at an elevation of 4,900 feet, approximately three miles west of U. S. Highway 95. The radar sight lines were excellent, and the civil engineering firm of F. W. Millard and Son prepared a detailed survey of the area. Only 1.75 miles of new gravel road would be required to connect the site to Highway 95 at the cost of $30,500, including the installation of culverts and ditches. The site was 20 miles by road from Beatty, and an additional five miles to the Beatty airfield. No commercial power or water was available at the site, so the EECo again installed diesel generators. Water (at no cost,

initially) from the Beatty city water supply was trucked to the site.-1281

RANGE FUNCTIONAL DIAGRAM

r^n

RADAR,

TELEMETER.

VOICE

MICROWAVE AND TELEPHONE LWt INTERCONNECTING CIRCUITS

TRANSMITTED BETWEEN STATIONS

TELEMETERING DATA

TIMING

TELEMETERING DATA

The High Range consisted of three stations: one at Beatty, Nevada, one at Ely, Nevada, and the main station at the High-Speed Flight Station at Edwards. All three sites were interconnected by a sophisticated (for 1955) communications network. Each of the Nevada sites had a "local plot" that could track the course of the X-15 if needed. The general concept of the High Range formed the basis of the later manned spaceflight control networks, not surprising since the same man – Gerald Truszynski – was responsible for the High Range and the initial Mercury network. (U. S. Air Force)

The third site, an extension added to the back of the third floor of building 4800 at the HSFS, was the easiest to select. The construction would extend the building toward the airfield ramp from the existing "Flight Control" room using the exterior doorway as the entrance to the new addition. Initial estimates indicated that 1,200 square feet would be adequate for the intended purpose, but further investigation showed that structural constraints required the addition of at least 1,500 square feet. The additional 300 square feet was necessary to take advantage of the existing second-floor columns for greater support of the third-floor addition. After reviewing the plans, the Air Force and NACA requested that EECo further enlarge the addition to 2,500 square feet, which was the maximum the building could accommodate. The addition contained four rooms of roughly equal size: a monitor room with plotting boards, a radar room, a telemetry and communications room, and a utility/work area. No plumbing was required in the addition since the main building housed adequate restroom facilities and photographic dark rooms.-1291
competitive bid would perform the actual construction after the Air Force secured the land for the two remote sites. For unexplained reasons, the acquisition was not as straightforward as expected. For instance, the original schedule showed completion of the access road to the Ely site by 15 December 1956, but the Air Force ran into unexpected difficulties in withdrawing the site from the public domain, which delayed construction. In the end, it was October 1957 before the road was completed.-1301

At both remote sites, a 100-by-100-foot area was graded and hard-surfaced with asphalt paving and a sealant coat. This graded area was large enough to accommodate the radar shelter, vehicle parking area, and such items as the diesel generator, fuel tanks, etc. Because of the remote locations, officials decided to station permanently a Dodge Power Wagon four-wheel-drive truck at each site to provide transportation to the airfield. These trucks had sufficient towing capability to haul the water trailers, and the four-wheel drive allowed access to the site during inclement weather.-1311

Interestingly, the way the Air Force had written the High Range contracts, EECo was responsible for constructing 800 square feet of each shelter to house the telemetry equipment and "housekeeping" rooms, but the Reeves Instrument Company was responsible for constructing another 800 square feet at each shelter to house the radar equipment. Smartly, in order to avoid too much duplication of effort and to ensure a uniform appearance, the companies decided that one or the other should build the entire shelter. Since Reeves was not interested in facility construction, the honor fell to EECo. This was probably not the optimum solution, however, since Reeves retained the responsibility to construct the radar pedestal itself (which was an integral part of the building structure) because the exact position of the radar antenna was important to the final operation of the radar, and both contractors believed that the radar contractor should build the pedestal.-1321

