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Space Systems Design Laboratory (SSDL)
Space Systems Design Laboratory (SSDL)
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ItemRapid Simultaneous Hypersonic Aerodynamic and Trajectory Optimization Using Variational Methods(Georgia Institute of Technology, 2011-08) Grant, Michael J. ; Clark, Ian G. ; Braun, Robert D.Traditional multidisciplinary design optimization methodologies of hypersonic missions often employ population-based global searching methods that rely on shooting methods to perform trajectory optimization. In this investigation, a rapid simultaneous hypersonic aerodynamic and trajectory optimization methodology is constructed based on variational methods. This methodology is constructed from two enabling advancements in analytic hypersonic aerodynamics and rapid trajectory optimization. Comparisons made with a single and multi-objective particle swarm optimizer highlight the computational advantages and improved solutions obtained through continuation of variational methods. The incorporation of trajectory constraints into the particle swarm optimization process through penalty functions or as additional objectives is shown to greatly increase the complexity of the design process. Alternatively, variational methods are able to precisely satisfy trajectory constraints without this added complexity. Examples demonstrate that Pareto frontiers in both vehicle and trajectory objectives can be constructed using variational methods. For convex frontiers, this is performed using a weighted sum of the objectives. For non- convex frontiers, the optimization is performed through continuation of a set of constrained objectives.
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ItemRapid Design Space Exploration for Conceptual Design of Hypersonic Missions(Georgia Institute of Technology, 2011-08) Grant, Michael J. ; Clark, Ian G. ; Braun, Robert D.During conceptual design, multidisciplinary optimization is often performed using computationally intensive direct methods. Prior work has shown that rapid design studies can be performed using fast indirect methods, but several optimization techniques including discrete dynamic programming, nonlinear inversion, and pseudo spectral methods are required to construct a suitable initial guess within the design space. In this investigation, a simplified methodology is developed to eliminate these optimization techniques, enabling rapid design space exploration using continuation of indirect methods alone. This is made possible by initially converging to a simple solution that is outside of the design space of interest, and solutions within the design space of interest are quickly accessed through continuation from this initial solution. As an initial step to automate this continuation process, state transition tensors are used to predict optimal solutions throughout the design space. A methodology is developed to provide accurate predictions of trajectories with varying flight times, and the error of these predictions is controlled to regulate the continuation process. This approach provides flexibility to adapt to future computational capabilities and serves as an initial step to bridge the gap between conceptual trajectory design and onboard trajectory planning.
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ItemRapid Entry Corridor Trajectory Optimization for Conceptual Design(Georgia Institute of Technology, 2010-08) Grant, Michael J. ; Clark, Ian G. ; Braun, Robert D.During conceptual design, an entry configuration is chosen to provide an envelope of vehicle performance throughout the entry corridor that satisfies mission requirements. In many applications, this process is performed using computationally intensive direct methods. In this investigation, an automated process has been developed to perform rapid trajectory optimization using indirect methods. This process combines and advances disparate trajectory optimization techniques developed over the previous century into a unified framework that is capable of solving a wide range of design problems. Specifically, this framework implements discrete dynamic programming, nonlinear inversion, pseudospectral methods, indirect methods, and continuation. The results from pseudospectral methods identify challenges in the formulation of corner conditions and switching structure associated with indirect methods. Examples demonstrate that families of optimal trajectories can be quickly constructed for varying trajectory parameters, vehicle shape, atmospheric properties, and gravity. These results validate the hypothesis that many entry trajectory solutions are linked through indirect methods. This framework enables rapid trajectory optimization and design space exploration, rapid sensitivity and robustness analysis, and rapid vehicle requirements definition.
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ItemHigh Mass Mars Entry, Descent, and Landing Architecture Assessment(Georgia Institute of Technology, 2009-09) Steinfeldt, Bradley A. ; Theisinger, John E. ; Korzun, Ashley M. ; Clark, Ian G. ; Grant, Michael J. ; Braun, Robert D.As the nation sets its sight on returning humans to the Moon and going onward to Mars, landing high mass payloads (>/= 2 t) on the Mars surface becomes a critical technological need. Viking heritage technologies (e.g., 70degrees sphere-cone aeroshell, SLA-561V thermal protection system, and supersonic disk-gap-band parachutes) that have been the mainstay of the United States' robotic Mars exploration program do not provide sufficient capability to land such large payload masses. In this investigation, a parametric study of the Mars entry, descent, and landing design space has been conducted. Entry velocity, entry vehicle configuration, entry vehicle mass, and the approach to supersonic deceleration were varied. Particular focus is given to the entry vehicle shape and the supersonic deceleration technology trades. Slender bodied vehicles with a lift-to-drag ratio (L=D) of 0.68 are examined alongside blunt bodies with L=D = 0.30. Results demonstrated that while the increased L=D of a slender entry configuration allows for more favorable terminal descent staging conditions, the greater structural efficiencies of blunt body systems along with the reduced acreage required for the thermal protection system affords an inherently lighter vehicle. The supersonic deceleration technology trade focuses on inflatable aerodynamic decelerators (IAD) and supersonic retropropulsion, as supersonic parachute systems are shown to be excessively large for further consideration. While entry masses (the total mass at the top of the Mars atmosphere) between 20 and 100 t are considered, a maximum payload capability of 37.3 t results. Of particular note, as entry mass increases, the gain in payload mass diminishes. It is shown that blunt body vehicles provide sufficient vertical L=D to decelerate all entry masses considered through the Mars atmosphere with adequate staging conditions for the propulsive terminal descent. A payload mass fraction penalty of approximately 0.3 exists for the use of slender bodied vehicles. Another observation of this investigation is that the increased aerothermal and aerodynamic loads induced from a direct entry trajectory (velocity ~6.75 km/s) reduce the payload mass fraction by approximately 15% compared to entry from orbital velocity (~4 km/s). It should be noted that while both IADs and supersonic retropropulsion were evaluated for each of the entry masses, configurations, and velocities, the IAD proved to be more mass-efficient in all instances. The sensitivity of these results to modeling assumptions was also examined. The payload mass of slender body vehicles was observed to be approximately four times more sensitive to modeling assumptions and uncertainty than blunt bodies.