Integrated modeling and dynamics simulation for the next generation space telescope

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1999.Includes bibliographical references (p. 216-220).The Next Generation Space Telescope (NGST) represents a challenging problem from the point of view of maintaining a milli-arcsecond level pointing accuracy and diffraction limited wavefront performance in the presence of dynamic onboard disturbances during science observations. This is due to the fact that NGST will make extensive use of deployable, inflatable and lightweight components, which leads to high modal density and light damping of the structural plant. An integrated model comprising multiple disturbance sources, structures, optics and control systems was developed in order to predict the expected dynamic performance of the observatory in terms of wavefront error and line-of-sight jitter. A simple three degree-of-freedom system is used to motivate the analysis and to develop the equations before applying them to the full order observatory. An initial performance assessment showed that the conceptual design model based on the NASA Goddard "Yardstick" concept for NGST cannot meet performance assuming a set of nominal design parameters stipulated by the author. This performance assessment is carried out using time domain, frequency domain and Lyapunov techniques. Two strategies for dealing with large order models using numerical conditioning and singular value decomposition of the optics linear sensitivity matrices have been demonstrated. A sensitivity analysis revealed which parameters contribute significantly to the root-mean-square errors. This information was subsequently used to recommend performance improvements including stiffening the secondary mirror tower, isolating reaction wheel disturbances and adding passive damping treatments. A repeated performance analysis and time simulations using NASA's non-linear simulation model of NGST showed that performance can be met with comfortable design margins by applying a combination of the suggested redesign actions on the nominal design. This research is motivated by the fact that it will not be possible to test the fully deployed observatory in a 1g environment before launch.by Olivier Ladislas de Weck.S.M

Multivariable isoperformance methodology for precision opto-mechanical systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2001.Includes bibliographical references (p. 277-285).Precision opto-mechanical systems, such as space telescopes, combine structures, optics and controls in order to meet stringent pointing and phasing requirements. In this context a novel approach to the design of complex, multi-disciplinary systems is presented in the form of a multivariable isoperformance methodology. The isoperformance approach first finds a point design within a given topology, which meets the performance requirements with sufficient margins. The performance outputs are then treated as equality constraints and the non-uniqueness of the design space is exploited by trading key disturbance, plant, optics and controls parameters with respect to each other. Three algorithms (branch-and-bound, tangential front following and vector spline approximation) are developed for the bivariate and multivariable problem. The challenges of large order models are addressed by presenting a fast diagonal Lyapunov solver, apriori error bounds for model reduction as well as a governing sensitivity equation for similarity transformed state space realizations. Specific applications developed with this technique are error budgeting and multiobjective design optimization. The goal of the multiobjective design optimization is to achieve a design which is pareto optimal, such that multiple competing objectives can be satisfied within the performance invariant set. Thus, situations are avoided where very costly and hard-to-meet requirements are levied onto one subsystem, while other subsystems hold substantial margins.(cont.) An experimental validation is carried out on the DOLCE laboratory testbed. The testbed allows verification of the predictive capability of the isoperformance technique on models of increasing fidelity. A comparison with experimental results, trading excitation amplitude and payload mass, is demonstrated. The predicted performance contours match the experimental data very well at low excitation levels, typical of the disturbance environment on precision opto-mechanical systems. The relevance of isoperformance to space systems engineering is demonstrated with a comprehensive NEXUS spacecraft dynamics and controls analysis. It is suggested that isoperformance is a useful concept in other fields of engineering science such as crack growth calculations in structures. The isoperformance approach enhances the understanding of complex opto-mechanical systems beyond the local neighborhood of a particular point design.by Olivier L. de Weck.Ph.D

Engineering Design and Rapid Prototyping

This course provides students with an opportunity to conceive, design and implement a product, using rapid protyping methods and computer-aid tools. The first of two phases challenges each student team to meet a set of design requirements and constraints for a structural component. A course of iteration, fabrication, and validation completes this manual design cycle. During the second phase, each team conducts design optimization using structural analysis software, with their phase one prototype as a baseline. Acknowledgments This course is made possible thanks to a grant by the alumni sponsored Teaching and Education Enhancement Program (Class of '51 Fund for Excellence in Education, Class of '55 Fund for Excellence in Teaching, Class of '72 Fund for Educational Innovation). We gratefully acknowledge the financial support.The course was approved by the Undergraduate Committee of the MIT Department of Aeronautics and Astronautics in 2003. We thank Prof. Manuel Martinez-Sanchez and the committee members for their support and suggestions