PAGE CONTENTS
Objectives
The objective of the study was to develop a software tool for the analysis and optimization of high-fidelity transfer trajectories from launch orbit to GEO for spacecraft employing electric propulsion.
This activity takes a further step towards maturing such software for optimized electric propulsion manoeuvre planning and attitude guidance in order to prepare for support of actual spacecraft transfer planning.
Consequently, it has been a key objective of the activity to have proper validation of the optimization as well as the analysis features of the software and testing on commercially relevant telecom spacecraft scenarios.
SYSTEM ARCHITECTURE
As central hub of the LOTOS tool acts the GESOP (Graphical Environment for Simulation and OPtimization) core software which interfaces with the LOTOS model library and the optimizers to provide the required data flow. Some software main functionalities are handled by the GESOP core as well as many interactions to the user. Access to all software modes and functionalities is given via the graphical user interface and the command line interface.
The LOTOS model library computes the orbit transfers and all related data. It reads all scenario data defined by the user and stores it internally. With this data the orbit transfer trajectory dynamics can be computed as well as many additional outputs. Besides, the model library also computes and evaluates each user defined cost term (objective function) and constraint, which are both part of the optimal control problem. The model library of the LOTOS tool contains all relevant functions to provide mathematical computations, frame transformations, and all aerospace related features required for a very sophisticated evaluation and computation of electric orbit raisings and hybrid transfers under consideration of a comprehensive set of constraints.
Challenges
One aim of the project was the support of hybrid orbit transfers, a chemical orbit-raising followed by an electric orbit-raising. It had to be implemented within the already existing architecture of the software, which up to then only supported low-thrust orbit transfers. Further, the computation of the electric orbit-raising had to be solved without involving a user controlled optimization process.
Another challenge was the implementation of a software feature to support ground operations and spacecraft autonomy by means of re-optimization of reference trajectories with updated initial state. For example, orbit determination is providing an updated spacecraft state which is then combined with the pre-computed reference trajectory and attitude profile.
For more precise refinements of eclipses and thrust/coast arcs, for example, the product has to support the exact evaluation of begin and end of an eclipse event. These conditions are met by additional constraints added to the optimal control problem.
Plan
The project lasted 18 months with the following milestones:
Kick-off (KO): Project start
Software Requirements Review (SRR): All software requirements are consolidated.
Preliminary Design Review (PDR): A preliminary architectural design of the software is elaborated.
Critical Design Review (CDR): The design process is completed and the software tool is fully developed.
Acceptance Review (AR): All verification and validation tests have been successfully completed and the report finalized.
Current Status
The project has been completed and the product is available as COTS software LOTOS (Low-thrust Orbit transfer Trajectory Optimization Software).
