DSpace Repository :: Browsing by Author "Sood, Rohan"

Browsing by Author "Sood, Rohan"

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Analytical evaluation of effective solar sail attitude control using metasurfaces for deep space exploration
(University of Alabama Libraries, 2018) Ullery, Dylan; Kim, Seongsin; Kung, Patrick; University of Alabama Tuscaloosa
We examine the theoretical implications of incorporating metasurfaces on solar sails, and the effect they can have on the forces and torques applied to the sail. This would enable a significant enhancement over state-of-the- art attitude control by demonstrating a novel, propellant-free and low mass approach to induce a roll torque on the sail, which is a current limitation in present state-of-the-art technology. We do so by utilizing anomalous optical reflections from the metasurfaces to generate a net in-plane lateral force. This can lead to a net torque along the roll axis of the sail, in addition to the other spatial movements exhibited by the sail from solar radiation pressure. We characterize this net lateral force as a function of incidence angle. In addition, the influence of the phase gradients and anomalous conversion efficiency characteristics of the metasurfaces are independently considered. The optimum incidence angle that corresponded with the maximum net lateral-to-normal force ratio was found to be -30° for a metasurface exhibiting 75% anomalous conversion efficiency with a phase gradient of 0.71k0. Upon comparison with the force and torque plots from current reflection control devices such as those that were utilized on IKAROS, the anomalously reflecting metasurfaces offer a considerable increase in torque along the roll axis. This is important because up until this point, roll control has been a particularly difficult aspect of solar sail attitude control to manipulate without the use of reaction wheels or propellant. The torque along the roll axis can reach values of torque as high as 358 μN m under ideal conditions.
Application of Numerical Methods to the Hamilton-Jacobi-Isaacs Equation in Various Dynamical Systems
(University of Alabama Libraries, 2021) Ledbetter, William Gordon; Sood, Rohan; University of Alabama Tuscaloosa
The field of differential games has broad applicability to topics of economics, engineering, business, and warfare. Given the increasing levels of autonomy implemented in man-made systems in these fields, competition-based analysis may be the best option for understanding behavioral bounds when such systems interact. Differential games are governed by the Hamilton-Jacobi-Isaacs PDE, and many solution techniques are explored before identifying a gap in the existing literature. This dissertation develops a new approach to analyzing differential games based on a saddle-point solution technique. In a 2D system, the standard algorithmic approach produces both a value function interpolation and an approximate control map. Additionally, analysis of the observation error indicates that future analysis should prefer a problem formulation with relative motion. In the Circular Restricted Three-Body Problem, the same algorithms are applied to a system with real-world implications. The value and control interpolations produce a near-optimal trajectory, but the radial basis function approach suffered from high data density and did not exactly recreate the nominal solution. A perturbation analysis indicated that any mid-flight disturbance to the game state is most likely to benefit the pursuer, extending the works of Isaacs to a new domain. Ultimately, the proposed method is demonstrated to be a valuable tool for future differential games research.
A CubeSat Train for Radar Sounding and Imaging of Antarctic Ice Sheet
Gogineni, Prasad; Simpson, Christopher R.; Yan, Jie-Bang; O'Neill, Charles R.; Sood, Rohan; Gurbuz, Sevgi Z.; Gurbuz, Ali C.; University of Alabama Tuscaloosa
Design of Missed Thrust Resilient Trajectories Using Expected Thrust Fraction
(University of Alabama Libraries, 2022) Rubinsztejn, Ari; Sood, Rohan; University of Alabama Tuscaloosa
Low-thrust propulsion methods, and the order of magnitude reduction in required fuel, is an enabling technology for deep-space exploration. Unfortunately, spacecraft with low-thrust levels require long-duration thrusting arcs, making them susceptible to missed thrust events. This investigation describes a new method for designing low-thrust spacecraft trajectories resilient to missed thrust events. The method, called expected thrust fraction, embeds the stochastic nature of missed thrust events into a time-dependent thrust penalty, which can be utilized inside a traditional optimal control solver. Additionally, the theory of expected thrust fraction is expanded to include solar sailing missions and their unique missed thrust behavior. The performance of expected thrust fraction is then characterized on a variety of low-thrust transfers, and its resilience to missed thrust events is compared to previous mitigation techniques. Finally, the technique is applied to NASA's Mars Sample Return mission concept, where the spacecraft's resilience to missed thrust events is improved from 65.4% to >99%.
