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]

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

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

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

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

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

Douglas summarized the advantages of HK31 as follows:1831

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

off the drawing board.-1104!

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

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

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

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

TIME FROM LAUNCH – SECONDS

TIME FROM LAUNCH – SECONDS

mm

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

DESIGN MISSIONS

… femperafure vs. time

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

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

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

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

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

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

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

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