Digital control laws for the space shuttle orbiter

The purpose of this paper is to describe the Space Shuttle Orbiter manual command augmentation (CAS) control law. The paper addresses the configuration as presently baselined for the approach/land test (ALT) program. The requirements and problems associated with the ALT CAS control mode as well as the design/analysis approach used with it are representative of those associated with the operational atmospheric flight. The pertinent stability and performance requirements are summarized from the specifications. These considerations then lead to the selection of feedbacks, gains and filters where the last opens up the question of digital filter design. Special features of the resultant control laws include provision for command signal limiting (CSL) and the mechanization of the pitch axis proportional plus integral control. Digital filter design techniques used in obtaining bending filters differ from those used in obtaining lower frequency filters for rigid body control. These techniques and the rationale for their choice are summarized.