Analysis of the X-15 Flight 3-65-97 Divergent Limit-Cycle Oscillation

A brief history of the X-15-3 adaptive control system and an analysis of the destructive limit-cycle oscillation that occurred during its final November 1967 flight are presented. The X-15 was a piloted single-seat rocket-propelled hypersonic research aircraft operated by the NASA Flight Research Center from 1959 until 1968. Due to the limited information previously available in the public domain and the 1968 decision by the accident investigation board to forego detailed analysis of the adaptive control system's role in the accident, it was widely assumed in the adaptive controls community that an anomalous behavior of the adaptive component caused the loss of control. Notwithstanding the complex human factors and subsystem failures that contributed to the accident, it is shown that the adaptation dynamics were not a causal factor. The limit cycle observed in the flight data is reproduced in a nonlinear time-domain simulation. Describing function analysis reveals that the instability was caused by a latent design error in the inner-loop structural filters that did not account for the nonlinear behavior of the X-15 servoactuator under rate saturation when coupled with the lightly damped aircraft longitudinal mode at high Mach numbers.