Adaptive Control on the X15

Nothing pressed issues of control more than the innovative adaptive control system installed in the third X-15. In 1958, engineers at Minneapolis Honeywell, a leading manufacturer of control systems, built an analog computer that ran a model of a desired aircraft's response and handling qualities, and then programmed the computer to adapt as necessary to match the model. The analog computer literally ran a simulation of the ideal aircraft while installed in the real one. For the X-15, Honeywell's system could use both the reaction controls and the aerodynamic surfaces, thus combining the two separate flight regimes into one, and allowing the pilot to fly the reentry with a single control stick. The adaptive system could fly ''outer loop'' functions like holding a particular pitch angle or angle of attack (useful for reentry) or moving at particular angular rates in pitch or roll. The computer could theoretically give the X-15 ideal handling qualities under a wide variety of flight regimes.

In 1960, before it flew, the third X-15 blew up during an engine test on the ground, propelling pilot Scott Crossfield out of the aircraft but leaving him uninjured. NASA sent the vehicle back to North American for repair, and took advantage of the overhaul to install the Honeywell adaptive controller, dubbed the MH-96.

The MH-96 contained within it an analog computer running a simulation of the X-15, the very airplane it was supposed to be flying. This model represented an ideal X-15 under ideal conditions. When the pilot commanded the stick to move the airplane, he actually commanded the electronic model. Then a feedback control system, ''the fighting heart of the adaptive system,'' gave whatever commands were necessary to the airplane itself to make it respond like the idealized model.33

The MH-96 could cause the ''real'' X-15 to fly like an ''ideal'' one, which would make it behave exactly the same under all flight conditions, from the vacuum of space right down to the ground. It would automatically mix the reaction controls and aerodynamic controls, so the pilot only needed one control stick, whether flying in the atmosphere or in space, or during reentry. Also included were several autopilot modes: the pilot could command the MH-96 to hold the X-15 constant in roll, pitch, or yaw, as well as to keep a constant angle of attack, which was most useful during reentry. Rather than fighting with three different control sticks, the pilot could usually fly by twiddling a few knobs. It seemed the ideal solution to extend the airmen's manual skills to the vacuum of space.

In 1961, with this system installed aboard the third and reconstructed X-15, Neil Armstrong, who among the active test pilots had come to be something of a specialist in control systems, flew the first three flights. On the first MH-96 flight (the forty-sixth X-15 flight overall), on December 20, 1961, at a conservative Mach 3.76 and 81,000 feet, the system had a variety of problems as the axes switched in and out, and Armstrong landed with the adaptive system engaged only in roll. Still, he flew the entire flight with a single side-stick controller. By the third test, he got the vehicle up to 180,000 feet, and noticed that the controller was holding the aircraft unusually stable. ''At this time, I took a time out to look around and see out the window,'' said Armstrong, which he had barely been able to do before.34 The adaptive control system relieved him of enough workload that he could enjoy his environment. He then flew the entire approach and landing with the side stick.

By the fourth flight, things were well tuned up and Armstrong was reporting to the ground: ''The reaction control damping is exceptionally good. It flies as good as the air plane does on aerodynamic controls at low altitudes.'' He took the X-15 to Mach 5.3 and above 200,000 feet. While Armstrong was concentrating on the controls and evaluating their various modes, however, ''I did not properly appreciate the altitude I was at... which caused me to go sailing merrily by the [landing] field.'' Upon reentry, he bounced off the atmosphere and reentered far down range. Armstrong banked to turn, but he was still high enough that the wings were ineffective, so the vehicle continued going straight;gradually he dropped the nose to gain speed, and the aircraft banked. Finally, the wings bit the air and he entered a supersonic turn, far south of Edwards, nearly to Pasadena. After the overshot turn, Armstrong just made it back to the Edwards lake bed. ''Handling qualities during the flare were considered to be less desirable than on previous similar approaches,'' he reported. At more than twelve minutes, this was the longest ever (in endurance and distance) X-15 flight. It would be Armstrong's second-to-last before he was selected to join the astronaut corps.35

Despite this unusual close call, the pilots built up confidence in the MH-96 and preferred it to control for the high-altitude flights. ''In theory, the pilots should have been completely satisfied. In real life, however, this was not necessarily true,'' Armstrong wrote, for the pilots had reservations.36 For them, the MH-96 presented an unusual situation. Thompson found it ''somewhat unnerving'' to fly, because ''some electrons were now moving the control system without any inputs from the pilot. . . . As a pilot, you hope the guy who designed this electronic control system knew what he was doing. In fact, you would like him to be in the airplane with you to be exposed to any adverse results.''37 Some pilots didn't like to use the pitch or angle-of-attack hold modes during reentry, because in case of an emergency, they preferred to be actively in the loop and ready to respond.38 Said Thompson: ''Such [automatic] modes can greatly reduce the pilot's concentration and workload, but this can boomerang.''

By automatically making the aircraft's response invariant, the adaptive system could mask the effect of changes. Normally, when the nose goes up, the airspeed decreases, for example, but not on the X-15 with adaptive control. Thus ''the pilot has the impression that the aircraft does not want to land. . . . The aircraft feels completely solid to the pilot right up to the point at which loss of control occurs.''39

On one flight, Thompson felt the adaptive autopilot go ''berserk.'' The stabilizers began limit-cycling (a kind of oscillation), at Mach 5.5. ''The aircraft was essentially out of control'' in at least two of its three axes, he reported dryly. ''It was quite a ride.'' The problem would come back to bite them in a fatal crash.

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