Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

The Douglas Proposal

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]

The Douglas Proposal

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 Douglas Proposal

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 Douglas ProposalThe 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

The Douglas Proposal Подпись: 100 Подпись: 200 Подпись: S0Q The Douglas Proposal

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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

The Douglas ProposalElectrical 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

The Douglas Proposal

… 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

THE SECOND INDUSTRY CONFERENCE (1958)

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

THE SECOND INDUSTRY CONFERENCE (1958)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.

Подпись:ANHYIMJUS AWMON A

ІДКК (FULL:

QIC OXYGEN

.■ TANK (OXCIZER)

lIQJO NTKOGFN

і AUXILIARY

Подпись: POWER UN 15

А ТІ I ULli

HVCROGEW

THE SECOND INDUSTRY CONFERENCE (1958) THE SECOND INDUSTRY CONFERENCE (1958)

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

THE HIGH RANGE

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,

THE HIGH RANGEinitially) from the Beatty city water supply was trucked to the site.-1281

RANGE FUNCTIONAL DIAGRAM

r^n

RADAR,

TELEMETER.

THE HIGH RANGE Подпись: і EDWARDS THE HIGH RANGE Подпись: BEATTY Подпись: LOCAL PLOT Подпись: ELY

VOICE

THE HIGH RANGE

MICROWAVE AND TELEPHONE LWt INTERCONNECTING CIRCUITS

Подпись: RECORDED AT EACH STATION

Подпись: PRECISION RADAR DATA Подпись: RADAR ACQUISITION DATA

TRANSMITTED BETWEEN STATIONS

Подпись: VOICETELEMETERING DATA

Подпись:Подпись: TIMINGTIMING

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

THE HIGH RANGE

THE HIGH RANGE

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

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

Initial Cost

20-Year

10-Year

5-Year

Microwave (Philco)

$396,000

$21.90

$30.10

$46.60

Telephone (PT&T)

$55,000

$48.44

$49.57

$51.81

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

20-Year

10-Year

5-Year

Microwave (Philco)

$52,825

$72,650

$112,299

Telephone (PT&T)

$118,680

$121,434

$126,947

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 HIGH RANGE

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]

ACKNOWLEDGMENTS

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.

The Republic Proposal

Republic also seemed at a disadvantage in the X-15 competition, for many of the same reasons North American was. However, the company was working on a Mach 3+ interceptor, the XF-103, and had developed the first supersonic combat-type aircraft, the experimental XF-91. With the XF-91, the company had gained experience in integrating a liquid-fueled rocket engine into a manned aircraft. The XF-103 was providing a wealth of experience (most of it unhappy), including information concerning the effects of high-speed heating on aircraft structures. In addition, Republic had Alexander Kartveli, one of the most innovative aircraft designers in the world.11271

The Republic AP-76 was the heavyweight of the competitors, with a launch weight of 39,099 pounds. Nevertheless, Republic expected the design to exceed very slightly the speed specification at 6,619 feet per second, although it fell somewhat short of the altitude requirement at only 220,000 feet.11281

Like Bell, Republic opted for XLR81-BA-1 engines, although the heavyweight AP-76 used four of them. Each of the engines produced 14,500 lbf, so a total of 58,000 lbf was available at 40,000 feet. Republic justified their choice by noting that "a sacrifice in weight was made in order to use these four units in place of a single thrust chamber engine. The increased safety of numbers as well as the increased reliability of starting one or more units influenced this choice." The engines used a fuel called JP-X that consisted of 40% unsymmetrical dimethylhydrazine (UDMH) and 60% jet fuel. The oxidizer was red, fuming nitric acid. The combination was hypergolic, so no ignition system was required. The thrust line of each engine chamber passed through the center of gravity of the airplane, eliminating any directional component of single – or multiple-chamber operations.12^

A switch panel at the normal throttle location on the left console controlled the engines, based on experience gained on the XF-91 interceptor. The XF-91 had both switches and a conventional throttle quadrant, but the pilots preferred using the switches. A fixed handgrip next to the switches ensured that the pilot’s hand would be near the switches at all times. There were nine two-position switches on the panel: a "master arm" switch, four individual "arm" switches, and four "on" switches. Igniting varying numbers of the engines varied the thrust, just as it had on the X-1 and D-558. Republic did not seem to incorporate the ability to use the "half-thrust" feature of the XLR81.130

Much like the XF-103, Republic eliminated the conventional canopy enclosure and submerged the pilot inside the fuselage. Three glass panels on each side of the fuselage provided side vision from launch until the airplane had descended to approximately 25,000 feet. Once the AP-76 had slowed to Mach 0.7, a hatch on the upper surface of the cockpit raised 13 degrees at its leading edge to expose a mirror system that provided forward vision during approach and landing. The system used two mirrors—one in the front of the hatch reflected an image downward to a second mirror on top of the instrument panel. The pilot looked at the second image. This system was similar to the one that had been developed for the XF-103 and had received favorable comments from the pilots during simulations. Surprisingly, the system offered good depth perception and minimal loss of brightness. Republic chose this unique system "because the problem of protecting the pilot from the high temperatures and, if need be, from cosmic radiation in a [conventional] canopy arrangement seem almost impossible." The cockpit and forward instrument compartment used gaseous nitrogen to maintain 40-100°F at a 5-psi differential, while the aft compartment had a 2.5-psi differential.131

The Republic Proposal

The Republic Proposal

The Republic AP-76 was large, heavy, and although Republic indicated it could slightly exceed the velocity requirements, it fell about 15 percent short of the desired altitude capability. In reality, very few believed it could attain the performance numbers generated by Republic, especially given the weight gains that seem to occur during any development exercise. The Republic entry placed last in the evaluation. (Republic Aviation)

To assist the pilot in flying the predetermined trajectory, Republic proposed installing a "flight program indicator." This display presented the pilot with a second-by-second trace that showed the proper speed, altitude, angle of attack, and path angle during powered flight. The pilot simply guided the airplane to match the cues on the display. It would have been a useful tool.-1132!

Normal Air Force fighter standards (+7.33/-3.00 g at burnout weight, but a great deal lower at full gross weight) provided the structural requirements for the AP-76, in contrast to the other competitors that only stressed their designs for +5 g. To accomplish this, and to withstand the expected heating environment, Republic proposed a novel structure for the fuselage. The main structure consisted of longitudinal titanium "Z" stringers. The structural titanium skin attached to the inner leg of the stringers, and the outer leg held a series of 0.020-inch-thick corrugated Inconel X shingles that formed a heat shield. The corrugations were very mild, with a 0.08 depth – to-length ratio, and permitted circumferential growth resulting from high transient temperatures. In between the heat shield and inner skin were 0.5-inch-thick blocks of Marinite insulation made by the Johns Manville Company. The 2-foot-wide Inconel outer skin sections stretched over three frames and used elongated attachment holes that allowed the sheets to expand and contract without warping. With the Inconel outer skin at its full 1,200°F, the interior titanium structure would never exceed 300°F.[133]

of the propellant tanks, and storage bottles were located below the wing carry-through structure. To the rear of the second nitric acid tank was the JP-X tank. The titanium oxidizer and fuel tanks were an integral part of the fuselage, but because nitric acid reacts with titanium at elevated temperatures, the acid tanks had removable aluminum liners.[134]

The trapezoidal wing used a slightly rounded leading edge with a flat airfoil between the 20% and 80% chord lines and a blunt trailing edge. Unlike the fuselage, Republic did not attempt to insulate the wing structure, and designed it to carry the design loads at elevated temperatures without developing high thermal stresses. The wing used three main sections: 1) the main wing structure, 2) the leading edge, and 3) the trailing edge, which consisted of a conventional single-slotted landing flap and a conventional aileron. The primary load-carrying structure was a tapered multi­cell box that ran from tip to tip and attached to the fuselage at four points (two per side). Intermediate spars were located on 5.5-inch centers with 15 spars at the root and four at the tip. The Inconel X skins were on average 0.10 inch thick. The leading edges were made of kentanium (a titanium carbide alloy) castings segmented into six parts per wing.[135]