EECo developed a generic 1,760-square-foot floor plan for the remote sites, although each would diverge somewhat from the ideal due to site-specific considerations. In essence, each building consisted of four large rooms: a radar room, a telemetry room, a room for data transmitting and receiving equipment, and a utility/work area. The building also included a smaller telephone- equipment room and dark room, and even smaller restrooms and closets. Oddly, the telephone room could only be accessed from outside the building. EECo calculated that each site would use approximately 155 gallons of water per day (5 gallons for personnel use, 50 for the dark room, and 100 gallons for the flush-type toilet). A 1,000-gallon tank meant that each site would need weekly water deliveries if it was manned continuously. Extreme weather conditions at Ely dictated that the water be stored inside the shelter to keep it from freezing. The shelters consisted of a metal exterior over an insulated framework and drywall interior, with a wooden false floor installed above a concrete slab to provide a location to run wires and cables.1331

The Ely, Beatty, and Edwards tracking stations had radar and telemetry tracking with oscillograph recording, magnetic-tape data collection, and console-monitoring services. Especially early in the flight program, a backup "communicator" was located at each station in case the communication links went down. Each ground station overlapped the next, and communications lines allowed voice communication, timing signals, and radar data to be available to all. Each station recorded all acquired data on tape and film, and strip charts and plotting boards displayed some of the data locally for the backup communicator.1341

Instrument Corporation modified the three Model II radars (generally called Mod II) and the Air Force supplied them to the EECo as government-furnished equipment. The radars had two selectable range settings: 768,000 yards (436 miles) and 384,000 yards (218 miles). The normal method for acquiring the initial target was to use a remote optical tracker. The antenna pedestal also had provisions for mounting an 80-inch boresight camera. Using a unique (for the period) range-phasing system, two or more Mod 2 radars could simultaneously track the same target without mutual interference.-1351

The radar used a 10-foot parabolic dish that transmitted a 2.5-degree wide beam. Peak power was 350 kilowatts with a pulse width of 0.8 microsecond and a selectable pulse-repetition frequency between 205 and 1,707 pulses per second. The maximum slewing rates were approximately 5 degrees per second in azimuth and 2.5 degrees per second in elevation. These were considered adequate for the X-15, although these limitations were considerations during the selection of launch and contingency landing lakes.-351

Precision azimuth and elevation information was obtained from two optical encoders, and range data came from one electromechanical encoder attached directly to the radar. The optical encoders were 16-digit analog-to-digital converters produced by the Baldwin Piano Company that used coded glass disks to produce a reflected binary (Gray)-371 code. The output of these units was a 16-digit parallel code produced by an internally synchronized flashlamp actuated 10 times per second by the master timing signal. This was the primary precision tracking information obtained from the radar system, and an Ampex FR-114 magnetic tape recorder recorded it in digital format. In addition, a data camera photographed the selsyn dial indications of azimuth, elevation, and range for coarse trajectory information.381

The AFFTC Project Datum system at Edwards provided automated processing for the radar and telemetry data recorded on the magnetic tapes. This was a general-purpose data-reduction computer system developed by the Air Force to accept a variety of input data tapes and generate output tapes compatible with the IBM 704 computers used for data processing. The IBM computer, in turn, provided data on factors such as the geometric altitude, plan position, trajectory position, and velocity. Project Datum was a post-test analysis tool, not a real-time system. Another IBM 704 computer was located at the FRC for processing the oscillograph data from the X-15. Operators transferred the raw data on the oscillograph and photorecorders to IBM punched cards by using manually operated film recorders, and the punched cards generated magnetic tapes.-391

Each of the three tracking sites had a "local" Electronic Associates Model 205J plotting board that showed the position of the X-15 as reported by its local radar, and the station at Edwards had a "master" board that correlated all of the results and plotted the vehicle along the entire trajectory. The local boards at each site could alternately display parallax-corrected data from another station. It is interesting to note that the technology of the day did not allow the parallax from the Ely station to be corrected digitally at Edwards because the results would cause the data receiver register to overflow (i. e., the resulting number would be too large for the available space). Since it was necessary to correct the parallax before displaying the data on the master plotting board, engineers devised a method to alter the analog voltage signals at the input to the polar-to – Cartesian coordinate converter. It was an innovative solution to a technological limitation. The coordinate converter itself was an Electronics Associates Model 484A computer.-401

