Qualitative Differences Between Shuttle On-Orbit and Transition Control

Abstract

Two separate reaction control system digital autopilots evolved from one original Space Shuttle orbital autopilot concept. A computer overload forced this evolution. Part of the overload problem was due to unique performance requirements imposed on the reaction control system during each of several different flight regimes. The two resultant reaction control system digital autopilots yield different effector responses because they rely on different sources of sensory input and they process data differently. This paper describes the evolution of the two and illustrates their behavioral differences. The transition autopilot, used in orbital insertion and deorbit, is sensitive to Orbiter flexure due to its feedthrough character. The on-orbit autopilot is sensitive to transient rate control degradation from large disturbances due to feed-forward rate estimation. Simulation results and flight data are used to illustrate performance differences between the two autopilots under various conditions. These include computer failures where electronic stringing and procedural reconfiguration differences affect autopilot behavior.