On my second altitude flight, I missed my intended peak altitude of 220,000 feet by 6,000 feet due to an improper setting of my climb indicator. I was upset by this because I had hit my intended altitude on my first flight almost right on—179,600 feet actual versus 180,000 feet planned. The 6,000-foot error on my second flight seemed huge by comparison. Bikle calmed me down though by reminding me that it was less than a 3 percent error. Our measurement accuracy was not a great deal better than that. In fact, before launch it was not uncommon to have 2,000 or 3,000 feet discrepancies between X-15 pressure altitude and inertial altitude. Radar altitude may or may not agree with either of the two. We kind of had to average everything out.
My next altitude flight was scheduled to be a flight to 265,000 feet, high enough to qualify for astronaut wings. I did not get a chance to make that flight, because I was reassigned full time to the lifting body program. I would not have gotten astronaut wings anyway. NASA did not award astronaut wings to NASA pilots.
SOME MINOR DISTRACTIONS
Joe Engle seemed to have a charmed relationship with the X-15. He flew sixteen flights without any major emergencies, or significant deviations from his planned flight conditions. He always managed to get an engine light on the first try and he made it back to Edwards for landing on every flight. Joe seemed to have more than his share of uneventful flights while other pilots reaped more than their share of emergencies to compensate for Joe’s good luck.
Joe did have his share of stability augmentation system problems. On his sixth flight, a checkout flight in the number three aircraft with the MH-96 system, Joe lost all of his stability augmentation 10 seconds after launch. The flight was an altitude buildup flight for Joe in the number three aircraft to an altitude of 180,000 feet. Joe did not realize that he had lost stability augmentation until after engine burnout during the climb. He was committed to ballistic flight without the essential high-frequency damping required for safe flight outside the atmosphere.
After engine burnout, the aircraft began oscillating rapidly in all three axes reaching peak amplitudes of ±10 degrees in pitch, ±6 degrees in yaw and over 90 degrees in roll. It was a wild ride for 40 seconds during the climb. Joe finally managed to reengage the MH-96 system after several attempts and the oscillations stopped. The MH-96 system remained engaged and operating properly during reentry. Joe had no further problems during the flight.
On Joe’s ninth flight, a checkout flight of the Dyna-Soar inertial system installed in the number one X-15, Joe lost his pitch augmentation 10 seconds after launch. The flight was a relatively low-altitude flight to 110,000 feet altitude, so pitch damping was not critical. The damper appeared to reengage when Joe reset the switch during boost but it did not seem to work properly because Joe had serious controllability problems during a planned pullup to high angle of attack after engine burnout. The remainder of the flight was uneventful.
Joe’s most noteworthy flight was his fifteenth flight. The flight was a planned altitude flight to 266,000 feet, in the number three aircraft. On that flight, the yaw damper failed immediately after launch. Joe managed to reset the damper and continued flying the profile. Mission rules dictated that the pilot revert to an alternate, low altitude profile if the yaw damper failed within the first 32 seconds after launch. Joe did not feel obligated to fly the alternate profile since the damper reset satisfactorily. However, the yaw damper failed again, 19 seconds later, and again, 10 seconds later. It failed three times within the critical 32 seconds after launch. Joe managed to reset it each time, but the system was trying to tell Joe something. Joe should have reverted to the alternate profile.
During the last of these resetting attempts, he inadvertently engaged the control-stick-steering control mode. This control mode was engaged while Joe was still pulling up into his climb. The control system logic then assumed that Joe wanted to continue the pullup, so it began to apply more nose up control. During the next 49 seconds prior to engine burnout, the yaw damper failed three more times. Joe reset it each time, but now he was also having to fight the tendency of the control system to continually pull the nose up.
At burnout, he was committed to exoatmospheric flight to more than 50 miles altitude. The yaw damper was quite critical for a successful reentry from that altitude. Without it, he could lose control and tumble or spin back into the atmosphere. During exoatmospheric flight, the yaw damper failed nine more times. Joe reset the damper each time within a second and a half of the failure, indicating that he was keeping one hand on the damper switch as he flew the aircraft with the other hand. Joe managed to keep the airplane at the desired pitch attitude while fighting the nose-up command of the autopilot. All of this had to be horribly distracting, but Joe seemed to take it all in stride. He did not report each of the yaw damper failures, only the first one. He did comment on the apparent out of trim condition in pitch during exoatmospheric flight. The control room received indications of each yaw damper failure in the telemetered data, but since Joe did not confirm the failures, they assumed that the data were erroneous.