The X-15 made extensive use (for that time) of telemetry data from the vehicle to the ground. As originally installed, the telemetry was a standard pulse duration modulation (PDM) system capable of receiving up to 90 channels of information in the FM frequency band. A servo-driven helical antenna was located at each range station to receive telemetry data. The antenna was slaved to

the radar to track the vehicle, although it could also be positioned manually using a hand crank. Later in the program, NASA installed auto-tracking telemetry antennas at each site. Ampex FR – 114 magnetic tape machines recorded 40 analog real-time outputs from an Applied Science Corporation Series M telemetry decommutator. Immediately after each flight, the receiving station processed the recorded information onto strip chart recorders. At the very end of the flight program, X-15-3 received a modern pulse-code modulation (PCM) telemetry system, and NASA modified the Ely and FRC sites to process the data (NASA had decommissioned Beatty by that time).[41]

Engineers and researchers on the ground needed to look at some of the telemetry data in real time to assist the X-15 pilot if necessary. They could look at this information in various forms on the data monitor consoles located at all three stations, although Edwards generally conducted the critical analyses. All parameters were presented in the form of vertical bar graphs on two center – mounted oscilloscopes, which allowed rapid assessment of a group of parameters to determine whether the operation was within predetermined limits. Of the total parameters transmitted, researchers could look at any 40 at one time, and the strip charts could display an additional 12 channels.-421

When thinking about radar operators, generally a large "radarscope" comes to mind. However, that was not the case during the 1950s, and the output from a radar was generally a small set of oscilliscopes as shown here on the Mod II unit. (It takes a computer to convert raw radar data into a plan-view for display on a radarscope, and such computers largely did not exist during the late 1950s.) For the most part, on the High Range the radar data was processed and displayed on a set of large paper charts that traced the flight progress on a pre-printed map. The position was plotted using one color of ink for position and another for altitude. (NASA)

Standard military ground-to-air AN/GRC-27 UHF equipment provided voice communications with the X-15. Originally, the Air Force indicated that it would provide the radios as government – furnished equipment; however, the long lead times caused the AFFTC to ask EECo to bid on supplying them separately. EECo found a Collins unit with 1,750 channels that it could acquire within nine months. The radio was fully compatible with the AN/ARC-34 UHF transmitter-receiver set that North American would install in the X-15.[43]

To ensure positive contact between any of the tracking sites and the X-15 regardless of its location over the High Range, EECo installed a network communications system. Each range station contained two UHF transmitters and receivers (one of each was a spare) and a specially designed communication amplifier and switching unit. When an operator keyed a transmitter at any location, all three stations transmitted the same information simultaneously. The receivers at all three stations fed their outputs onto a telephone line and, regardless of which station received the information, all stations could hear the transmission. The EECo also installed dedicated station-to-station communications links.[44]

simultaneously, the airborne receiver experienced an "audible beat or tone" interference. The solution to this heterodyne interference problem was to offset each transmitter frequency by a small amount without drifting outside the frequency bandwidth of the receiver. Experimentation led the team to adopt offsets of 0.005-0.010% of the operating frequency as nearly ideal. It was also determined that each transmitter should be offset by an unequal amount to avoid creating a noticeable "beat" in the audio. In the end, technicians tuned the Edwards transmitter 22 kilocycles below the center frequency, while the Ely site transmitted at 14 kilocycles above the center frequency. Beatty, being in the middle, used the center frequency for its transmitter.*451

Since a microphone at any one of the stations modulated all three transmitters simultaneously, the signal arrived at the aircraft at slightly different times because of differing distances from the station to the aircraft. In addition, signals originating on the aircraft took slightly different times to reach each of the ground stations. Consequently, some slightly different delays affected each signal. Given that such signals travel at the speed of light (186,000 miles per second), the time difference for an actual transmission was a maximum of approximately 4 milliseconds. A slightly longer delay was encountered in sending the keying signals between stations, resulting in a total delay of about 12 milliseconds between the two outermost sites (Edwards and Ely).*461

