Real-Time Orbiter Abort Guidance

This paper describes a real-time abort guidance algorithm which determines the time sequence of the powered maneuvers and the orientation of the thrust vector throughout an abort-mission action initiated during the orbiter ascent phase. It involves guiding a heavily loaded Space Shuttle vehicle, passing through severe environmental conditions, back to a designated landing area. A graphical example and estimates of the computer requirements are included. Introduction The function of a real-time guidance scheme for an orbiting space vehicle, or orbiter, is to compute guidance commands that define a trajectory profile from a given (existing) orbiter state to a state determined by certain desired end conditions. Since the orbiter state, especially in the abort mode, is continually perturbed by effects that cannot be incorporated in the guidance model, any real-time guidance scheme requires the ability to take into account all such variations and still produce a satisfactory trajectory profile. An “abort” is defined as the premature or abnormal termination of a mission as a result of either a system or a human failure which, however, does not incapacitate the vehicle or its control system. For example, suppose that after launching the Shuttle it is discovered that a support system or a docking port on the in-orbit space laboratory has malfunctioned, or that an astronaut has suddenly become ill. In contrast with previous space vehicles that have been recovered, postmission, by a pre-planned ballistic-trajectory ocean drop, the Space Shuttle is a winged, guidable, and reusable vehicle. Thus, there is good reason to consider the possible aborting of a mission without assuming an intrinsic malfunction of the Shuttle itself, and there i’s considerable potential and motivation for the undamaged recovery of the vehicle in such a case. Among all the flight phases of the Space Shuttle vehicle, abort guidance during the ascent phase is the most critical because it involves guiding a heavily loaded vehicle that is passing through severe environmental conditions [ I ] . This paper describes a predictive guidance algorithm for the the Space Shuttle, which is intended to return the orbiter to the launch site when its nominal mission is aborted during the ascent phase. The predictive guidance algorithm uses knowledge of the current orbiter state and the desired terminal state, in relation to the landing site, and determines the time sequence of the powered maneuver and the orientation of the thrust vector throughout the abort mode. For example, the magnitude of the down-range component of acceleration, the time at which to begin to return toward the desired landing site, and the radial acceleration necessary to attain the desired terminal altitude (while remaining in the low dynamic-pressure environment) are computed. The key formulations in the predictive guidance algorithm are the analytical expressions for the terminal state that the vehicle will reach for a given sequence of thrust orientations, along with the associated Jacobian matrix; the set of equations and associated definition of symbols may be obtained from the author. In the terminal phase of the abort mode, the predictive guidance reduces to a velocity-to-be-attained scheme. This has the desirable effect of reducing those deviations in the end-state prediction which result from variations and uncertainties in the vehicle characteristics. Graphical results for a representative abort case and estimates of the computer requirements are also included in this paper.