Category X-15 EXTENDING THE FRONTIERS OF FLIGHT

John Manke was the last NASA pilot assigned to the X-15 program, but he never flew the airplane. Manke was born on 13 November 1931 in Selby, South Dakota. He attended the University of South Dakota before being selected for the NROTC program in 1951, and graduated from the Marquette University in Milwaukee in 1966 with a bachelor of science degree in electrical engineering. Following graduation, Manke entered flight training and served as a fighter pilot with the Marine Corps. He left the service in 1960 and worked for Honeywell for two years.

NASA hired Manke on 25 May 1962 as a flight research engineer, and he served as an X-15 flight planner. Along with Mike Adams, Manke completed X-15 "ground school" and conducted a test run of the XLR99 in the Rocket Engine Test Facility. Manke left the X-15 program after the X-15- 3 accident that claimed Mike Adams’s life. On 28 May 1968 he flew the HL-10, the first of his 42 flights in a heavyweight lifting body.

After the X-15 program ended, Manke became chief of flight operations at the FRC in October 1981 and continued in that capacity until he retired on 27 April 1984.

As it turned out, the initial flight plan was modified somewhat as the program progressed. The envelope-expansion program was eventually broken into two parts: the basic research program and the basic program extension. The first category consisted mostly of the original plan that covered the aerodynamic, stability and control, and structural aspects of the basic X-15. The government expected that it would take only 17 flights to reach the design conditions of Mach 6 and 250,000 feet; the rest of the early flights would be for pilot familiarization.

Nevertheless, intermediate progress deviated considerably from the plan, since during the course of the program observations sometimes indicated the need for extreme caution and at other times permitted larger increments than planned. In the end, partially because of the delay resulting from X-15-3 blowing up at the Rocket Engine Test Facility, it took 45 flights to reach Mach 6, and 52 flights to reach 246,700 feet (close enough to 250,000).

The basic program extension was essentially similar but was concerned with answering a few lingering questions and conducting the same evaluations of the "advanced" X-15A-2. In the meantime, a separate program began that used the X-15 as a flying test bed and as a carrier for a variety of follow-on experiments.-^

It is interesting to note that the X-15 program lacked much of the drama of the earlier X-planes. Although it was pushing performance levels and the state of art further than any previous airplane, the X-15 did not experience the catastrophic technical problems that had plagued earlier programs. The XLR99 worked, if not perfectly, well enough for its intended purpose, unlike the Curtiss-Wright XLR25 in the X-2. The Inconel X hot structure seemed to suffer little ill effect from its prolonged exposure to high temperatures and dynamic pressures. The inertial coupling phenomena that had caused the loss of the X-2, and almost the X – 1A, had been addressed by a combination of aerodynamic design, an efficient damper system, and some restrictions on flight maneuvers. The explosive effects of Ulmer leather and liquid oxygen were well understood and avoided.-491

However, these conclusions were not obvious as the envelope-expansion program began. The researchers-and pilots-worried about many things. Would the hot structure survive the tremendous heating rates? Would the wings remain attached to the fuselage during a 6-g pullout from high altitude after the structure was heated to 1,200°F? Would the ballistic control system provide sufficient control while outside the atmosphere?

The flight program expanded speed and altitude concurrently. Normally, the speed flights came first to ensure that the airplane was controllable at the velocity necessary for the next altitude flight. During the high-speed flights, the pilot pulled up to an angle of attack that simulated the expected pullout from the next high-altitude flight, allowing a relatively safe evaluation of the effects of the pullout. It took only 12 flights for the X-15 to expand its envelope from the Mach

3.5 and 136,500 feet attained with the XLR11 (and basically representative of the best the earlier

X-planes had managed) to Mach 6.06 and 246,700 feet. It was an amazing feat.

Perhaps not so amazingly to the designers, John Becker and the researchers at Langley had done a lot of basic research, and Charlie Feltz and his team at North American had taken that, added to it, and developed a very robust airframe. North American took Hartley Soule’s comments to Harrison Storms about making errors on the strong side seriously. The airplane ended up a bit overweight, resulting in slightly diminished performance, but it could take a great deal of punishment and survive. The simulation program run by North American and later by the FRC and AFFTC flight planners correctly predicted almost every nuance of the flight program. As the pilots learned to trust the simulator, most of the initial worries disappeared. Still, it was incredible that the program accomplished the envelope expansion so apparently effortlessly.

This is not to say the program did not experience problems. As Bob Hoey remembers, "[T]he X-15 had a significant inertial coupling problem for roll rates that were easily within the capability of the control system. The boundaries were reasonably well established on the simulator, and everyone recognized that there was no need to perform rapid rolls on an X-15 mission, so the pilots were advised ‘don’t do that!’ and they didn’t." The auxiliary power unit provided more than its share of challenges early on, and was never completely satisfactory. The stable platform got off to a marginal start, got better, and then got a lot worse. In the end, a more modern unit originally designed for the canceled X-20 Dyna-Soar replaced it. The ballistic control system was particularly troublesome during the initial flights, so much so that researchers purposely turned it off on some of the early altitude buildup flights. Fortunately, the bugs had been worked out and it performed satisfactorily by the time it was really needed.-50

The XLR99 had its share of minor problems (mainly sensitivity to throttling) and a worrisome habit of shedding some of the insulating coating inside its exhaust nozzle. Then there was the landing gear, which underwent a constant set of modifications right up until the final year of the flight program. In this case, it was not the components’ fault, at least not completely. The airplane was overweight when North American delivered it, and it continued to get heavier over the years. Upgraded struts, skids, nose wheels, tires, and stronger supporting structures never caught up with the weight increases. Still, few of the problems were show-stoppers, and the X-15 program continued at a blistering pace.

Each of the initial X-15 pilots had spent many hours in the fixed-base simulator at North American and had undergone centrifuge training at NADC Johnsville. Prior to his first flight in the X-15, each pilot went through a ground dry run with the X-15 mated to the NB-52 to familiarize himself with the complete prelaunch checklist and cockpit procedures. Each pilot also performed engine runs at the Rocket Engine Test Facility prior to his first X-15 flight. In addition, the pilots flew missions in the NT-33 and JF – 100C variable-stability trainers to become familiar with the low-speed handling characteristics of the X-15. The pilots practiced landings in F-104s, including approaches to each of the uprange lakebeds in service at the time. There should be no surprises.-1511

There had always been questions about exactly how to apply an ablative coating over the surface of an entire airplane, even one as small as the X-15. Even more questions existed on how to maintain the airplane after applying the coating, and how difficult it would be to refurbish the coating between flights. There appears to have been little actual concern about the effectiveness of the ablator; if it was applied correctly, everyone was relatively sure the concept would work.