It was found, however, that the time delay was not totally undesirable. The human voice contains a multitude of continuously varying harmonic frequencies. The time delay canceled out a small number of these frequencies since they were 180 degrees out of phase with each other. The only effect this had was to introduce a slight flutter in the reproduced sound that did not seriously degrade speech intelligibility. The second effect the time delay brought was a slight echo effect. Due to the acuity of the human ear, there must be a spacing of approximately 30 milliseconds between signals for the ear to detect that an echo is present. Researchers discovered that a small echo effect actually increases the intelligibility of a voice because of the slight lengthening of word syllables. Analysis indicated that the maximum predicted 12-millisecond time delay would not be sufficient to cause undesirable effects, so the X-15 program elected to ignore the issue.*471

In the course of determining solutions to the various communications challenges, EECo discovered that it was not the first to confront these issues. Commercial airlines had been using similar systems (operating in VHF instead of UHF) for approximately five years after they had installed communications networks under their frequently traveled routes to allow aircraft to be in constant touch with their home offices. Each of these networks was composed of several transmitter – receiver sets that contained between two and six stations tied together by a transmission link. Several groups made up a complete network.*481

United Airlines had designed a similar communications system and contracted its operation to the Aeronautical Radio Company to make it available for other airlines. As Aeronautical Radio expanded and upgraded the original network, it contracted the work to Bell Telephone. Aeronautical Radio leased the system from Bell, and in turn leased the services to the airlines. Collins Radio worked with the service providers and airlines to create a series of radios specifically tailored to operate in the multiple-transmitter environment. Aeronautical Radio, Bell Telephone, Collins Radio, and United Airlines all provided information and assistance to EECo at no charge.*491

In order to evaluate a working communications system of this type before committing to the use of one on the High Range, EECo arranged for a demonstration using one of the airline VHF networks that ran in a line between Oceanside near San Diego to San Francisco, California. The NACA flew a Boeing B-47 Stratojet from Los Angeles to San Francisco at an altitude of 15,000 feet, returning to Los Angeles at 40,000 feet. The pilot made contact with the ground at 10- minute intervals while Air Force, NACA, and EECo representatives located at the Los Angeles

International Airport monitored the two-way communications.1501

The network spanned a distance of 400 miles, but used six stations (instead of the three planned for the High Range) to provide communications down to an altitude of 1,000 feet. Coverage for the High Range was concentrated above 7,000 feet, and one of the goals of the evaluation was to determine how the concept worked at high altitudes. On the return flight at 40,000 feet, it was likely that the B-47 received signals from all six ground stations, and that all six ground-stations received signals from the aircraft. Thus, potential interference was even greater than it would be with the three-station network planned for the High Range. The only effect noted during the evaluation was a flutter or warble at certain locations in the flight path. Researchers played tapes recorded during the flight for numerous pilots and ground personnel at Edwards, and nobody voiced any serious objections. This validated the concept for the High Range, and the EECo began procurement of the various radios, switching units, and other components.1511

The three High Range stations could share radar and telemetry data to automatically direct the next radar in line to the target, and to plot radar data from a remote station on a local plotting board if desired. It was necessary to convert the data from each station into the correct relative position using a set of fixed translation equations, which is one reason why the exact position of each radar antenna had to be precisely determined during construction.1521

The High Range stations were positioned on top of mountains to provide the best look angles for the radar and telemetry receivers. The Beatty, Nevada station was closed when the X-15 program ended and nothing remains at the site except for the concrete slabs where the buildings once stood. (NASA)

There were three likely ways to transmit data between the three sites: a leased wire facility, a scatter propagation system, or microwave transmission.1531 The contract with EECo specifically stated that "the contractor shall investigate the possibility of using a microwave service link for

radar data transmission originating at the Ely site, passing through the Beatty site, and terminating at Edwards Air Force Base." To satisfy this requirement, EECo personnel discussed possible microwave solutions with the Collins Radio, Pacific Telephone & Telegraph Company (PT&T), Philco Corporation, and Raytheon Manufacturing. EECo also discussed the possibility of a scatter propagation system with the same companies, although only Collins provided any meaningful data.[54]

