CHANGES

The engineers never expected that the design proposed by North American would be the one actually built—it seldom works that way even for operational aircraft, much less research vehicles. True to form, the design evolved substantially over the first year of the program, and on 14-15 November 1955 researchers gathered in Inglewood to resolve several issues. For instance, the North American proposal used 1,599 psf for the minimum design dynamic pressure, while the NACA wanted at least 2,100 psf and preferably 2,500 psf. It would take 100 pounds of additional structure to accommodate the higher pressure. On the other hand, increasing the design load factor from 5.25 g to 7.33 g would cost another 135 pounds, but everybody agreed that raising the design dynamic pressure was a better use of the weight. Nevertheless, as built, the X-15 was rated at 7.33 g, and the change was incorporated when it became obvious that the additional weight was rather trivial after various other upgrades were incorporated.-1191

Researchers also spent considerable effort on evaluating the structural materials proposed by North American, but a lack of detailed information made it impossible to reach a final decision on the wing leading-edge material. The group discussed various ceramic-metallic (cermet), copper, fiberglass, plastic, and titanium carbide materials without conclusion. North American had proposed a wing leading edge that was easily detachable, and the researchers considered this a desirable capability even though it drove a slightly more complex structure and a little additional weight. A weight increase of 13 pounds allowed the use of Inconel X sandwich construction for the speed brakes and provided additional speed brake hinges to handle the higher dynamic pressure already approved. The use of 0.020-inch titanium alloy for the internal structure of the wings and stabilizers instead of 24S-T aluminum gained support, although it involved a weight increase of approximately 7 pounds.

Other structural discussions included changing the oxygen tank to Inconel X due to the low – impact strength of the original titanium at cryogenic temperatures. At the same time, researchers reviewed the need to include a pressurization system to stabilize the propellant tanks. Initially the engineers had considered this undesirable, and North American had not provided the capability in the original design. However, the additional stresses caused by increasing the design dynamic pressure made it necessary to accept a large increase in structural weight or include a pressurization system, and the attendees endorsed the latter. In fact, during the flight program, pilots routinely repressurized the propellant tanks after they jettisoned any remaining propellants to provide an extra margin of structural strength while landing.-128

When the researchers considered a random-direction, 1-inch thrust misalignment, it became obvious that the original large dorsal vertical stabilizer was unsatisfactory for the altitude mission profile. Based on experience with the X-1, the researchers knew that an installed engine could be a couple of degrees out of perfect alignment, although aerodynamic trim easily corrected this. However, in the case of the X-15, the thrust of the engine and the extreme velocities and altitudes involved made the issue a matter of some concern, and the government and North American agreed to include provisions correcting potential thrust misalignment. Along with several other issues, this caused engineers to modify the configuration of the vertical stabilizer.-121

Researchers also concluded that the design would suffer from some level of roll-yaw coupling, and agreed upon acceptable limits. The government also pointed out the need for a rate damping (stability augmentation) system in pitch and yaw for a weight increase of 125 pounds. The need to make the dampers redundant would be the subject of great debate throughout the development phase and early flight program, with the initial decision being not to. Attendees also decided the ballistic control system did not require a damping system, something that would change quickly during the flight program.-122

North American agreed to provide redundant ballistic control systems and to triple the amount of hydrogen peroxide originally proposed. Engineers agreed to provide separate sources of peroxide for the ballistic controls and auxiliary power units (APUs) to ensure that the power units always had propellant. These changes added about 117 pounds.-123

The configuration of the pilot’s controls was finally established. A conventional center stick mechanically linked to a side-controller on the right console operated the aerodynamic control surfaces, while another side-controller on the left console above the throttle operated the ballistic control system. These were among the first applications of a side-stick controller, although these were mechanical devices that bore little resemblance to the electrical side-sticks used in the much later F-16.[24]

In an unusual miscommunication, the attendees at the November meeting believed the WADC had already developed a stable platform and would provide this to North American as government – furnished equipment. Separately, the NACA agreed to supply a "ball nose" to provide angle-of – attack and angle-of-sideslip data. The ball nose, or something functionally similar, was necessary because the normal pitot-static systems would not be reliable at the speeds and altitudes envisioned for the X-15. Although North American proposed a system based on modified Navaho components, the NACA believed that the ball nose represented a better solution.-123

Per a recent service-wide directive, the Air Force representative had assumed that the X-15 would be equipped with some sort of encapsulated ejection system. On the other hand, North American had proposed a rather simple ejection seat. The company agreed to document their rationale for this selection and to provide a seat capable of meaningful ejection throughout most of the expected flight envelope, although all concerned realized that no method offered escape at all speeds and altitudes.-1261

