At the Edge of Space

by Milton O. Thompson

  This, of course, is typical of experimental aircraft testing. Something violent happens, but no one has enough information or they cannot assimilate it fast enough to tell the pilot what the problem is. The pilot has to cope with it immediately on his own. The control room is not much help in real time unless they have anticipated an emergency and have practiced it.

  Bob managed to regain and maintain control of the aircraft and get the nose back up to maintain altitude. His major concern now was to get the aircraft home since the nose gear added a lot of drag. He did not want to land the aircraft at one of the emergency lakes, but he was losing energy much faster than planned. NASA-1 recommended that he keep the airplane at high altitude until he slowed to subsonic speeds, and then begin his descent into the Edwards area. The control room continued to provide Bob with heading information, energy status, and position along the planned ground track. Bob requested that the chase join up as quickly as possible to inspect the aircraft and confirm that the nose gear was deployed. The chase finally spotted him as he was descending about 20 miles northeast of Edwards and climbed on up to join him.

  The chase quickly confirmed that the nose gear was extended, but he had to move in close to determine the extent of the damage. He informed Bob and the control room that the tires were badly burned, but they appeared to be intact. The nose gear strut, door, and wheel well appeared to be charred but otherwise normal in appearance. There was, however, a question of whether the tires were deflated and whether the nose gear strut was damaged sufficiently to compromise the oleo function. If the oleo was badly damaged, the aircraft could break as the nose slammed down. Bob was now in the landing pattern and less than 3 minutes from touchdown. A major decision was required. Should a landing be attempted under these conditions?

  This kind of potential life-and-death decision was frequently required during the X-15 program and during every other experimental flight program. Decisions were needed in real time in a matter of minutes and sometimes seconds. Luckily, we had the kind of people who could readily make such decisions. The decision was made to land the airplane. It was not a dramatic pronouncement, it was just a quiet comment to the controller, “Let’s go ahead and bring it on in.” The landing was a good one, but the rollout was rough. The tires failed at touchdown and rapidly disintegrated during the high-speed rollout, but the gear survived.

  From flight data, we learned that the nose gear had deployed at Mach 4.4. The temperature created by the impact of the air at that speed was over 1,000° F. That obviously accounted for the burned tires and charred nose gear. During the slideout after landing, Bob was heard to comment sarcastically, “Thank you, Mr. North American.”

  Following the flight, the landing gear system was carefully inspected in an attempt to determine why the nose gear had deployed. There was no obvious answer. The landing gear system appeared to be properly rigged and functioning normally during ground checkout.

  The landing gear deployment system then became a prime suspect. The deployment system was a relatively simple system consisting of cables attached to the various uplock hooks on the gear and nose gear door which terminate at a T-handle on the lower left side of the instrument panel. To release the gear, the pilot simply pulled the T-handle and the gear deployed by gravity, with some aerodynamic assistance. The gear release handle opened a small scoop door on the nose gear door which then pulled the nose gear door open and allowed the nose gear to fall free. The cable release system had some built-in slack to accommodate the expansion of the aircraft as it got hot.

  Some of the engineers suspected that the slack allowance may not have been adequate for the longer, modified airplane. They recalculated the expansion of the aircraft and found that indeed the aircraft would expand almost three-quarters of an inch more than the available slack. Thus, as the airplane got hot, it caused the cables to tighten up and finally release one of the hooks. The landing gear cable release system was subsequently modified to increase the slack to a total of 3 1/2 inches. This would provide ample margin, but it also required a longer pull of the release handle to release all the hooks. The pilots were not too happy with the additional T-handle motion required. It almost required two hands to pull the cable all the way out. We could not really argue, though, because we really did not want the gear coming out prematurely.

  The next flight was finally scheduled about 6 weeks later and Rushworth was again scheduled to fly it. The flight plan called for a flight up to Mach 5. Additional stability and control data was to be obtained at roughly 100,000 feet. The flight proceeded as planned up through burnout. As the aircraft decelerated through Mach 4.5, Bob heard another bang and the aircraft pitched down and began to oscillate in roll and yaw.

