An initial guess generator for launch and reentry vehicle trajectory optimization

Guidance laws are presented for three different kinds of aerospace vehicles: rocket launchers, air-breathing launchers and reentry vehicles. The guidance laws allow to describe a control history close to the optimum with a small number of parameters. Therefore these guidance laws can be used for optimization in their own right. The resulting savings of a huge number of discretization parameters reduces optimization time considerably. For conventional launchers, the guidance laws are implemented as controls of yaw and pitch. Two simple guidance laws for the yaw angle are available: one is to set it equal to the heading angle. The second option uses the inclination of a target orbit and sets the yaw to the heading of this orbit. For the vertical guidance, a sequence of guidance strategies is provided: a push-over maneuver with optimizable pitch rate and a subsequent gravity turn. Outside the dense atmosphere, the required velocity concept by Battin and a bi-linear tangent law based optimal control theory are available. For air breathing launcher a guidance concept is presented that is based on the dynamic pressure, which replaces the variables of the vertical motion. Altitude and flight path angle are considered 'fast' as compared to the variables of the horizontal motion and removing them not only reduces the size of the state vector, but also reduces the number of integration steps needed. Also for reentry vehicle a dynamic pressure control is proposed. In this way the relation of the most important path constraints are simplified and thus optimization time reduced. Diese Arbeit prasentiert Lenkgesetze fur drei verschiedene Arten von Raumfahrzeugen: Raketen, luftatmende Fahrzeuge und Wiedereintrittsfahrzeuge. Diese Lenkgesetze erlauben es, einen fast-optimalen Verlauf der Steuerungen mit wenigen Parametern zu beschreiben. Deshalb konnen diese Lenkgesetze auch selbst bei der Optimierung verwendet werden. Durch die Einsparung einer grosen Anzahl an Diskretisierungsparametern wird die Optimierungszeit wesentlich beschleunigt. Fur Raketentrager wurden zwei einfache Lenkgesetze fur den Gierwinkel implementiert: eines tangential zur augenblicklichen Flugbahn, das andere setzt den Gierwinkel gleich dem Azimuth einer Flugbahn mit vorgegebener Inklination. Sobald die Startrampe verlassen wird, wird ein Push-Over Maneuver mit optimierbarer Nickrate eingeleitet, mit anschliesenden Gravity-Turn. Auserhalb der dichten Atmosphare stehen das Required-Velocity-Konzept von Battin und das Bi-Linear-Tangent zur Verfugung. Fur luftatmende Trager wird ein Lenkgesetz auf Grundlage des Staudrucks vorgeschlagen. Da diese Variable proportional zur Luftdichte ist, kann mit dieser Steuerung die Vertikalbewegung ersetzt werden. Hohe und Flugbahnneigungswinkel sind 'schnelle' Grosen verglichen mit der Horizontalbewegung. Durch ihre Umwandlung in eine Steuerung wird nicht nur der Zustandsvektor verkurzt, sondern auch die Anzahl der notigen Integrationsschritte wird verringert. Auch fur Wiedereintrittsfahrzeuge wird die Staudrucklenkung vorgeschlagen. Diese Art der Steuerung vereinfacht die Darstellung der wesentlichen Pfadbeschrankungen und erleichtert so die Optimierung.

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