169 lines
8.7 KiB
TeX
169 lines
8.7 KiB
TeX
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\chapter{Introduction}
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Model rocketry is a sport that involves designing, constructing and
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launching self-made rockets. Model rockets vary greatly in size,
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shape, weight and construction from detailed scale models of
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professional rockets to lightweight and highly finished competition
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models. The sport is relatively popular and is often cited as a
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source of inspiration for children to become engineers and
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scientists.
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The hobby started as amateur rocketry in the 1950's when hobbyists
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wanted to experiment their skill with building rockets. Designing,
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building and firing self-made {\it motors} was, however, extremely dangerous,
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and the American Rocket Society (now the American Institute of
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Aeronautics and Astronautics, AIAA) has estimated that about one in seven
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amateur rocketeers during the time were injured in their hobby. This
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changed in 1958 when the first commercially-built model rocket
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motors became available. Having industrially-made, reasonably-priced
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and safe motors available removed the most dangerous aspect of amateur
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rocketry. This along with strict guidelines to the design and
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launching of model rockets formed the foundation for a safe and
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widespread hobby.~\cite[pp.~1--3]{stine}
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Since then model rocketry has spread around the globe and among all
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age groups. Thousands of rockets ranging from 10~cm high miniatures
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to large models reaching altitudes in excess of 10~km are launched
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annually. Model rocket motors with thrusts from a few Newtons up to
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several kilo-Newtons are readily available. Since its forming in
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1957, over 90\s000 people have joined the National Association of
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Rocketry (NAR) in the U.S. alone.
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% Model rocketry is used as an
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%educational device in numerous of schools and by many youth
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%organizations.
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In designing rockets, the {\it stability} of a rocket is of central
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priority. A stable rocket corrects its course if some outside
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force disturbs it slightly. A disturbance of an unstable rocket
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instead increases until the rocket starts spinning in the
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air erratically. As shall be discussed in
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Section~\ref{sec-stability}, a rocket is deemed
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{\it statically stable} if its center of pressure (CP) is aft of its
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center of gravity (CG)\footnote{An alternative term would be
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{\it center of mass}, but in the context of model rocketry, we are
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interested in the effect of gravity on the rocket. Thus, the term
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center of gravity is widely used in model rocketry texts, and this
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convention will be followed in this thesis.}.
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The center of gravity of a rocket can be easily calculated in advance
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or determined experimentally. The center of pressure, on the other
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hand, has been quite hard to determine either analytically or
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experimentally. In 1966 James and Judith Barrowman developed an
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analytical method for determining the CP of a slender-bodied rocket at
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subsonic speeds and presented their results as a research and
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development project at the 8th National Association of Rocketry Annual
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Meeting (NARAM-8)~\cite{barrowman-rd}, and later as a part of James
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Barrowman's Master's thesis~\cite{barrowman-thesis}. This method has
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become known as the {\it Barrowman method} of determining the CP of a
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rocket within the model rocketry community, and has a major role in
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determining the aerodynamic characteristics of model rockets.
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Another important aerodynamic quantity of interest is the
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{\it aerodynamic drag} of a rocket. Drag is caused by the flow of air
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around the rocket and it can easily reduce the maximum altitude of a
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rocket by 50--80\% of the otherwise theoretical maximum. Estimating
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the drag of a model rocket is a rather complex task, and the effects
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of different design choices are not always very evident to a
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hobbyist.
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Knowing the fundamental aerodynamic properties of a rocket allows one
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to simulate its free flight. This involves numerically integrating
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the flight forces and determining the velocity, rotation and position
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of the rocket as a function of time. This is best performed by
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software designed for the purpose of model rocket design.
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RockSim~\cite{rocksim} is one such piece of software. It is a
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commercial, proprietary program that allows one to define the geometry
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and configuration of a model rocket, estimate its aerodynamic
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properties and simulate a launch with different rocket motors. It has
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become the {\it de facto} standard software for model rocket
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performance estimation. However, as a proprietary program, it is
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essentially a ``black-box'' solution. Someone wishing to study or
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validate the methods will not be able to do so. Similarly extending
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or customizing the functionality or refining the calculations methods
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to fit ones needs is impossible. The software is also only available
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on select operating systems. Finally, the cost of the software may be
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prohibitive especially for younger hobbyists, voluntary organizations,
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clubs and schools.
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Open Source software, on the other hand, has become an increasingly
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competitive alternative to proprietary software. Open Source allows
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free access to the source code of the programs and encourages
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users with the know-how to enhance the software and share their
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changes~\cite{oss-principles}. Success stories such as the Linux
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operating system, the OpenOffice.org office suite, the Firefox web
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browser and countless others have shown that Open Source software can
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often achieve and even exceed the quality of expensive proprietary
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software.
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\section{Objectives of the thesis}
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The objectives of this thesis work are to:
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%
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\begin{enumerate}
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\item Develop and document relatively easy, yet reasonably accurate
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methods for the calculation of the fundamental aerodynamic
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properties of model rockets and their numerical simulation;
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\item Test the methods developed and compare the results with other
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estimates and actual experimental data; and
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\item Implement a cross-platform, Open Source model rocket design and
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simulation software that uses the aforementioned methods, is at the
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same time easy to use and yet versatile, and which is easily
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extensible and customizable for user requirements, new types of rocket
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components and new estimation methods.
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\end{enumerate}
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The methods presented will largely follow the methods developed by
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Barrowman~\cite{barrowman-rd,barrowman-thesis}, since these are
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already familiar to the rocketry community. Several extensions to the
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methods will be added to allow for more accurate calculation at larger
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angles of attack and for fin shapes not accounted for in the original
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paper. The emphasis will be on subsonic flight, but extensions will
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be made for reasonable estimation at transonic and low supersonic
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velocities.
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The software developed as part of the thesis is the OpenRocket
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project~\cite{openrocket}. It is an Open Source rocket development
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and simulation environment written totally in Java. The program
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structure has been designed to make full use of object oriented
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programming, allowing one to easily extend its features. The software
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also includes a framework for creating user-made
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{\it listener components} (discussed in Section~\ref{sec-listeners})
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that can listen to and interact with the simulation while it is
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running. This allows a powerful and easy way of interacting with the
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simulation and allows simulating for example guidance systems.
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One possible future enhancement that has also specifically been
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considered throughout the development is calculating the aerodynamic
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properties using computational fluid dynamics (CFD). CFD calculates
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the exact airflow in a discretized mesh around the rocket. This would
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allow for even more accurate calculation of the aerodynamic forces for
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odd-shaped rockets, for which the methods explained herein do not
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fully apply.
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It is anticipated that the software will allow more hobbyists the
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possibility of simulating their rocket designs prior to building them
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and experimenting with different configuration, thus giving them a
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deeper understanding of the aerodynamics of rocket flight. It will
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also provide a more versatile educational tool since the simulation
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methods are open and everyone will be able to ``look under the hood''
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and see how the software performs the calculations.
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In Chapter~\ref{chap-basics} a brief overview of model rocketry and
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its different aspects will be given. Then in
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Chapter~\ref{chap-aerodynamics} methods for calculating the
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aerodynamic properties of a general model rocket will be presented.
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In Chapter~\ref{chap-simulation} the aspects of simulating a rocket's
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flight are considered. Chapter~\ref{chap-software} then explains how
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the aerodynamic calculations and simulation are implemented in the
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OpenRocket software and presents some of its features. In
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Chapter~\ref{chap-experimental} the results of the software simulation
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are compared with the performance of constructed and launched rockets.
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Chapter~\ref{chap-conclusion} then presents a summary of the
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achievements and identifies areas of further work.
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