Divert capability analysis and subsequent guidance design for mars landing

Defined as the largest horizontal distance that the lander can cover during the powered descent phase, the divert capability determines whether the lander can reach the distant landing target safely from the parachute jettison point. In this paper, previously unrevealed engineering constraints on divert capability are revealed, including initial powered descent conditions, path constraint, thrust bounds, and residual fuel amount. Next, methods to obtain the divert capability using different guidance laws under engineering constraints are developed. For open-loop guidance based on trajectory optimization, since the former three constraints are considered in the optimization problem, the optimized largest horizontal distance allowed by the residual fuel is taken as the divert capability. For explicit closed-loop guidance, an iterative searching algorithm is developed to obtain the divert capability while satisfying all constraints. The divert capability under various initial conditions are obtained. The coupled effect of all constraints on divert capability are analyzed. The divert capability can be improved by increasing residual fuel or thrust bounds. Thereafter, the guidance parameter design based on divert capability analysis is studied. It's found that the flight time is a key parameter in the performance tradebetween divert capability and fuel consumption. Therefore, two methods for selecting flight time are proposed. One is to obtain the maximum divert capability with a limited residual fuel amount, and anther is to minimize the fuel consumption while satisfying the divert capability requirement. Finally, the expected flight time for two mission objectivesare calculated and depicted.