The November meetings ended with a presentation by Douglas engineer Leo Devlin detailing their second-place proposal. A presentation on the advantages of HK31 magnesium alloy for structural use was interesting but provided no compelling reason to switch from Inconel X. Afterwards, Rocketdyne presented a 50,000-lbf rocket engine concept based on the SC-4 being designed for a high-altitude missile; this was a matter of only passing interest, given that a modified XLR30 was already under contract. Separately, Hartley Soule and Harrison Storms discussed the proposed wind-tunnel program, attempting again to agree on which facilities would be used and when.-123

The research instrumentation for the X-15 was the subject of a two-day meeting between personnel from Langley and the HSFS on 16-17 November. The group concluded that strain gauges would be required on the main wing spars for the initial flights, where temperatures would not be extreme, but that wing pressure distributions were not required. The HSFS wanted to record all data in the aircraft, while Langley preferred to telemeter it to the ground. Unfortunately, a lack of funds prevented the development of a high-speed telemetry system. The day following the NACA meeting, representatives from North American drove to the HSFS and participated in a similar meeting. Charlie Feltz, George Owl, and D. K. Warner (North American chief of flight test instrumentation) participated along with Arthur Vogeley, Israel Taback, and Gerald M. Truszynski from the NACA. The participants quickly agreed that the NACA would provide the instruments and North American would install them. The first few flights would use a more or less standard NACA airspeed boom on the nose of the X-15 instead of the yet-to-be-completed ball nose. North American desired to have mockups of the instrumentation within nine months to facilitate the final design of the airplane, and the NACA indicated this should be possible.[28]

The debate regarding engine fuels flared up again briefly at the end of November when John Sloop at Lewis wrote to Captain McCollough recommending the use of a hydrocarbon fuel instead of ammonia. Lewis had concluded that it would be no more difficult to cool a hydrocarbon fuel than ammonia, and the fuel would be cheaper, less toxic, and easier to handle. No information was available on repeated starts of a JP-4-fueled rocket engine, but researchers at Lewis did not expect problems based on recent experience with a horizontally mounted 5,000-lbf engine. The researchers repeated their warning that anhydrous ammonia would attack copper, copper alloys, and silver, all of which were standard materials used in research instrumentation. At the same time, the HSFS wrote that tests exposing a standard NACA test instrument to anhydrous ammonia vapor had proven disastrous. Both NACA facilities repeated their request for a change to a hydrocarbon fuel.[29]

Later the same day, Captain McCollough notified Hartley Soule that the Power Plant Laboratory had reviewed the data submitted by Reaction Motors on the relative merits of substituting a hydrocarbon fuel for ammonia. The laboratory concluded that Reaction Motors had grossly underestimated the development time for conversion, and recommended the continued use of anhydrous ammonia as the most expeditious method of meeting the schedule. A meeting on 1 December at Wright Field brought all of the government representatives together to finalize the fuel issue. The conclusions were that 1) one fuel had no obvious advantage over the other insofar as performance was concerned, 2) the corrosive character of anhydrous ammonia was annoying but tolerable, 3) it would take 6 to 12 months to switch fuels, and 4) the engine development program should continue with anhydrous ammonia. This finally put the issue to rest, although the NACA facilities still believed the requested change was justified.[30]

November also saw an indication that Inconel might have unforeseen problems. A test of the tensile strength of the alloy was published by Langley, and the results differed significantly (in the wrong direction) from the specifications published by the International Nickel Company, the manufacturer of Inconel. NACA Headquarters asked Langley to explain the discrepancies. The reason was unknown, but researchers though it could be related to variations in the material, milling procedures, heat treatment, or testing procedures. Fortunately, further testing revealed that the results from the first test were largely invalid, although researchers never ascertained the specific reasons for the discrepancy. Still, the episode pointed out the need to precisely control the entire life cycle of the alloy.-131

In December, North American engineers visited both Ames and Langley to work out details of the wind-tunnel program. The participants agreed that Langley would perform flutter tests on the speed brakes using the 1/15-scale model. The PARD would make a second flutter investigation, this one of the wing planform, since North American required data from a large-scale model at Mach 5 and a dynamic pressure of 1,500 psf—something no existing tunnel could provide. North American was supplied with additional requirements for a rotary-derivative model to be tested at Ames, and NACA personnel suggested that two 1/50-scale models be constructed—one for testing at Ames and one for Langley. The North American representatives agreed to consider the suggestion, but pointed out that no funds existed for two models. Ames also announced that they would take the 10 by 14-inch hypersonic tunnel out of service on 1 May for several months of modifications. The location was important since the tunnels were not identical and researchers could not directly compare the results from the two facilities.-1321

