At the Edge of Space

by Milton O. Thompson

  Flight 43 was the next envelope expansion flight. (Flights 41 and 42 were research flights flown by Forrest Petersen and Bob Rushworth.) Bob White made this flight, which was an altitude flight to 217,000 feet. The planned altitude was 200,000 feet. The left-hand windshield shattered during reentry due to aerodynamic heating. Fortunately, only one pane of the dual-panel window broke. This flight was otherwise very uneventful and didn’t stimulate any significant pilot comments.

  Flight 44 was another speed flight by Joe to a Mach number of 5.74. The planned Mach number was 5.70. The only unusual thing about this flight was a severe aircraft vibration attributed to the flight control system. Joe again complained about the poor controllability at high angles of attack without stability augmentation. Joe’s evaluation of the controllability at high angles of attack during these flights verified a predicted stability and control problem. As a result of his evaluation, an all out effort was initiated to find a solution since we could not make the pullout during entry from the design altitude without the ability to fly at high angles of attack.

  Bob White made the last speed flight to a Mach number of 6.04 on Flight 45. On this flight, the right-hand windshield shattered and Bob commented, “Good Lord, I hope the other one doesn’t go.” The windshield was completely opaque. If the other one had shattered, Bob considered jettisoning the canopy and trying to land the airplane without a windscreen. The odds would have been against him though, due to the potentially severe buffeting at approach speeds. Luckily, the other windshield remained intact. Bob was not happy with the visibility during landing with only one windshield available. He had trouble lining up with the runway and judging height. This concluded the speed envelope expansion for the standard aircraft. He had achieved design speed.

  The last envelope expansion flight was Flight 52, which was an altitude flight to 247,000 feet. Joe Walker made this flight. Previously, a backup stability augmentation system was added to the flight control system to ensure that stability augmentation would always be available to provide good controllability during entry from high altitude. This was a partial solution to the controllability problem that Joe had verified on previous flights. Flight 52 was very uneventful. Joe commented, “Up on top I thought that Los Angeles was already under me and I was mentally bracing myself for an unfortunate flight call. L.A. was really down under the nose and as a matter of fact, I seemed to be rapidly coming up on the shoreline.” I, Milt Thompson, was not privy to these comments when I made my first altitude flight which resulted in similar anxious moments for me.

  The maximum altitude was ultimately increased to 354,200 feet on Flight 91 by Joe Walker. The airplane had the power to fly to higher altitudes, but it was questionable whether a successful reentry could be made without exceeding a structural limit. The simulator predicted that altitudes as high as 450,000 feet could be achieved. Successful reentries could also be made from these altitudes on the simulator, but these simulator reentries were accomplished under ideal conditions. The slightest error in control technique under such conditions would result in the loss of the airplane. Wiser heads decided that it was not worth the risk to attempt higher altitudes. The maximum Mach number was ultimately increased to Mach 6.7 during the X-15A-2 flight program.

  This phase went surprisingly fast, considering that we were flying into a totally new flight regime–the hypersonic flight region. This was the flight regime where theory predicted that a potential thermal barrier may exist. The most pessimistic predictions indicated that the expected heating rates would soften the best available structural steels. Any airplane entering this region would become a red hot limp noodle, according to some experts. The flight results proved them wrong.

  As the results of these flights would indicate, there were no major problems encountered during this envelope expansion phase. The airplane stayed together and the pilots could control it throughout the design speed and altitude envelope. The airplane had survived flights into the hypersonic flight region to speeds more than twice as fast as any previous aircraft.

  The airplane had also penetrated and survived the so-called thermal barrier. Amazingly, it had accomplished all this so effortlessly. On the other hand, the feat probably was not so amazing to the designers. They obviously anticipated the major problems and designed the airplane to cope with them. It did. It came through with flying colors—a little bit scorched but completely intact.

  The more significant problems cropped up as we exceeded the design envelope. We also had problems with the aerodynamic heating expansion flights during which the airplane was flown at high speed at lower altitudes to intentionally heat the structure of the airplane. These flights were made during Phases VII and VIII. The airplane really did get hot on those flights. Temperatures in excess of 1,300° F were recorded. Parts of the airplane glowed cherry red and softened up a bit during those flights. The airplane got so damned hot that it popped and banged like an old iron stove. It spewed smoke out of its bowels and it twitched like frog legs in a skillet. But it survived.

  Forrest Petersen’s last flight occurred during this phase of the program. It was the forty-seventh flight of the program. Pete was scheduled to launch at Mud Lake on January 10, 1962. The engine failed to light after launch. Pete recycled the engine by retarding the throttle to the off position. He then pushed the engine reset button, primed the engine for 5 seconds until the igniter ready light came on and then advanced the throttle to the on position again. This procedure required about 10 seconds to accomplish, but it seemed more like 10 minutes or even 10 hours.