A typical solution to the microwave system provided three main terminals at Ely, Beatty, and Edwards linked together by 10 repeater stations located approximately 30 miles apart. Each location had complete standby power and radio frequency (RF) equipment to ensure reliability. Engineers estimated the propagation delay from Ely to Edwards at 1.8 milliseconds. There were, however, substantial costs to build the system. For instance, each of the repeater sites needed power generators (at least primary, and probably backup). Then there was the cost to build roads to each repeater site; at an average cost of $3,000 per mile for an estimated three miles per site, this came to $90,000. The roads were to be of the same quality as a typical "pole maintenance" road not intended for regular vehicle traffic. The estimated cost of the microwave system was $396,000, and estimated operating expenses were $33,000 per year, not including amortization of the initial installation costs.-55

The propagation scatter system would have involved placing 28-foot-diameter antennas at each of the three sites and bouncing signals off the troposphere. Collins Radio recommended using a UHF system for distances up to 350 miles, and VHF for distances up to 1,200 miles. At the time, the Federal Communications Commission (FCC) had not made any licensing provisions for tropospheric scatter systems since it appeared only the government would be interested in using them. Collins pointed out that each system was custom-made, and the only way to determine whether such a system would work between any two or more locations was to try a Collins Transhorizon System in a van setup between each of the sites. Collins estimated the original system cost at $287,600, not including installation or spare parts. Collins also pointed out that the system was very susceptible to atmospheric disturbances and weather.-155

The leased wire facility would provide telephone lines from Edwards through Los Angeles and Sacramento to Reno, Nevada. From Reno the lines would branch off through Tonopah, Nevada, to the Beatty site, and through Wendover, Utah, to the Ely site. The estimated propagation delay from Edwards to Ely was 10 milliseconds. The standard telephone facilities at Ely and Beatty would be "semi public toll service stations," meaning that they would be on a party-line hookup with the towns of Ely and Beatty. All calls from these telephones would be toll calls (10 cents minimum) with a minimum charge of $5.00 per month. The transmission links were semi-permanently connected lines that would not go through an operator’s patch panel, avoiding the chance of accidental disconnections. Pacific Telephone would provide all of the maintenance.-1571

Ma Bell, being Ma Bell, had charges for everything. The initial construction charge (running the necessary land lines and terminal equipment) would be $55,000, but there was also an "installation charge" of $95 per site to have a technician actually connect the equipment. The total annual operating costs would be $113,790, not including the cost of two standard telephones at Ely and Beatty, which would run an additional $5 each per month. Pacific Telephone also informed the government that if it selected a microwave system, the telephone company would not find it profitable to provide only standard telephone service to the two remote sites-this would be economically practical only if Pacific Telephone provided the entire data transmission contract.-581

system since it did not seem to offer any great cost advantage and represented a largely unknown operational quantity. The microwave system offered low annual operating costs, assuming the system continued to be used for at least six or seven years to amortize the installation costs. Additional channels were readily available with minor expenditures, and engineers considered the link more secure since it was unlikely anybody would attempt to "tap" it. The principal disadvantages of the microwave system were its high initial costs, the possibility that the repeater sites would be inaccessible during bad weather, and that maintenance was the responsibility of the end user (the NACA).[59]

On the other hand, leased telephone facilities offered high reliability and low initial costs, and the telephone company would provide all maintenance. Its principal disadvantages were high annual operating costs and the inability to easily add more channels, particularly high-bandwidth ones.*60*

EECo conducted a cost analysis that included amortization of the initial costs over 5-, 10-, and 20-year periods. The results of this analysis for the "Cost per Channel per Mile per Year" were as follows:*611

The total annual operating costs, also based on the three possible amortization options were:*62*

The microwave cost curve dropped sharply in the early years and then leveled off to some degree after 10 years. Additional channels, however, dropped the per-channel cost considerably. This was because the basic investment in a microwave system was in the initial installation; additional channels only required more relatively low-cost multiplex equipment. This reduction, however, only extended until expansion filled the full bandwidth of the microwave system. At this point, the cost would increase greatly because additional microwave equipment would be required. This was not a major concern since the proposed system provided a bandwidth of 100 kilocycles, and the seven required channels only used 21 kilocycles.-*63*