The vertical and horizontal stabilizers were "of conventional size made possible by the use of double wedge type sections with rounded leading edges." The included angles were 10 and 12 degrees, respectively. The horizontal surfaces were all moving, but the airplane used conventional ailerons instead of the differentially moving horizontals found on the North American design. The vertical surfaces consisted of a dorsal stabilizer and a jettisonable ventral stabilizer. Wind-tunnel data from the XF-103 provided data for the rudder design, although the overall shape was different. The rudder consisted of the upper 46% of the surface and the entire trailing edge aft of the 70% chord line. Spilt flaps, consisting of the trailing 30% and 35% of the vertical and horizontal stabilizers, respectively, opened through a maximum angle of 50 degrees to increase drag and reduce the speed of the aircraft during reentry. Like the ailerons, these split flaps were each divided into three sections to permit operation while under thermal stresses. The stabilizers were generally of the same construction as the wings, and, like the wing, the leading edges of the empennage were made of cast kentanium.[136]

The landing gear consisted of two main skids and one tail skid. The 48-by-5-inch main skids, installed externally on the side of the fuselage bottom just ahead of the center of gravity, extended 18.5 inches using pneumatic shock absorbers. Just before landing, the tail skid automatically extended when the pilot jettisoned the ventral stabilizer. The landing gear could accommodate descent velocities of only 6 feet per second, considerably less than the 9 fps that tactical aircraft were design to absorb. The rationale was that "highly experienced pilots only are expected to fly this airplane." In fairness to Republic, the NACA had conducted an analysis of earlier research airplane landings and found that the majority were well below the 6-fps figure.-113^

Two hydrogen peroxide auxiliary power units each drove an alternator and hydraulic pump. A 60- gallon supply of the monopropellant could drive the power units for 30 minutes and operate the reaction control system for 3 continuous minutes. The reaction control system used six 90-lbf thrusters (one on each wing tip and four at the rear of the fuselage). Republic linked the thrusters to the same control column that the aerodynamic controls used, and a switch in the cockpit activated them when necessary. At the time of the proposal, the thrusters were throttleable, but Republic noted that "studies of a ‘bang-bang’ system, that is ‘full-on’ or ‘full-off…appear very promising."[138]

AP-76. "Consideration was given to the use of a pilot’s escape capsule in the AP-76. It was found to be extremely difficult to design a capsule which would have the necessary stability characteristics in the low density air of the high altitudes attained by the AP-76." Similarly, Republic found it was almost impossible to provide drag devices that would retard the capsule’s descent to the degree necessary to prevent excessive skin temperatures. In its place was an escape seat with leg (but no arm) restraints; the pilot would rely on his partial-pressure suit for protection during ejection.-1139!

Not surprisingly, given the weight of the AP-76, Republic chose a Convair B-36 bomber as the carrier aircraft. Republic had some experience in using the B-36 since the company manufactured the RF-84K parasite fighter used in the FICON project. The AP-76 was sufficiently large that it took up the majority of all four B-36 bomb bays. The lifting frame and main attach points were mounted on the B-36 wing box and attached to the AP-76 on top of the fuselage over its wing. It was necessary to modify two main bomb bay frames on the B-36 to clear the research airplane, and to add sway braces to "suitable strong points on the lower longerons of the bomb bay truss." A fairing with a soft gasket sealed the bomb bay when the B-36 was carrying the AP-76.!140!

Unfortunately, Republic appears to have misread the intentions of NACA and the Air Force, and its proposal stated that "the achievement of the speed [6,600 fps] is paramount whereas flight at very high altitudes has a secondary role." Because of this, Republic concentrated on designing an aircraft that would be capable of meeting the velocity requirement, while ignoring the altitude requirement to some degree. Although the proposal listed 220,000 feet as the maximum altitude of the aircraft, other data submitted with the proposal indicated that the company believed the aircraft could achieve almost 300,000 feet if necessary.-141!

The typical high-speed flight profile for the AP-76 began with the airplane being carried aloft by a B-36H142 carrier from Edwards AFB. The research airplane pilot would be riding in the comparative comfort of the pressurized compartment of the bomber. The B-36 would carry the AP-76 to a predefined release point approximately 540 miles from Edwards and launch the airplane at an altitude of 40,000 feet and a true air speed of 350 knots. After the AP-76 dropped clear of the B-36, the pilot would ignite all four rocket engines and pull into a 20-degree climb before running out of propellants after 105 seconds at approximately 140,000 feet. The AP-76 would then continue a free-flight trajectory to a peak altitude of 220,000 feet about 69 seconds after burnout. During the climb through 100,000 feet, the pilot would activate the switch that armed the reaction control system; thereafter, the movement of the control column and/or rudder pedals would activate the thrusters in addition to the now-useless aerodynamic controls.

The airplane would continue on a ballistic trajectory until it reached an altitude of 150,000 feet, where the aerodynamic controls would regain effectiveness. The airplane would go through a series of pull-ups and glides while the pilot maintained the angle of attack at a constant 6 degrees. The speed brakes on the horizontal and vertical stabilizers would open as needed. When the descent reached 25,000 feet and the speed reduced to Mach 0.7, the pilot would jettison the ventral stabilizer since it was no longer required for directional stability, and raise the hatch to expose the mirror system to provide forward visibility. Finally, the airplane would glide to a landing on its skids on Rogers Dry Lake.143

The Republic approach to the required two-seat engineering study was a little different from and decidedly more useful than the other proposals. All of the other competitors had simply deleted all of the research instrumentation and installed accommodations for an observer, although North American, at least, had provided a proper canopy arrangement. Republic, however, stretched the constant-section of the fuselage just ahead of the forward propellant tank by 29 inches. On the single-seat aircraft, two compartments held the research instrumentation (550 pounds ahead of the pilot and 250 pounds behind the pilot). For the two-seat airplane the 250 pounds in the rear compartment were deleted, and, combined with the 29-inch extension, this provided a full-size cockpit for the observer. The airplane could still carry the other 550 pounds of instrumentation – in fact, it was the only proposed two-seat aircraft that could carry any. The empty weight of the airplane increased 380 pounds and the launch weight increased 610 pounds, resulting in a degradation of performance of 170 fps.-1144

Comparison of Physical Characteristics

Bell

Douglas

NAA

Republic

D-171

Model 684

ESO-7487

AP-76

Fuselage:

Length (feet):

44.42

46.75

49.33

52.58

Frontal area (square feet):

25.00

21.00

?

?

Maximum diameter (feet):

5.15

5.16

4.50

5.00

Fineness ratio:

8.62

9.06

?

10.5

Wing:

Airfoil:

biconvex (mod)

Clark Y (mod)

66005 (mod)

hexagonal

Span (feet):

25.67

19.50

22.36

27.66

Root section (percent):

5.0

7.0

5.0

5.0

Tip section (percent):

6.0

4.5

1.0

7.5

Root chord (feet):

13.16

10.40

10.80

16.00

Tip chord (feet):

3.86

2.75

3.00

2.25

Area (square feet):

220.0

150.3

200.0

254.0

Flap area (square feet):

15.25

14.44

?

28.80

Aileron area (square feet):

16.00

9.88

n/a

15.80

Angle of incidence (degrees):

0

0

0

0

Dihedral (degrees):

0

0

0

0

Aspect ratio:

3.00

2.53

2.50

3.00

Taper ratio:

0.30

0.22

?

0.14

Aileron deflection (degrees):

15

20

n/a

+ 17/-12

Flap deflection (degrees):

-45

-45

-40

-38

Leading-edge sweep (degrees):

37.0

40.0

25.0

38.4

MAC (inches):

112.50

105.26

123.23

130.87

Horizontal Stabilizer:

Airfoil:

biconvex (mod)

5° wedge

66005 (mod)

10° wedge

Span (feet):

13.75

11.83

17.64

15.70

Root chord (feet):

7.05

7.66

7.02

7.08

Tip chord (feet):

2.11

1.66

2.10

1.83

Area (square feet):

63.00

55.20

51.76

69.70

Aspect ratio:

3.00

2.54

2.81

3.48

Taper ratio:

0.30

0.22

0.22

0.26

Leading-edge sweep (degrees):

35.5

40.0

45.0

22.3

Deflection (degrees):

+ 10/-20

+ 5/-20

+ 15/-45

+ 7/-20

Bell

Douglas

NAA

Republic

D-171

Model 684

ESO-7487

AP-76

Dorsal Stabilizer:

Airfoil:

biconvex (mod)

diamond (mod)

10° wedge

12° wedge

Area (square feet):

45.30

39.25

38.14

47.60

Rudder area (square feet):

13.5

7.85

?