  Again, Bob was not certain of what had happened, but he suspected the landing gear system had malfunctioned again. The bang was not as loud as on the previous flight, so he assumed that it might be just the nose gear scoop door opening, rather than the entire gear extending. He advised the control room of his suspicions, but again the control room could not identify the problem nor really help him other than to offer sympathy. Bob had to wrestle with the airplane to regain complete control and fly it on to Edwards.

  On arrival at Edwards, the chase joined up and confirmed that the nose gear scoop door was open. As Bob came level following the landing flareout, he pulled the gear handle and nothing happened. He immediately pulled the handle again and heard the nose gear deploy. Joe Engle, who happened to be the chase pilot, noted that the nose gear was extending very slowly. He called out to Bob to, “hold it off” to allow the nose gear time to extend and lock. Bob managed to hold the airplane in the air just long enough to let the nose gear lock before the main skids touched the lakebed. The nose slammed down hard, but the tires stayed intact for the first thousand feet of the slideout and then disintegrated.

  A visual inspection of the nose gear and the nose gear wheel well showed extensive heating damage. The scoop door opening allowed the hot air to enter the nose gear compartment, which burned the tires, melted some aluminum tubing and caused the nose gear to bind up during extension. This incident had the potential to be more catastrophic than the previous one. Luckily, it was not. Rush worth was beginning to have mixed emotions about the joys of flying the X-15. He was getting upset with these surprises and, was also getting gunshy. What might happen the next time?

  Postflight inspection revealed that the nose gear scoop door uplock hook was distorted, indicating excessive loads on the hook. Again, the engineers assumed that a heating problem had caused the door to open. They ultimately heated up the nose section in the hangar to simulate the in-flight heating to confirm their suspicions. As a result of this test, they redesigned the uplock mechanism and prepared the aircraft for flight.

  This modification was subsequently made to each of the X-15s, since they were all susceptible to this problem. Somehow, we had only experienced it three times previously. I was elected to make a captive flight to demonstrate proper gear deployment after this modification since I was scheduled to make the next X-15 flight. The captive flight was scheduled to be made the day before my drop flight. It was late afternoon before the airplane was mated and serviced. We got airborne just before five o’clock and climbed to altitude to cold soak the airplane. By the time we completed the climb, cold soaked the airplane, and then deployed the landing gear, it was dark. That was the first and only night X-15 operation. I would have really had a problem if I had to be launched in an emergency.

  The flight planners took pity on Bob and decided to give him a breather. They scheduled Jack McKay for the next flight. Jack flew an uneventful flight and then it was Bob’s turn again. Bob’s flight plan was very similar to his last two flights. A climb and acceleration to about 100,000 feet at slightly over Mach 5.

  The flight proceeded normally until the aircraft decelerated through Mach 4.4. At that time, Bob felt, rather than heard, something let go. The airplane pitched down and then yawed violently to the right. This incident
occurred at a particularly bad time since, just prior to the event, Bob had deactivated the stability augmentation system to do stability and control maneuvers. With the stability augmentation system off, the airplane was very loose and it would diverge and oscillate wildly in response to any external force.

  Bob was subjected to a few seconds of terror as the aircraft oscillated violently until he could reengage the SAS. Again, there was no obvious indication of what had happened, either in the cockpit or in the control room. This time Bob could not deduce what had happened. He had to wait until the chase joined up to be informed that his right main gear was down. This caused some concern that the skid might not function properly on touchdown and would result in a failure of the gear itself. Bob made a very smooth landing and the gear remained intact. During the slideout Bob commented, “Boy, I’ll tell you, I’ve had enough of this!” When Bob finally got out of the airplane, he turned around and kicked it.