  The pilot did not have much time to waste after launch. He either had to get the engine lit or abort the flight and make a landing at the launch lake. The problem was that he was losing altitude rapidly (about 12,000 feet per minute) while waiting for the engine to light. If he lost too much altitude before deciding to abort, he could not accomplish the normal approach pattern or finish jettisoning propellants before landing. Also, if he descended too far before getting an engine light, he negated all of the preplanned energy management calculations. This was not an airplane that could be flown on guess-work. The pilot would not know which emergency lake to head for if the engine quit prematurely. Another possible problem in descending too low was that the pilot might overstress the airplane by flying at high speed through the dense, lower altitude before he managed to get the plane pointed uphill. And finally, if he descended too low before getting an engine light, he may not regain enough energy to make it home. All of these considerations dictated that the pilot make a decision to abort the flight before descending through 30,000 feet altitude.

  This cutoff altitude provided enough time to attempt two restarts. The engine failed to light on Pete’s first restart attempt. He again recycled the engine and waited for that big kick in the butt. No luck. The engine failed to light again. That was it. Now, he had to turn toward Mud Lake and start to jettison the aircraft’s unused propellants. Pete continued to jettison until he turned on final approach. Pete made a nice landing to conclude his participation in the X-15 program. His departure from the X-15 program was necessary for him to get back in the mainstream of naval life to advance his career. Pete began and ended his X-15 flight activity with an emergency landing.

  When an X-15 landed at an emergency lake, the only way to get it home was to load it on a flatbed truck and bring it back on the highway. The airplane had a short wingspan, less than 25 feet, so it could be transported legally on the highway without removing the wing. A factor in our favor was the lack of traffic and obstructions along the highways in the remote desert regions where we were operating. We could load the airplane, put a wide load sign on the truck bumper and haul ass down the road. Our truck drivers did conscientiously try to pull way over to one side to avoid opposing traffic.

  On one trip coming back from Mud Lake, our driver noticed a big camper coming at him at a high rate of speed. Our driver vainly attempted to warn the camper driver to slow down and stay over to the side of the road by blinki
ng his lights. The camper driver went by our truck doing about 70 MPH. Our driver felt a jolt and slowed to a stop. The camper also came to a stop. A quick examination of the two vehicles revealed a huge gash through the side of the camper about a foot below the top, from the front to the rear of the camper. The wing of the X-15 had almost sliced the top off that camper. The X-15 was not damaged at all. Steel wings do have their advantages.

  THE LONGEST FLIGHT

  The MH-96 checkout phase was a brief phase involving the first four flights in the number three aircraft to check out the MH-96 adaptive flight control system. Neil Armstrong was the expert on this system and he flew all four flights. These flights were primarily altitude buildup flights starting at 81,000 feet and progressing to 133,000 feet, 180,000 feet, and finally to 207,000 feet. The benefits of the MH-96 system were predicted to be more obvious during altitude flights, and thus, altitude flights were used to check out the system.

  The MH-96 system was the first command augmentation system with an adaptive gain feature. This type of system is intended to provide invariant aircraft response throughout the flight envelope of the aircraft. The MH-96 system utilized a rate command control mode wherein a given control stick deflection produced a specific rate response of the aircraft. For example a one-inch pitch stick deflection would result in a 5-degree-per-second pitch rate. This response would be the same regardless of airspeed within the normal flight envelope. In a more conventional aircraft, the pitch rate response would vary with airspeed. In an aircraft with a large speed envelope, and a conventional control system, a one-inch stick input could produce aircraft responses that varied from no response at low speed to a violent response at high speed; possibly violent enough to tear the wings off. Thus for a high-speed aircraft, it would seem desirable to have invariant response to prevent the pilot from inadvertently destroying the aircraft.

  Nothing comes free however. With invariant response, you lose some of the cues that you may have depended on to warn you of impending disaster. The controls do not become sloppy or ineffective as you approach a stall, for example. Everything feels fine until the aircraft departs. With rate command, you also lose speed stability unless it is artificially provided. The aircraft nose does not tend to drop as airspeed decreases since the control system wants to maintain a zero pitch rate unless commanded otherwise. Without speed stability, it is quite easy to fly into a stall.

  A good rate command system also eliminates trim or altitude changes as a result of configuration changes such as gear, flap, or speed brake deployment. The system also masks any shift in center of gravity. In one sense, these features are desirable. You do not have to retrim. You do, however, lose the cue that confirms that the gear or flaps deployed as commanded. You can also lose control due to an undetected center of gravity change. Overall, however, these systems are generally pleasant to fly and they do have some distinct advantages.

  This system proved to be superior to the basic flight control system during altitude flights for a number of reasons. For example, it combined the aerodynamic and reaction control commands on one control stick. This eliminated the need to fly with both hands on an altitude flight. The system also offered several autopilot modes, such as roll hold, pitch attitude hold, and angle of attack hold. These modes could be used to reduce significantly the pilot workload during altitude flights.

  This system also minimized any extraneously induced aircraft motions due to its much higher system gain. The pilots appreciated this feature particularly on altitude flights since extraneous motions were very undesirable and hard to damp out using manual control, particularly during reentry. This system was later used for all maximum altitude attempts.

