V. S. Sohoni

Peer Reviewed

Abstract: Abort is defined as a premature or abnormal termination of a mission as a result of either a system or human failure. The abort guidance during the ascent phase, among all flight phases of the shuttle vehicle, is most critical; because it involves guiding a heavily loaded shuttle vehicle that is passing through severe environmental conditions. This paper describes a predictive guidance algorithm for the abort mode initiated during the orbiter ascent flight phase. Contrary to the concept of the minimization of fuel consumption during a nominal ascent, flight phase, the Space Shuttle orbiter is required to deplete its fuel tank in order to be separated from the vehicle during the abort mode. The guidance objective during the abort mode is to attain a proper terminal energy state in relation to the landing site, consistent with the aerodynamic heating and loading constraints. In the absence of a well-defined payoff function for optimization during this mode the abort guidance algorithm presented in this paper is formulated to minimize aerodynamic forces. Minimization of aerodynamic forces was considered because of the possibility that the failure that caused the initiation of the abort mode may have weakened the orbiter structure. Immediately after the abort mode initiation, consistent with the philosophy of the minimization of the aerodynamic forces, the orbiter is forced to fly a lifting trajectory to attain higher altitude, thereby reaching a low dynamic pressure environment. The predictive guidance algorithm uses the 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 the 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 analytical expressions for the terminal state that the vehicle will reach for a given sequence of thrust orientations, and the associated Jacobian matrix. Therefore, these have been carefully developed in the paper. In the terminal phase of the abort mode, the predictive guidance reduces to a velocity to be gained scheme which has the desirable effect of reducing the deviations in the end-state prediction that results from variations and uncertainties in the vehicle characteristics. Detailed results for representative abort cases and estimates of the computer requirements are also included in the paper.
Published in: NAVIGATION: Journal of the Institute of Navigation, Volume 20, Number 3
Pages: 252 - 261
Cite this article: Sohoni, V. S., "ORBITER ABORT GUIDANCE", NAVIGATION: Journal of The Institute of Navigation, Vol. 20, No. 3, Fall 1973, pp. 252-261.
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