Mission Planner for Heating-Optimal Re-Entry Trajectories with Extended Range Capability

Abstract

Atmospheric entry is a process defined by extremes, from the great velocity at the point of entry to the high deceleration loads and aerodynamic heating in the lower layers of the atmosphere. In addition to the vehicle maintaining structural integrity, an equally important aspect of the re-entry mission is safety. This safety can only be guaranteed if the vehicle is controllable throughout the re-entry process. Mission design plays a role in adhering to operational and safety requirements in the form of trajectory planning: the development of a re-entry trajectory wherein the vehicle remains within its operational constraints and is controllable throughout. The focus of this thesis lies on the design-time trajectory development part of mission design, i.e., the development and analysis of feasible trajectories under certain requirements to be flown for a specific mission. The research goal of this thesis is formulated as follows: to what extent can optimal re-entry trajectories be developed in the design-time phase of mission development for a winged entry vehicle that provide a maximum-range capability under the objective of minimizing heat loads and adhering to operational constraints? To answer this question, a mission design tool is developed in four successive stages: 1) Development of the re-entry simulator, 2) Design of the guidance algorithm, 3) Development of the mission planner, and 4) Mission planner testing. The capacity to quickly and reliably simulate re-entry trajectories is paramount to a mission planner. For this purpose, a simulator was developed with the specific goal of later integration with the mission planner. Steering is achieved by modulating the vehicle’s attitude in terms of its angle of attack and bank angle. The guidance profile is based on the specification of attitude commands at specific points in the trajectory related to the instantaneous energy of the vehicle. The course of an entire trajectory can be specified by its guidance profile. The purpose of the mission planner is to develop guidance profiles for trajectories that keep the vehicle within its operational constraints, minimize the heat load, and provide the largest possible range under these conditions. The objectives of minimum heat load and maximum range are conflicting. Trajectories with minimum heat load requirements are generally short in duration with smooth heat flux profiles where the heat flux is maintained close to its constraint value. The total heat load is minimized by keeping the duration of the re-entry as short as possible. This, however, is in direct opposition to the objective of maximizing range, where keeping the vehicle aloft for as long as possible is beneficial. The mission planner develops trajectories by specifying the individual parameters in this guidance profile by performing multiobjective optimization to determine the combinations of parameters that result in optimal trajectories in terms of the mission objectives. The mission planner consistently provides a set of optimal trajectories over a diverse range of objective values and under the provision that operational constraints are met. Optimal trajectories are determined based on these conflicting objectives over a range of objective values, wherein the relative priorities of the objectives are varied. In all cases is the mission planner able to provide trajectories with an extended- range capability, even when minimizing the heat load is considered the main priority.