Orion is NASA’s newest spacecraft designed to takes humans farther than ever into space.  With the launch and splash down of Orion’s first unmanned flight test on December 5, 2014, we look to the future of space travel and the potential of manned missions to Mars and asteroids.  With such forward looking and grand plans it is ironic that NASA has gone back to the 1960s formulations for the heat shield material; see Fig. 1. NASA is using ablative materials for the heat shield in contrast to the tiles used on the Space Shuttles.

These ablative heat shields are not reusable whereas tiles used on the Space Shuttles were reusable. The ablative heat shield functioning is in distinct contrast to the Space Shuttle tiles that were thermally stable during the heating that occurred during reentry.  Although the black Space Shuttle tiles are part of Orion’s thermal management system making up the back shell, the material exposed to the highest temperatures will be an ablative material that dates back to the Apollo program.

The assessment of the heat shield, and more generally the thermal protection system, is one of the critical tests on this initial flight of the Orion spacecraft.  Specifically, a flight objective as set by NASA and Lockheed Martin reads (1)

“Demonstrate thermal protection system performance during a high-energy return, when Orion will travel near 20,000 mph (32,000 kph), generating 4,000 degrees Fahrenheit (2,200 degrees Celsius) on its heat shield and 3,150 degrees Fahrenheit (1,730 degrees Celsius) on its backshell”

Figure 1  As Orion pushes air particles out of its way, those particles heat up. Temperatures around the vehicle reach 2200 °C.  A heat shield protects occupants and equipment from this extreme heat generated during atmospheric reentry.  Engineered voids play an important role in the functioning of this heat shield. (Source: http://www.nasa.gov/externalflash/orionfirstflight/)

We would like to discuss the importance of voids in the general design of heat shields as well as the formulation of Orion’s.  A syntactic foam is a class of material that is widely used as a heat shield material.  They are made by incorporating hollow particles (microballoon or hollow microsphere) in a binder phase.  Syntactic foams can be subdivided into two-, three-, or multi­-phase foams, see Fig. 2.

A two-phase syntactic foam has only the binder and hollow sphere phase. The void is present inside the shell of the hollow sphere and is sometimes referred to as a reinforced void.

A three-phase syntactic foam does not have enough binder to fill the spaces between hollow microspheres.  In addition to the reinforced voids, a three-phase syntactic foam has voids in the binder, which constitute the third phase. These voids in the binder are referred to as unreinforced or interstitial voids.

A multiphase syntactic foam is made by additions of fibers and/or other particles to a two- or three-phase syntactic foam.  Figure 3 illustrates a multiphase syntactic foam by the addition of discontinuous fibers to a three-phase syntactic foam.

This heat shield that glows red-hot during reentry undergoes a process called ablation.   This process is characterized by the removal of surface material by vaporizing it. This is somewhat analogous to “evaporation leaving a cooling effect.” Think of what happens when a volatile liquid is placed on your skin or the use evaporative coolers.  During atmospheric entry of a spacecraft, extreme ablative conditions are present; these include high velocity, high temperature potentially in an oxi­dizing environment, and erosion from friction with the atmospheric gases. The important material properties required for ablative resistance are low density, low thermal conductivity, high temperature resistance, formation of a stable char, and high char shear strength. To meet these requirements, the multiphase syntactic foams are used extensively as ablative materials.

Voids play an important role in the functioning of an ablator. Space travel requires low density materials to minimize weight, thus reducing the energy requirement to lift a vehicle either into orbit or to escape the pull of Earth’s gravity. Designing voids into materials reduces density. Other important properties that voids bring to ablative materials are low thermal conductivity and low coefficient of thermal expansion.

The ablative material shown in Fig. 3 is a multiphase syntactic foam containing both reinforced and unreinforced voids as well as discontinuous fibrous phase. The unreinforced void is the interstitial void between the filler mate­rials. Common reinforced voids are a hollow glass, phenolic or carbon microspheres.  Common binder phase materials for ablative materials are phenolics and silicones because of their high char yield.

Figure 2 A schematic of a two-phase,  three-phase, and multiphase syntactic foam. Note that in the three-phase foam, there is both reinforced (microballoon) and unreinforced (interstitial) void present.

Figure 3  A micrograph of a multiphase syntactic foam  having hollow particles, binder, interstitial voids and fibers.  This is similar to the syntactic used on Orion’s heat shield. Note the presence of reinforced and unreinforced voids. The binder, not labeled, is only present in a small volume fraction. This binder phase coats the fibers and hollow spheres, bonds them together, and chars upon entry into the planetary atmosphere. SEM.  (Courtesy of ARA Ablatives Laboratory, Centennial CO, USA.)

