In this paper, the tangent guidance law, which is derived from optimal control methods, was considered with a closed-form solution approached through simplification of the equations of motion by expanding the thrust acceleration and gravitational acceleration, and the cosine of the vertical attitude angle to polynomials. The coefficients of the closed-form solution were solved from the boundary conditions. It has been shown that the developed closed form solution is a good match to the numerical solution through comparison with numerical results. It is noted that the closed-form nature of the derived solution makes it suitable for on-board implementation within the guidance system. Subsequently, a robust DAC (direct adaptive control) law was presented which is proved to be convergent using Lyapunov stability theory, thus overcoming the drawback of convergence in the tangent guidance law. Furthermore, the DAC law is shown to be resilient to variations in the available thrust level. This resilience, in combination with on-board implementation of the derived high-accuracy closed-form polynomial solution within the guidance system introduces and enables the concept of embedded autonomy to mission critical control systems as traditional on-board monitoring systems are made redundant. A sample lunar mission scenario was presented in which initial navigation error and thruster failure were considered. Numerical results showed that both fixed gain and DAC were capable of tracking a required profile if there was enough design margin for the thrusters. Furthermore, DAC exhibits fast transient response and stability, thus if thruster failure occurs near the terminal time the lander is shown to crash under FGC methods, but survive under DAC methods. As such, DAC increases the probability of lander survival while simplifying system level failure, detection, isolation and recovery requirements through the implementation of autonomy in the control loop.
Analytical landing trajectories for embedded autonomy
Analytische Landungsbahnen mit umfassender Autonomie
2010
15 Seiten, 9 Bilder, 6 Tabellen, 23 Quellen
Article (Journal)
English
Raumfahrttechnik , Landeverfahren , Randbedingung , Bewegungsgleichung , Schubkraft , Raketentriebwerk , Gravitationskraft , numerische Lösung , Führungssystem , adaptive Steuerung , Lyapunow-Methode , Unabhängigkeit , Bordelektronik , Beobachtungssystem , Navigation , Versagensart , Recovery-Management
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