32.0

Aspect ratio:

0.8

1.277

1.25

1.6

Leading-edge sweep (degrees):

45.0

40.0

52.0

27.9

Rudder deflection (degrees):

20

30

45

20

Ventral Stabilizer:

Airfoil:

10° diamond

7° edge

15° wedge

10° wedge

Area (square feet):

22.70

12.08

11.42

12.30

Leading-edge sweep (degrees):

45.0

60.0

52.0

45.0

Weights:

Launch (pounds):

34,140

25,300

27,722

39,099

Burnout (pounds):

12,942

10,600

10,433

15,300

Landing (pounds):

12,595

10,450

10,200

14,800

Empty (pounds):

11,964

9,208

9,959

14,388

Propellants (pounds):

21,600

14,700

16,410

23,660

Propulsion:

Number of engines:

3

1

1

4

Engine type:

XLR81

XLR30

XLR30

XLR81

Total thrust (lbf):

43,500

57,000

57,000

58,000

Fuel type:

JP-X

NH3

NH3

JP-X

Fuel quantity (gallons):

704

1,142

1,239

710

Oxidizer type:

RFNA

LOX

LOX

RFNA

Oxidizer quantity (gallons):

1,358

816

907

1,430

Jl________________ II________________ II_____________ II

Performance (estimated):

Maximum speed (fps):

6,850

6,655

6,950

6,619

Maximum altitude (feet):

400,000

375,000

800,000

220,000

Cost and Schedule:

R&D plus three aircraft (millions):

$36.3

$36.4

$56.1

$47.0

Estimated First flight:

Jan. 59

Mar. 58

Nov. 57

Feb. 58

The Million-Horsepower Engine

Although the hypersonic research airplane concept developed at Langley had met with almost unanimous endorsement by the Air Force, the lack of a suitable powerplant was a major shortcoming in the eyes of the WADC Power Plant Laboratory. The Langley study had determined that an engine (or engines) that could produce roughly 50,000 lbf was needed for the research airplane. The flight profiles developed by John Becker and his researchers showed that the ability to vary the thrust during flight would provide much better data and allow pilots to repeat maneuvers with some precision. The laboratory thought the Hermes A1 engine used in the Becker study was not capable of evolving into a man-rated engine, and suggested several engines it believed were "more suitable" for a manned aircraft. Despite these suggestions, however, the laboratory believed further study was required before any engine could be selected.-^

By October 1954, researchers from the Air Force, Navy, and the NACA had selected four existing or proposed power plants for possible use in the X-15. These included the Aerojet XLR73, Bell XLR81, North American NA-5400, and Reaction Motors XLR10. Despite the tentative selections, the Power Plant Laboratory thought that any engine would require major modifications to meet the needs of the X-15. The laboratory also believed the Air Force needed to "accept responsibility for development of the selected engine and…provide this engine to the airplane contractor as government furnished equipment." The primary consideration, for both the laboratory and the NACA, was that the engine be able to operate safely under any condition (acceleration in any axis) the X-15 was likely to experience. Maintenance and reliability (as defined by time between overhauls) did not need to be up to production standards.-21

The 30 December 1954 invitation-to-bid letter from the Air Materiel Command included summaries of the four engines recommended by the Power Plant Laboratory. However, although the stated preference to use one of these engines did not forbid bidders from using other engines, it did require the bidder and engine manufacturer to justify the selection. The bidder needed to present the justification to the X-15 Project Office for approval.

The powerplant that was ultimately selected for the X-15 was not one of the four recommended ones, but became known during discussions with Reaction Motors concerning the XLR10 from the Viking missile. During a meeting with the Air Force, the company promoted "a larger version of the Viking engine" that was under development for the Navy as the XLR30. After these discussions, the Power Plant Laboratory estimated that Reaction Motors could develop the XLR30 into a suitable engine for less than $5,000,000 in approximately two years. It was not even close.-31

On 25 January 1955, the Air Force requested additional information from Reaction Motors. The company replied on 3 February 1955 with details on the XLR10 and XLR30, and recommended four possible combinations for the X-15 program. These included an oxygen-ethanol XLR10, an oxygen-ammonia XLR30, an oxygen-hydrocarbon XLR30, and an oxygen-ethanol engine using two XLR10 chambers fed by a single XLR30 turbopump. Each of the engines used hydrogen peroxide to drive the turbopump. After it was briefed on the Becker study, Reaction Motors doubted that a single XLR10 was "adequate to perform the objectives of this type of aircraft."-41

Although it suggested a combination of XLR10 thrust chambers and an XLR30 turbopump, Reaction Motors believed this engine would be overly complicated and predicted it would weigh 815 pounds (compared to 420 pounds for either of the XLR30 configurations). The company suggested that relatively minor modifications to the XLR30 would allow throttling between 17,000 and 57,000 lbf with a specific impulse of 278 seconds. The XLR30 installation required a

space 70 inches long and 30 inches in diameter, considerably less than that required for the larger XLR10-XLR30 combination.-51

The Million-Horsepower Engine

between 1949 and 1955. (Reaction Motors Inc.)

Independently, Reaction Motors determined that the two most important safety requirements were the propellant combination and the means of achieving combustion during ignition and shutdown. The company reviewed seven propellant combinations in depth, and eventually narrowed the choices to liquid oxygen and anhydrous ammonia. Reaction Motors based this choice largely on its significant experience with this combination, which had shown that ammonia had fewer critical starting characteristics than most hydrocarbon fuels. Additionally, the propellants were ideal for the regenerative cooling of the proposed engine’s thrust chamber.

The Air Force, however, was still more interested in the XLR10, and on 4 February 1955 it asked Reaction Motors for additional information on that engine. On the same day, however, Reaction Motors and the X-15 Project Office held a meeting during which the company detailed a significant development program to man-rate the XLR-10 for the X-15. Given the development effort required for either engine, the company believed the XLR30 would ultimately be a better engine. After a meeting between the Air Materiel Command and the X-15 Project Office, the government advised Reaction Motors to "make all further estimates on the basis of the XLR30’s development." [6]

Concurrently, the Air Materiel Command had also been in discussions with the other three engine manufacturers. The fact that the other manufacturers showed a somewhat lower level of interest than Reaction Motors is understandable-after all, Reaction Motors engines had powered most of the rocket-equipped X-planes since the original XS-1. In fact, by this time North American had already requested that the Air Force withdraw the NA-5400 from consideration. On 18 March 1955, the Air Force supplied the prospective airframe contractors with the specifications on the three remaining engines. The Air Force expected that a flight engine would be available to the winning contractor within 30 months.-^

The X-15 Project Office released its analysis of the data provided by the engine manufacturers on 22 March 1955. One of the comments was that generating the necessary 50,000 lbf would require multiple Bell and Aerojet engines. The X-15 Project Office made clear that the final engine was not a production item, and that the amount of available propellants was the only limit to the operating time of the engine.[8]

After much discussion, the Air Force decided to release a request for proposal for the X-15 engine that was separate from the airframe competition. On 26 April, Headquarters ARDC requested that "the engine program be subjected to a final critical review apart from, but concurrent with the evaluation of the airframe proposals." The Power Plant Laboratory, NACA, and Navy would complete their engine evaluations by 12 July. The evaluation was to come to one of three conclusions: 1) that one engine was so superior to the others that its use would be mandated, 2) that one engine was so inferior that its use would be forbidden, or 3) that all of the engines were so nearly comparable that the choice would be left to the airframe contractor. The WADC scheduled the final engine evaluation meeting for 28 June, although this later slipped to 6­7 July.[9]

DRY LAKES

Although they had one of the most ideal test locations in the world, the Air Force and NACA could not simply go out and begin conducting X-15 operations. Several hurdles had to be overcome before the X-15 could ever do more than just conduct short flights over the Edwards reservation.

It had been recognized early during planning for the X-15 flights that suitable contingency landing locations would need to be found in the event of an abort after separation from the B-52 carrier aircraft, or if problems during the flight forced the pilot to terminate the mission before reaching Edwards. Since North American had designed the X-15 to land on dry lakebeds, the logical course of action was to identify suitable lakebeds along the flight path-in fact, these

lakebeds had been one of the factors used to determine the route followed by the High Range.

