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Master's Projects

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Exploring the F6 Fractionated Spacecraft Trade Space with GT-FAST

(Georgia Institute of Technology, 2009-11-12) Lafleur, Jarret M.

Released in July 2007, the Broad Agency Announcement for DARPA’s System F6 outlined goals for flight demonstration of an architecture in which the functionality of a traditional monolithic satellite is fulfilled with a fractionated cluster of free-flying, wirelessly interconnected modules. Given the large number of possible architectural options, two challenges facing systems analysis of F6 are (1) the ability to enumerate the many potential candidate fractionated architectures and (2) the ability to analyze and quantify the cost and benefits of each architecture. This paper applies the recently developed Georgia Tech F6 Architecture Synthesis Tool (GT-FAST) to the exploration of the System F6 trade space. GT-FAST is described in detail, after which a combinatorial analysis of the architectural trade space is presented to provide a theoretical contribution applicable to future analyses clearly showing the explosion of the trade space as the number of fractionatable components increases. Several output metrics of interest are defined, and Pareto fronts are used to visualize the trade space. The first set of these Pareto fronts allows direct visualization of one output against another, and the second set presents cost plotted against a Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) score aggregating performance objectives. These techniques allow for the identification of a handful of Pareto-optimal designs from an original pool of over 3,000 potential designs. Conclusions are drawn on salient features of the resulting Pareto fronts, important competing objectives which have been captured, and the potential suitability of a particularly interesting design designated PF0248. A variety of potential avenues for future work are also identified.
Analysis of Human-System Interaction For Landing Point Redesignation

(Georgia Institute of Technology, 2009-05-26) Chua, Zarrin K.

Despite two decades of manned spaceflight development, the recent thrust for increased human exploration places significant demands on current technology. More information is needed in understanding how human control affects mission performance and most importantly, how to design support systems that aid in human-system collaboration. This information on the general human-system relationship is difficult to ascertain due to the limitations of human performance modeling and the breadth of human actions in a particular situation. However, cognitive performance can be modeled in limited, well-defined scenarios of human control and the resulting analysis on these models can provide preliminary information with regard to the human-system relationship. This investigation examines the critical case of lunar Landing Point Redesignation (LPR) as a case study to further knowledge of the human-system relationship and to improve the design of support systems to assist astronauts during this task. To achieve these objectives, both theoretical and experimental practices are used to develop a task execution time model and subsequently inform this model with observations of simulated astronaut behavior. The experimental results have established several major conclusions. First, the method of LPR task execution is not necessarily linear, with tasks performed in parallel or neglected entirely. Second, the time to complete the LPR task and the overall accuracy of the landing site is generally robust to environmental and scenario factors such as number of points of interest, number of identifiable terrain markers, and terrain expectancy. Lastly, the examination of the overall tradespace between the three main criteria of fuel consumption, proximity to points of interest, and safety when comparing human and analogous automated behavior illustrates that humans outperform automation in missions where safety and nearness to points of interest are the main objectives, but perform poorly when fuel is the most critical measure of performance. Improvements to the fidelity of the model can be made by transgressing from a deterministic to probablistic model and incorporating such a model into a six degree-of-freedom trajectory simulator. This paper briefly summarizes recent technological developments for manned spaceflight, reviews previous and current efforts in implementing LPR, examines the experimental setup necessary to test the LPR task modeling, discusses the significance of findings from the experiment, and also comments on the extensibility of the LPR task and experiment results to human Mars spaceflight.
Experimental Determination of Material Properties for Inflatable Aeroshell Structures

(Georgia Institute of Technology, 2009-05-26) Hutchings, Allison L.

As part of a deployable aeroshell development effort, system design, materials evaluation, and analysis methods are under investigation. One specific objective is to validate finite element analysis techniques used to predict the deformation and stress fields of aeroshell inflatable structures under aerodynamic loads. In this paper, we discuss the results of an experimental mechanics study conducted to ensure that the material inputs to the finite element models accurately predict the load elongation characteristics of the coated woven fabric materials used in deployable aeroshells. These coated woven fabrics exhibit some unique behaviors under load that make the establishment of a common set of test protocols difficult. The stiffness of a woven fabric material will be influenced by its biaxial load state. Uniaxial strip tensile testing although quick and informative may not accurately capture the needed structural model inputs. Woven fabrics, when loaded in the bias direction relative to the warp and fill axes, have a resultant stiffness that is quite low as compared with the warp and fill directional stiffness. We evaluate the experimental results from two load versus elongation test devices. Test method recommendations are made based on the relevance and accuracy of these devices. Experimental work is conducted on a sample set of materials, consisting of four fabrics of varying stiffness and strength. The building blocks of a mechanical property database for future aeroshell design efforts are constructed.
Computational Fluid Dynamics Validation of a Single, Central Nozzle Supersonic Retropropulsion Configuration