As part of its initial contract, Martin Marietta developed a comprehensive procedure for applying the coating, maintaining it, and removing it if necessary. Martin accomplished the first complete application of the ablator in general agreement with the schedule and procedures published earlier. Simply because it represents one of the few attempts to use an ablative coating on an entire airplane, it is appropriate to review the application in detail.-1322

The process began with cleaning the airplane, and Martin admitted the preparatory cleaning was "somewhat overdone" for the first application. Technicians masked all joints, gaps, and openings before the cleaning began to prevent solvent from getting into the airplane. The surface condition of the airplane, with its accumulation of contamination and overabundance of lacquer, necessitated the use of a great deal of solvent during the initial cleaning. Technicians accomplished the final cleaning with powdered cleanser and water using a "water-break-free" test to ascertain when the surface was properly clean. Some areas of the aircraft, especially around fastener heads and skin joints, never did achieve a completely water-break-free condition, and Martin noted that "these areas continually bleed hydraulic fluid or other contamination."[323]

Next, technicians used polyethylene tape to mask all of the seams between panels to keep the ablative material out of the aircraft compartments. The only problem encountered in the initial ablator application was that nobody had anticipated masking the gap between the fixed portion of each vertical stabilizer and the rudders. The installation crew then improvised a solution that was mostly successful. As a means of checking the adequacy of the masking during all phases of ablator operation, technicians placed airborne contamination collectors in nine aircraft compartments before beginning the application process. At the end of the process, quality inspectors from Martin Marietta and NASA checked these collectors and found very little contamination, indicating that the masking worked as expected.-1324

Before turning the airplane over to Martin Marietta, NASA had made a few minor changes to accommodate the ablator installation. The retractable pitot tube (or "alternate pitot" as it was called) was installed, as was a new retaining ring around the ball nose that had a step at its aft end. When the ablator was built up during the application, it would fill up to the top of the step, resulting in a smooth surface.[325]

Next up was installing the molded ablator "details" on the aircraft. This included premolded leading-edge covers made from ESA-3560-NA for the wing and horizontal stabilizers, and covers for various antennas, the canopy leading edge, and the vertical stabilizer leading edge. Although it was not provided as part of the kit, the installation team fabricated a detail for the leading edge of the dummy ramjet instrumentation rake from a spare piece of the vertical stabilizer leading-edge detail.[326]

After technicians glued the details onto the surface of the leading edges, they covered the majority of the airplane with polyethylene sheeting to protect cleaned areas from overspray during the sequential ablator applications. The airplane was broken down into nine distinct areas that technicians would spray in sequence. Technicians installed marker strips (a vinyl foam tape) over the contamination masking and applied a layer of DC93-027 RTV over fastener heads and peripheral gaps of the seldom-removed panels. The installation team then sprayed the MA-25S ablator using a commercial paint spray gun. Controlling the thickness of the ablator was the most significant difficulty encountered during the application process, but the team got much better toward the end as they became more familiar with the deposition characteristics of the material. Some areas, particularly the middle of the wing root and the crown centerline of the fuselage, proved to be too much of a stretch for the technicians standing on the ground. This condition resulted in a "somewhat cheezy" ablator application in those areas, but the layer was deemed adequate to protect the airframe.[327]

Once the entire surface was covered, the next task was to go back and remove the trim marker strips. This proved more difficult than had been expected because the tape was "too thick and possessed too high an adhesive tack." Nevertheless, the team eventually accomplished the task, but decided to use a different tape next time. It was important to avoid disturbing the sealing tape under the marker strips, since it would have to protect the compartments from the effects of the sanding operation still to come.[328]

Applying the MA-25S ablator was more involved than most expected. The airplane had to be scrubbed clean, and then each individual panel had to be taped to ensure ablator did not get into the airplane. The ablator was then sprayed, sanded to a consistent finish, and its depth measured. The amount of time required to coat the relatively small X-15 did not bode well for a large Space Shuttle. (NASA)

The ablator was left to cure at room temperature for a few hours, and then technicians sanded the entire surface to remove overspray and irregularities, and to bring the ablator layer down to within 0.020 inch of its design thickness. This proved to be a very tedious operation. First, the team had to draw grid lines on the airplane to establish precise monitor locations, and a penetrating needle dial gage determined the thickness at each point on the grid. Technicians then sanded the surface. Since this removed the grid lines, they would have to be redrawn and the thickness rechecked. The process continued until the desired thickness was reached. It was evident that there was a need for a better way to establish the grid on the airplane.[329]

When the sanding was finished, the team glued 10 test plugs to the ablator surface and cut through the ablator layer around their periphery. A pull test was performed on the plugs to determine whether the ablator had properly bonded to the skin. The first application successfully passed all of its pull tests. Various "hard point inserts" were then installed around the external tank inboard sway brace attach points and the aircraft jacking points. Inserts of MA-25S-1

material also covered the ram air door in the fuselage nose and the engine compartment fire doors on the aft fuselage.-1330!

MA-25S had a natural pinkish color and somehow this seemed inappropriate for the world’s fastest airplane. Fortunately, the specification called for a layer of Dow Corning DC90-090 RTV over the entire airplane to provide a wear coating and to seal the ablator. The DC90-090 was translucent white and did not completely hide the pink, so NASA asked Martin Marietta to apply an extra coat (or two, in some areas) so that the airplane would have a uniform white finish. This exhausted the available supply of the coating; however, Dow Corning had replaced DC90-090 with a similar product called DC92-007. Martin requested samples of the new product to determine its suitability as a substitute.-333-

At this point, the team applied a limited number of hazard and warning markings to the exterior using standard high-temperature aircraft lacquer paint. The last step was to remove the polyethylene tape that sealed the service panels and install strips of MA-25S-1 around their periphery to provide extra durability during panel removal and replacement. Martin then returned the airplane to the X-15 maintenance crews, who installed instrumentation and prepared it for flight.-1332-

As completed, each X-15 had four "bug-eye" structural camera bays, named for their odd shape.-65 Two were located on top of the fuselage just behind the cockpit, and two were under

the center-of-gravity compartment. Originally, each bay held a 16-mm motion picture camera that ground personnel could aim through a limited field of view to observe the fuselage, wings, or stabilizers. Over the course of the program, researchers used these bays to house a variety of other equipment. Sometimes the bug-eye fairings above the fuselage provided a viewing port for the experiments or simply provided extra volume, and at other times flush plates covered the area. The lower bays were usually faired over later in the flight program with the internal space used by experiments or data recorders.

