\chapter{Introduction}

Model rocketry is a sport that involves designing, constructing and
launching self-made rockets.  Model rockets vary greatly in size,
shape, weight and construction from detailed scale models of
professional rockets to lightweight and highly finished competition
models.  The sport is relatively popular and is often cited as a
source of inspiration for children to become engineers and
scientists.

The hobby started as amateur rocketry in the 1950's when hobbyists
wanted to experiment their skill with building rockets.  Designing,
building and firing self-made {\it motors} was, however, extremely dangerous,
and the American Rocket Society (now the American Institute of
Aeronautics and Astronautics, AIAA) has estimated that about one in seven
amateur rocketeers during the time were injured in their hobby.  This
changed in 1958 when the first commercially-built model rocket
motors became available.  Having industrially-made, reasonably-priced
and safe motors available removed the most dangerous aspect of amateur
rocketry.  This along with strict guidelines to the design and
launching of model rockets formed the foundation for a safe and
widespread hobby.~\cite[pp.~1--3]{stine}

Since then model rocketry has spread around the globe and among all
age groups.  Thousands of rockets ranging from 10~cm high miniatures
to large models reaching altitudes in excess of 10~km are launched
annually.  Model rocket motors with thrusts from a few Newtons up to
several kilo-Newtons are readily available.  Since its forming in
1957, over 90\s000 people have joined the National Association of
Rocketry (NAR) in the U.S. alone.
%  Model rocketry is used as an
%educational device in numerous of schools and by many youth
%organizations.

In designing rockets, the {\it stability} of a rocket is of central
priority.  A stable rocket corrects its course if some outside
force disturbs it slightly.  A disturbance of an unstable rocket
instead  increases until the rocket starts spinning in the
air erratically.  As shall be discussed in
Section~\ref{sec-stability}, a rocket is deemed 
{\it statically stable} if its center of pressure (CP) is aft of its
center of gravity (CG)\footnote{An alternative term would be 
  {\it center of mass}, but in the context of model rocketry, we are
  interested in the effect of gravity on the rocket.  Thus, the term
  center of gravity is widely used in model rocketry texts, and this
  convention will be followed in this thesis.}.
The center of gravity of a rocket can be easily calculated in advance
or determined experimentally.  The center of pressure, on the other
hand, has been quite hard to determine either analytically or
experimentally.  In 1966 James and Judith Barrowman developed an
analytical method for determining the CP of a slender-bodied rocket at
subsonic speeds and presented their results as a research and
development project at the 8th National Association of Rocketry Annual
Meeting (NARAM-8)~\cite{barrowman-rd}, and later as a part of James
Barrowman's Master's thesis~\cite{barrowman-thesis}.  This method has
become known as the {\it Barrowman method} of determining the CP of a
rocket within the model rocketry community, and has a major role in
determining the aerodynamic characteristics of model rockets.

Another important aerodynamic quantity of interest is the 
{\it aerodynamic drag} of a rocket.  Drag is caused by the flow of air
around the rocket and it can easily reduce the maximum altitude of a
rocket by 50--80\% of the otherwise theoretical maximum.  Estimating
the drag of a model rocket is a rather complex task, and the effects
of different design choices are not always very evident to a
hobbyist.

Knowing the fundamental aerodynamic properties of a rocket allows one
to simulate its free flight.  This involves numerically integrating
the flight forces and determining the velocity, rotation and position
of the rocket as a function of time.  This is best performed by
software designed for the purpose of model rocket design.

RockSim~\cite{rocksim} is one such piece of software.  It is a
commercial, proprietary program that allows one to define the geometry
and configuration of a model rocket, estimate its aerodynamic
properties and simulate a launch with different rocket motors.  It has
become the {\it de facto} standard software for model rocket
performance estimation.  However, as a proprietary program, it is
essentially a ``black-box'' solution.  Someone wishing to study or
validate the methods will not be able to do so.  Similarly extending
or customizing the functionality or refining the calculations methods
to fit ones needs is impossible.  The software is also only available
on select operating systems.  Finally, the cost of the software may be
prohibitive especially for younger hobbyists, voluntary organizations,
clubs and schools.

Open Source software, on the other hand, has become an increasingly
competitive alternative to proprietary software.  Open Source allows
free access to the source code of the programs and encourages
users with the know-how to enhance the software and share their
changes~\cite{oss-principles}.  Success stories such as the Linux
operating system, the OpenOffice.org office suite, the Firefox web
browser and countless others have shown that Open Source software can
often achieve and even exceed the quality of expensive proprietary
software.


\section{Objectives of the thesis}

The objectives of this thesis work are to:
%
\begin{enumerate}
\item Develop and document relatively easy, yet reasonably accurate
  methods for the calculation of the fundamental aerodynamic
  properties of model rockets and their numerical simulation;

\item Test the methods developed and compare the results with other
  estimates and actual experimental data; and

\item Implement a cross-platform, Open Source model rocket design and
  simulation software that uses the aforementioned methods, is at the
  same time easy to use and yet versatile, and which is easily
  extensible and customizable for user requirements, new types of rocket
  components and new estimation methods.
\end{enumerate}

The methods presented will largely follow the methods developed by
Barrowman~\cite{barrowman-rd,barrowman-thesis}, since these are
already familiar to the rocketry community.  Several extensions to the
methods will be added to allow for more accurate calculation at larger
angles of attack and for fin shapes not accounted for in the original
paper.  The emphasis will be on subsonic flight, but extensions will
be made for reasonable estimation at transonic and low supersonic
velocities.

The software developed as part of the thesis is the OpenRocket
project~\cite{openrocket}.  It is an Open Source rocket development
and simulation environment written totally in Java.  The program
structure has been designed to make full use of object oriented
programming, allowing one to easily extend its features.  The software
also includes a framework for creating user-made 
{\it listener components} (discussed in Section~\ref{sec-listeners})
that can listen to and interact with the simulation while it is
running.  This allows a powerful and easy way of interacting with the
simulation and allows simulating for example guidance systems.

One possible future enhancement that has also specifically been
considered throughout the development is calculating the aerodynamic
properties using computational fluid dynamics (CFD).  CFD calculates
the exact airflow in a discretized mesh around the rocket.  This would
allow for even more accurate calculation of the aerodynamic forces for
odd-shaped rockets, for which the methods explained herein do not
fully apply.

It is anticipated that the software will allow more hobbyists the
possibility of simulating their rocket designs prior to building them
and experimenting with different configuration, thus giving them a
deeper understanding of the aerodynamics of rocket flight.  It will
also provide a more versatile educational tool since the simulation
methods are open and everyone will be able to ``look under the hood''
and see how the software performs the calculations.

In Chapter~\ref{chap-basics} a brief overview of model rocketry and
its different aspects will be given.  Then in
Chapter~\ref{chap-aerodynamics} methods for calculating the
aerodynamic properties of a general model rocket will be presented.
In Chapter~\ref{chap-simulation} the aspects of simulating a rocket's
flight are considered.  Chapter~\ref{chap-software} then explains how
the aerodynamic calculations and simulation are implemented in the
OpenRocket software and presents some of its features.  In
Chapter~\ref{chap-experimental} the results of the software simulation
are compared with the performance of constructed and launched rockets.
Chapter~\ref{chap-conclusion} then presents a summary of the
achievements and identifies areas of further work.