The Air Force and NACA had to identify lakebeds that would enable the X-15 to always be within gliding range of a landing site. In addition, the flight planners always selected a launch point that allowed the pilot a downwind landing pattern. Normally, the launch point was about 19 miles from the lakebed runway and the track passed the runway 14 miles abeam. To establish the proper launch point, flight planners used the fixed-base simulator to determine the gliding range of the airplane, including both forward glides and making a 180-degree turn and returning along its flight path. Another consideration was that the flight planners needed to selected lakes that would provide an overlap throughout the entire flight.[69]

The first hurdle for the Air Force was to secure permission from the individuals and several government agencies that owned or controlled the lakebeds. Next was seeking permission from the Federal Aviation Agency (FAA-it became an administration later) to conduct flight operations over public land.

Although responsibilities concerning the lakebeds continued throughout the life of the X-15 program, there were several spurts of activity (two major and one minor) concerning them. The first occurred, logically enough, just before the beginning of the flight program when efforts began to secure the rights to the lakebeds needed for the initial flight tests. The second involved securing the lakes needed for the higher-speed and higher-altitude flights made possible by the introduction of the XLR99 engine. One final push later in the program tailored the set of lakes for the improved-performance X-15A-2 and its external tanks.

Eventually, 10 different launch locations would be used, including eight dry lakes: Cuddeback supported a single launch; Delamar was the most used, with 62 launches; Hidden Hills saw 50 launches; Mud hosted 34; Railroad was used for only 2; Rosamond was used for 17, Silver hosted 14, and Smith Ranch was used for 10. In addition, the Palmdale VOR (OMNI) hosted eight launches, and a single flight originated over the outskirts of Lancaster. Hidden Hills was usually the intended site for the abortive 200th flight. The vast majority of these flights (188) would land on Rogers Dry Lake. 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.[70]

Rosamond Dry Lake, several miles southwest of Rogers, offered 21 square miles of smooth, flat surface that the Air Force used for routine flight test and research operations and emergency landings. This dry lakebed had served as the launch point for many of the early rocket-plane flights at Edwards. It is also the first lakebed that most visitors to Edwards see, since the road from Rosamond (and Highway 14) to Edwards crosses its northern tip on its way to the main base area. Scott Crossfield would make the X-15 glide flight over Rosamond Dry Lake, and no particular permission was necessary to use Rosamond since the lakebed was completely within the restricted area that made up the Edwards complex. Unfortunately, the lake was only 20 miles away from the base, so it did not allow much opportunity for high-speed work.

The Rogers and Rosamond lakebeds are among the lowest points in Antelope Valley, and they collect seasonal rain and snow runoff from surrounding hills and from the San Gabriel Mountains to the south and the Tehachapi Mountains to the west. At one time, the lakebeds contained water year-round, but changing geological and weather patterns now leave them wet only after infrequent rain or snow. A survey of the Rosamond lakebed surface showed its flatness, with a curvature of less than 18 inches over a distance of 30,000 feet.[71]

Beginning in early 1957, North American, AFFTC, and NACA personnel conducted numerous evaluations of various dry lakes along the High Range route to determine which were suitable for X-15 landings. The initial X-15 flights required 10 dry lakes (five as emergency landing sites near launch locations, and five as contingency landing sites downrange) spaced 30-50 miles apart.^72

The processes to obtain permission to use the various lakebeds outside the Edwards complex were as diverse as the locations themselves. For instance, permission to use approximately 2,560 acres of land at Cuddeback Lake as an emergency landing location was sought beginning in early 1957, with first use expected in January 1959. The lakebed was within the land area reserved for use by the Air Force at George AFB, California, but the Department of the Interior controlled the lakebed itself. Since the Air Force cannot acquire land directly, officials at the AFFTC contacted the Los Angeles District of the Army Corps of Engineers, only to find out that George AFB had already requested the Corps to withdraw the land from the public domain. The Bureau of Land Management controls all land in the public domain, although control may pass to other government agencies (such as the military) as stipulated in various laws (U. S. Code Title 43, for example). At the time, the Corps of Engineers acted as the land management agent for the U. S. Air Force, and John J. Shipley was the chief of the real estate division for the Los Angeles District.

DRY LAKES

This map shows the general location of the lakebeds as well as the radar coverage afforded by the three High Range stations. The two primary restricted airspace areas are shaded, although the entire flight path of the X-15 was restricted on flight day. (Dennis R. Jenkins)

Officials at George intended to use the lakebed as an emergency landing site. In turn, on 17 May 1957 the Corps wrote to the Bureau of Land Management on behalf of the Secretary of the Air Force, requesting a special land-use permit for Air Force operations at the lake. When the Los Angeles District received the request from the AFFTC, Shipley contacted Lieutenant Colonel C. E. Black, the

installations engineer at George AFB, requesting that a joint-use agreement be set up that would permit sharing the lake with the AFFTC for X-15 operations.-1731

By the end of July 1959, the Bureau of Land Management had approved the permit, and George AFB had agreed in principle to the sharing arrangement. The special-use permit gave George AFB landing rights for several years, and permitted the lakebed to be marked as needed to support flight operations. John Shipley, very intelligently, decided that the joint-use agreement between the AFFTC and George was an internal Air Force affair and bowed out of the process after the issuance of the Bureau of Land Management permit. Although there seemed to be no particular disagreement, the joint-use agreement had a long gestation period. The special-use permit was granted at the beginning of August, but at the end of September Colonel Carl A. Ousley, the chief of the Project Control Office at the AFFTC, questioned why a written joint-use agreement had not been signed. Major Resiner at George replied on 14 October that he had received verbal approval from all parties, but written approval was required from two separate Air Force commands (the ARDC and the Tactical Air Command (TAC)), the Corps of Engineers, and the Bureau of Land Management. He foresaw no difficulties in obtaining the signatures, and apparently the process worked itself out within a suitable period since there appears to have been no further correspondence on the matter. The joint-use agreement with George AFB essentially stated that the AFFTC was responsible for any unique preparations and marking of the lakebed required to support X-15 operations, although George did offer to supply emergency equipment and personnel as needed.-741

Simultaneously with the request to use Cuddeback, the AFFTC issued a similar request for Jakes Lake and Mud Lake, both in Nevada. Originally, the X-15 program had wanted to use Groom Lake, Nevada, as a launch site instead of Mud Lake. However, the security restrictions in place at Groom Lake (also known as "The Ranch") to protect the CIA-Lockheed reconnaissance programs led the AFFTC and NASA to abandon plans to use this facility. Officials at Nellis suggested Mud Lake as a compromise between the needs of the X-15 program and the highly classified CIA programs.751

The AFFTC asked for approximately 2,500 acres of land in the public domain at Jakes Lake; at Mud Lake, the request was for 3,088 acres. The indefinite-term special-use permits sought the right to install fencing to keep cattle from grazing in certain areas. Several ranchers had grazing rights on the public domain land, so this required modifying these agreements and compensating the ranchers with Air Force funds. In this case the Air Force did not want to remove the land from the public domain, but it did want to use approximately 9,262 acres of land at Mud Lake that had already been withdrawn from the public domain for use as part of the Las Vegas Bombing and Gunnery Range.761

October 1957 for approval and funding. By the end of January 1958, however, Lieutenant Colonel Donald J. Iddins at the AFFTC began to worry that the process was taking too long. The X-15 needed the lakes in July 1959, and there was no evidence of final action. Part of the problem was that land actions involving over 5,000 acres (which the two actions together did) required approval from the House Armed Services Committee. The AFFTC reminded the chief of engineers that they did not want to remove the land from the public domain, which seemingly eliminated the need for congressional approval, and brought the situation to the attention of the X-15 Project Office during a management review at Wright Field on 5 February 1958. The result was a renewed effort to ensure that all three lakes (Cuddeback, Jakes, and Mud) were available for X-15 use on schedule, including the right to build roads to the lakes, marking approach and landing areas, and fencing certain areas if necessary to ensure the safety of the X-15.-177