Nevertheless, the Air Force was in the position to make the final decision, and it selected the telephone system. There were four reasons for this choice: 1) the high reliability offered by a utility-maintained system, 2) the high initial cost of the microwave system, 3) the distance and inaccessibility of the microwave repeater sites for maintenance, and 4) the fact that the telephone company maintained all telephone facilities. These reasons were unquestionably valid. However, in reality, the more likely rationale was the simple fact that although the Air Force was responsible for funding the installation of the chosen system, the NACA was responsible for maintaining the system once it was operational. The Air Force, therefore, chose the system that would cost it the least amount of up-front money, with little consideration given to future capabilities or operating costs. By March 1961, even before the Ely station came on line (in April 1961), NASA had opted to install a microwave system between the stations on the High Range. The microwave capability from Beatty was operational in June 1961, with Ely following in January 1962.[64]

A master timing system at Edwards provided a constant time reference for all the tracking stations using three separate timing signals: 1,000 parts per second (pps), 100 pps, and 10 pps. An operator at any station could record timing marks on recordings at all three stations to indicate a significant event for later reference.-65

Early in the program, a pilot staffed each of the High Range sites in addition to the engineers and technicians necessary to run the equipment. The pilot at Beatty used the call sign NASA-2, and the one at Ely used NASA-3. For later flights, pilots often did not staff the remote sites as the communications links between the sites acquired more bandwidth and all involved gained more confidence in the reliability of the systems. Normally, important information from the control room passed to the pilot through the NASA-1 controller, who was usually another X-15 pilot. However, other ground-control personnel had the capability to transmit directly to the pilot in the event of an emergency where there might be insufficient time to relay information through NASA – 1, or, as happened on several occasions, the radio at Edwards did not work properly.-66

Although they were not designed as part of the original control room, researchers added various specialized devices during the flight program. For instance, engineers programmed a small analog computer to take radar-derived altitude, velocity, and vertical velocity measurements and compute the resulting range footprint to assist ground personnel in understanding which contingency landing sites were available at every moment during the flight. A scope-type map display presented the data in the control room. The analog flight simulator generated the data to program this computer. The flight surgeons also gained a dedicated biomedical console.-671

The station at Ely was functionally identical to the one at Beatty, although the physical layout of the two sites differed somewhat due to local environmental conditions. At the end of the X-15 program, the Ely station reverted to the Air Force and continued to play a part in test operations until 1992 when it was finally closed. (NASA)

The High Range underwent a series of modifications over the years. For instance, on 10 March 1967, NASA replaced the Mod II radar at Ely with an improved Reeves Instrument Corporation MPS-19C unit that became operational on 2 May. Wallops Island shipped another MPS-19C during March 1967 for installation at the FRC. At the FRC, the original Mod II had been located on top of building 4800, but engineers deemed this unacceptable because the increased accuracy of the new radar required a firmer base to eliminate vibration and flex. As a result, the new radar was installed a mile or so west, primarily in a new facility with a stiffer base. In addition, in early 1967 NASA upgraded the microwave relay system from Ely to Edwards to handle the higher-bandwidth PCM data from X-15-3. The first successful test (at 144 Kbs) was on 29 March 1967, and the system successfully supported flight 3-58-87 on 26 April.[68]

Robert S. Houston, a historian at the Air Force Wright Air Development Center, wrote the most frequently quoted X-15 history in 1959. This narrative, unsurprisingly, centered on the early Air Force involvement in the program, and concentrated mostly—as is normal for Air Force histories— on the program management aspects rather than the technology. Dr. Richard P. Hallion, later the chief historian for the U. S. Air Force, updated Houston’s history in 1987 as part of volume II of The Hypersonic Revolution, a collection of papers published by the Aeronautical Systems Office at Wright-Patterson AFB. Hallion added coverage of the last nine years of the program, drawing mainly from his own On the Frontier: Flight Research at Dryden, 1946-1981 (Washington, DC: NASA, 1984) and "Outline of the X-15’s Contributions to Aerospace Technology," written in 1977 by Ronald G. Boston. These historians did an excellent job, but unfortunately their work received comparatively limited distribution.