The yaw damper failed a total of twenty-one times during the 10-minute flight. In the postflight debriefing, Joe did not consider the damper failures to be a real problem but rather a distraction. In the flight report, the flight planner enumerated and described all of the system failures and discussed the piloting problems created by the inadvertent activation of the control-stick-steering mode. He summarized his discussion by saying, “It is difficult to understand why no pilot comments were made during the flight and very few made during the postflight [de]briefing.” Joe apparently did not want to abort the planned flight just because of a damper failure. He could be quite stubborn at times. For some pilots, that flight could have been very suspenseful. For Joe, it was uneventful with only minor distractions.
Joe was lucky that he encountered this kind of a problem while flying the number three aircraft. Aerodynamic and reaction controls were combined in one control stick. If he had encountered this problem in one of the other aircraft, he would have been flying with both hands and would not have had a free hand to re-engage the damper. That could have been potentially catastrophic.
On Joe’s last X-15 flight, he had yaw damper failures at launch and during reentry from an altitude in excess of 50 miles. The airplane oscillated wildly in yaw during the entry, but Joe completed the pullout successfully. By this time, Joe had become an expert, by necessity, on damper off controllability. He was ready for something more challenging, like flying the space shuttle.
A SLIGHT OVERSHOOT
On the first day of November 1966, Bill Dana in the number three X-15, was carried aloft on the B-52 to make an altitude flight to 267,000 feet. The purpose of the flight was to collect micrometeorites, measure wing tip pod accelerations, measure the optical background at altitude using a dual channel radiometer, check out the precision attitude indicator, check out the Alert computer, measure sky brightness, and evaluate a new cockpit display panel. This was the 174th flight of the program. At this late stage of the program the X-15s were carrying a significant number of experiments on each flight.
The flight had been delayed over a month due to wet lakebeds, weather, lack of a C-130 support aircraft, several system problems, and having repeated aircraft and engine functional checks due to the long delay before flight. The prelaunch checklist proceeded without any major problems, and the launch occurred on schedule.
Bill got the engine lit on the first try and then pulled up to establish his climb angle of 39 degrees. He reached his climb angle slightly early, but his cross-checks seemed to indicate that he was on his planned profile. Bill was not reading NASA-1 on the radio during the early portion of the climb and thus did not receive any altitude checks for verification of his climb profile. Post-flight checks indicated that he was climbing at a steeper angle than planned due to an error in the climb attitude indicator. He was climbing at 42 degrees rather than 39 degrees.
When Bill finally began receiving NASA-1 as he cl
imbed through 110,000 feet altitude, NASA-1 did not alert him to any error in his profile. In fact, at the time that Bill shut the engine down, NASA-1 indicated that Bill’s track and profile were looking very good. Postflight analysis would indicate that Bill was high on energy at engine shutdown and would overshoot his planned peak altitude. The engine burned longer than planned, so Bill shut it down about a second later than the predicted burnout time. This extra engine burn time significantly increased the impending altitude overshoot.
As Bill climbed through 230,000 feet altitude, he got the first confirmation that he was going to go high on peak altitude. NASA-1 said, “Roger, and we have you going a little high on profile. Outside of that, it looks good.” Several seconds later NASA-1 said, “Rog, we got you going through 280,000 now.” Dana responded, “How about that.” At this time, Dana was already 13,000 feet higher than his planned peak altitude and he was still climbing.
