MULTI-SCALE THERMAL and STRUCTURAL CHARACTERIZATION of CARBON FOAM for the PARKER SOLAR PROBE THERMAL PROTECTION SYSTEM by Eliza
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MULTI-SCALE THERMAL AND STRUCTURAL CHARACTERIZATION OF CARBON FOAM FOR THE PARKER SOLAR PROBE THERMAL PROTECTION SYSTEM by Elizabeth Ann Congdon A dissertation submitted to the Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy Baltimore, Maryland February 2021 ABSTRACT Since the founding of the National Aeronautics and Space Administration (NASA), a top mission priority has been to study the Sun in situ. For sixty years, this mission remained just an aspiration of heliophysicists due to the inherent limitations of the engineering technology required to make the mission possible. One critical spacecraft element is the thermal protection system (TPS) or heat shield, a low mass self- supporting structure that required implementation of a structurally integral insulation sandwich panel. For this multifunctional sandwich panel approach to be successful, the thermal and structural properties of the main insulative core, carbon foam, needed to be understood to validate that the TPS could survive the launch loading and the extreme temperatures at its perihelion. An extensive test program including coupon, subscale and full-scale evaluation was undertaken to determine the properties of this low-density lattice and to compliment full-scale modeling. Candidate foams, ranging from 3% to 10% relative density, were evaluated using both the guarded hot plate method and thermal diffusivity methods for effective thermal conductivity. The effective thermal conductivity was primarily driven by solid conductivity around room temperature and shown to follow simple Gibson and Ashby predictions. At temperatures beyond 900°C, radiation was much more important and the test data diverged from the model. These coupon-level tests were also used to show the influence of relative density and foam architecture on the effective thermal conductivity of these foams. Subscale thermal evaluation of a representative thickness was performed at Oak Ridge National ii Laboratory (ORNL). The subscale results from this testing at temperatures above 900°C diverged from the coupon results and model predictions, indicating that thickness plays an important role in the overall effective thermal conductivity at these higher temperatures. Structural testing of coupon, subscale and full-scale specimens were conducted in concert with the thermal experiments. Tension, compression and shear loading were executed in the three primary directions of the foam. The coupon results revealed anisotropy, which could not be explained by purely geometric structural anisotropy as advocated by Gibson and Ashby. Subscale bending tests were consistent with coupon test data but size effects were observed; the smaller region of elevated stress, associated with bending, resulted in lower variability than was seen in the uniaxially loaded coupons. The data obtained in these experiments formed the baseline material properties database that was used for full scale modeling, which employed a margining approach and was dependent on material properties consistent with the success criteria for spaceflight missions. All full-scale units successfully survived acoustic, vibration and cold thermal testing and were used to certify the flight design. On August 12, 2018, the Parker Solar Probe (PSP) spacecraft lifted off from Cape Canaveral, FL on its way to the Sun. To date, it has successfully completed six passes around the Sun, and it has broken the record for being the closest manmade object to the Sun three times. The heat shield is performing as expected, and the mission has already returned groundbreaking science. iii Primary Reader and Advisor: Professor Kevin Hemker Secondary Readers: Professor Jamie Guest and Professor Jaafar El-Awady iv ACKNOWLEDGEMENTS To begin, I would like to thank my Ph.D. advisor, Professor Kevin J. Hemker for his support, guidance, patience, encouragement and mentoring over the last ten years. I was an unusual Ph.D. student and he saw my abilities even when I could not see them in myself. He was constantly encouraging me and supporting me throughout my time at Hopkins. I would also like to thank my readers Professor Jamie Guest and Professor Jaafar El-Awady for their time and support during this process. If there is one thing that I have learned in the last 10 years, it is that I am someone who loves working with people. I have been lucky enough to be surrounded by wonderful, dedicated people who have generously taught me about materials, testing, life, and friendship. I like to say that my favorite part of my job is that no one person with all the time and knowledge can build a spacecraft. It takes hundreds of thousands of people working towards a singular goal. Before launch during press events, I would say we are going to rewrite science textbooks and that my children would learn about the Sun differently due to the work that we did. What is amazing is that is already true. I am humbled to be a small part of that. It is an understatement to say that this work would not have been possible without hundreds of people who supported me in big and small ways along the way. Specifically, I would like to thank Doug Mehoke for his mentoring, support, kindness and leadership. I would not be where I am today without you. Ed Schafer has taught me a lifetime of knowledge about structural materials and spacecraft and much of that is reflected here. Shelly Conkey has always supported me, guided me and taught me about technical matters and friendship. I am lucky to be v working with her currently on my second spacecraft. Luke Becker has been a constant supporter of mine and read through every chapter of this dissertation and for that I cannot thank him enough. Others at APL that contributed to this work and my growth are Liz Abel, Elizabeth Heisler, Ted Hartka, Andy Driesman, Neal Bachtell, Erin LaBarre, Andy Lennon, Bruce Trethewey, Ryan Deacon, Craig Leese, Tony Ahan, Daniel Eby, and Kelles Gorges. At C- CAT and Plasma Processes, Aaron Brown, James Thompson, Erika Rensden, Johnny Menidola, Chris McKelvey and Daniel Butts were important friends and colleagues without whom this work would not have been possible. This work was done as part of the Parker Solar Probe program for NASA and I would be amiss to not thank them for their support. I would also like to thank friends both at WSE and without for all their support throughout my graduate studies. There have been many additions to the Hemker group throughout my time and everyone has been supportive and encouraging. I am honored to be a part of such a wonderful group of people. Specifically, Dr. Justin Jones was an early collaborator on Solar Probe and Dr. Gianna Valentino came into the group later in my studies. Both provided great friendship and council. Outside of Hopkins, Simmie Berman, Shelby Martin, David Solomon, Patrick Sanders, Jennifer Glicoes, Kevin Fox, and Miranda Selover, thank you all for your unwavering support through this dissertation and your friendship. There are too many other people to name but all the friends and members of the Mechanical Engineering department that helped me through my studies and research were important. vi Finally, I would like to thank my family for all their support and love. My parents, Lee and Kathy, my mother-in-law, Patti and my brother-in-law Deny, all supported and cheered for me throughout. To my husband, Billy Gallagher, I know that I started this Ph.D. before I met you, but it would not have been completed without you. Thank you for always believing in me, loving me and supporting my goals. I dedicate this work to my parents. To my mother, Kathy Bissell who taught me that women can do anything and to always fight the good fight for what you believe in. To my father, Lee Congdon who gave me a curiosity about the world that has always taken me to great places. vii TABLE OF CONTENTS ABSTRACT .................................................................................................................. ii ACKNOWLEDGEMENTS ............................................................................................... v TABLE OF CONTENTS ................................................................................................ viii LIST OF TABLES ........................................................................................................... x LIST OF FIGURES ........................................................................................................ xi CHAPTER 1. Introduction ......................................................................................... 1 1.1. Motivation-Parker Solar Probe .................................................................... 2 1.2. Overview of Dissertation ........................................................................... 21 CHAPTER 2. Background ........................................................................................ 23 2.1. Overview of Historical Thermal Protection Systems ................................... 24 2.1.1. Overview of Ablative Systems ............................................................ 25 2.1.2. Overview of Reusable Systems ........................................................... 27 2.1.3. Impact of Historical TPS Approaches on the PSP TPS ........................... 30 2.2. Overview of Carbon Foams Under Study .................................................... 32 2.3. Historical References of Foam Behavior ..................................................... 37 2.4. Thermal Properties of Cellular Materials