On 14 February 1958, the chief of engineers responded that he had initiated the process to grant special-use permits, but had terminated the effort when he noted that the AFFTC wanted to fence off the land. However, the law did not permit fencing to be erected on special-use permitted land. This meant that the land would have to be withdrawn from the public domain after all, or go unfenced. It appears that the answer to the problem was obtained by the AFFTC agreeing to a reduction in the Mud Lake acquisition to just under 2,500 acres (versus the original 3,088), bringing the total to under 5,000 and circumventing congressional approval. This allowed the land to be withdrawn from the public domain, and some of it was fenced as needed to keep stray cattle from wandering onto the marked runway.-178

Simply getting access to the lakebeds was not always sufficient. For instance, Mud Lake was in the extreme northwest corner of Restricted Area R-271, meaning that Sandia Corporation, which controlled R-271 for the Atomic Energy Commission (AEC), had to approve its use. A "Memorandum of Understanding between the Air Force Flight Test Center and Sandia Corporation" allowed AFFTC support aircraft to operate in the immediate vicinity of Mud Lake during X-15 flights. The AFFTC had to furnish flight schedules to Sandia one week before each anticipated mission, and Sandia made the point that it had no radar search capability and could not guarantee that the area was clear of traffic. Sandia also agreed not to schedule any tests within the restricted area that might conflict with X-15 flights. Once approved by Sandia, the AFFTC sought additional approval from Nellis AFB since Mud Lake was also within the Las Vegas Bombing and Gunnery Range. This approval was somewhat easier to negotiate because it was obtained from another Air Force organization.-1791

On 3 November 1958, a team from the AFFTC visited Mud Lake to conduct a preliminary study of lakebed conditions and to determine what action would be required to clear areas of the lakebed for use as a landing strip. When the group from the Flight Test Operations Division and Installations Engineer Division arrived over the lake, the pilot made several low passes to orient the group and obtain a general knowledge of the various obstructions that might conflict with landing on the lakebed. What the group saw was a general pattern of obstructions running east to west in a straight line across the center of the lakebed. The team landed at the Tonopah airport and proceeded by car to the lake, 16 miles away, for a closer inspection.-1801

They found that the obstructions observed down the center of the lake were a series of old gunnery-bombing targets dating from World War II. Practice bodies, wooden stakes, and good­sized rocks used to form bull’s-eyes for bombing practice littered the lakebed. The targets were in a narrow straight band down the center of the lake from west to east, but the debris covered a considerably wider area. As would become standard practice on all the lakes, the group dropped an 18-pound steel ball from a height of 6 feet and measured the diameter of the resulting impression. This gave a good indication of the relative hardness of the surface and its ability to support the weight of the X-15 and other aircraft and vehicles. At the edges of the lake, the ball left impressions of 3.25 inches or so, while toward the center of the lake the impressions were only 2.25-3.0 inches in diameter. At the time, the Air Force believed that impressions of 3.125 inches or less were acceptable. The general surface condition of the lakebed varied from relatively smooth and hard to cracked and soft. Although it was not ideal, the group thought the lakebed could be made useable with minor effort.-1811

More lakebed evaluations followed on 13-14 July 1959. X-15 pilot Bob White and the AFFTC chief of flight test operations, Colonel Clarence E. "Bud" Anderson, used a Helio L-28 Super Courier aircraft to visit 12 dry lakes along the High Range route. At each lake, Anderson and White dropped the "imperial ball" from six feet and measured the diameter of the resulting impression.

By this time, the Air Force had changed the criteria slightly: a diameter of 3.25 inches was acceptable, and anything above 3.5 inches was unacceptable. The survey included an evaluation of the surface hardness, surface smoothness, approximate elevation, length and direction of possible runways, and obstacles. Anderson remembers that there was "only one lake where we had to make a full power go-around as we watched the tires sink as we landed." Many future surveys would take personnel from AFFTC, NASA, and North American to most of the larger dry lakes along the High Range route.-1821

In addition, on 13 July 1959, four FAA representatives and two members of the AFFTC staff held a meeting at the FAA 4th Region Headquarters in Los Angeles to discuss using Silver Lake as a launch site for the X-15. Since some of the X-15 flight corridor would be outside existing restructured airspace, FAA approval was necessary. The FAA claimed jurisdiction under Civil Aeronautics Regulation 60.24, but was anxious to assist the Air Force within the limits of the law. The Air Force intended to use Silver Lake launches for early X-15 flights with the XLR11 engines. The proposed 100-mile flight path consisted of Silver Lake, Bicycle Lake, Cuddeback and/or Harpers Lake, and then on to landing at Edwards. The FAA had no particular problem with the concept, but since its charter was to protect the safety of all users of public airspace, it believed that certain restrictions needed to be in place before the flights could be approved. The participants spent most of the meeting discussing possible operational problems and concerns, and then developing limitations or restrictions that mitigated the concerns.1881

For Silver Lake launches, both the launch and the landing were performed in a restricted airspace called a "test area." Silver Lake was inside Flight Test Area Four, while Edwards was at the center of Flight Test Area One. However, none of the test areas surrounding Edwards were restricted 24 hours per day, or seven days per week. In fact, they were open to civilian traffic most of the time, and their closure had to be coordinated with the FAA (the airspace immediately around Edwards was always closed to civilian traffic). In addition, the flight path from Silver Lake to Edwards would take the X-15 out of restricted airspace and into civilian airspace for brief periods. Future flights using the northern portion of the High Range would also be outside normal test areas. The FAA, therefore, needed to approve the plans and procedures for using that airspace.1841

On 1 September 1959, L. N. Lightbody, the acting chief of the General Operations Branch of the Los Angeles office (4th Region) of the FAA wrote to Colonel Roger B. Phelan, deputy chief of staff for operations at the AFFTC. The letter contained a "certificate of waiver covering the release of the X-15 research vehicle over Silver Lake" subject to some special limitations. The FAA imposed the limitations to ensure "maximum safety not only to your AFFTC personnel and equipment, but also to other users of the immediate airspace. Further, the communications requirements will insure the blocked airspace may be returned to its normal use with minimum delay." The FAA approved the certificate of waiver (form ACA-400) on 1 September 1959 and listed the period of waiver as 1

October 1959 to 31 March 1961, although it was subsequently extended to 1 July 1963, and later still through the end of 1969.[85]

Given the effort that accompanied the acquisition of Cuddeback Lake in late 1957 and early 1958, it is surprising that the first serious survey of the lake does not appear to have taken place until 7 October 1959. Of course, conducting detailed surveys significantly ahead of the anticipated use was not a particularly useful exercise since the periodic rains that kept the lakebeds useable also changed their character each time, as did the effects of other vehicles (such as cars). By this time, the X-15 had already made its first two flights from over Rosamond Dry Lake, landing each time at Rogers. Since the Air Force expected the X-15 to begin rapidly to expand its flight envelope, North American sent George P. Lodge to Cuddeback in an Air Force Piasecki H-21 Shawnee helicopter.1861

Lodge conducted the standard hardness tests by dropping the same "imperial ball" used in the other surveys. He found that the ball left an impression of about 3 inches (which was considered acceptable) at the southern end of the proposed runway, but quickly degraded to 4-4.5 inches by the northern end. He noted that these measurements compared unfavorably to tests on Rogers (2 inches) and Rosamond (2 inches) conducted after the last rains. A note emphasized that there were a set of deep ruts running the length of the runway made by a vehicle when the lake was wet, and that although it was only a single set of ruts, they "wander around to some extent." The nature of the lakebeds was such that grading or other mechanical methods could not repair major damage-only nature could do that. Lodge recommended that "Cuddeback lake, in its present condition, not be considered as an alternate landing site for the X-15 airplane and should be used only as a last resort in an extreme emergency." He warned that "should a landing be attempted with the X-15 airplane on Cuddeback lake in its present condition, there would be more than a 50-50 chance of wiping out the nose gear." It was clear that the lakes had both good and bad qualities: they were largely self-repairing each time it rained, but they could also be self­destroying by the same process.-1871

Two weeks later, Lodge, who was a flight safety specialist for North American, performed a survey of Silver Lake and nine other lakes to determine their suitability as emergency landing sites. At Silver, Lodge found that the prevailing wind was out of the north, with the best landing heading estimated at 200-310 degrees magnetic. The southern portion of the lake was soft with numerous sinkholes, and not satisfactory for touchdown. Lodge also found an abandoned railroad bed, approximately 2 feet high and 10 feet wide, running north to south across the east side of the lakebed. There was also a dirt road with deep ruts running east to west across the northern part of the lake, a paved road going from Baker to Death Valley along the eastern perimeter, and another dirt road (this time with no ruts) running diagonally northwest to southeast.-1881

Despite these obstacles, there was approximately 16,000 feet of satisfactory lakebed between the soft southern portion and the northern road. There were a few sinkholes, most measuring about 7 inches across and 3-4 inches deep, but the Air Force would fill these before use. The usual imperial-ball tests resulted in impressions between 2.9 and 3.7 inches in diameter, although the main area was on the lower end of that range. In addition, Lodge pounded both 3/8-inch and 1/2-inch steel rods into the ground with 200 pounds of force to determine what the condition of the soil was under the upper crust. The 3/8-inch rod generally penetrated between 1 and 3 inches, while the 1/2-inch rod penetrated between 0.25 and 1.5 inches. The results of the tests led Lodge to recommend a location for a marked runway. Of the other nine lakebeds visited,

Lodge landed only on the east and west lakes in the Three Sisters group, and determined that both were satisfactory for emergency use despite having "a few rocks and ammo links strewn 1891

about.