  The first flight was flown on December 20, 1961. The flight launched at Silver Lake. The flight plan called for a climb at 50 percent thrust to 62,000 feet altitude and then a pushover to come level at 75,000 feet. The engine was to be shut down at a speed of 3,500 feet per second after 104 seconds of burn time. Various control system pulses were to be performed throughout the flight to evaluate the performance of the MH-96 flight control system.

  During the actual launch, the stability augmentation in all three axes disengaged and a severe right roll occurred with accompanying yaw and pitch excursions. Neil recovered from the rolloff, lit the engine, and then managed to successfully reengage all three axes of stability augmentation. The flight progressed as planned from that point on, with only minor flight control system problems and some severe radio problems.

  The second flight was made on January 17, 1962. This flight launched at Mud Lake. NASA-2 at Beatty, Nevada controlled this flight until the X-15 passed China Lake en route to Edwards, at which time NASA-1 at Edwards took control. This transfer of control from Beatty to Edwards was normal during the early envelope expansion flights before telemetry and tracking data were transmitted in real time between tracking sites.

  Nothing significant occurred on this flight. The flight plan called for numerous maneuvers to evaluate the flight control system and, as a result of pilot preoccupation with these maneuvers, the planned speed and altitude were substantially exceeded. The aircraft landed without incident after it obtained all the desired data.

  The third flight occurred on April 5, 1962. The aircraft was launched at Hidden Hills and the flight plan called for a peak altitude of 170,000 feet. When Neil attempted to light the engine after launch, he said he saw the igniter pressure go to zero and then failed to hear the engine start. Neil checked the engine instruments noting that everything looked good, so he attempted a relight. The engine restarted successfully, but he noted that it seemed like an awful long time for the engine to light.

  By the time the engine came up on thrust, Neil had lost almost 10,000 feet altitude. He had to mentally recompute his climb schedule to get back on his desired climb profile. At 25 seconds after launch when finally back on profile, he advanced the throttle to 100 percent thrust. Neil reached the desired climb attitude of 35 degrees and then noted as he maintained that attitude, that he felt the airplane was continuing to rotate upward. He thought that he was going straight up or even going over on his back. He attempted to crosscheck his attitude by referring to his backup attitude indicator, but it was on the stop at 35 degrees. He said he asked NASA-1 (Joe Walker), “How is my trajectory?” in a casual, offhand way, not wanting to admit that he thought he was climbing straight up. Joe indicated that his climb angle was good.

  In the postflight debriefing, someone asked Neil, “Did it occur to you that since you thought you were going straight up that you would be coming straight down?” Neil admitted that the thought had occurred to him and that is why he had asked Joe about his trajectory. He was very happy to hear that his trajectory was normal.

  Neil peaked at 180,000 feet altitude and then set up for the entry. For some reason he could only command 11 degrees angle of attack, instead of the planned 15 degrees, but he felt that it was adequate to make the reentry. As the normal acceleration began to rise during the entry, several engine malfunction lights came on. For a moment Neil thought the engine was trying to light up. He commented that, “it seemed like an awful lot of stuff was going on at the same time and it was difficult to evaluate something like the engine trying to light up. This could get you in trouble if it really did. If that engine lights up when you’re going downhill, that’s all.” It would have been catastrophic. Neil finally cycled the throttle on and then off and the lights went out. Neil completed the entry without any further incidents and proceeded to a successful recovery at Edwards.

  Neil’s fourth flight in the number three aircraft occurred on April 20, 1962. He made the launch at Mud Lake. The plan called for a flight to a peak altitude of 205,000 feet, in which Neil would perform maneuvers during ascent and descent to evaluate the flight control system. The flight proceeded as planned with only minor deviations in speed and altitude. Neil commented, “In general, the aircraft control and damping during ballistic flight and entry were outstanding and considerably more
smooth than had been expected. Unfortunately, this may be at the expense of excessive reaction control fuel consumption.” The number one APU and BCS peroxide-low lights did light up at approximately 160,000 feet during the descent, verifying a higher than normal consumption of peroxide. The peroxide transfer system used to transfer unused engine turbo-pump peroxide was immediately energized and the light was extinguished as he descended through 115,000 feet altitude. Radio communications were intermittent during this period and a potential new problem surfaced as indicated in the radio communications.

  NASA-1: “OK, brakes out.”

  Neil: “Rog, and we’re getting a little … oh, that head bumper.”

  NASA-1: “OK, and a hard left turn, check the RCS off.”

  Neil: “Say again.”

  NASA-1: “Hard left turn, check the RCS off.”

  Neil: “RCS off, brakes are out and I have the base in sight.”

  NASA-1: “OK, lots more left there, retract the brakes, 25 degrees stabilizer, Neil.” [NASA-1 is concerned because Neil is not turning the airplane toward Edwards.]

  NASA-1: “We show you ballooning, not turning, Neil. Six- seven-two, hard left turn.” [Neil is beginning to bounce back out of the atmosphere, rather than descend into it. As a result, he is unable to pull g to turn.]

  Neil: “Rog, I’m reading.”

  NASA-1: “Hard left turn, Neil.”