FLIGHT DAY

X-15 flights generally began early in the morning; indeed, most flight-testing at Edwards began early in the morning when the temperatures and winds in the high desert were lower. The ground crew had mated the X-15 to the NB-52 the day before and stayed all night or arrived early to prepare the airplane for the flight. Floodlights lit the scene as propellants and gases were loaded onto both the carrier aircraft and research airplane, and liquid-oxygen vapor drifted around the area, lending a surreal fog. When the X-15 pilot arrived, he generally went straight to the physiological support van to get into the David Clark full-pressure suit. Getting the suit on and hooking up the biomedical instrumentation took about 15 minutes once the program switched to the A/P22S-2 suits; the MC-2 suits had taken considerably longer.-1^5

When the ground crew was ready for the pilot to enter the cockpit, two technicians carried a portable cooling system and other equipment while they escorted the pilot from the van to the airplane-a scene vaguely similar to Cape Canaveral before a space flight. Oddly, the driver of the physiological support van in which the pilot donned the pressure suit made no particular effort to park near the X-15, forcing the pilot to walk across the ramp. A large ladder and platform were located alongside the X-15 to allow the pilot and his handlers easy access to the cockpit. The cockpit itself was large for a single-seat airplane, but the bulk of the pressure suit made it seem somewhat smaller. Nevertheless, most pilots found it had more than adequate room and some of the smaller pilots even had difficulty reaching all of the controls mounted far forward, since the seat was not adjustable. Once the pilot was in the cockpit, the ground crew hooked up a myriad of lines, hoses, and straps that provided life support and monitored the pilot’s biomedical data.

While this was happening, the pilot began going through the preflight checklist to verify the status of all the aircraft systems. Once this was completed (usually a 30-minute process), the ground crew closed the X-15 canopy. The cockpit suddenly seemed smaller since the canopy fit snugly around the pressure-suit helmet.

While this was happening, the ground crew was disconnecting the servicing carts used to prepare the NB-52 and X-15 for flight. At this point, the NB-52 started its engines and the carrier aircraft pilots went through their preflight checklist, taking about 10 minutes to complete the activity. The ground crew then closed up the NB-52 hatches and the mated pair taxied toward the runway accompanied by a convoy of a dozen or so vehicles. Edwards is a large base, and the aircraft had to taxi for 2 or 5 miles depending on which runway was active. One of the H-21 helicopters took off and performed a visual check of the runway to make sure no debris was present, then took up a position beside and slightly behind the bomber, preparing to follow it down the runway for as long as possible.

At the end of the runway, the ground crew removed the safety pins from the X-15 release hooks. When everybody signaled they were ready, the NB-52 took off and climbed to 25,000 feet while circling over Edwards to make sure the X-15 could make an emergency landing on Rogers Dry Lake. Once above 25,000 feet, the NB-52 turned toward the launch lake and began climbing to 45,000 feet, since at this altitude the X-15 could glide to an alternate lakebed if necessary. The NB-52 supply topped off the X-15 liquid-oxygen tank, and the inertial platform was receiving alignment data, but otherwise things were quiet. Chase-1 flew in formation with the B-52, observing the X-15 for leaks or other anomalies that might signal a potential problem.

The mission rules dictated that if a serious problem occurred on the NB-52 while the mated pair was on the way to the launch lake, the carrier aircraft would jettison the research airplane since the extra 30,000 pounds of dead weight under the right wing would undoubtedly be detrimental to saving the NB-52. Similarly, if something happened on the X-15 that looked like it would endanger the NB-52, the research airplane would be jettisoned. As Scott Crossfield later observed, "It was not heroics; it was simple mathematics. Better to lose one man than four." In reality, the X-15 stood a chance of surviving if it was jettisoned, especially if the X-15 pilot had some advanced notice. The major problem was that neither of the APUs aboard the X-15 was running during captive carry since there was not enough propellant to last more than about 30 minutes. During the climb-out, the NB-52 supplied all of the X-15’s electrical needs, as well as breathing oxygen and pressurization gas. If the carrier aircraft jettisoned the X-15, the pilot would have his hands full trying to get the APUs started using a small emergency battery since without the APUs the pilot had no flight controls, no radio, no anything. If the APUs started, the pilot could try to fly (with or without the engine) to a lakebed. Of course, the ejection seat was always an option. Fortunately, the program never had to find out what would happen in that scenario.-1161

The need for the various lakebeds was largely driven by a program requirement to always have a landing site available to the X-15 pilot if he needed it for any reason. Therefore, each flight was planned such that the X-15 could glide to an emergency landing site from any point on the flight path, although frequently the nearest site was behind the airplane and required the pilot to turn around, as illustrated in this drawing. The program used the emergency landing sites 10 times. (NASA)