Although it was not truly an experiment, the National Geographic Society occasionally provided cameras for the upper bug-eye camera bays. Photos looking back at the vertical stabilizer of the X-15 with the curve of the Earth in the background are more often than not ones taken by the Society’s cameras.[66]

Early Experiments

During the early portion of the flight program, various small experiments were piggybacked onto the airplanes as time and space permitted. These usually required little, if any, support from the airplane or pilot during the mission since the flight program was concentrating on acquiring aero – thermo and stability and control data.-67

Despite the HRE debacle, Marquardt did not give up easily. Although NASA had ruled out a ducted rocket in 1963, Marquardt managed to generate enough interest in the concept to get a study contract from the FRC in early 1964, and the AARPS came back as a separate study. The designers of the AARPS contemplated the use of advanced air-breathing propulsion cycles, such as ducted rockets and ejector ramjets. NASA awarded Marquardt a small contract to define a research and development plan for AARPS and to determine the feasibility and usefulness of flight-testing the system on X-15A-2.2821

On 3 January 1967, the company proposed a different ramjet installation for X-15A-2, actually providing power to allow a "cruise capability of approximately Mach 5." The company proposed installing an "ejector ramjet on X-15A-2 in the area now occupied by the rocket engine." The ramjet would use jet fuel and liquid oxygen as propellants, although hydrogen peroxide was listed as an alternate oxidizer.-12831

The gross weight of the airplane would increase 2,571 pounds (from 35,735 to 38,306 pounds). The amount of liquid oxygen would remain constant at 10,533 pounds, but 9,400 pounds of jet fuel would replace the normal 8,199 pounds of anhydrous ammonia. The propulsion system weight would increase from 910 pounds (XLR99) to 2,280 pounds (1,380 pounds for the engine and 900 pounds for the inlet). On the airplane itself, the liquid-oxygen tank would remain unchanged, but the proposal modified the existing ammonia tank to allow room for the inlet ducting. Unfortunately, Marquardt did not specify how it would accomplish this, given that the ammonia tank was a full-monocoque structural member of the fuselage.-2841

XLR99 (but well above the 16,000 Ibf provided by the interim XLR11s). Marquardt estimated that the acceleration to Mach 5 would take 4.21 minutes at 1.04 g (much slower than the 90 seconds or so it normally took to get to Mach 6), covering approximately 150 miles in the process. Once at Mach 5 the ramjet would provide 14.8 minutes of steady-state cruise, covering 840 miles. This

1,0- mile flight would have necessitated a major extension to the High Range, and might well have exceeded the heat-sink capability of the Inconel structure, even with an ablative coating.[285]

Marquardt also suggested that the engine could be adapted to the delta-wing airplane, and that in the future a modified engine could provide additional cruise performance. In either case, the engine featured a large rectangular inlet located under the fuselage that started slightly ahead of the wing. The inlet duct swept upward into the fuselage just ahead of the ventral stabilizer, explaining the required modifications to the fuel tank. The inlet faired into the ventral, and the ramjet engine was located where the normal XLR99 had been.[286]

It appears that NASA did not take any action based on the study results.

Jack McKay flew the X-15 for 70 months from 28 October 1960 until 8 September 1966, making 29 flights. These included two flights with the XLR11 and 27 with the XLr99. McKay reached Mach 5.65, a maximum speed of 3,938 mph, and an altitude of 295,600 feet. He made three emergency landings in the X-15, and although was seriously injured on one of them, he returned to fly 22 more X-15 missions.

John Barron "Jack" McKay was born on 8 December 1922 in Portsmouth, Virginia, and graduated from Virginia Polytechnic Institute in 1950 with a bachelor of science degree in aeronautical engineering. During World War II he served as a Navy pilot in the Pacific, earning the Air Medal with two oak leaf clusters and a Presidential Unit Citation while flying F6F Hellcats.

He joined the NACA on 8 February 1951 and worked at Langley as an engineer for a brief period before transferring to the HSFS, where he flew the F-100, YF-102, F-102A, F-104, YF-107A, D – 558-1, D-558-2, X-1B, and X-1E. With the exception of Scott Crossfield, McKay accumulated more rocket flights than any other U. S. pilot (46 flights before he joined the X-15 program). As Milt Thompson remembers, "Jack was an excellent stick and rudder pilot, possibly the best of the X-15 pilots." McKay retired from the NASA on 5 October 1971 and died on 27 April 1975, mostly from late complications resulting from his X-15 crash. On 23 August 2005, NASA presented McKay’s family with a set of astronaut wings, honoring MacKay’s high-altitude flight in the X-

15.ШЛ

As conceived at the time of the rollout in 1958, the contractor flight program consisted of four phases. The first was called "B-52 and X-15 Lightweight Captive Flight Evaluation" and was intended to verify the mated flight characteristics with an unfueled X-15, operational procedures, jettison characteristics (using dye), systems (APU, hydraulic, heat and vent, and electrical) operations, B-52 communications and observation, and carrier aircraft performance at launch speed and altitude. The second phase was the "X-15 Lightweight Glide Flight Evaluation" and involved launching an unfueled X-15 on an unpowered glide flight to verify the launch procedure, low-speed handling characteristics, and landing procedures.-1521

The third phase was the "B-52 and X-15 Heavyweight Captive Flight Evaluation," which was intended to replicate the first series of tests with a fully loaded X-15, demonstrate topping off the liquid-oxygen system, and verify that a full propellant load could be jettisoned using actual propellants. The last phase involved the initial "X-15 Powered Flight Evaluation" using the interim XLR11 engines. The schedule showed the first captive flight on 31 January 1959, with the first glide flight on 9 February 1959 and the first powered flight on 2 April 1959. Somehow, it would not work that way.-1531

X-15-1 arrived at Edwards on 17 October 1958, trucked over the foothills from the North American Inglewood plant. The second airplane joined it in April 1959, and the third would arrive later. As Air Force historian Dr. Richard P. Hallion later observed, "In contrast to the relative secrecy that had attended flight tests with the XS-1 a decade before, the X-15 program offered the spectacle of pure theater." It was not that the X-15 program necessarily relished the limelight – it simply could not avoid it after Sputnik.-1541

Beginning in December 1958, North American conducted numerous ground runs with the APUs installed in X-15-1 at Edwards. The company intended this to build confidence in the units before the first flights, but it did not turn out that way. Bearings overheated, turbines seized, and valves and regulators failed, leaked, or did not regulate. The mechanics would remove the failed part, rebuild it, and try again. More failures followed. Scott Crossfield later described this period as "sleepless weeks of sheer agony." Harrison Storms eventually got together with the senior management at General Electric, who sent Russell E. "Robby" Robinson to Edwards to fix the problem. For instance, Robinson noted that invariably after one APU failed, the other would follow within a few minutes. The engineers finally deduced that a sympathetic vibration transferred through the shared mounting bulkhead caused the second one to fail. North American devised a new mounting system that separated the APUs onto two bulkheads.-1551

The North American pilot was Scott Crossfield, the person who arguably knew more about the airplane than any other individual did. After they performed various ground checks, technicians mated X-15-1 to the NB-52A and then conducted additional ground tests. All of this delayed the original schedule by about 60 days. When the day for the first flight arrived, Crossfield described it as a "carnival at dawn." Things were different during the 1950s. Crossfield and Charlie Feltz shared a room in the bachelor officer quarters (BOQ) at Edwards; there was no fancy hotel in town. They each dressed in a shirt and tie before driving to the flight line-nothing casual, even though Crossfield soon changed into a David Clark MC-2 full-pressure suit. When they got to the parking lot next to the NB-52 mating area, more than 50 cars were already waiting. The flight had been scheduled for 0700 hours. Based on his previous rocket-plane experience, Crossfield predicted they would take off no earlier than noon, and maybe as late as 1400.1561