Game Theory Applications in Astrodynamics and Space Domain Awareness
(University of Alabama Libraries, 2021) Schoenwetter, Luke; Sood, Rohan; University of Alabama Tuscaloosa
As the number of nations possessing space launch capabilities increases, Earth orbit inherently becomes a competitive environment. Furthermore, each competing agent possesses unique objectives that may or may not align with the objectives of other agents. The competitive dynamics presented by this system are well suited for the application of game theory: the study of rational competitors from a mathematical perspective. The presented work combines the disciplines of game theory, optimal control, and astrodynamics to form generic game solution methods. These solution methods are used to obtain optimal open-loop strategies for an interceptor and an evasive target. A game involving an interceptor, a defender, and a ballistic target is also studied. Parameter space analyses are performed across a wide range of initial conditions to identify and visualize trends in the solution spaces. Additionally, a framework for testing strategies in a closed-loop format is developed to evaluate the consequences of sub-optimal actions. The various trends and characteristics found in the solution spaces are discussed, as is the relevancy of the results to modern space security and contingency planning.
Hollow Cathode Materials in an Iodine Plasma Enviroment
(University of Alabama Libraries, 2023) Rogers, James Daniel; Branam, Richard D
Current electric propulsion devices have proven their worth for large, conventional satellites. Trade studies using iodine as a propellant show superior system-level performance when compared to existing xenon-based systems. As a halogen, iodine introduces chemical reactivity issues not present in xenon plasmas, which are especially relevant for the extreme environment of hollow cathodes. An RF inductively coupled plasma source was designed, built, and characterized to simulate the environment found in a hollow cathode. Langmuir probe measurements were taken for a range of plasma conditions in iodine and argon plasmas. The iodine plasma required measurement with a double probe due to the occurrence of significant negative ionization. A combination of low-resolution survey spectra and high-resolution spectra were measured to measure plasma composition.Material samples of tungsten, tantalum, and molybdenum were tested for a range of temperatures in argon plasma, iodine plasma, and iodine vapor; their associated erosion ranges were measured. Molybdenum's erosion rate showed a roughly exponential relationship with respect to temperature, and a maximum erosion rate at 2000 K in iodine plasma of 70.1 µm/hr. Tungsten's erosion rate showed two distinct linear relationships with respect to temperature, and a maximum erosion rate at 2000 K in iodine plasma of 36.8 µm/hr. Tantalum experienced significant oxidation and expanded in size but had heavy cracking present which weakened the samples. Molybdenum was found to be the best material in iodine vapor, but tungsten was superior in iodine plasma. Tantalum demonstrated very unfavorable changes in surface structure and is not recommended for a system that contains any iodine or oxygen/water contamination. A significant challenge of this experiment was eliminating oxygen due to the oxygen and water presence in the iodine source. The primary improvements suggested for future studies are careful design of the iodine source to minimize moisture and oxygen contamination, a larger plasma source and sample loading area, and use of a differential vacuum system.
Loitering and Heliocentric Transfer Strategies for Low-Thrust Spacecraft in a Near Rectilinear Halo Orbit
(University of Alabama Libraries, 2026) Sandel, Carrie Grace; Sood, Rohan
With plans to establish a long-term cislunar presence through NASA's lunar space station, Gateway, there is significant interest in exploring a wide range of potential missions that either originate from, operate in, or conclude at the vicinity of Gateway. The lunar Gateway will be placed in an Earth-Moon L2 southern near rectilinear halo orbit and will act as a staging outpost for Moon-based missions and beyond. Since several missions may end up centered around Gateway, this creates an opportunity for secondary payloads with smaller spacecraft, such as CubeSats, to use Gateway as a starting point for cislunar or deep space missions. However, as most prior trajectory design has leveraged Earth as a starting location, it is critical for mission designers to understand the design space of missions that originate from Gateway. As such, this work performs a reachability analysis of a CubeSat equipped with electric propulsion departing from Gateway and explores mass-optimal and time-optimal transfers to families of Lagrange point orbits in the Sun-Earth system. An uncertainty analysis is performed to understand the departure dynamics from Gateway when targeting heliocentric space to mitigate re-contact risks due to the low acceleration levels of electric propulsion. Additionally, on-orbit servicing, inspection, and docking in a multi-body environment will need to take place in highly perturbed orbits for the first time in order for CubeSats to operate near Gateway. Thus, the investigation of these relative motion trajectories between two spacecraft is critical to successfully performing rendezvous and proximity operations (RPO) in a multi-body environment. In this work, mass-optimal and time-optimal trajectories are generated for a low-thrust chaser spacecraft entering into natural and forced loiter sequences about a target on a near rectilinear halo orbit. Constraints on the allowable region for safe RPO are applied to investigate the effects on feasible loitering maneuvers. Fuel costs are compared for transfers to xboth forced and natural loitering trajectories to determine the propellant reduction from leveraging the relative dynamics of NRHO manifolds. Finally, a passive safety analysis is performed, and a novel method is introduced to ensure passively safe low-thrust trajectories.
