REVISIONS

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