Crossfield was pleasantly surprised. At 1000 hours on 10 March 1959, the mated pair took off on its scheduled captive-carry flight (retroactively called program flight number 1-C-1). It had a gross take-off weight of 258,000 pounds, and lifted off at 172 knots after a ground roll of 6,085 feet. During the 1 hour and 8 minute flight, Captains Charlie Bock and Jack Allavie found that the NB-52 was an excellent carrier for the X-15, as was expected from numerous wind-tunnel and simulator tests.-1571

Scott Cross field spent many hours in his David Clark full-pressure suit while the X-15-1 was prepared for its initial flights. Despite the daytime temperatures in the desert, Crossfield believed that his presence, ready to go, kept ground crew morale high. (NASA)

During the captive flight, Crossfield exercised the X-15 flight controls, and the recorders gathered airspeed data from the flight test boom to calibrate the instrumentation. Bock and Allavie found that the penalties imposed by the X-15 on the NB-52 flight characteristics were minimal, and flew the mated pair up to Mach 0.85 at 45,000 feet. Part of the test sequence was to make sure the David Clark full-pressure suit worked as advertised, although Crossfield had no doubts. This was a decidedly straightforward test. The suit should inflate as soon as the altitude in the cockpit went above 35,000 feet. As the mated pair passed 30,000 feet, Crossfield turned off the cabin pressurization system and opened the ram air door to equalize the internal pressure with the outside air. Once the airplanes climbed above 35,000 feet, Crossfield felt the suit begin to inflate, and "from that point on [his] movements were slightly constrained and slightly awkward." Still, Crossfield could reach all of the controls, including the hardest control in the cockpit to reach: the ram air door lever. Crossfield closed the door, and as the cockpit repressurized, the suit relaxed its grip. Pilots repeated this test on every X-15 flight until the end of the program. Near the end of the flight, Crossfield lowered the X-15 landing gear just to make sure it worked, even if it looked a little odd while still mated to the NB-52.[58]

The next step was to release the X-15 from the NB-52 to ascertain its gliding and landing characteristics. North American rescheduled the first glide flight for 1 April 1959, but aborted it when the X-15 radio failed. The NB-52A and X-15 spent 1 hour and 45 minutes airborne conducting further tests in the mated configuration. A combination of radio failure and APU problems caused a second abort on 10 April. Yet a third attempt aborted on 21 May 1959 when the X-15 stability augmentation system failed and a bearing in the no. 1 APU overheated after approximately 29 minutes of operation.-1591

The problems with the APU were the most disturbing. All of these flights encountered various valve malfunctions, leaks, and speed-control problems with the APUs, all of which would have been unacceptable during research flights. Tests conducted on the APU revealed that extremely high surge pressures were occurring at the pressure relief valve (actually a blowout plug) during the initial peroxide tank pressurization. The installation of an orifice in the helium pressurization line immediately downstream of the shut-off valve reduced the surges to acceptable levels. Engineers decided that other problems were unique to the captive-carry flights and deemed them of little consequence to the flight program since the operating scenario would be different. Still, reliability was marginal at best. The APUs underwent a constant set of minor improvements during the flight program, but continued to be a source of irritation until the end.*60

On 22 May, North American conducted the first ground run of the interim XLR11 engine using X – 15-2 at the Rocket Engine Test Facility. Scott Crossfield was in the cockpit for the successful test, clearing the way for the eventual first powered flight-if X-15-1 could ever make its unpowered flight. Another attempt at the glide flight on 5 June 1959 aborted even before the NB-52 left the ground, when Crossfield reported smoke in the cockpit. Investigation showed that a cockpit ventilation fan motor had overheated. The continuing problems with the first glide flight were beginning to take their toll, both physically and mentally, on all involved.*61

Because of the lessons learned on the aborted glide flights and during the XLR11 ground runs, engineers modified numerous pieces of equipment on the X-15. These included the APUs, their support brackets, the mounting bulkheads, and the bearings inside them. North American also improved the flight control system’s mechanical responsiveness. In addition, technicians accomplished a great deal of work on the various regulators and valves, particularly in the hydrogen-peroxide systems. Storms remembered, "[I]n the final analysis, the regulators and valves were the most troublesome hardware in the program insofar as reliability was concerned.’,[62]

Because so many unknowns still existed, the Air Force and NASA decided to conduct a thorough post-flight inspection of the entire airplane surface for each mission, and to monitor the ablator char depth and back-surface temperatures throughout the performance buildup. The ablator weighed 125 pounds more than planned and, taken with the expected increase in drag, the maximum speed of airplane was expected to barely exceed Mach 7.-333-

Pete Knight made the first flight (2-52-96) in the ablator-coated, ramjet-equipped X-15A-2 without the external tanks on 21 August 1967. The flight reached Mach 4.94 and the post-flight inspection showed that, in general, the ablator had held up well. The leading-edge details on the wings and horizontal stabilizers had uniform and minor charring along their lengths. A careful examination revealed only minor surface fissuring with all char intact, and good shape retention. The char layers were approximately 0.050 inch deep on the wing leading edge and 0.055 inch deep on the horizontal stabilizer-well within limits.-334-

The ablator details for the canopy and dorsal vertical stabilizer showed almost no thermal degradation, and local erosion and blistering of the wear layer were the only evidence of thermal exposure. The leading edge of the forward vane antenna suffered local erosion to a depth of 0.100 inch because of shock-wave impingement set up by an excessively thick ablator insert over the ram air door just forward of the antenna. The remainder of the leading-edge detail on the aft vane antenna showed "minor, if not insignificant degradation."!335-

The most severe damage during the flight was to the molded ablator detail on the leading edge of the modified ventral, which showed heavy charring along its entire length. "The increased amount of thermal degradation was directly attributable to the shock wave interactions from the pressure probes and the dummy ramjet assembly." Additional shock waves originated from the leading edges of the skids. The shock impingement had apparently completely eroded the lower portion of the detail, and very little char remained intact. However, it was difficult to ascertain how much of the erosion had taken place during the flight since sand impact at landing had caused similar, although much less significant, erosion during an earlier ablator test flight. Still, this should have been a warning to the program that something was wrong, but somehow everybody missed it.-336-

The primary ablative layer over the airplane experienced very little thermal degradation as the result of this Mach 5 flight. On the wings and horizontal stabilizers, only the areas immediately adjacent to the molded leading edges were degraded. These areas exhibited the normal random reticulation of the surface, there was no evidence of delaminating, and all material was intact. Some superficial blistering of the ablative layer was evident on the outboard lower left wing surface. Martin Marietta believed the blistering was most probably the result of an excessively thick wear-layer application. Not surprisingly, the speed brakes exhibited some wear, but there was no significant erosion or sign of delamination. The only questionable area of ablator performance was the left side of the dorsal rudder. A number of circular pieces of ablator were lost during the flight. A close examination of the area revealed that all separation had occurred at the spray layer interface, and significant additional delamination had occurred. The heavy wear layer, however, had held most of the material in place. The program had seen similar delamination during some of the earlier test flights and traced it to improper application of the material. It then changed the installation procedures to prevent reoccurrence.[337]