  Postflight examination revealed a bent right-hand main gear uplock hook. Further analysis indicated that aerodynamic heating caused the new longer main gear strut to bow outward more than normal, which failed the uplock hook and allowed the gear to deploy. The uplock hook was subsequently modified to compensate for the extra deflection of the longer strut.

  It is amazing how easy it is to overlook something that can be catastrophic, during a major redesign of an aircraft. After this modification, we had no further incidents. Some say the fact that we had no further incidents can be attributed to an extensive re-analysis of the entire landing gear system to determine if there were any more lurking problems that had not been considered in the initial redesign of the number two airplane. Others said, we had no more landing gear incidents because the crew put a small sign above the landing gear T-handle that said, “Do not extend landing gear above Mach 4.”

  Jack McKay flew the next two flights to complete the modified basic airplane envelope expansion. Then the aircraft began a series of flights to evaluate a star tracker experiment. The objective of this experiment was to photograph the sky, to locate the brightest stars in the ultraviolet spectrum in preparation for the launch of the Orbital Astronomical Observatory Satellite. A camera was mounted on a gyrostabilized platform in the instrument bay behind the cockpit. As the airplane ascended above 200,000 feet altitude, two doors opened up allowing the camera to photograph the sky. Four flights were made for this purpose during the next 4-month period.

  Following those four flights, the next was the first flight with external tanks. The intent of this flight was to demonstrate jettison of empty tanks at the design jettison conditions of Mach 2 at 70,000 feet. Rushworth made the flight. The flight was planned to be flown from Cuddeback Lake to Edwards. It was planned as a very benign flight to minimize the potential complications in case of malfunction.

  This flight was, however, a potentially hazardous flight. The tanks were enormous compared to the airplane. They were each about half the size of the airplane and they were not aerodynamically stable. The major concern was that they might not separate cleanly and as a result might critically damage the X-15. Other possibilities included such things as one tank jettisoning and the other not jettisoning, or a hang-up of one hook on a tank and a release of the other.

  The other extreme malfunction would be no release at all. It was theorized that the airplane could be landed with the tanks since they would crush as the aircraft nose slammed down and the aircraft would slide out on its gear with the tanks dragging along. All of this optimistic fantasy provided some small measure of confidence to the pilot, but there was no question that this flight was a “biggie.” Fortunately, the flight went as planned. The tanks separated cleanly and Bob made an uneventful recovery at Edwards.

  The next flight was a stability and control flight out of Mud Lake by Rushworth with no unusual occurrences. The following flight was the first flight with fully loaded external tanks. Bob was again the lucky pilot selected to make the flight. It was to be a launch out of Mud Lake and a flight to Edwards. The major concern on this flight was the possibility of an unplanned partially full tank jettison due to an emergency. For a number of reasons, the tank jettison analysis had only included the empty tank and full tank conditions. A partial fuel jettison scenario had an infinite number of CG (center of gravity) and inertia combinations which were not readily solvable in that era of analog computers, so they made no attempt to calculate any intermediate fuel tank jettison trajectories. You either jettisoned them full or empty or you suffered the consequences.

  As you might guess, on this flight the tanks had to be jettisoned in a partially full condition. Shortly after X-15 launch and engine light, Bob could not confirm that he was using fuel out of the external tank. His propellant flow meters indicated that he was using LOX out of the external tank, but no fuel. If this were indeed true, Bob would soon have an uncontrollable airplane with a large asymmetry in internal and external fuel tank loading. Bob jettisoned the tanks at 1.6 Mach at 41,000 feet and then jettisoned his internal fuel and made an emergency landing at Mud Lake. That was Rushworth’s last X-15 flight. He told me later that he was glad to finally leave the program. He wanted to try something less hazardous for a while, like going into combat in Vietnam.