NASA-1 called again to say, “Right on track, Bill. Looking real good. Track is real good. We have you peaking out around 310,000 feet.” Dana responded, “OK. How many alpha would you like on re-entry?” NASA-1 replied, “Twenty-three degrees.” Dana said, “OK.” NASA-1 then said, “Track looks real good. You’re going to be in good shape for Eddy.” Dana responded, “Roger,” and then asked if Jack McKay was sending his congratulations. About a year earlier, Bill had been the controller in NASA-1 on one of Jack McKay’s flights. Jack had overshot his planned altitude by over 30,000 feet due to an accumulation of small variations in climb angle, engine burn time, and engine thrust. Bill really bugged Jack about that overshoot in altitude. In fact, for a whole year he needled Jack about that gross altitude error. Bill was merciless at times.
Now it was Jack’s turn. Bill had overshot his planned altitude by almost 40,000 feet. Forty thousand feet! Most airplanes could not even climb 40,000 feet, let alone overshoot by that much. I could just imagine the conversation if Bill Dana had to report the altitude overshoot to an FAA controller. It would have gone something like this:
Dana: “Las Vegas Control, this is NASA 672.”
FAA: “Roger, NASA 672, this is Las Vegas Control, go ahead.”
Dana: “This is NASA 672. I’d like to report that I overshot my altitude a little bit.”
FAA: “Roger, NASA 672. It doesn’t appear to be a problem right now. We have no conflicting traffic in your area within 5,000 feet of your assigned altitude. By the way, how much did you overshoot?”
Dana: “Forty thousand feet.”
FAA: “Oh.”
On this flight, Dana established an almost unbeatable world overshoot record for an airplane. To add insult to injury, as Dana shut the engine down, he apparently bumped his kneepad with his arm and released the clip holding the checklist pages to his kneepad. All of a sudden he had twenty-two pages of checklist floating around the cockpit during the entire time that he was outside the atmosphere at 0 g. He could not see his cockpit instruments without brushing away the checklist pages. Bill commented after the flight that it was “like trying to read Shakespeare [while] sitting under a maple tree in October in a high wind.”
Bill managed to make a very nice reentry and then proceeded to make an uneventful glide back to Edwards and a nice landing. He had to have some help from NASA-1 to complete the post landing checklist because he could not find the right page of his checklist. It was somewhere down on the floor of the cockpit.
This altitude overshoot did not really compromise the objectives of the flight. In fact, the overshoot actually enhanced the data obtained on some of the experiments. When precise altitude was required to achieve the flight objectives, Bill showed that he could hit the desired altitude precisely. A cold-wall experiment to measure aerodynamic heating rate required a very precise altitude and Mach number combination. Bill was scheduled to fly that experiment on his next flight.
This experiment was an attempt to measure aerodynamic heating rate at a specific Mach number. A skin panel on the upper vertical fin was instrumented to measure temperature, skin friction and static pressure at various locations on the panel. A cover was then fastened over this panel to prevent the airstream from impinging on the skin panel. This cover would keep the skin panel cool during the climb and acceleration to the desired Mach number. At the specified Mach number the panel was blown off and the instantaneous heating rate and skin friction were measured as the airstream impinged on the instrument panel.
On the first attempt to conduct this experiment, Bill had to make an emergency landing at the launch lake due to a low fuel line pressure indication. On the second attempt, Bill came within 100 feet of the desired altitude and was right on the desired Mach number. It was a beautiful data point. As mentioned earlier, these heating flights were extremely tough to fly precisely. They were a real challenge.
Bill was somewhat surprised when he shut the engine down on this flight. It was his first low altitude, heating flight. He said, “Then I got my surprise for the flight and that was that I was reading my heading indicator from a distance of 2 inches.” The aircraft drag at this flight condition was extremely high and when the engine was shutdown Bill was thrown violently forward into his shoulder straps, due to the rapid deceleration. An unanticipated problem surfaced when the panel was blown off. The rudder oscillated violently for several cycles in response to the detonation of the explosive bolts.
The next flight was planned to a lower dynamic pressure to ensure that no structural damage would occur as a result of this problem. On that flight, Bill again nailed the desired flight conditions, coming within a few feet of the intended altitude. When the chips were down, Bill came through. Bill will have to work on his statistics, though. It will take a lot of precise flying to compensate for that 40,000-foot overshoot.
