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Daniel Guggenheim School of Aerospace Engineering

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https://hdl.handle.net/1853/70742

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College of Engineering

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Aerospace Design Group

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Aerospace Systems Design Laboratory (ASDL)

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Air Transportation Laboratory

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Cognitive Engineering Center

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Space Systems Design Laboratory (SSDL)

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Publication Search Results

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Design space pruning heuristics and global optimization method for conceptual design of low-thrust asteroid tour missions

(Georgia Institute of Technology, 2009-11-13) Alemany, Kristina

Electric propulsion has recently become a viable technology for spacecraft, enabling shorter flight times, fewer required planetary gravity assists, larger payloads, and/or smaller launch vehicles. With the maturation of this technology, however, comes a new set of challenges in the area of trajectory design. Because low-thrust trajectory optimization has historically required long run-times and significant user-manipulation, mission design has relied on expert-based knowledge for selecting departure and arrival dates, times of flight, and/or target bodies and gravitational swing-bys. These choices are generally based on known configurations that have worked well in previous analyses or simply on trial and error. At the conceptual design level, however, the ability to explore the full extent of the design space is imperative to locating the best solutions in terms of mass and/or flight times. Beginning in 2005, the Global Trajectory Optimization Competition posed a series of difficult mission design problems, all requiring low-thrust propulsion and visiting one or more asteroids. These problems all had large ranges on the continuous variables - launch date, time of flight, and asteroid stay times (when applicable) - as well as being characterized by millions or even billions of possible asteroid sequences. Even with recent advances in low-thrust trajectory optimization, full enumeration of these problems was not possible within the stringent time limits of the competition. This investigation develops a systematic methodology for determining a broad suite of good solutions to the combinatorial, low-thrust, asteroid tour problem. The target application is for conceptual design, where broad exploration of the design space is critical, with the goal being to rapidly identify a reasonable number of promising solutions for future analysis. The proposed methodology has two steps. The first step applies a three-level heuristic sequence developed from the physics of the problem, which allows for efficient pruning of the design space. The second phase applies a global optimization scheme to locate a broad suite of good solutions to the reduced problem. The global optimization scheme developed combines a novel branch-and-bound algorithm with a genetic algorithm and an industry-standard low-thrust trajectory optimization program to solve for the following design variables: asteroid sequence, launch date, times of flight, and asteroid stay times. The methodology is developed based on a small sample problem, which is enumerated and solved so that all possible discretized solutions are known. The methodology is then validated by applying it to a larger intermediate sample problem, which also has a known solution. Next, the methodology is applied to several larger combinatorial asteroid rendezvous problems, using previously identified good solutions as validation benchmarks. These problems include the 2nd and 3rd Global Trajectory Optimization Competition problems. The methodology is shown to be capable of achieving a reduction in the number of asteroid sequences of 6-7 orders of magnitude, in terms of the number of sequences that require low-thrust optimization as compared to the number of sequences in the original problem. More than 70% of the previously known good solutions are identified, along with several new solutions that were not previously reported by any of the competitors. Overall, the methodology developed in this investigation provides an organized search technique for the low-thrust mission design of asteroid rendezvous problems.
Aerodynamic design, analysis, and validation of a supersonic inflatable decelerator

(Georgia Institute of Technology, 2009-07-06) Clark, Ian G.

Since the 1970's, NASA has relied on the use of rigid aeroshells and supersonic parachutes to enable robotic mission to Mars. These technologies are constrained by size and deployment condition limitations that limit the payload they can deliver to the surface of Mars. One candidate technology envisioned to replace the supersonic parachute is the supersonic inflatable aerodynamic decelerator (IAD). This dissertation presents an overview of work performed in maturing a particular type of IAD, the tension cone. The tension cone concept consists of a flexible shell of revolution that is shaped so as to remain under tension and resist deformation. Systems analyses that evaluated trajectory impacts of a supersonic IAD demonstrated several key advantages including increases in delivered payload capability of over 40%, significant gains in landing site surface elevation, and the ability to accommodate growth in the entry mass of a spacecraft. A series of supersonic wind tunnel tests conducted at the NASA Glenn and Langley Research Centers tested both rigid and flexible tension cone models. Testing of rigid force and moment models and pressure models demonstrated the new design to have favorable performance including drag coefficients between 1.4 and 1.5 and static stability at angles of attack from 0º to 20º. A separate round of tests conducted on flexible tension cone models showed the system to be free of aeroelastic instability. Deployment tests conducted on an inflatable model demonstrated rapid, stable inflation in a supersonic environment. Structural modifications incorporated on the models were seen to reduce inflation pressure requirements by a factor of nearly two. Through this test program, this new tension cone IAD design was shown to be a credible option for a future flight system. Validation of CFD analyses for predicting aerodynamic IAD performance was also completed and the results are presented. Inviscid CFD analyses are seen to provide drag predictions accurate to within 6%. Viscous analyses performed show excellent agreement with measured pressure distributions and flow field characteristics. Comparisons between laminar and turbulent solutions indicate the likelihood of a turbulent boundary layer at high supersonic Mach numbers and large angles of attack.
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
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.