  Bob did an outstanding job as an X-15 pilot. He flew a total of thirty-four flights—more than any other pilot—and more than twice the average number of flights per pilot. He flew the X-15 almost 6 years. Bob Rushworth and Jack McKay were the dog soldiers of the program. They did the grunt work. Walker and White were the pioneers and received most of the glory. Bob received very little recognition in reward for his contributions, but he gained tremendous respect from his peers and coworkers. Paul Bikle felt that Bob was one of the best research pilots that he had ever been associated with, and Bikle had worked with the cream of the crop during his many years in the flight test business. I would agree with Bikle. I really liked and respected Bob. He was my hero.

  Pete Knight inherited the program from Rushworth and flew all of the remaining flights in the airplane. His first three flights, without the external tanks, were high-altitude ultraviolet stellar photography flights or star tracker flights. These flights also served to familiarize Pete with the modified basic airplane. It flew somewhat differently than the other two aircraft.

  The flight following the star tracker flights, his fourth in this airplane, was made to obtain data for another experiment to investigate the effects of the hypersonic flight environment on high-resolution photography. Pete also evaluated the aircraft’s handling qualities on this flight.

  Pete’s fifth flight was his first flight with external tanks. The flight was launched at Mud Lake and flown almost exactly as planned. Pete reached a maximum Mach number of 6.33 at 97,000 feet altitude. Tank jettison was clean and both recovery chutes deployed. The tanks were slightly damaged at touchdown, but were refurbishable.

  Pete did not sense a big change in aircraft handling qualities on this flight. He had to use more aileron to counteract the asymmetric lateral CG, which was a result of the difference in weight of the LOX and ammonia tanks. Overall, though, the airplane appeared to handle surprisingly well with the tanks on. The airplane was not aesthetically pleasing with the tanks on, but apparently it flew all right. Once the tanks were gone, the airplane handled about the same as the standard airplanes.

  Several experiments were conducted on this flight to obtain data for the higher-speed flights. In one experiment, ablative material was installed on the lower ventral to determine its ability to minimize the aerodynamic heating of the structure. No data was obtained on this experiment, owing to a malfunction of the thermocouple temperature measurement system. Another experiment was conducted to determine the effects of ablator residue on windshield visibility. A piece of high-temperature glass was installed on the ventral behind the ablative material to determine whether the residue would stick to the glass and compromise the transparency of the glass. The ablator residue did cloud the glass, which meant that we would have
to protect the windshield with an eyelid at high speeds to ensure that the pilot could see out the window for landing. Another experiment measured the local flow conditions in the lower ventral area to define the aerodynamic flow environment for the scramjet engine. Good data was obtained on this experiment.

  Due mainly to wet lakebeds resulting from the seasonal rains, the next flight, number 180, did not occur until 6 months later. On that flight, the new canopy eyelid was installed on the left windshield to protect it from ablator residue and a dummy scramjet was installed on the lower ventral. The scramjet was installed to evaluate its effect on aircraft handling qualities and to check out the scramjet jettison system. Ablative material was installed on the ventral behind the inlet spike of the dummy scramjet.

  The flight was launched out of Hidden Hills and was flown as planned. The maximum Mach number of 4.75 and maximum altitude of 97,600 feet were slightly higher than planned, due to an error in the inertial system computer. However, all the flight objectives were achieved. The scramjet had no significant effect on aircraft handling qualities and the eyelid worked as advertised. One significant result of this flight was the complete erosion of the ablator material from the leading edge of the ventral behind the spike of the dummy scramjet. The temperature measuring thermocouples in the ventral failed to work properly again, so we were unable to determine how hot the ventral skin got during the flight.

  Later events would indicate that we used poor judgment in proceeding to high speeds without having temperature data in this region. The missing ablator on the lower ventral was a result of much higher than expected heating in that area due to the presence of the dummy scramjet. Shock waves off the dummy scramjet were impinging on the ventral and lower fuselage and creating local hot spots of extremely high temperatures. We did not appreciate the significance of this clue because we had no thermocouple data to confirm the high heating rates and we continued to expand the flight envelope to higher speeds.