After the inspection, Martin Marietta set about repairing the ablator for the next flight. With the exception of the leading-edge details for the ventral stabilizer and the forward vane antenna, refurbishment was minimal. Only the wings and horizontal stabilizers had experienced any degree of charring, and technicians refurbished them by sanding away the friable layer. They did not attempt to remove all of the thermally affected material, and sanding continued only until they exposed resilient material. The canopy leading-edge detail required no refurbishment, but Martin lightly sanded its surface to remove the wear layer to minimize deposition on the windshield during the next flight.[338]

As it finally rolled out of the paint shop at the Flight Research Center, the MA-25S-covered X – 15A-2 was the polar opposite of what an X-15 normally looked like. Instead of the black Inconel X finish, the airplane had a protective layer of white Dow DC90-090. Actually, this was a relief to the pilots since the MA-35S has a natural pink color. (NASA)

Martin repaired local gouges in the main MA-25S ablator using a troweled repair mix and kitchen spatulas, eliminating most of the sanding usually required for the patches. Technicians sanded the speed brakes to apparent virgin material and resprayed the areas to the original thickness. They also completely stripped the left side of the dorsal rudder and reapplied the ablator from scratch using the revised procedure.-1339

Although the ablator around the nose of the airplane had experienced no significant thermal degradation, a malfunction of the ball nose necessitated its replacement, and the ablator was damaged in the process. Unfortunately, Martin had depleted the supply of ablator during the refurbishment process, forcing the use of a batch of MA-25S left over from a previous evaluation. For the final preparation the entire surface of the airplane was lightly sanded and a new coating of the replacement DC92-007 wear layer was applied.-349

In what was perhaps the first true "follow-on" experiment, in 1961 researchers used X-15-2 to test a coating material designed to reduce the infrared emissions of the North American B-70 Valkyrie bomber. One of the complaints frequently voiced against the B-70 was that it was a large target. Although the concept of "stealth" (a term not yet applied to the idea) was not far advanced in 1960, engineers at Lockheed and North American both understood that reducing the radar and infrared signatures of strategic aircraft would at least delay their detection by the enemy. For instance, Lockheed specifically intended the shape and materials of the A-12/SR-71 Blackbird to lower its radar signature. North American conducted several detailed studies into the infrared and radar signatures of the B-70 to provide a basis for reduction attempts.-68

During the very short YB-70 development period, the Air Force directed North American to investigate ways to reduce the probability that the B-70 would be detected. The company made preliminary investigations into applying various radar absorbing materials to the airframe, particularly the insides of the air intakes. However, most of the North American effort appears to have concentrated on reducing the infrared signature of the aircraft. Exhausting cool air around the J93 engines was one means of reducing the infrared signature of the B-70.

As part of its research, North American developed a "finish system" (i. e., paint) that provided a low emittance at wavelengths used by Soviet infrared detecting devices, and allowed most of the excess heat to be radiated from the surface in wavelengths that were not normally under surveillance. The finish used a low-emittance basecoat with an organic topcoat that was transparent to energy in the 1-6-micron range. The topcoat was strangely opaque and highly emissive at wavelengths between 6 and 15 microns. This finish was relatively invisible to infrared detecting equipment and still allowed the skin to radiate excess heat overboard to maintain its structural integrity.-^

was applied a 1-mil-thick mixture of 85% Ferro Enamaling no. AL-8 Frit and 15% Hommel no. 5933 Frit. The Type II basecoat was a mixture of 40% Hanovia silver resinate and 60% Hanovia L. B. coating no. 6593 applied 0.004 mil thick. The topcoat was a mixture of 74% 3M Kel-F no. 2140, 24% 3M Kel-F no. 601, and 2% Al2O3 applied 1 mil thick. Most probably, the topcoats would have been opaque silver instead of the white finish used on the two XB-70A prototypes.-1701

The finish system was somewhat difficult to apply to an aircraft as large as the B-70, but the engineers expected that further development would yield improvements in the process. The most difficult problem was that the underlying surface had to be highly polished prior to applying the basecoat. In addition, the basecoat of both finishes had to cure at 750°F, while the topcoat of the Type II finish had to cure at 1,000°F (creating almost a ceramic finish). Accelerated environmental tests indicated that the surface would prove durable on the stainless-steel sections of the B-70, but its long-term adhesion to titanium appeared to be weak. Both finishes were relatively immune to exposure to hydraulic fluid, fuels, oils, and other substances encountered during operational service.-1711

To obtain real-world flight experience, the Type I coating was applied to one panel on the vertical stabilizer of X-15-2 in March 1961 and flown on flight 2-13-26 by Bob White. Since Inconel X is a type of stainless steel, the test was relatively representative of the proposed B-70 installation.

No observable physical changes occurred during the Mach 4.43 flight, during which the aircraft’s exterior reached 525°F, and the engineers made no attempt to measure the infrared qualities of the coating during this single flight. It was made simply to determine whether the coating would survive the aero-thermo environment, and appears to have been successful.-721

During the mid-1960s, a proposed delta-wing modification to X-15-3 might have kept the program flying until 1972 or 1973. Unlike many proposals, such as the "orbital X-15," the delta wing was a real project and was the subject of a great deal of research and engineering.

The delta-wing X-15 grew out of hypersonic cruise research vehicle studies conducted during the early 1960s. A "hypersonic cruise" vehicle would spend minutes or tens of minutes at hypersonic velocities, in contrast to the original X-15 that spent only a few tens of seconds at that velocity. The delta-wing X-15 configuration used the third airplane with the MH-96 and the basic modifications made to X-15A-2. Proponents of the concept, particularly John Becker at Langley, found the idea very attractive. Becker opined that "the highly swept delta wing has emerged from studies of the past decade as the form most likely to be utilized on future hypersonic flight vehicles in which high lift/drag ratio is a prime requirement i. e., hypersonic transports and military hypersonic cruise vehicles, and certain recoverable boost vehicles as well."[287]

Researchers held a meeting at the FRC on 9-10 December 1964 to determine exactly what research could be undertaken with a delta-wing hypersonic-cruise X-15. Attendees included researchers from Ames and Langley, and of course groups from the AFFTC and FRC. Not surprisingly, Ames and Langley did not necessarily agree on the exact nature of the research, and NASA Headquarters asked each center to submit a position paper outlining its preferences.

