She did. Now I'm done!

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