(Georgia Institute of Technology, 2009-05) Cordell, Christopher E., Jr.

Supersonic retropropulsion provides an option that can potentially enhance drag characteristics of high mass entry, descent, and landing systems. Preliminary flow field and vehicle aerodynamic characteristics have been found in wind tunnel experiments; however, these only cover specific vehicle configurations and freestream conditions. In order to generate useful aerodynamic data that can be used in a trajectory simulation, a quicker method of determining vehicle aerodynamics is required to model supersonic retropropulsion effects. Using computational fluid dynamics, flow solutions can be determined which yield the desired aerodynamic information. The flow field generated in a supersonic retropropulsion scenario is complex, which increases the difficulty of generating an accurate computational solution. By validating the computational solutions against available wind tunnel data, the confidence in accurately capturing the flow field is increased, and methods to reduce the time required to generate a solution can be determined. Fun3D, a computational fluid dynamics code developed at NASA Langley Research Center, is capable of modeling the flow field structure and vehicle aerodynamics seen in previous wind tunnel experiments. Axial locations of the jet terminal shock, stagnation point, and bow shock show the same trends which were found in the wind tunnel, and the surface pressure distribution and drag coefficient are also consistent with available data. The flow solution is dependent on the computational grid used, where a grid which is too coarse does not resolve all of the flow features correctly. Refining the grid will increase the fidelity of the solution; however, the calculations will take longer if there are more cells in the computational grid.
Fully-Propulsive Mars Atmospheric Transit Strategies for High-Mass Missions

(Georgia Institute of Technology, 2009-04-29) Marsh, Christopher L.

A systems analysis focused on the use of propulsion during the EDL sequence at Mars for high-payload missions is presented. Trajectory simulation and mass sizing are performed to analyze the feasibility of a fully-propulsive descent. A heat rate boundary and associated control law are developed in an effort to limit the heating loads placed on the vehicle. Analysis is performed to explore the full-propulsive EDL strategy’s sensitivity to the vehicle’s propulsive capabilities and aero-propulsive and vehicle models. The EDL strategy is examined for ranges of initial masses and heat rate constraints, outlining an envelope of feasibility. The proposed architecture is compared against EDL systems in which significant aeroassist technology is employed. With this information, an overview of the impact of a fully-propulsive EDL system on spacecraft design and functionality is offered
Resonance Hopping Transfers Between Moon Science Orbits

(Georgia Institute of Technology, 2009-04-22) Brinckerhoff, Adam T.

Resonance hopping transfers between science orbits around two circular, coplanar moons of a common planet are designed using series of alternating V-infinity leveraging maneuvers and zero-point patched conic gravity assists. When this technique is combined with an efficient global search based on Bellman’s Principle, the end result is an exhaustive set of fuel and time optimal trajectories between the two moons in question. The associated Pareto front of solutions represents the classic fuel versus flight time trade study sought in preliminary mission design. Example numerical results are produced for orbital transfers between scientifically interesting moons in the Jovian system due to NASA and ESA’s particular interest in executing future tour missions in this environment. Finally, resonant transfers between neighboring pairs of moons are patched together to obtain fuel and flight time estimates for a full Jovian system tour between intermediate previously discovered circulating eccentric science orbits. Results from this fast, preliminary design procedure are intended to serve as useful starting points for higher fidelity multi-body mission design. In general, the resonant hopping design approach and the associated design procedure are found to be most relevant for missions with short flight time requirements.
Parametric Analysis and Targeting Capabilities for the Planetary Entry Systems Synthesis Tool

(Georgia Institute of Technology, 2008-12-05) Smith, Patrick J.