As 1959 ended, George Lodge was a busy man, and at the end of November he conducted yet another lake survey, this time of approximately 50 lakes in California, Nevada, and Utah. Again, the intent was to find suitable emergency landing sites for the X-15 as it expanded its flight-test program. The test methods Lodge used on the lakes were the same as he had used the previous month at Silver Lake.-90

The Air Force and NASA continued to survey the established and previously used lakebeds periodically, particularly after it rained to determine that the lakebed was dry enough to support operations and that no sinkholes or gullies existed. Changing the direction of the available runways on a lakebed also required a revised survey. For instance, in early December 1959 Lodge conducted a new survey of Rosamond Dry Lake to determine whether the lake would support a marked runway running northeast to southwest. Marked runways already existed on headings of 10-190 degrees and 70-250 degrees. Starting from a location in the southwest corner of the lakebed, Lodge inspected a heading of approximately 30 degrees, roughly toward the telemetry station located on the edge of the lake. He found that the lakebed was hard and smooth for 2 miles, moderately smooth at 2.5 miles, smooth again at 3 miles, moderately rough at 3.5 miles, and rough from 4 miles to the edge of the lakebed. Imperial-ball drop tests yielded diameters of about 2.5 inches across the route. The conclusion was that the runway was practical, and, as viewed from above, would result in a runway approximately halfway between the two existing runways, with all three converging at the southwest edge of the lakebed.-1911

The second round of lake acquisitions began when the XLR99 engine came on line. First up was securing rights to use Hidden Hills dry lake, slightly west of the Hidden Hills Ranch airstrip. Simulator studies had confirmed that Hidden Hills would be ideal as an emergency landing site during the launches for the initial XLR99 flights that needed to be conducted further uprange than the XLR11 flights. The lakebed would continue to be used as a contingency site as the program continued to launch further uprange into Utah. At the beginning of 1960, it was expected that the program would need access to the lake by 1 October 1960.[92]

However, schedules change, and the XLR99 flight dates kept slipping. A revised plan showed that the XLR99 research buildup flights would use Silver Lake and Hidden Hills Lake in California, Mud Lake in Nevada, and Wah Wah Lake in Utah as launch sites. The program needed various intermediate lakes along the upper portion of the High Range to provide complete coverage for emergency landings along the route. The Air Force would staff the intermediate lakes with crash and emergency personnel during flights. Additional contingency lakes would have runways marked on them, but would not be staffed with support personnel. At first the AFFTC and NASA had wanted to mark "all lakes with a satisfactory 10,000 feet landing surface" to provide an additional factor of safety for the X-15 program. Although no plans existed to use these lakes, the planners believed that marking them would also allow continued X-15 operation when a primary intermediate lake was wet. However, legal personnel indicated that there was "NO possibility" (emphasis in original) of marking any lake unless a right-to-use permit was obtained. Since personnel and funds did not exist to negotiate all the required permits, this plan was abandoned and a list of essential contingency sites was drawn up.[93]

The 30 September 1960 plan included launching immediate flights from Silver Lake, with the west lake at Three Sisters and Cuddeback acting as intermediate emergency sites. By 1 February 1961, operations would move to Hidden Hills, with Cuddeback as the intermediate site. On 1 April, Mud Lake would become the primary launch lake, with Grapevine and Ballarat as the intermediate sites, and contingency sites located at Panamint Springs and Racetrack. Two months later the launches would move to Wah Wah Lake, with Groom Lake, Delamar, and Hidden Hills becoming the intermediate sites, and Dogbone and Indian Springs the contingency sites. The AFFTC sought permission from Nellis to use the last two sites because they were located on the Las Vegas range, as was Mud Lake.[94]

Planners had always considered Smith Ranch Lake as a backup site to Wah Wah Lake, using Mud Lake as the intermediate site and the same contingency sites used during Mud Lake launches. This was still true at the end of February 1961. The program expected to begin launches from Hidden Hills in March 1961, and the launch lake still needed to be surveyed and marked. NASA expected to begin using Mud Lake in April 1961 and two of the support lakes (Grapevine and Panamint) still required use permits, while Ballarat had replaced Racetrack as the second contingency site. The program still needed to survey and mark all three of the support lakes. Launches from Wah Wah would begin in June 1961, and all of the sites along that route (except for Hidden Hills) still had to be "acquired," surveyed, and marked. As the program continued, however, it abandoned plans to use Wah Wah Lake, in part because of difficulties in obtaining permission to use the Nellis contingency sites (particularly Groom Lake) and airspace rights over Nevada’s restricted areas. Instead, the government eventually acquired the alternate launch site at Smith Ranch Lake, although flights from this point did not begin until June 1963.[95]

DRY LAKES

Determining if a lakebed could support the weight of an X-15 and its support airplanes was a relatively non-technical endeavor. A large steel ball, nicknamed the "imperial ball" was dropped from a height of six feet and the resulting impression was measured. For most of the program, a diameter of 3.25 inches or less was considered acceptable to support operations. Neil Armstrong is kneeling beside the ball in this June 1958 photo at Hidden Hills. (NASA)

Special Thanks

I owe a particular mention of Jay Miller, author of the popular The X-planes: X-1 to X-45, (Hinckley, England: Midland Publishing, 2001), among many other works. Anybody interested in reading about the other X-planes should pick up a copy of this excellent book. Jay was responsible for the first photograph I ever had published, and published my first book—a short monograph on the Space Shuttle. Somehow, I feel I have him to blame for the quagmire of aerospace history I find myself embroiled in. I truly appreciate the help and friendship from Jay and his lovely wife Susan over the past 25 years or so.

Thankfully, my mother, Mrs. Mary E. Jenkins, encouraged me to seize opportunities and taught me to write and type—such necessary attributes for this endeavor. As for so many things, I owe her a great deal of gratitude, along with my everlasting love and admiration. After listening to my trials and tribulations about this project for a decade, she passed away before publication. I hope she has found the peace and rest she so richly deserves.

A note regarding terminology: In the days before being politically correct became a prime influence on engineering and history, engineers called piloted vehicles "manned" aircraft, and the process of making them safe enough to fly was termed "man-rating." This work continues to use these terms since they are what were in use at the time.

[1] The Armstrong quote is in the foreword to Milton O. Thompson, At the Edge of Space: the X – 15 Flight Program (Washington, DC: Smithsonian Institution Press, 1992), p. xii.

[2] John V. Becker, "The X-15 Program in Retrospect," 3rd Eugen Sanger Memorial Lecture, Bonn, Germany, 5 December 1968, pp. 1-2

[3] Harrison A. Storms, "X-15 Hardware Design Challenges," a paper in the Proceedings of the X – 15 30th Anniversary Celebration, Dryden Flight Research Facility, Edwards, California, 8 June 1989, NASA CP-3105, p. 27.

[4] In the 3rd Eugen Sanger Memorial Lecture in 1968, John Becker stated that 109 flights exceeded Mach 5. A reevaluation of the flight data shows that only 108 actually did. See Becker, "The X-15 Program in Retrospect," p. 3 for Becker’s original numbers.

[5] Despite all that is written, the program held very few "official" records, mainly because it seldom invited the FAI out to witness the flights. In fact, it appears that the 314,750-foot altitude record set by Bob White is the only official record ever set by the program.