The Langley paper included an evaluation of possible aerodynamic and heat-transfer experiments, in-flight engine-inlet tests, and various structural recommendations that might be applicable to the delta-wing X-15. The entire paper was remarkably short-only 11 pages, or about the same size as the original 1954 paper that had been the genesis of the X-15 program.*288

John Becker and David E. Fetterman, Jr. led the Langley group that defined the aerodynamic and heat-transfer experiments primarily concerned with evaluating the differences between data gathered during full-scale flight tests and the results of wind-tunnel and analytical data. They noted that "[s]ince air-breathing hypersonic cruise vehicles will fly at such large Reynolds numbers that turbulent conditions will occur over the entire wing, it is obviously of great importance to establish the turbulent heat-transfer characteristics of delta wings. The lack of any rigorous theory to turbulent heat transfer places great emphasis on experimental determinations.

It has proved impossible, however, to achieve natural turbulent boundary layers on delta wing models in hypersonic ground facilities except over the rearward regions." Becker and Fetterman used similar logic to justify investigations into areas such as turbulent skin friction and Reynolds analogy, turbulent boundary-layer profile surveys, interference heating, and pressure distributions.-128^

The discussion of engine-inlet testing led to a rather surprising conclusion: "an inlet flight test program on the X-15 is not recommended." J. R. Henry believed that ground facilities such as the 20-inch hypersonic tunnel at Langley and the 3.5-foot hypersonic tunnel at Ames were more than adequate for research into inlet configurations up to Mach 8. Henry’s case was convincing, and despite the ongoing interest in the HRE experiment, Langley dropped engine-inlet testing from further consideration for the delta wing.[290]

Structural considerations were not so much possible experiments, but rather recommendations prepared by J. C. Robinson representing the views of the Structures Research Division at Langley. The primary recommendation was to manufacture the main wing structure from one of the nickel – or cobalt-base "superalloys" using simple construction methods such as "corrugated webs welded to machined cap members with a machined waffle plate skin between the cap members and welded or riveted to them." It was also recommended that the leading-edge material "should be refurbishable, fabricated of thoria-dispersed nickel or a refractory metal, radiation cooled, and should have expansion joints to accommodate differences in thermal expansion between it and the main structure."[291]

The Ames position paper submitted by J. Lloyd Jones was more extensive, consisting of 18 pages using much smaller typeface. The paper also levied some criticism of the existing X-15 program.[292]

Ames agreed with Langley that the primary area of research should be turbulent boundary layers "because of the difficulty of providing turbulent boundary layer flow on models in wind tunnel tests at hypersonic speeds." Ames noted that the existing X-15 had already validated wind-tunnel results up to Mach 5, and the Mach 8 data would be a natural extension. Ames also pointed out that the delta-wing X-15 "is more representative of hypersonic cruise aircraft configurations" and would therefore yield more useful data. Ames also believed that "knowledge that will accrue of the handling qualities of a delta winged vehicle representative of current concepts of airbreathing cruise configuration will certainly be of value."[293]

The researchers at Ames seemed to want to take the lead on the delta-wing configuration, much as Langley had done on the original X-15, and their list of potential research areas was a good deal longer. Besides turbulent-boundary-layer research, Ames wanted to look at skin-friction surveys, pressure distributions and local flow fields, boundary-layer-transition definitions, ablative studies, panel-flutter studies, and low-speed and hypersonic-handling qualities. To accompany these experiments, Ames believed that "an increased program of ground based research relating to hypersonic cruise aircraft technology should be initiated."[294]

Although Ames acknowledged that "the X-15 program to date unquestionably has been very successful" and had turned into "a research facility with which to conduct studies never envisioned at its inception," it believed the program was open to some criticism. "For example, measurement of heat transfer was one of the major research experiments conducted on the airplane, but design inflexibility resulted in the acquisition of heat transfer data in an environment which compromised their value to the extent that they cannot be fully explained nor understood."[295]

To overcome this fault on the new airplane, Ames indicated, "the key to the potential benefits…

for a delta wing X-15 program lies in the design of the airplane to accomplish well defined experimental tasks as primary objectives." To this end, Ames made several recommendations for the vehicle configuration:12961

1. Provision for removable test panels in the areas selected for measurements… to provide for heat transfer, boundary layer and structural tests on both experimental and prototype panels. Design of the test panels and supporting structure should be guided by the requirement to avoid localized heat sinks that withdraw heat away from the boundary layer in a non-uniform manner.

2. Provide a smooth primary test area along the bottom centerline of the airplane free of all protuberances. This consideration may well lead to the requirement for a low-wing configuration to insure a test region with a definable uniform flow field.

3. A removable fuselage nose section ahead of the wheel well with an instrument compartment and a removable tip. This feature would provide for replacement nose sections having different geometry.

4. An instrumentation bullet located at the wing-fin juncture may be advantageous.

Ames went on to describe an elaborate research program that began with wind-tunnel studies, followed by flights using X-15A-2, and finally tests with the delta-wing aircraft. The program would concentrate most of the theoretical and wind-tunnel work at Ames. Based on the scope of the work described, it probably would have been expensive, although the report did not include a cost estimate.

After reviewing the papers submitted by Ames and Langley, with additional input from the Air Force and FRC, NASA defined two primary objectives for the delta-wing X-15 program:12921

1. Aerodynamic research-The flight tests would provide realistic aerodynamic data under fully developed turbulent flow conditions to supplement ground-based research where such conditions cannot be achieved. Answers would be obtained to key questions relating to hypersonic aerodynamics of delta wings, large-scale behaviors of flap-type controls, tip-fin interference effects, and handling qualities of a configuration typical of present thinking for a future hypersonic air-breathing vehicle. Aerodynamic research on this vehicle would be unclouded by propulsion effects, inasmuch as most of the data would be taken under gliding conditions.

2. Structural research-The delta-wing proposal would permit the evaluation in a practical flight application of a hot radiation-cooled structure designed for repeated flights at temperatures between 1,500 degrees and 2,200 degrees Fahrenheit. It would also focus technical effort on a refurbishable, hot-wing leading edge design.