Modeling and simulation has led to major advances in the design of complex systems largely because it provides designers with an affordable method of testing new ideas. This report describes recent improvements to a modeling and simulation tool, known as the Planetary Entry Systems and Synthesis Tool or PESST, that allow a designer to quickly conduct parametric and targeting studies. PESST has been used in several conceptual design studies and the improvements to this tool allow a user to complete several cases quickly and gain valuable insight to a larger region of the design space. It would be impossible for designers to create truly robust systems without the ability to fully grasp the design space. By testing the effect of many different input variable values, the designer gains valuable insight to overall system response. As an example of the improvements added to PESST, hypothetical parametric and targeting studies have been completed for the Orion Crew Entry Vehicle.
Guidance, Navigation, and Control Technology System Trades for Mars Pinpoint Landing

(Georgia Institute of Technology, 2008-05-01) Steinfeldt, Bradley A.

Landing site selection is a compromise between safety concerns associated with the site’s terrain and scientific interest. Therefore, technologies enabling pinpoint landing (sub-100 m accuracies) on the surface of Mars are of interest to increase the number of accessible sites for in-situ research as well as allow placement of vehicles nearby prepositioned assets. A survey of various guidance, navigation, and control technologies that could allow pinpoint landing to occur at Mars has shown that negligible propellant mass fraction benefits are seen for reducing the three-sigma position dispersion at parachute deployment below approximately 3 km. Four different propulsive terminal descent guidance algorithms were analyzed with varying applicability to flight. Of these four, a near propellant optimal, analytic guidance law showed promise for the conceptual design of pinpoint landing vehicles. In addition, subsonic guided parachutes are shown to provide marginal performance benefits due to the timeline associated with Martian entries, and a low computational-cost, yet near fuel optimal propulsive terminal descent algorithm is identified. This investigation also demonstrates that navigation is a limiting technology for Mars pinpoint landing, with landed performance being largely affected by sensor accuracy.
Supersonic Retropropulsion Technology for Application to High Mass Mars Entry, Descent, and Landing

(Georgia Institute of Technology, 2008-04-30) Korzun, Ashley M.

As vehicle masses continue to increase for missions involving atmospheric entry, supersonic deceleration is challenging the qualifications and capabilities of Viking-heritage entry, descent, and landing (EDL) technology. At Mars, high entry masses and insufficient atmospheric density often result in unacceptable parachute deployment and operating conditions, requiring the exploration of alternative approaches to supersonic deceleration. Supersonic retropropulsion, the initiation of a retropropulsion phase while the vehicle is still traveling supersonically, may be an enabling technology for systems with high ballistic coefficients operating in thin atmospheres such as at Mars. The relevance of this technology to the feasibility of Mars EDL has been shown to increase with ballistic coefficient to the point that it is likely required for human Mars exploration. In conjunction with a literature review of supersonic retropropulsion technology as it applies to blunt body entry vehicles, a systems study was performed to assess the impact of supersonic retropropulsion on high mass Mars EDL. This investigation addresses the applicability, limitations, and performance implications of supersonic retropropulsion technology in the context of future human and robotic Mars exploration missions.
Initial Implementation of an Adjoint CFD Code for Aeroshell Shape Optimization

(Georgia Institute of Technology, 2008-03-05) Flaherty, Kevin W.

Application of computational fluid dynamics to the optimization of aeroshell shapes usually entails high computational cost. Many converged solutions are required to generate gradients and optimize a shape with respect to very few design variables. The benefits of high-fidelity aerodynamic analysis can be reaped early in the design cycle with less computational cost if the traditional direct optimization problem is transformed to an indirect optimization, using optimal control theory. The indirect gradient formulation decouples the effects on the objective function of the design variables and the flow solution. Meaning, all derivatives used to compute the gradient can be generated from a single converged flow solution. Involved in the computation of the gradient is the solution of an adjoint system of PDEs. An incremental approach is developed for the implementation of an adjoint equation solver. The phased approach begins using inexact and computationally costly finite difference derivative calculations. Results are presented for a transonic airfoil and a supersonic wedge to demonstrate that the finite difference gradient is reasonably accurate, providing a meaningful validation as exact numerical derivatives are substituted later in the development cycle. Finally, a roadmap is presented for future implementation of indirect optimization capability for the Euler/Navier-Stokes CFD code, NASCART-GT.