On the Applications of Signed Distance Fields to Spacecraft Trajectory Design and Station Keeping
(University of Alabama Libraries, 2024) Sikes, Jared Daniel; Sood, Rohan
As computational resources have continued to improve in both performance and accessibility in recent years, the usage of high-fidelity ephemeris models in the preliminary phase of mission design has likewise increased. From a practicality standpoint, it is necessary to use these high-fidelity models, as they better represent the real-world dynamics encountered by the spacecraft during flight and paint a more accurate picture of the mission design space and costs associated with the mission.However, preliminary trajectory design in ephemeris models presents several challenges that may not be apparent when dealing with lower-fidelity models, such as the circular restricted three body problem (CR3BP).Whereas the CR3BP admits periodic solutions in an autonomous reference frame, the ephemeris model only contains quasi-periodic, time dependent solutions, forcing mission designers to carefully consider the mission's epoch when transitioning from the CR3BP to the ephemeris model.To overcome some of these challenges, mission designers typically use trajectories computed in the CR3BP as reference solutions which are patched into the ephemeris model for re-convergence. However, depending on the chosen epoch, the patching process may result in decreased mission performance or failure to converge a valid solution, particularly in highly chaotic regimes like cis-lunar space. To address these issues, this work seeks to develop a more general framework for transitioning CR3BP reference solutions into the ephemeris model through the use of signed distance fields (SDFs) as boundary constraints on the trajectory optimization problem for preliminary mission design and station keeping assessments. SDFs are a scalar field that indicate the closest distance of a point to some pre-defined surface.Typically, SDFs are computed using a triangulated mesh along with the fast marching method to efficiently sweep the area of interest and obtain values for the distance, which are signed depending on whether the point is inside (negative) or outside (positive) the mesh.SDFs may be computed about a nominal CR3BP orbit by creating a 3D tube of a specified radius and used as a constraint in station keeping and trajectory design problems, irrespective of the ephemeris epoch of interest.By constraining the spacecraft to target and maintain its path through a 3D tube around the nominal orbit, perturbations in the local dynamics due to the time-dependent nature of the ephemeris frame may be exploited within optimization schemes, without being tied to a particular pre-computed reference path.Further, the incorporation of SDFs as a trajectory design boundary condition in ballistic trajectories results in decreased delta-V usage as compared to more traditional stable manifold targeting schemes when used in an ephemeris model.This investigation will further explore the performance of SDFs in trajectory optimization problems across the commonly used Sun-Earth and Earth-Moon ephemeris reference frames for a variety of regimes near the Lagrange points and central bodies.Lastly, this work will extend the applications of SDFs to computing station-keeping maneuvers about CR3BP reference orbits in high-fidelity models and recontact mitigation when departing from nearly-stable cis-lunar orbits.
On the Implementation and Applications of a High Performance Trajectory Optimization Toolkit
(University of Alabama Libraries, 2024) Pezent, James Brannan; Sood, Rohan
With cost, mass, and time at the highest premium, trajectory optimization is critical to all phases of the design and operation of aerospace vehicles. For non-real-time applications, this typically involves transcribing the dynamics, controls, and constraints of the vehicle either directly or indirectly into finite-dimensional optimization problems, which can then be solved with a variety of powerful general-purpose algorithms. However, like the vehicles themselves, the realization of this transcription and optimization process involves many tradeoffs that must be carefully considered in order to deliver the most effective design tool to the engineer. In pursuit of this goal, this dissertation presents the theoretical background, development, and applications of a new integrated trajectory optimization toolkit. The software, the Astrodynamics Software and Science Enabling Toolkit (ASSET), is an open-source Python library with a C++ back-end. ASSET implements a vector function modeling language, new sparse non-linear optimizer, and full featured optimal control interface that have all been co-designed to provide maximum performance and usability for practical trajectory optimization problems. In this dissertation, the performance of the full software suite is tested on industry standard benchmark problems. Results indicate that ASSET can deliver order of magnitude run-time improvements over state-of-the-art commercial alternatives. Additionally, this dissertation leverages ASSET to aid in the investigation of two novel problems in attitude dynamics and solar sail mission design.