Paul Bikle concluded that "[i]n general, a delta-wing X-15 program could establish a baseline of confidence and technology from which decisions regarding the feasibility and design of advanced air-breathing vehicles could be realistically made. The proposed time for the delta X-15 fits well with that for an overall hypersonic research vehicle program and the cost does not appear to be unreasonable."12981

Things seemed to be progressing rapidly. In January 1965 the FRC drafted a statement of work for North American to conduct a detailed study of the delta-wing concept. This document indicated that "NASA is considering a hypersonic cruise vehicle research program which involves a modification of the basic X-15 configuration for study of various aerodynamic, structural, and flight control problems. The program will also include limited investigations of flight to altitudes extending to about 180,000 feet." Extreme high-altitude research was not a requirement.12991

NASA suggested that an existing "X-15 airplane would be modified to incorporate a representative slender hypersonic wing substituted for the present wing and horizontal tail. The wing structure would be designed for sustained hypersonic flight at a Mach number of 7, but would also be capable of flight to Mach number 8 for limited time periods. The basic X-15 fuselage structure, rocket engine, flight control and other systems would be retained with minimum modification, and the present B-52 launch system and high range facility would be utilized."13001

The work statement went on to indicate that the airplane should have a load limit of 5 g and a

2,0- psf design dynamic pressure. Potential "improvements" included relocating the wing to be flush with the bottom of the fuselage, increasing the dynamic pressure to 2,500 psi and the load factor to 7.33 g, including external propellant tank(s), and relocating the nose landing gear. Other possibilities included the addition of a permanent thermal-protection system (in lieu of ablative coatings) over the fuselage to prevent contamination of the wing with ablative products.-13011

The delta wing seemed to languish at the FRC for the remainder of the year while engineers put together a project development plan. By the end of the year, the FRC had released the second draft of the plan for internal review, providing more detail on how things might progress. The opening paragraph provided the justification for the program:13021

Three of the most probable uses for hypersonic airbreathing aircraft are transport over long ranges, military reconnaissance, and as maneuverable reusable first-stage boosters. There are currently no military or civilian requirements of over-riding importance for any one of these. Their potential, however, constitutes a clear justification to proceed with comprehensive programs to develop the required hypersonic technology.

Unfortunately, the justification also included a rationale for not supporting the program, given the budget crunch NASA was experiencing as it continued the Apollo program to the detriment of the aeronautics budget. Nevertheless, the FRC pressed on with detailed plans. The FRC considered the delta-wing project an extension of the X-15 program, and assumed that all existing agreements between the Air Force and NASA would continue. Researchers believed that the manpower requirements for the program could be satisfied "with the present complements of the Langley, Ames, and Flight Research Center."13031

Researchers at the FRC estimated the program would cost $29,750,000 spread between FY67 and FY73. Of this, the airframe contractor would receive $24,600,000 to build the flight vehicle, while the other $5,150,000 would be "in-house" expenses. The second year represented the largest annual expenditure: $14,500,000 to the contractor and a little over $1,000,000 in-house. The planners warned, however, that "if the military withdraws their operational support from the general X-15 program, this project would be responsible for additional expenses over the 4-1/2 year operational period. These expenses could amount to as much as 17 million dollars." The preliminary schedule showed a request for proposals in August 1966, a contract award in March 1967, modifications to X-15-3 between December 1967 and October 1968, and a first flight in January 1969. The 37-flight research program continued until December 19 72.13041

In the project plan, the FRC had expected the X-15 Project Office at Wright-Patterson AFB to handle the procurement of the modifications, although NASA would pay the bills. Although it would seem logical for North American to perform the modifications, several other contractors (Lockheed, Northrop, and Republic) had expressed interest in the program, so the FRC proposed to make it a competitive process.13051

The Air Force was not totally in favor of this since they saw their involvement with the X-15 winding down, and had little apparent interest in the delta-wing program. Despite negotiations between the Air Force and NASA at various levels, the X-15 Project Office declined to participate in the expected procurement of the delta-wing airplane; however, it did agree to transfer the X – 15-3 airframe to NASA for the modification at the end of its flight program.

After nearly two years of delay, on 13 May 1967 the delta-wing program had progressed far enough for the FRC to issue a request for proposals (PR-7-174) for a formal conceptual design study. The primary objectives of the study were to 1) develop a preliminary design for evaluating the modification of X-15-3 to a delta-wing configuration, and 2) formulate an accurate estimate of performance, weight, cost, and schedule for such modification. A secondary objective was to analyze alternate approaches, such as unsegmented leading edges; eliminate the use of ablatives; and incorporate a fly-by-wire control system, multiplane airfoils, symmetrical tip fins, and different propulsion systems.-1306

There appears have been only a single respondent to the request for proposals. North American submitted a two-volume, 500-page proposal containing detailed engineering concepts and cost data-and that was just the proposal to do the study! By this time, North American had already been testing the delta-wing X-15 in wind tunnels for over a year. The North American low-speed and hypersonic tunnels and the Langley 20-inch hypersonic tunnel had tested 1/15-scale and 1/50-scale models at Mach numbers between 0.2 and 6.9 and Reynolds numbers up to 10,000,000 (equivalent, based on model length).-307

The proposed North American delta-wing X-15-3 was not a simple conversion. The 603-square – foot delta-wing planform had a 76-degree leading-edge sweep, but the span was the same as that of the original X-15 to ensure that there would be no clearance issues with the NB-52 carrier aircraft. Elevons (30.8 square feet each) at the trailing edge provided longitudinal and roll control by deflecting up to 4.5 degrees up or 5.0 degrees down. The existing dorsal and ventral stabilizers provided directional stability with the addition of wing-tip fins, and the existing dorsal rudder provided directional control. Engineers could adjust the removable tip fins on the ground for cant and tow-in, which allowed them to change the relative levels of directional and lateral stability to investigate the handling qualities of the vehicle. The tip fins compensated for the blanking of the normal centerline vertical stabilizers by the fuselage at hypersonic speeds and large angles of attack.308

The position of the wing was the subject of a great deal of study since the HRE was expected to be carried on the ventral stabilizer (as the dummy was on X-15A-2), and the aircraft also had to be stable in flight without the 1,000-pound engine. It proved to be a difficult problem. The final answer before NASA terminated the program was to position the wing in the best location to compensate for the HRE. On flights without the engine, as much research equipment or ballast as possible would be located in a new aft experiment compartment. Likewise, the shape of the wing leading edge was of some concern, but North American noted that "the effect of leading edge radius on the low-speed aerodynamic characteristics of highly swept delta wings is not well understood." Engineers did not think the effect on the leading-edge shape at supersonic speeds would be significant, because positive pressure on the wing lower surface would produce most of the lift. Nevertheless, since the leading-edge shape significantly influenced the landing characteristics, North American investigated various configurations in its low-speed wind tunnel. The company had not found a satisfactory answer, and was awaiting further data from NASA wind-tunnel tests, when the program ended.309

During the mid-1960s, a proposed delta-wing modification to X-15-3 might have kept the program flying until 1972 or 1973. Unlike many proposals, such as the "orbital X-15," the delta wing was a real project and was the subject of a great deal of research and engineering. Despite endorsements from the Flight Research Center and John Becker at Langley, support remained lukewarm within the Air Force and NASA. The FRC was still evaluating the proposals for the delta­wing study on 15 November 1967 when the crash of the X-15-3 effectively ended all thought of such a modification. (NASA)

North American was also somewhat uncertain about the leading-edge material, mainly because of the expected 2,200°F temperatures encountered on the design mission. North American built a segmented leading edge made from columbium alloy that successfully passed tests at 2,400°F. This appeared satisfactory, at least for initial use. The company expected that no available material would prove satisfactory for the lower surface of the wing, and that some form of thermal protection system would have to be developed. Unsurprisingly, many of the North American ideas looked similar to concepts the company was investigating for the space shuttle. One of the most promising ideas was to use metallic heat shields supported by standoff clips with a layer of low – density insulation sandwiched between the shield and the wing skin. Only an area about 2 feet wide just behind the leading edge needed this type of protection since the airflow further aft smoothed out sufficiently to keep temperatures within the ability of alloys such as TD nickel to