At 12 minutes before the scheduled launch time, things began to happen. The X-15 pilot started both APUs and began to run through the prelaunch checklists. The pilot checked all of the X-15 systems, exercised the flight controls, tested the ballistic control system, and set all switch positions. Chase-1 observed the results of these tests and reported them back to the X-15 pilot. During this time, radar and radio communication with NASA-1 guided the carrier aircraft into position near the launch lake. Eight minutes before launch the NB-52 began a long sweeping turn back toward Edwards, coming onto the final heading about 4 minutes later. At the same time, the X-15 pilot began activating the propulsion system. At 2 minutes prior to launch the X-15 pilot started the data recorders, checked the ball nose one last time, and turned on the cameras. One minute prior to launch the XLR99 was set to precool and the igniter was set to idle. More checks were performed to make sure the engine looked ready to fire. The X-15 pilot took a deep breat h.[l7]

Three, two, one: launch. The X-15 separated from the NB-52 and began to fall. The launch was harder than most pilots initially expected because the X-15 went from normal 1-g flight while attached to the carrier aircraft to 0-g flight instantaneously. The X-15 also wanted to roll to the right because of downwash from the NB-52 wing and interference from the fuselage. The X-15 pilots usually had left roll input applied at the moment of launch, but the airplane still rolled – more so on some occasions than others.

The XLR99 start sequence was remarkably simple, a necessary attribute in the days before computerized control systems. The first step was to pressurize the propellant tanks with gaseous helium to ensure a smooth flow of propellants to the turbopump. Then the oxidizer system was precooled to ensure that the liquid oxygen did not vaporize between the propellant tank and the turbopump (vaporized liquid oxygen caused the turbopump to cavitate and go into an overspeed condition that resulted in an automatic shutdown). It required about 10 minutes to chill the oxidizer system. Next was the engine prime sequence that fed a small amount of liquid oxygen and ammonia to the turbopump. The igniter-ready light came on when the prime cycle began and the pilot turned on the igniter switch. This all happened before the X-15 dropped away from the NB-52. As the X-15 was falling, the pilot continued the engine start procedure. There were about 10 seconds available to light the engine before the pilot had to abort to the launch lake; that was time for two ignition attempts.-118

Pressing the pump-idle button to start the turbopump initiated the ignition cycle. The turbopump spooled up quickly and forced propellants into the first-stage igniter, where a spark plug ignited them. The propellants were then fed into the second-stage igniter chamber, where the flame generated by the first-stage igniter caused them to combust. The second-stage igniter produced 1,500 lbf-as much as one chamber on the XLR11.

The throttle quadrant was a "backwards L" slot located on the left console. The outer corner was the "idle" position, the bottom inside corner of the L corresponded to minimum throttle, and the most forward position was 100% power. Moving the throttle inboard opened the main propellant valves and forced 30 gallons per second of propellants into the main chamber, where they were ignited by the flame from the second-stage igniter. The pilots found early in the flight program that the engine did not ignite reliably at low-power settings, so they usually immediately

advanced the throttle to the 100% position. Although the XLR99 proved to be a remarkably reliable engine, it really did not like to throttle. Still, the capability provided a certain amount of research utility that would not otherwise have been available, although it also contributed to several in-flight emergencies when the engine decided it no longer wanted to work as its designers had intended.-1191

In an attempt to ensure that the entire propulsion system was in working order, NASA conducted a ground run before almost every flight. Although it was comforting to the X-15 pilot to know the engine indeed seemed to work, these tests also added a great deal of wear and tear to the engines and other systems. During ground runs, the pilot would allow the engine to stabilize at 100% thrust for 8 seconds, and then retard it to idle for 5 seconds before shutting the engine down. The pilot would then perform an emergency restart sequence that relit the main chamber at 75% thrust. The pilot would stabilize the engine for 5 seconds, reduce it to idle for a couple of seconds, and then shut it down. It all became routine.

Energy management started the instant the NB-52 released the X-15. If the XLR99 did not light in two attempts, the pilot would make an emergency landing at the launch lake. If the engine died within the first 30-40 seconds of flight, the pilot would turn around and make an emergency landing at the launch lake. After about 40 seconds of burn, the airplane would be too far away to make it back to the launch lake, but if the engine burned less than 70 seconds, it was unlikely the pilot could make it to Rogers Dry Lake. The 30-second period in between was why the program had a large assortment of intermediate lakebeds available.

Unfortunately, the technology did not exist to provide the pilot with an energy-management display, although NASA installed a rudimentary unit in X-15-3 toward the end of the flight program. It was up to the ground controller (NASA-1) to advise the pilot where to land if a problem developed during the flight. As flight attendants are fond of saying on every commercial airline flight, the nearest exit may be behind you. In many cases, the best landing site was an intermediate lake that the airplane had already passed at hypersonic velocities.