Ultimately, funds were found to build two 1/50-scale models—one for use at Langley in the 11- inch hypersonic and 9-inch blowdown tunnels, and one for the North American 16-inch wind tunnel. It was decided not to use the Ames tunnel prior to its closing. Langley also tested a 1/15- scale high-speed model while Ames tested a rotary-derivative model. The wind-tunnel investigations included evaluating the speed brakes, horizontal stabilizers, vertical stabilizer, fuselage tunnels, and rolling-tail. Interestingly, the tests at Langley confirmed the need for control system dampers, while North American concluded they were not necessary. This was not the final answer, and researchers would debate the topic several more times before the airplane flew.[33]

Various wind tunnels around the country participated in the X-15 development effort. This 1956 photo shows an original "high tail" configuration. Note the shock waves coming off the wing leading edge and a separate showck wave just behind it coming off the front of the landing skid. Very soon, this configuration would change substantially as the fuselage tunnels were made shorter, the vertical surfaces reconfigured, and the skids moved further aft. (NASA)

North American had based its design surface temperatures on achieving laminar flow during most of the flight profile. However, most of the heat-transfer theories in general use at the time assumed fully turbulent flow on the fuselage. Researchers had previously raised the same issue with no particular solution. Ultimately, researchers used the Unitary Plan tunnel at Langley and the Air Force Arnold Engineering Development Center at Tullahoma, Tennessee, to resolve the discrepancy. These tests provided heat-transfer coefficients that were even higher than the theoretical values, particularly on the lower surface of the fuselage. Because of these results, the Air Force directed North American to modify the design to withstand the higher temperatures.

This proved particularly costly in terms of weight and performance, adding almost 2,000 pounds of additional heat-sink material to the airframe. This is when the program changed its advertising. Instead of using 6,600 fps (Mach 6.5) as a design goal, the program began talking about Mach 6; it was obvious to the engineers that the airplane would likely not attain the original goal. Later, measurements from the flight program indicated that the skin temperatures of the primary structural areas of the fuselage, main wing box, and tail surfaces were actually several hundred degrees lower than the values predicted by the modified theory; in fact, they were below predictions using the original theories. However, resolving these types of uncertainties was part of the rationale for the X-15 program in the first place.[34]

By January 1956, North American required government guidance on several issues. A meeting on 18 January approved the use of a removable equipment rack in the instrument compartment.

North American would still permanently mount some instrumentation and other equipment in the fuselage tunnels, but everybody agreed that a removable rack would reduce the exposure of the majority of research instruments and data recorders to ammonia fumes during maintenance.135

It soon became evident, contrary to statements at the November meeting, that no suitable stable platform existed, although the WADC had several units under development. It was a major blow, with no readily apparent solution.-1361

Other topics discussed at the 18 January meeting included the speed brake design and operation. Full extension of the speed brakes at pressures of 2,500 psf would create excessive longitudinal accelerations, so North American revised the speed brakes to open progressively while maintaining 1,500-psf pressure until they reached the full-open position. All in attendance thought that this was an appropriate solution.-1371

Pilot escape systems came up again during a 2-3 May 1956 meeting at Wright Field among Air Force, NACA, Navy, and North American personnel. WADC personnel pointed to a recent Air Force policy directive that required an encapsulated escape system in all new aircraft. Researchers from the WADC argued that providing some sort of enclosed system would comply with this policy and allow the gathering of research data on such systems. (This seemed an odd rationale in that it appeared to assume that the pilot would use the capsule at some point—an entirely undesirable possibility.) Those opposed to the Air Force view objected to any change because it would add weight and delay development. The opposing group, including Scott Crossfield, believed that the safety features incorporated in the X-15 made the ejection seat acceptable. After the meeting, the Air Force directed North American to justify its use of an ejection seat, but did not direct the company to incorporate a capsule.138

During a 24 May meeting at Langley, representatives from Eclipse-Pioneer briefed researchers from the NACA, North American, and the WADC on a stable platform that weighed 65 pounds and could be ready in 24 months. Later events would show that these estimates were hopelessly optimistic.-1391

On 11 June 1956, the government approved a production go-ahead for the three X-15 airframes, although North American did not cut metal for the first aircraft until September. Four days later, on 15 June 1956, the Air Force assigned three serial numbers (56-6670 through 56-6672) to the X-15 program. The Contract Reporting and Bailment Branch furnished this data by phone on 28 May and confirmed it in writing on 15 June.1401