Г3101

Originally, North American had envisioned using upswept wing tips to replace the directional stability lost by the removal of the ventral rudder from the delta-wing configuration. Although the X-15 program seldom used the ventral rudder, this was because most missions flew at high angles of attack, where the lower rudder was detrimental to stability. The delta-wing program, on the other hand, wanted to fly sustained high-speed cruise missions that would require little high – angle-of-attack work. The initial round of tests in the North American hypersonic tunnel revealed that the upswept wing tips were inadequate above Mach 6. Researchers tested various configurations in both the North American hypersonic tunnel and the 20-inch Langley hypersonic tunnel until they found a set of tip fins that extended both above and below the wing centerline to be adequate. Nevertheless, engineers decided to make it easy to replace the fins just in case the wind-tunnel tests proved to be inaccurate.13111

The large-angle-of-attack capability of the basic X-15s was no longer required since researchers did not intend the mission to go to high altitudes. Given that the maximum angle of attack envisioned for the new airplane was less than 15 degrees, engineers decided to use a fixed-flow direction sensor to sense the angle of attack and sideslip. A hemispherical nose with five pressure taps could provide an air-data computer with sufficient information to derive the necessary angles without the complexity and weight of the ball nose. Conceptually, this was identical to the fixed alpha nose flown during six of the last X-15-1 flights.13121

North American proposed to stretch the fuselage 10 feet to 62.43 feet overall, and to manufacture what was essentially a new fuselage from the cockpit rearward. The company would stretch the propellant tank section 91 inches and provide new mounting provisions for the delta wing. North American manufactured test specimens from Rene 41 and Inconel 718 to determine which would be the best material for this area; these tests were in progress when NASA canceled the program. The space between the liquid-oxygen and ammonia tanks accommodated the standard center-of – gravity instrumentation compartment, and North American added a new 29-inch-long compartment behind the fuel tank but ahead of the engine to hold fuel and gases for the HRE. Designers also wanted to replace the existing ogive forward fuselage (in front of the canopy) with a 20-degree, included-angle-cone section. This semi-monocoque structure would use Rene 41 outer skin and Inconel X (or Inconel 718) frames, and titanium would be used for the inner skin of the equipment compartment.13131

North American investigated several different powerplants for the airplane, with the leading challenger being an Aerojet YLR91-AJ-15 from the second stage of a Titan II ICBM. This engine used unsymmetrical dimethylhydrazine and nitrogen tetroxide as propellants, and had already been man-rated for the NASA Gemini program. When equipped with a 25:1 nozzle, this engine completed the reference mission without the use of external propellant tanks. In fact, at a launch weight of 52,485 pounds and a burnout weight of 18,985 pounds, the YLR91 would have allowed a maximum velocity of 8,745 fps, well in excess of the 7,600 fps required by NASA. The effect of carrying the 1,000-pound HRE would have reduced this by about 400 fps.13141

North American briefly investigated the idea of using a separate "sustainer" engine to provide thrust to overcome drag during hypersonic cruise. Although the integration issues involved with incorporating a second engine and its propellants into the airframe eventually convinced all concerned that it would be too difficult, the particular engines investigated show how a program could come full circle. One of the engines investigated was the Bell YLR81-BA-11, a variation of the one of the engines proposed to power the X-15 in 1954. North American also investigated several variants of the Reaction Motors LR11 family that had been used for the initial X-15 flights,

along with the Aerojet LR52 (AJ-10).-1315

The engine that was ultimately selected was a modified XLR99 that provided 83,000 Ibf at 100,000 feet and was throttleable down to 8,000 lbf for sustained cruise. This was the version of the XLR99 that used a single thrust chamber and nozzle, not the Reaction Motors concept that used a second, remotely located alternate chamber. The increased internal fuel would permit sustained flights at Mach 6.5 using the low-thrust "sustainer" capability of the modified XLR99 to overcome drag but not produce any acceleration. The addition of a single centerline external tank would allow Mach 8 flights.-316

The main landing gear would be a version of the gear developed for the X-15A-2, appropriately strengthened for the almost 19,000-pound normal landing weight of the delta-wing design. As on the X-15-A-2, North American proposed to use both short and long versions of the rear shock struts; the long ones would provide clearance for the HRE under the ventral, while flights that did not carry the HRE would use the short ones. The nose gear would be moved to the instrument compartment behind the pilot, and the recorders and other research instrumentation normally carried there would be moved to a new compartment in front of the pilot where the original nose gear well was.-1317!

Although it was not part of the delta-wing baseline, North American was investigating the use of a fly-by-wire control system on the delta-wing X-15. Engineers believed that this would reduce the overall system size, weight, and volume, and provide better overall performance. This system would have used an analog flight-control system, not a digital one. The MH-96 adaptive control system was capable of accepting electrical inputs that were equivalent to flying in a fly-by-wire mode, and Honeywell designed the MH-96 to interface to the fly-by-wire flight-control system in the Dyna-Soar. In X-15-3 these were paralleled with mechanical linkages, and the delta wing could eliminate these mechanical linkages altogether. Given the facts that a fly-by-wire system had never flown, and the delta-wing airplane was flying in a new performance envelope anyway, NASA was not supportive of this effort.316

The final delta-wing design spent a considerable amount of time in this NASA Langley wind tunnel before the program was cancelled. (NASA)

Although the delta-wing airplane was generally described as a modification of X-15-3, about the only structure from the original airplane that would remain would be the cockpit and the aft thrust structure. Most of the electronics (i. e., the inertial system and MH-96) would also remain.

However, at least by weight, the majority of the aircraft would be new. Since the North American study had not been completed when X-15-3 was lost, the company had not estimated the final cost, but the amount would probably have been substantial.

Although no formal contracting arrangement existed, North American pressed on with a great deal of research into the delta-wing configuration. By March 1967, wind-tunnel models had accumulated over 300 hours of testing, with a 1/50-scale model used for high-speed tests and a 1/15-scale model used for low-speed tests. According to a North American news release, "the four year research program has also enabled North American to check out the integrity of components using new super alloys that will be required at hypersonic speeds. Tanks, wing sections, and other components have been fabricated of such materials and put through exhaustive thermal and structural tests."[319]

Despite endorsements from the FRC and John Becker at Langley, support remained lukewarm within the Air Force and NASA. The FRC was still evaluating the proposals for the delta-wing study on 15 November 1967 when the crash of the X-15-3 effectively ended all thought of such a modification. Since the general concept depended upon the use of the electronic systems that were unique to X-15-3, most researchers did not consider it readily possible to convert one of the other airframes; besides, the accident effectively sealed the fate of the entire X-15 program.-132^