[6] Ronald G. Boston, "Outline of the X-15’s Contributions to Aerospace Technology," 21 November 1977. Unpublished preliminary version of the typescript available in the NASA Dryden History Office. For those interested in Boston’s original paper, the easiest place to find a copy is in the Hypersonic Revolution, republished by the Air Force History and Museums program. It constitutes the last section in the X-15 chapter; Letter, William H. Dana, Chief, Flight Crew Branch, DFRC, to Lee Saegesser NASA History Office, transmitting a copy of the SETP paper for the file. A slightly rewritten (more politically correct) version of the paper was later published as The X-15 Airplane-Lessons Learned (American Institute of Aeronautics and Astronautics, a paper prepared for the 31st Aerospace Sciences Meeting, Reno Nevada, AIAA-93-0309, 11-14 January 1993). Boston listed 1,300°F as the maximum temperature, but Bill Dana reported 1,350°F in his SETP and AIAA papers. Boston also listed the max-q as 2,000 psf, but in reality it was 2,202 psf on Flight 1-66-111.

[Z] Storms, "X-15 Hardware Design Challenges," pp. 32-33

[8] Becker, "The X-15 Program in Retrospect," pp. 1-2

[9] J. D. Hunley, "The Significance of the X-15," 1999, unpublished. Typescript available at the DFRC History Office.

[10] Officially, Johnny Armstrong (who is now the chief engineer in the Hypersonic Flight Test Team) maintains the AFFTC Hypersonic Flight Test Team Project Files and is, fortunately, something of a pack rat. However, to everybody at Edwards and Dryden, this wonderful collection is simply the Armstrong Memorial Library.

THE AIRFRAME EVALUATION

The airframe evaluation process lasted from mid May until late July, with the Air Force, NACA, and Navy conducting independent evaluations based on a number of preestablished criteria. The preliminary NACA evaluation of the proposals consumed the better part of three weeks before each of the laboratories forwarded preliminary results to Hartley Soule. On 3 June 1955, Ames tentatively ranked the submissions as 1) Douglas, 2) North American, 3) Bell, and 4) Republic. The Douglas ranking resulted from "the completeness and soundness of design study, awareness of factors in speed and altitude regime, and relative simplicity of approach." Ames, however, expressed skepticism over the Douglas magnesium hot-structure wing because it would preclude the study of problems associated with insulated-type structures that would potentially be used in future aircraft intended for greater flight duration. This seemed to be a major disconnect between Ames and Langley. It appears that Ames wanted to test a structure that would be representative of some future production aircraft; Langley just wanted to test a structure that would survive.

Another problem that worried the Ames evaluators was the flammability of magnesium. It seemed that "only a small area raised to the ignition temperature would be sufficient to destroy the aircraft." The researchers at Ames held that if Douglas should win the competition, the company should build two aircraft with the proposed HK31 structure, but a third aircraft "should have a wing based upon the alternative higher temperature insulated type of design approach." The Ames report continued to stress the need for a wing of greater leading-edge sweep angle (at least 53 degrees) "for the purpose of minimizing the rate of heat transfer to the leading edge."145

At Langley, on 6 June, researchers rated the North American proposal number one, followed by Douglas, Bell, and Republic. According to the Langley assessment, led by John Becker, the research utility of the North American hot-structure approach outweighed the advantages of the simplicity of the magnesium structure proposed by Douglas. Slightly rebuffing Ames, Langley noted that the 21% reduction in heat transfer gained by increasing the leading-edge sweep from the proposed 40 degrees to 53 degrees did not seem to justify the alteration of the planform. This was particularly true because the structure appeared capable of handling the heat load.-1146!

In a reminder to the evaluation teams, also on 6 June, Arthur Vogeley and Captain McCollough reiterated that the purpose of the evaluation was "to select a contractor rather than a particular design." Although certain features of the winning design could be unsatisfactory, it was the basic design approach as described in the proposal that might best be relied upon to produce an acceptable research airplane.147-

On 10 June 1955, the HSFS sent its airframe results to Soule, detailing the design approach and research utility aspects of the airframe, flight control system, propulsion unit, crew provisions, handling and launching, and miscellaneous systems. Researchers at the HSFS ranked the proposals as 1) Douglas, 2) North American, 3) Bell, and 4) Republic, although the proposals from Douglas and North American were essentially equal.148

The final evaluation by Ames, on 10 June, ranked the proposals as 1) North American, 2) Douglas, 3) Bell, and 4) Republic. This represented a change from the earlier Ames evaluation, based largely on researchers considering the North American structure superior in terms of research utility—an opinion voiced earlier by Langley. The Ames evaluators had apparently changed their minds about wanting to test a production-representative structure. The laboratory had also finally given up on advocating an insulated structure since no serious support for their earlier recommendation of equipping the third aircraft with a different wing structure had materialized (sufficient funds to construct an alternate wing were simply not available).-1149-

The final evaluation from Langley on 14 June ranked the proposals as 1) North American, 2) Douglas, 3) Republic, and 4) Bell. Although researchers at Langley thought the magnesium wing structure of Douglas was feasible, they feared that local hot spots caused by irregular aerodynamic heating could weaken or destroy the structure. The use of Inconel X by North American presented an advantage with regard to thermal limits—not only from the standpoint of margins for maneuverability within the design temperatures, but also from a safety viewpoint if the airplane ever exceeded its design temperature.

A few days after receiving all of the final evaluations, Soule sent copies of each to the WADC Project Office, along with a consolidated result. The final NACA ranking was (points based on a scale of 100) as follows:150-

Design Approach

Research Utility

B

D

N

R

B

D

N

R

Airframe

70

80

85

75

70

80

90

80

Flight controls

70

80

75

70

70

75

75

75

Propulsion

80

80

90

30

75

40

40

75

Crew provisions

55

85

80

40

55

85

80

35

Handling/launching

95

65

75

65

90

70

70

70

Miscellaneous

70

85

70

70

70

85

70

70

Average

73

79

79

58

72

73

71

68

Oddly, the final order representing the overall NACA evaluation was 1) North American, 2)

Douglas, 3) Bell, and 4) Republic, despite the fact that Douglas scored slightly more points in the evaluation (152 versus 150 for North American). Soule pointed out that although Ames, Langley, and the HSFS did not rank the four proposals in the same order, the final ranking did represent an overall NACA consensus. All of the laboratories involved in this portion of the evaluation considered both the Douglas and North American proposals to be much superior to those submitted by Bell and Republic. While researchers preferred the Inconel X structure of the North American proposal, the design was not without fault. For instance, the NACA thought that the landing-gear arrangement was undesirable, the differentially-operated horizontal stabilator design in lieu of ailerons was an overly complicated arrangement, and (at least at Langley) the replaceable fiberglass leading edges were unacceptable.

John Becker wrote to Hartley Soule on 16 June attempting to clarify why the North American design was superior to that of Douglas. The letter listed the thermal limits expected for the new aircraft, and showed that the Inconel X structure on the North American design was "impressively superior" to the magnesium alloy used by Douglas. The data were shown for three categories: 1) performance within the design temperature limits in terms of allowable velocity, altitude, and dependence on speed brakes; 2) reserve heat capacity (in case the design temperatures were exceeded by a moderate margin) such that the structure would still have a fair possibility of remaining intact; and 3) the possibility of melting or burning in case the design temperatures were greatly exceeded in local hot spots. There appears to be no further correspondence on this subject, so Becker’s explanation seems to have answered whatever unasked questions existed.-1151-

During the first two weeks in July, the WADC evaluation teams sent their final reports to the WADC Project Office. As with the NACA evaluations, the Air Force found little difference between the Douglas and North American designs, point-wise, with both proposals considered significantly superior to those of Bell and Republic.