Passive drag sail applications for the accelerated deorbit and targeted reentry of spacecraft
(University of Alabama Libraries, 2021) Sweeten, Andrew Michael; Sood, Rohan; University of Alabama Tuscaloosa
In the relatively short time that space has been an asset to humans, the amount of debris occupying the region has become a noticeable concern. Maintaining the usability of space for future generations requires consideration of novel methods to remove debris and otherwise prevent space from becoming further congested. One such proposed method is aerodynamic drag sails to accelerate the natural deorbit process caused by the high-altitude atmosphere. The method, properly implemented, could cause the spacecraft to reenter the atmosphere and burn up without requiring the planning of additional maneuvers, potentially saving time and money while still meeting international requirements. Analysis of the technique requires solving the expected times in orbit and selecting a sail that optimizes cost relative to the spacecraft's orbital lifetime, initially using CubeSats. Atmospheric drag, however, is only one of many forces that may perturb a spacecraft along its trajectory. Solar radiation pressure is another source of perturbing forces acting on large surfaces in the direction of the Sun. After including these forces in high-fidelity deorbit analysis, one can predict where the spacecraft would likely impact the Earth if components do not burn up in Earth's atmosphere. To prioritize the safety of life and property on the surface, legal requirements dictate the location where the spacecraft may impact the surface. Since a passive drag sail does not have active control authority, the sail's initial deployment timing, orientation, and altitude dictate the final reentry point for a given gravitational and atmospheric drag model. Based on specific initial conditions, it is possible to show that a drag sail is an effective and efficient method of safely deorbiting a spacecraft while optimizing cost and conforming to legal requirements.
Path Planning Multi-Vehicle Missions with Random Finite Set Based Tracking
(University of Alabama Libraries, 2023) Thomas, Ryan; Larson, Jordan
Multi-vehicle missions pose many problems including, Multi-Object Tracking (MOT) with non-Gaussian noise, optimal vehicle to target assignment, and optimal path planning. The contributions of this work are two fold, (1) non-Gaussian extensions to MOT filters, and (2) creating algorithms for multi-vehicle mission planning; both for a centralized architecture. To achieve (1), non-Gaussian extensions to existing Random Finite Set (RFS) based MOT filters are derived focusing on heavy-tailed distributions. The RFS-based MOT filters were selected due to their ability to rigorously model vehicle birth, death, and spawn, and measurement clutter. Student's t, and a Gaussian Scale Mixture (GSM) versions of the Generalized Labeled Multi-Bernoulli (GLMB) filter were developed. The Sequential Monte Carlo (SMC) GLMB was improved by utilizing Unscented Particle filters with a Markov Chain Monte Carlo movement step. Performance was compared via simulations and non-Gaussian filters performed better in the presence of non-Gaussian noise. Finally, the Student's t and GSM-GLMB were run on hardware, with the Student's t version running in real time. The key takeaway was real time performance, despite an un-optimized implementation. For (2), a hierarchical Guidance Navigation and Control (GNC) architecture was proposed, allowing a large team to navigate through static debris with vehicle death. Additionally, single vehicle Extended LQR (ELQR) was recast for the multi-vehicle case in two variants; analogous to Iterative LQR (ILQR). The first variant is like ILQR with an additional automatic transformation of target states into a distribution. Compared to ILQR, ELQR had better run time performance. The second used the Optimal Sub-Pattern Assignment (OSPA) for its cost and was compared to LQR-Rapidly-exploring Randomized Tree* (LQR-RRT*). The OSPA ELQR had better runtime and trajectories than either RRT* or density ELQR. Finally, an exploratory scenario, a Rendezvous and Proximity Operation (RPO), was formulated to combine tracking and planning; a spacecraft must track a dynamic debris field, calculate safe paths to a Resident Space Object (RSO), and track RSO features to estimate attitude and rate. RFS-based MOT filters performed tracking and attitude estimation conducted via a coupled sequential Extended KF and GLMB. ELQR was used for planning and predicted forward debris states assuming Clohessy-Wiltshire-Hill dynamics.