Chapter 7
Results and Unanticipated Problems
Flight research is conducted for a number of reasons. One reason is to obtain data that cannot be obtained by any other means. The X-1 flight program was an excellent example of this. The X-1 was developed and flown to obtain data that existing wind tunnels were incapable of providing—transonic and supersonic aerodynamic data. Another reason to do flight research is to validate the results obtained from wind tunnels, computational fluid dynamics, or other predictive techniques. There have been many examples in this area. Two that come to mind are the supercritical wing and the forward swept wing programs. The wind tunnels predicted the benefits of these two new innovations but, because of their somewhat radical nature, flight was deemed necessary for confirmation.
Still another reason for flight research is to stimulate the application of new technology. In this case, a flight demonstration can be worth a thousand paper studies. A good example of this type of flight research is fly-by-wire flight controls. There was no real question that fly-by-wire controls were feasible, but no one was willing to design an airplane with such a system until a flight program verified the practicality and reliability of fly-by-wire controls. A more basic reason to do flight research is “to separate the real from the imagined problems and to make known the overlooked and the unexpected problems,” according to Dr. Hugh L. Dryden.
At the time that the X-15 was developed, there were ground facilities capable of providing good hypersonic data. These facilities were lacking in some respects, but they were adequate to design a research aircraft. With this predictive design capability, the X-15 would not be probing the unknown like the X-1. It would instead be validating the results obtained from these new facilities which had a limited track record. There were some areas of great concern when the aircraft began its flight program, but there were no areas completely void of information.
The flight program provided the following results. The basic aircraft stability and control characteristics measured in flight compared quite well with predictions. There were no surprises or problems in this disciplinary area. The X-15 flight results confirmed that existing wind tunnels were capable of accurately predicting the stability and control characteristics
of hypersonic flight vehicles. The excellent agreement between wind tunnel and flight results obviated the need for another, higher-speed, research aircraft since hypersonic aerodynamics do not change significantly from Mach 6 to orbital speeds of Mach 25. And, in fact, the space shuttle was designed and successfully flown based primarily on the credibility of the results obtained by the X-15.
Without the lower ventral fin, the X-15 had some limitations such as low directional stability at moderate angles of attack at subsonic speeds. Large speed brake deflections further reduced directional stability at these conditions. Overall, however, the stability and control characteristics were completely adequate throughout the flight envelope.
The augmented handling qualities of the aircraft within the atmosphere were generally very good—equivalent to those of the best fighter aircraft. A pilot-induced oscillation was encountered on the first flight due to inadequate actuator rate limits on the horizontal stabilizer, but once these rates were increased, there were no further PIO problems.
The major handling quality problem unaugmented was a lateral-directional divergence at moderate to high angles of attack at high Mach numbers. This problem was significant because it compromised the pilot’s ability to make a successful atmosphere entry if the augmentation system should fail during the attempt. This problem was a result of the negative dihedral resulting from the extended lower ventral fin. Removal of the lower ventral fin eliminated this problem and the latter half of the flight program was flown without the lower ventral. Directional stability was still positive and adequate at high Mach numbers without this segment of the ventral.
Exoatmospherically, the major handling quality problem was the requirement to fly using both hands during the transition from atmospheric to exoatmospheric flight and vice versa. The X-15 had one controller, the right-hand, for aerodynamic controls and another, the left-hand, for reaction controls. This was aggravated by the fact that there was no static stability so to speak during exoatmospheric flight. The pilot had to manually counter any induced aircraft motion. There was some rate damping in the reaction control system, but it was not as crisp as it might have been to convince the pilot that the aircraft was indeed stable. The threshold at which the rate reaction control system triggered was large enough to allow the aircraft to drift off the desired attitude at a disturbing rate causing additional pilot anxiety. The solution would have been to add an attitude hold feature to the system to lock the aircraft attitude except during control inputs. This type of system was available in the MH-96 flight control system and it proved to be very good. The MH-96 system also blended both aerodynamic and reaction controls on one controller which significantly improved the exoatmospheric handling qualities. The MH-96 system also provided increased damping at low dynamic pressures due to its higher gain capability. Overall, the MH-96 system provided much better handling qualities during the transition from atmospheric to exoatmospheric flight and back again.
At the Edge of Space Page 27