She did. Now I'm done!
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\chapter{Introduction}
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Continuous low-thrust engines utilizing technologies such as Ion propulsion, Hall thrusters, and
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others can be a powerful system in the enabling of long-range interplanetary missions with fuel
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efficiencies unrivaled by those that employ only impulsive thrust systems. The challenge in
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utilizing these systems, then, is the design of trajectories that effectively utilize this
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technology. Continuous thrust propulsive systems tend to be particularly suited to missions
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which require very high total change in velocity ($\Delta V$) values and take place over a
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particularly long duration. Traditional impulsive thrusting techniques can achieve these changes
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in velocity, but typically have a far lower specific impulse and, as such, are much less fuel
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efficient, costing the mission valuable financial resources that could instead be used for
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science. Because of their inherently high specific impulse (and thus efficiency), low-thrust
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propagation systems are well-suited to interplanetary missions.
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others enable long-range interplanetary missions with fuel efficiencies unrivaled by those that
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employ only impulsive thrust systems. The challenge in utilizing these systems, then, is the
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design of trajectories that effectively utilize this technology. Continuous thrust propulsive
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systems tend to be particularly suited to missions which require very high total change in
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velocity ($\Delta V$) values and take place over a particularly long duration. Traditional
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impulsive thrusting techniques can achieve these changes in velocity, but typically have a far
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lower specific impulse and, as such, are much less fuel efficient, costing the mission valuable
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financial resources that could instead be used for science. Because of their inherently high
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specific impulse (and thus efficiency), low-thrust propulsion systems are well-suited to
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interplanetary missions.
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The first attempt by NASA to use an electric ion-thruster for an interplanetary mission was the
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Deep Space 1 mission\cite{brophy2002}. This mission was designed to test the ``new'' technology,
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in October 2018 and is projected to perform a flyby of Earth, two of Venus, and six of
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Mercury before inserting into an orbit around that planet.
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A common theme in mission design is that there always exists a trade-off between efficiency
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(particularly in terms of fuel use) and the time required to achieve the mission objective. Low
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thrust systems in particular tend to produce mission profiles that sacrifice the rate of
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convergence on the target state in order to achieve large increases in fuel efficiency. Often a
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low-thrust mission profile in Earth orbit will require multiple orbital periods to achieve the
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desired change in spacecraft state. Interplanetary missions, though, provide a particularly
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useful case for continuous thrust technology. The trajectory arcs in interplanetary space are
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generally much, much longer than orbital missions around the Earth. Because of this increase,
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even a small continuous thrust is capable of producing large $\Delta V$ values over the course
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of a single trajectory arc.
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A common theme in mission design is that there is a trade-off between efficiency (particularly
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in terms of fuel use) and the time required to achieve the mission objective. Low thrust systems
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in particular tend to produce mission profiles that sacrifice the rate of convergence on the
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target state in order to achieve large increases in fuel efficiency. Often a low-thrust transfer
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in Earth orbit will require multiple orbital periods to achieve the desired change in spacecraft
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state. Interplanetary missions, though, provide a particularly useful case for continuous thrust
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technology. The trajectory arcs in interplanetary space are generally much, much longer than
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orbital missions around the Earth. Because of this increase, even a small continuous thrust is
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capable of producing large $\Delta V$ values over the course of a single trajectory arc.
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Another technique often leveraged by interplanetary trajectory designers is the gravity assist.
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Gravity assists utilize the inertia of a large planetary body to ``slingshot'' a spacecraft,
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routine for producing unconstrained, globally optimal trajectories for realistic interplanetary
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mission development that utilizes both planetary flybys and efficient low-thrust electric
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propulsion techniques. Similar studies have also been performed by a number of researchers
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including a team from JPL\cite{sims2006} as well as a Spanish team\cite{morante}, among several
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others.
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including a team from JPL\cite{sims2006}, among several others\cite{morante}.
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This thesis will attempt to develop an algorithm for the optimization of low-thrust enabled
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trajectories for initial feasibility analysis in mission design. The algorithm will utilize
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a non-linear programming solver to directly optimize a set of control thrusts for the
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user-provided flyby planets, for any provided cost function. A monotonic basin hopping algorithm
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(MBH) will then be employed to traverse the search space in an effort to find additional local
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optima. This approach differs from the work produced earlier by Englander and the other teams,
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but is largely meant to explore the feasibility of such techniques and propose a few
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enhancements. The approach defined in this thesis will then be used to investigate an example
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mission to Saturn.
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This thesis focuses on optimization of low-thrust enabled trajectories that use gravity assists.
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The approach uses a non-linear programming solver to directly optimize a set of control thrusts
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for the user-provided flyby planets, for any provided cost function. A monotonic basin hopping
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algorithm (MBH) is then employed to traverse the search space in an effort to find additional
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local optima. This approach differs from the work produced earlier by Englander and the other
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teams, but is largely meant to explore the feasibility of such techniques and propose a few
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enhancements. The approach defined in this thesis is then used to design low thrust trajectories
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with gravity assits from the Earth to Saturn.
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This thesis will explore these concepts in a number of different sections. Section
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\ref{traj_dyn} will explore the basic dynamical principles of trajectory design, beginning the
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with fundamental system dynamics, then exploring interplanetary system dynamics and gravity
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flybys, and finally the dynamics that are specific to low-thrust enabled trajectories. Section
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\ref{traj_optimization} will then discuss process of optimizing spacecraft trajectories in
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general and the tool available for that. Section \ref{algorithm} will cover the implementation
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details of the optimization algorithm developed for this paper. Finally, section \ref{results}
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will explore the results of some hypothetical missions to Saturn.
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This thesis is organized as follows: Section \ref{traj_dyn} will explore the basic dynamical
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principles of trajectory design, beginning the with fundamental system dynamics, then exploring
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interplanetary system dynamics and gravity flybys, and finally the dynamics that are specific to
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low-thrust enabled trajectories. Section \ref{traj_optimization} will then discuss process of
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optimizing spacecraft trajectories in general and the tool available for that. Section
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\ref{algorithm} will cover the implementation details of the optimization algorithm developed
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for this paper. Finally, section \ref{results} will explore the results of some hypothetical
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missions to Saturn.
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