Performance Measurements of a Radio Frequency Ion Thruster on Noble Gas Propellants
(University of Alabama Libraries, 2024) Tripoli, Tanner Joseph; Olcmen, Semih M
Radiofrequency Ion Thrusters are well suited to operating at a range of powers and are ideal candidates for scaling down for sub 100 W electric propulsion systems. Low power electric propulsion systems are an enabling technology for increased access to space for cubesats due to the low power availability on CubeSats. Xenon is still the primary propellant used in such propulsion systems, with iodine having been demonstrated as an alternative. Other noble gases such as krypton and argon may have an important niche as alternative propellants as well.A laboratory model Busek BIT-3 ion thruster was used to study the thruster's performance using krypton instead of xenon. A vacuum chamber with adequate pumping speed and base pressure was prepared for the operation of the BIT-3, and a translational and rotational probe arm installed in the chamber. The arm was used to take Faraday probe measurements of the thruster plume running on either xenon or krypton. The probe data was analyzed and compared to the beam current measured from the ion optics. The beam current data from the ion optics was used to determine thruster efficiencies, while the Faraday probe measurements determined thruster beam divergence.Keywords: Electric Propulsion, Faraday Probe, Ion Thruster Plume
The Self-Sustaining Processes in a Hollow Cathode to Produce a Steady Electron Emission
(University of Alabama Libraries, 2023) Boehm, Kirk Joachim; Branam, Richard D
Hollow cathodes are used in just about every plasma process, endemic throughout our lives (i.e., manufacturing, communications, health care, energy). Performance improvements of the hollow cathode seem to have plateaued, though. The two major characteristics are that the electron emission of the cathode reaches a self-sustaining mode, ion bombardment of the low work-function materials. This research proposes the theory that the self-sustaining mode is a complex result of a mix of ionization states (singly, doubly, ...), the thermodynamic state, the electron energy distribution, and different electron production in the plasma with a focus on thermionic and secondary emitted electrons. Most computational models assume that only singly ionized particles are present, but in practice no plasma is composed of only singly ionized particles. The temperatures in the quasi-neutral gas and plasma are often assumed to be the same, but evidence suggest these temperatures are very different. Evidence is needed to better describe the plasma physical phenomena (ion production, ion-surface impact, electron production at the surface) inside of the hollow cathode are the accurate measurements of plasma composition, individual species' temperatures, ionization states, and surface temperatures. Thermocouples can provide some information but not directly in the plasma or on the surface of the insert without changing the operation of the cathode. Langmuir probes have been used to query plasma properties, but only provide global values and are inherently limited by the sheath effect. To produce evidence in support of the proposed theory, this research effort is developed and used non-intrusive optical emission measurement techniques to quantify individual species properties and surface temperatures. This research was then be injected into electron energy distribution model combined with the emission model and collisional radiative model (CRM) to explain the actual physical processes. The results of this research showed distinct thermionic, ionization, and secondary (from ion bombardment) electron processes, as well as traces of primary and secondary ions. This informationcan was used to make recommendations to potentially reduce losses in energy production processes, increase communications bandwidth, and reduce energy consumption in manufacturing plating processes.
Strong Solar Radiation Forces from Anomalously Reflecting Metasurfaces for Solar Sail Attitude Control
(Nature Portfolio, 2018) Ullery, Dylan C.; Soleymani, Sina; Heaton, Andrew; Orphee, Juan; Johnson, Les; Sood, Rohan; Kung, Patrick; Kim, Seongsin M.; University of Alabama Tuscaloosa; National Aeronautics & Space Administration (NASA)
We examine the theoretical implications of incorporating metasurfaces on solar sails, and the effect they can have on the forces applied to the sail. This would enable a significant enhancement over state-of-the-art attitude control by demonstrating a novel, propellant-free and low-mass approach to induce a roll torque on the sail, which is a current limitation in present state-of-the-art technology. We do so by utilizing anomalous optical reflections from the metasurfaces to generate a net in-plane lateral force, which can lead to a net torque along the roll axis of the sail, in addition to the other spatial movements exhibited by the sail from solar radiation pressure. We characterize this net lateral force as a function of incidence angle. In addition, the influence of the phase gradients and anomalous conversion efficiencies characteristics of the metasurfaces are independently considered. The optimum incidence angle that corresponded with the maximum net lateral-to-normal force ratio was found to be -30 degrees for a metasurface exhibiting 75% anomalous conversion efficiency with a phase gradient of 0:71k(0).

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