The intermediate lakes were more important for the high-speed flights than for the altitude flights. Given enough altitude, the X-15 could glide for over 400 miles-more than enough distance to make it back to Edwards from almost any point on the High Range. Every flight was supposed to have excess energy as the airplane arrived over Edwards, allowing some flexibility during the final approach. Nevertheless, part of why the program had an excellent safety record was that the pilots and flight planners always had contingency plans—even for the contingency plans.

Most X-15 flights were essentially in the vertical plane, and it was important to establish the proper heading back toward Edwards during the first 20 seconds after launch. Once the engine shut down, the ballistics was pretty well established for the next few minutes of flight. If there was a wind at the launch altitude (and usually there was), the NB-52 would crab as necessary to maintain the proper ground track during the final 10-minute turn. At 1 minute to launch, the NB – 52 pilot would turn to the desired launch heading and allow the carrier aircraft to drift. Since winds at launch did not seriously affect the X-15 trajectory, this minimized the workload on the X-15 pilot to obtain, and hold, the desired heading. After launch, any necessary heading corrections were made by the X-15 pilot using small bank angles while performing a 2-g rotation and accelerating from Mach 1 to 2 in about 20 seconds. Once the X-15 reached the desired pitch angle, the g-level was less than one, and no further turning corrections could be made until after completion of the reentry.-120

The design speed mission was flown at relatively low altitudes – from 100,000 to 110,000 feet. These were the essential heating flights that were used to validate the various theoretical and experimental (wind tunnel) results. The time at maximum speed was not spent flying straight and level since the pilot was conducting a series of rudder pulses and other maneuvers to optimize the heating on the side of the aircraft that was instrumented. The ability to exactly repeat these maneuvers from one flight to the next was critical for the ultimate success of the flight program and a tribute to the flying skills of the pilots. (NASA)

The thrust from the XLR99 could be terminated by one of two routine ways at the nominal end of burn. The most frequently used method was called "shutdown." When a specific set of flight conditions had been reached, the pilot would manually shut down the engine. Normally the pilot did this after a precalculated amount of time based on a stopwatch in the cockpit that started when the main propellant valves opened. After NASA installed the X-20 inertial systems later in the program, the pilot could also use inertial velocity to shut down the engine, and several of the high-altitude flights used the altitude predictor installed in X-15-3. The other type of thrust termination was called "burnout." In this method the pilot just let the engine burn until the propellants were exhausted and the engine quit.-121

The high-speed flights were conducted at fairly low altitudes (a relative term since the altitudes would have been considered extraordinary before the X-15 program began). For these flights, the X-15 was essentially an airplane; its wings generated lift, maneuvering was accomplished via a set of aerodynamic control surfaces, and the atmosphere created a great deal of drag and friction on the airframe. The pilot would begin a 2-g rotation to the desired pitch angle immediately after the engine lit. During this rotation the primary piloting task was to adjust the bank angle to attain and hold the desired heading back to Edwards. As he approached 70,000 feet, the pilot initiated a gentle pushover to come level at something between 100,000 and 110,000 feet. As the airplane came level, the pilot either stabilized his speed at some preset value to conduct various research maneuvers, or continued to accelerate to attain more speed. The X-15 liked to accelerate; even at the top end, it took only 6 seconds to accelerate from Mach 5 to Mach 6. The research maneuvers continued after engine burnout until the airplane decelerated to the point that no more useful data were forthcoming. These were the essential heating flights.[22]

Altitude flights began much the same way, except that the pilot continued a steep climb out of the atmosphere. The engine shut down on the way up and the airplane coasted over the top on a ballistic trajectory. The pitch angle, in conjunction with the shutdown velocity, established both the range and maximum altitude of the arc that would follow. As the airplane continued on the ballistic trajectory, it was committed to a steep descent back into the atmosphere. The pilot set up the angle of attack for reentry, performed a pullout to level flight after reentry, and then began a shallow descent during the glide back to Rogers Dry Lake. A combination of dynamic pressure (q), load factor (g), and structural temperature limited the reentry since the relaxation of one parameter resulted in an excess of one of the others. These flights spent between 2 and 5 minutes outside the atmosphere, much of that time in a weightless (i. e., no accelerations) environment. The ballistic control system allowed the pilot to maintain attitude control, but he could not change the flight path of the airplane. Contrary to popular lore, as fast as it was, the X – 15 never flew anywhere near fast enough to attain orbital velocities or altitudes.-1231

