Guidance parameters and constraints for controlled atmospheric entry, Vol. I

Abstract

Entry of astronautical vehicles into planetary atmospheres is examined in this thesis with respect to interactions of the guidance function, vehicle performance, trajectory prediction, and mission objectives. All entry missions which originate from planetary reconnaissance orbits are classified into two broad categories: the direct entry profile and the degenerate orbital profile. These classifications are distinguished by the fact that some form of thrust generating mechanism is required to effect controlled entry in the direct entry profile, while engines are not required for controlled entry in the degenerate orbital profile. Approximate analytical solutions of guidance parameters and constraints are derived for both classes of profiles. As used in this thesis, guidance parameters include: (1) Predicted values of distance flown, range, range-to-go, altitude, velocity, time of flight, and specific force level; (2) Sensitivity of the foregoing quantities to errors and characteristics of the vehicle, density characteristics of the atmosphere, magnitude and direction of the engine thrust vector. Constraints are defined as trajectory limitations resulting from: (1) permissible specific force levels of the vehicle and its human occupants; (2) heat flow rates to the skin of the vehicle, and stagnation point temperatures; (3) radiation hazards. A major shortcoming of numerical studies performed in the conceptual and early design phases of astronautical entry systems is that an infinite number of possible trajectories and guidance schemes must be explored. In order that the engineer understand the the effect of changing various design parameters and in order to compare different guidance schemes, simple analytical results, even though only approximate, can be far more informative than a long series of machine computations. The philosophy advanced in this thesis, therefore, emphasizes the use of dynamical approximations, with specified limitations, to derive simple analytical solutions of important guidance quantities. It is shown in this thesis that the trajectories of both the direct and degenerate orbital profiles possess three distinct operational regimes, defined as the Keplerian, Intermediate and Gas-dynamic phases. A parameter, called the Conservation Parameter, is defined to specify precisely the boundary conditions between these operational phases. The Conservation Parameter is an index of the influence of the operating environment on vehicular motion and may be used as a switching function for the guidance computer and as an orbital aid for adaptive control of the vehicle. Three-dimensional dynamical equations of motion are developed for entry of lifting or non-lifting vehicles in banking or wings-level flight with variable thrust capabilities into the atmosphere of oblate, rotating planets with or without atmospheric winds. The figure of the planet, the gravitational model, and the atmospheric model for important bodies of the solar system are summarized. Special problems associated with first-time entry into the atmospheres of strange planets are discussed.