I began this history by using these earlier works as a basis, checking the sources, expanding upon them as appropriate, and adding a NACA/NASA and Navy perspective. Amazingly, almost all of the original source documentation still existed in one archive or another, allowing an evaluation of the tone and inflection of some of the earliest material. Although it is largely a new work, anybody who is intimately familiar with the earlier histories will recognize some passages—the original historians did a remarkably thorough job.

Many people assisted in the preparation of this work, and all gave generously and freely, well beyond any reasonable expectation an author might have. Foremost were Betty J. Love, Tony Landis at Dryden, and Dr. Roger D. Launius at the National Air and Space Museum. The surviving X-15 pilots—Neil A. Armstrong, A. Scott Crossfield, William H. Dana, Brigadier General Joe H.

Engle (USAF, Retired), Colonel William J. "Pete" Knight (USAF, Retired), and Major General Robert M. White (USAF, Retired)—contributed immensely, and several of them read the manuscript multiple times to ensure that nothing significant was missed or misrepresented. John V. Becker and Charles H. Feltz spent many hours explaining things I probably should have already known, greatly improving the manuscript. Then there are the flight planners—Johnny G. Armstrong,^ Richard E. Day, and Robert G. Hoey. I would have missed many subtleties without the patient tutoring from these engineers, all of whom read and commented on several versions of this manuscript and continued my education well past my two engineering degrees.

There was correspondence with many individuals who had been involved with the program:

William P. Albrecht, Colonel John E. "Jack" Allavie (USAF, Retired), Colonel Clarence E. "Bud" Anderson (USAF, Retired), Bill Arnold (RMD/Thiokol, Retired), Colonel Charles C. Bock, Jr., (USAF, Retired), Jerry Brandt, Richard J. Harer, Gerald M. Truszynski, and Alvin S. White. In addition, Jack Bassick at the David Clark Company, Stephen J. Garber and Colin A. Fries at the NASA History Office, Michael J. Lombardi at the Boeing Company Archives, Air Force Chief Historian Dr. Richard P. Hallion, Dr. James H. Young and Cheryl Gumm at the AFFTC History Office, and John D. "Jack" Weber at the AFMC History Office all provided excellent support. Friends and fellow authors Gerald H. Balzer, Robert E. Bradley, Benjamin F. Guenther, Scott Lowther, Mike Machat, Michael

Moore, Terry Panopalis, and Mick Roth also assisted.

Others who contributed include Lynn Albaugh at Ames, Jack Beilman, Rodney K. Bogue at DFRC, Anita Borger at Ames, John W. Boyd at Ames, Russell Castonguay at the JPL archives, Erik M. Conway at Langley and NASM, Mark L. Evans at the Naval Historical Center, Dr. Michael H. Gorn at the DfRc History Office, Matt Graham at DFRC, Fred W. Haise, Jr., Wesley B. Henry at the Air Force Museum, T. A. Heppenheimer, James B. Hill at the John Fitzgerald Kennedy Library, Dr. J. D. "Dill" Hunley at the DFRC History Office, Kenneth W. Iliff (DFRC, Retired), Bob James (DFRC, Retired),

Jack Kittrell (DFRC, Retired), Christian Ledet, F. Robert van der Linden at the National Air and Space Museum, Marilyn Meade at the University of Wisconsin, Roger E. Moore, Claude S. Morse at the AEDC, Karen Moze at Ames, Doug Nelson at the AFFTC Museum, Anne-Laure Perret at the Federation Aeronautique Internationale (FAI), Colonel Bruce A. Peterson (USMCR, Retired), Charles E. Rogers at the AFFtC, Mary F. Shafer (DFRC, Retired), Bonita S. Smith at GRC, Colonel Donald M. Sorlie (USAF, Retired), and Henry Spencer.

It all would never have seen the light of day had it not been for Tony Springer of the Aeronautics Research Mission Directorate at NASA Headquarters.