George Spangenberg was in charge of the Navy evaluations, which got off to a late start and ended up being cursory. In the end, the Navy found much the same thing as the NACA and ranked the airframe proposals as 1) Douglas, 2) North American, 3) Republic, and 4) Bell. Given the Navy’s long—and successful-association with Douglas airplanes, the order was not surprising. Most Navy concerns centered on the selection of an engine. As Clotaire Wood explained, "the airframe-engine combination was to be evaluated and not the engine alone, since it had been agreed that the engine of the winning design would be the engine supported by the special development program." This was not how the Power Plant Laboratory saw the process, but it seemed to put the Navy at ease. In addition, Wood indicated that "it would be of real value to have the Bureau’s [BuAer] recommendations regarding an engine development program once the winner of the competition is determined."-152

In early July the Navy began to raise questions about the various airframe proposals. For instance, the BuAer electronics group did not believe the Bell design had a satisfactory electrical power system, and Navy researchers rated the North American design last from an equipment (e. g., life support) perspective. The Douglas and Republic designs had the best potential flying qualities, and BuAer researchers felt that North American had incorrectly assumed laminar flow over much of their design, and had therefore underestimated the heating values. It was a bit late to be raising concerns, but most of the issues were minor and did not materially affect the outcome of the competition. After conferring with his Air Force and NACA counterparts, on 15 July George Spangenberg finalized the Navy’s position as Douglas, North American, Republic, and Bell.-1153!

On 26-28 July, the Air Force, NACA, and Navy evaluation teams met at Wright Field to select an airframe contractor. George Spangenberg stated that it was unfortunate that the point system used in the evaluation "appeared to give no conclusive winner," since a contractor could score highly in one area and low in another yet still have a winning score, while another that was satisfactory in all areas would be rated lower. He also indicated that the goals of the project seem to have shifted somewhat, resulting in a "firm requirement" for 1,200°F skin temperature research instead of the previous "desire" for high temperatures.-1154!

Presaging events to come, discussions ensued concerning the amount of work recently awarded to North American and Republic, and whether additional awards would spread their engineering groups too thin. Other discussions included the possibility of selecting Douglas but directing it to redesign its aircraft using an Inconel hot structure instead of magnesium. In the end, the Air Force and the NACA concluded that the North American proposal best accommodated their requirements. The Navy did not want to cast the only dissenting vote and, after short deliberation, agreed to go along with the decision.-11551

During the week of 1-5 August 1955, the WADC Project Office prepared the final evaluation summary and oral presentation: "the evaluation of the proposals submitted in competition was made in five areas: performance, technical design, research suitability, development capability, and cost." It is interesting to note that this competition was not about the "lowest bidder," and none of the proposals were anywhere near the original $12.2 million estimate. The results of these evaluations were as follows:!156!

Performance: The performance evaluation consisted of a check of the probability of the different designs, considering present uncertainties, of meeting the specified speed and altitude requirements. The probabilities were calculated to be best for the North American proposal, equal for the Bell and Douglas proposals, and least for the Republic proposal; but because of the assumptions of the analysis, all designs were judged able to meet the requirements.

Technical Design: This factor was judged on the awareness shown by the contractor of the problems of high-speed, high-altitude flight and of the means, as indicated by the airplane designs, the contractor proposed for exploring and studying these problems. The general design competency of the contractor also was judged from the designs submitted: North American 81.5 points; Douglas 80.1 points; Bell 75.5 points; and Republic 72.2 points. No design, as submitted, was considered safe for the use intended. The Douglas design was considered best in this regard, but did not include adequate margins for ignorance factors and operational errors.

Research Suitability: In this area, the fundamental differences in the proposed structures were examined and rated because of their decisive importance in the research uses of this aircraft. North American was rated acceptable because of the Inconel X "hot-structure" heat­sink, which was most suitable for research and which was potentially the simplest to make safe for the mission. Republic and Bell were considered unsatisfactory because of the hazardous aspects associated with the insulated structures used, and Douglas was considered unsatisfactory because of the low safety margins available and because of the limited future usefulness of the "cool" magnesium heat-sink principle.

Development Capability: Ratings were based on the physical equipment and manpower the contractor had available for pursuing the project, and the resulting time proposed for development. Evaluation of this factor resulted in the following ratings: (1) Douglas was acceptable; (2) North American was acceptable; (3) Bell was less acceptable; (4) Republic was less acceptable. North American, Republic, and Douglas estimated that the first flight date would be within 30 months, but the Republic estimate was not believed to be credible, hence their lower score. Bell promised a first flight date within 40 months.

Costs: Costs for three aircraft plus static test article, engines, and spares as adjusted by AMC to a comparable basis are: Bell, $36.3 million; Douglas, $36.4 million; Republic, $47.0 million; and North American, $56.1 million.

On 9 August, Captain McCollough presented the results of the evaluation to Brigadier General Howell M. Estes, then chief of the Weapons Systems Division, under whose jurisdiction the WADC Project Office fell, and a select group of senior Air Force officers. McCollough made a second presentation in Baltimore on 11 August for Generals John W. Sessums and Marvin C. Demler, who were the commanders of the WADC and ARDC, respectively, and Hartley Soule from the NACA.-157

The final briefing to a combined meeting of Air Force, NACA, and Navy personnel was at NACA Headquarters on 12 August. The attendees included Hugh Dryden, Gus Crowley, Ira Abbott,

Richard Rhode, and Hartley Soule from the NACA; Brigadier General Kelsey, Colonel Donald H. Heaton, Lieutenant Colonels Gablecki and Maiersperger, and Major Heniesse from the Air Force; and Captain R. E. Dixon, Abraham Hyatt, and George Spangenberg from BuAer. Following this, the Research Airplane Committee met, accepted the findings of the evaluation groups, and agreed to present the recommendation to the Department of Defense.-1158!

Because the estimated costs submitted by North American were far above the amount tentatively allocated for the project, the Research Airplane Committee included a recommendation for a funding increase before signing the final contract. A further recommendation-one that would later take on greater importance-called for relaxing the proposed schedule by up to 18 months. The committee approved both recommendations and forwarded them to the Assistant Secretary of Defense for Research and Development.

ENGINE PROPOSALS

Three companies-Aerojet, Bell, and Reaction Motors-submitted proposals for the X-15 engine on 9 May 1955, the same day as the airframe competitors. North American had already asked the Air Force and NACA to dismiss the NA-5400 as an alternative. A copy of the Aerojet XLR73 proposal could not be located.

Bell was conservative in its engine proposal and stated that "modifications have been limited to those necessary to permit the engine to be used in a piloted aircraft." The changes to the XLR81 were made primarily in the starting and control systems, mostly to provide additional safety margins. The modified engine would be capable of multiple starts with a safety system based on a similar device provided for use during ground testing. The modifications provided an engine that could operate at an 8,000-lbf thrust level in addition to the normal 14,500-lbf full thrust. The modifications included the addition of a propellant bypass valve just in front of the injector so that, at the reduced thrust level, approximately one-half of the propellants would return to the tanks instead of being injected into the thrust chamber. This eliminated the need to change the pump discharge pressures, and allowed the same amount of propellants to flow through the cooling system. Only one engine in each airplane would have the capability to provide the 8,000- lbf level, although this reflected the removal and capping of the bypass valve and not any major change in engine configuration. Bell also proposed changing the fuel as a safety measure. In an attempt to minimize the risk of mixed propellants accumulating and exploding, Bell wanted to exchange the jet fuel normally used in the XLR81 with a mixture of 40% unsymmetrical dimethylhydrazine (UDMH) and 60% jet fuel (Bell called this combination "JP-X"). This would make the two propellants hypergolic, eliminating the hazard. Bell also pointed out that these propellants would not need to be topped off from the carrier aircraft, since neither had an appreciable vaporization rate. Bell noted that "since tests of the major components of the XLR81-BA-1 engine have been successful, extensive development tests of these components will not be required for the X-15 engine program."10

Like the Bell proposal, the proposal from Reaction Motors was brief (Bell used 15 pages, and Reaction Motors used just 14). The XLR30 would be modified to "1) emphasize safety and minimum development time, 2) start, operate and shutdown at all altitudes and attitudes, and 3) be capable of at least five successive starts without servicing or manual attention other than cockpit controls." Instead of the thrust-stepping proposed by Bell, Reaction Motors offered an infinitely variable thrust ranging from 13,500 to 50,000 lbf at sea level. Reaction Motors believed that "the highly developed state of the major engine components, i. e., turbopump, thrust chamber and control valves allows RMI to meet the schedule…." Unlike Bell, which extensively discussed the modifications required to make its engine meet the X-15 requirements, Reaction Motors instead gave a technical overview of the XLR30, and it was not possible to determine what the modifications were. Nevertheless, the overall impression was that the state of XLR30 development was far along.-1111