For the next few minutes, the calls from NASA-1 were primarily comparisons between the planned profile (on the plot boards) and the actual radar track of the airplane. These let the X-15 pilot know how well he had flown the exit phase and, more importantly, what maneuvers might be required during reentry. If he was "high and long," he would expect to make an immediate turn and apply the speed brakes during the latter part of the reentry. If he was "low and short," he would expect a straight-ahead glide with brakes closed. A "left of course" call would alert him to expect a right turn to a new heading after reentry. The ability to comprehend some of these energy-management subtleties while simultaneously controlling the aircraft’s attitude and subsystems, and accomplishing test maneuvers was one of the goals of the X-15 simulator training.-1241

Perhaps surprisingly, the altitude flights required a longer ground track than the high-speed flights. This was primarily because the airplane covered many miles while it was outside the atmosphere. Let us use the two maximum performance flights as comparative examples. Joe Walker’s 354,200-foot altitude flight required a ground track of 305 miles to climb out of the atmosphere, coast to peak altitude, reenter, make the pullout, and then slow to land. On the other hand, Pete Knight’s 4,520-mph speed flight only took 225 miles, mainly because the airplane slowed quickly after engine burnout since the speed flights occurred in a relatively dense atmosphere.-1251

Surprisingly, the X-15 spent more time at higher Mach numbers during the altitude missions than it did during the speed missions. This is because there is less aerodynamic drag at high altitudes and the airplane coasted at high velocities for longer while it was outside the atmosphere. The altitude missions were particularly demanding on the pilot since even small deviations from the planned profile could result in overshooting the target altitude. The ballistic control system was required to control the attitude of the airplane during these missions, most of which were flown by X-15-3 since the MH-96 adaptive control system provided more redundancy. (NASA)

During the envelope expansion and heating flights, the pilots performed a specific set of maneuvers (rudder pulses, angle-of-attack changes, and rolls) to evaluate the stability and control of the airplane in various flight regimes. Many times these maneuvers were near the limits of controllability for the airplane, and well-practiced contingency plans were always at the ready. Other tests provided information on control effectiveness, aerodynamic performance, lift-to-drag ratio, and aero-thermo loads. All of these maneuvers required that the pilot fly at a specific speed, attitude, and altitude while gathering the data. Often, the program needed to exactly duplicate the profile on subsequent flights to eliminate variables from the data, a decided challenge before the advent of computerized flight-control systems.-126

As Milt Thompson later observed, "This is the kind of thing a research pilot is required to do to earn his money-accomplishing good maneuvers for data purposes. Flying the airplane is just something the pilot does to get the desired test maneuver. He can be the greatest stick and rudder pilot in the world, but if he cannot do the required data maneuvers, he is worthless as a research pilot." Most of the X-15 pilots were very good research pilots.-127

Assuming all went as planned, the X-15 arrived back at Edwards and set up a high key (the highest point of the final approach to a runway) at approximately 35,000 feet and 290 to 350 mph. As he approached Edwards, the X-15 pilot began dumping any residual propellants to lower the landing weight and to get rid of potentially explosive substances. It also made a convenient way for the chase planes to find the small X-15 in the vast skies over the high desert. The X-15

then entered a 35-degree banking turn while maintaining 250 to 300 knots. The pilots normally turned to the left, although each pilot seemed to develop a preference, and it really did not matter much. At the completion of the turn, the X-15 was approximately 4 miles abeam of the intended touchdown point at 18,000 feet altitude headed in the opposite direction of the landing runway (this was low key). The pilot then continued around 180 degrees, turned onto final at about 8,000 feet and 300 knots, and flared at around 1,000 feet. The pilot jettisoned the ventral rudder, lowered the landing flaps as the airplane came level at about 100 feet, and deployed the landing gear at 215-225 knots. Touchdown was generally made at around 190-200 knots. The pilot judged the possible crosswinds by the simple expedient of looking at the smoke from flares beside the runway.-1281

Unsurprisingly, not all flights arrived at high key exactly as planned. At least one flight arrived at high key at 80,000 feet and over Mach 3.5, another made high key at only 25,000 feet, and one made a straight-in approach because it was too low on energy when it arrived at Edwards. Despite these variances, the majority of X-15 touchdowns were made within 2,000 feet of the intended spot, although a couple of flights missed by over 4,000 feet. Neil Armstrong managed to miss by 12 miles-fortunately, Rogers is a large dry lake. The X-15 generally slid for 8,000-10,000 feet before coming to a stop, chased by a convoy of rescue and support vehicles.-291

The general concept was similar to that ultimately adopted as the terminal area energy – management maneuver used by the Space Shuttle. The proven ability of the X-15 (and later the heavyweight lifting bodies) to make unpowered approaches was one reason the Space Shuttle program decided it could eliminate the complexity of landing engines and make the Orbiter a glider. It is another enduring legacy of the X-15 program.-301