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  • POROUS EMITTER COLLOID THRUSTER PERFORMANCE

    CHARACTERIZATION USING OPTICAL TECHNIQUES

    THESIS

    Eric T. Wolf, Captain, USAF

    AFIT-ENY-13-M-36

    DEPARTMENT OF THE AIR FORCE AIR UNIVERSITY

    AIR FORCE INSTITUTE OF TECHNOLOGY

    Wright-Patterson Air Force Base, Ohio

    APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED.

  • The views expressed in this thesis are those of the author and do not reflect the official

    policy or position of the United States Air Force, Department of Defense, or the United

    States Government. This material is declared a work of the U.S. Government and is not

    subject to copyright protection in the United States.

  • AFIT-ENY-13-M-36

    POROUS EMITTER COLLOID THRUSTER PERFORMANCE

    CHARACTERIZATION USING OPTICAL TECHNIQUES

    THESIS

    Presented to the Faculty

    Department of Aeronautics and Astronautics

    Graduate School of Engineering and Management

    Air Force Institute of Technology

    Air University

    Air Education and Training Command

    In Partial Fulfillment of the Requirements for the

    Degree of Master of Science in Aeronautical Engineering

    Eric T. Wolf, BS

    Captain, USAF

    March 2013

    APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED.

  • AFIT-ENY-13-M-36

    POROUS EMITTER COLLOID THRUSTER PERFORMANCE

    CHARACTERIZATION USING OPTICAL TECHNIQUES

    Eric T. Wolf, BS

    Captain, USAF

    Approved:

    ___________________________________ __________

    Dr. David Liu, Capt, USAF (Chairman) Date

    ___________________________________ __________

    Dr. Carl R. Hartsfield, Lt Col (Ret), USAF (Member) Date

    ___________________________________ __________

    Dr. William E. Wiesel, Jr. (Member) Date

  • AFIT-ENY-13-M-36

    iv

    Abstract

    This research focuses on experimentally determining the performance parameters

    of a colloid thruster with porous emitters using optical techniques. Porous emitters

    contrast with traditional needles by allowing a variation in the number and size of Taylor

    Cone locations throughout a range of propellant flow rates. Droplet exhaust

    characteristics, thrust, and specific impulse specifications must be investigated across the

    span of flow rates to understand the thruster's operational envelope and potential mission

    capabilities. Optical methods, including image analysis, image correlation, and use of a

    fiber optic distance sensor, are evaluated to verify their applicability in obtaining the

    thruster's range of performance. Algorithms are created to calculate the estimated

    minimum and maximum number of emitter sites based on imaged porous emitter head

    topography. Then, theoretical performance models for the variable number of emitter

    sites are established. The variable and fixed numbers of emitter site models present a

    positive order of magnitude comparison demonstrating the initial accuracy of the variable

    model. This variable emitter baseline model can be utilized by future researchers to

    understand the expected performance range prior to laboratory testing. Experimental

    techniques for evaluating thrust through elasticity measurements and droplet exit velocity

    with image examination schemes are proposed and assessed for their compatibility with

    thruster testing. Calibration and sensitivity analyses show the elasticity procedure is a

    practical method for thrust determination. Comparisons explain why image correlation is

    a more promising approach to velocity determination than image streak tracking.

    Upcoming experimentalists may apply the predetermined elasticity and image correlation

  • AFIT-ENY-13-M-36

    v

    methods to obtaining the colloid thruster thrust and exit velocity, respectively. The

    research results provide theoretical thruster performance models and experimental

    procedural investigations laying a foundation for future investigational colloid thruster

    testing and characterization.

  • AFIT-ENY-13-M-36

    vi

    To my amazing wife and wonderful daughter

  • vii

    Acknowledgments

    I would like to thank my advisors Capt Liu and Lt Col (Ret) Hartsfield for their

    guidance in this endeavor. Also, I would like to thank my sponsor, Dr. Hargus for

    allowing me to work on this project and learn more about electric propulsion than I ever

    could in a classroom. I thank Dr. Reeder for his support and willingness to help me

    whenever needed. Lastly, I thank all of the ENY lab techs, Capt Ober, and everyone else

    who helped me along the way. I couldnt have done it without you.

    Eric T. Wolf

  • viii

    Table of Contents

    Page

    Abstract .............................................................................................................................. iv

    Acknowledgments............................................................................................................. vii

    Table of Contents ............................................................................................................. viii

    List of Figures ......................................................................................................................x

    List of Tables .....................................................................................................................xv

    Nomenclature ................................................................................................................... xvi

    List of Abbreviations ....................................................................................................... xix

    I. Introduction .....................................................................................................................1

    1.1 General Issue ....................................................................................................1 1.2 Problem Statement ............................................................................................3

    1.3 Research Objectives ..........................................................................................5

    II. Literature Review ...........................................................................................................6

    2.1 Electrostatic Propulsion Theory .......................................................................6 2.2 Colloid Thruster Theory ...................................................................................9

    2.3 Taylor Cone Formation ...................................................................................10 2.4 Droplet Theory ................................................................................................13

    2.5 Performance Equations ...................................................................................17 2.6 Colloid Thruster and Electrospray Research History .....................................24

    2.7 Current Colloid Thruster Research .................................................................25 2.8 Thrust Measurement .......................................................................................29

    2.8.1 Force Balance ................................................................................................ 29 2.8.2 Time of Flight ................................................................................................ 30 2.8.3 Elasticity Measurement ................................................................................. 31

    2.9 Flowfield Velocity Measurement ...................................................................33 2.9.1 Direct Visualization ....................................................................................... 34

    2.9.2 Planar Doppler Velocimetry (PDV) .............................................................. 35 2.10 Summary .........................................................................................................37

    III. Methodology ...............................................................................................................38

    3.1 1U Colloid Thruster ........................................................................................38 3.2 Vacuum Chamber ...........................................................................................43

  • ix

    Page

    3.3 Emitter Surface Imaging .................................................................................46

    3.4 Theoretical Performance Envelope .................................................................53 3.5 Thrust Measurement .......................................................................................57 3.6 Exit Velocity Measurement ............................................................................69 3.7 Taylor Cone Formation ...................................................................................79 3.8 Summary .........................................................................................................85

    IV. Analysis and Results ...................................................................................................86

    4.1 Emitter Number Range Development ............................................................86 4.2 Theoretical Performance .................................................................................93 4.3 Thrust experiment .........................................................................................101

    4.4 Exit Velocity Experiment