Orion’s heat shield receives the brunt of the heat during planetary reentry, reaching temperatures up to 2200 °C. The Orion heat shield material has its roots in the Apollo missions that first took humans to the Moon. Figure 3 shows a scanning electron micrograph of such an ablative material, similar to that of the Orion heat shield.  Orion’s four-phase syntactic foam heat shield is composed of:

1. hollow particles (reinforced voids) – hollow phenolic microspheres
2. binder phase – novolac epoxy
3. unreinforced interstitial voids – engineered porosity in the novolac epoxy
4. glass fibers

The hollow particles used in the Orion formulation are hollow phenolic spheres with an average diameter of a human hair, the binder phase is a novolac epoxy, there are interstitial voids in The Space Between hollow phenolic particles, and finally the glass fibers.  Providing additional structural support to the syntactic foam is a honeycomb made from glass fiber with a phenolic matrix.  Each cell of the honeycomb is filled with the syntactic foam material described above.  This heat shield formulation is called AVCOAT® and has a density of 0.51 g/cm³ (32 lb/ft³) which is about ½ the density of water.  The honey­comb adds compressive strength and helps maintain the structural integrity of the syntactic foam during service.  Figure 4 is a picture from our book and shows a heat shield that has been tested at the Sandia National Laboratory’s Solar Tower Facility Solar Facility, http://energy.sandia.gov/energy/renewable-energy/solar-energy/csp-2/nsttf/  , in a test designed to simulate planetary entry.  One can clearly see the honeycomb structure in Fig. 4, exposed after the test.

Figure 4  The posttest condition of a  heat shield after a solar tower test. Note the presence of honeycomb in the heat shield. (Courtesy of ARA Ablatives Laboratory, Centennial, CO, USA.)

We are looking forward to NASA’s assessment of the heat shield performance from this first flight test.  We discuss the role of reinforced and unreinforced voids, material design, and testing on ablative materials in our book, Voids in materials: From unavoidable defects to designed cellular materials.  The basic premise of the book is that, at some level, all solids contain voids, whether intrinsic or intentional.  Sometimes the voids are ignored, at other times they are taken into account, and other times they are the focal point of the research.  In the book, we take due notice of all of these occurrences of voids, whether designed or unavoidable defects. We define these voids (or empty spaces in materials), categorize them, characterize them, and describe the effect they have on material properties.

Read more about ‘The Space Between’ and materials voids by Gary:

• Biomaterials and Much Ado About Nothing(ness)
• Reverse-Engineering Bread
• The Space Between: How Voids in Materials Contribute to 21st Century Society
• To the Bottom of the Sea Using Buoyancy Voids
• A Cause and Solution to Global Warming

Gladysz and Chawla’s upcoming book Voids in Materials: From Unavoidable Defects to Designed Cellular Materials is now available for pre-0rder on the Elsevier Store. Use discount code “STC3014” and save up to 30% on your very own copy!

About the Authors

Dr. Gary Gladysz (Twitter: @GMGladysz) is an Associate at Empyreus Solutions, LLC, Seattle WA, USA, where he consults and leads university and government technical interactions. He received his PhD from the New Mexico Institute of Mining & Technology where he participated in the NATO Collaborative Program with the German Aerospace Institute (DLR).   Since receiving his PhD, he has led research efforts in university, government and industrial settings. He has extensive research experience designing and characterizing fibrous composite materials, ceramic composites, polymers, composite foams, and thin films.

As a Technical Staff Member at Los Alamos National Laboratory (LANL), he was technical lead for Rigid Composites and Thermoset Materials. In 2005 he was awarded the LANL Distinguished Performance Group Award for his work leading materials development on the Reliable Replacement Warhead Feasibility Project. He has served on funding review boards for LANL, National Science Foundation, ACS, and the Lindbergh Foundation. He has been guest editor on four issues of leading materials science journals, including Journal of Materials Science and Materials Science & Engineering: A. Dr. Gladysz has organized five international conferences/symposia on syntactic foams and composite materials. He started and currently chairs the ECI international conference series on Syntactic & Composites Foams. Dr. Gladysz currently lives near Boston, Massachusetts, USA.

Krishan Chawla is Professor Emeritus of Materials Science and Engineering at the University of Alabama at Birmingham, USA. He received his Ph.D. from the University of Illinois at Urbana-Champaign. His research interests encompass processing, microstructure, and mechanical behavior of materials. He has taught and/or done research at several universities around the world. Professor Chawla has served as a Program Director for metals and ceramics in the Division of Materials Research, National Science Foundation.

He is a fellow of ASM International. Among his other awards are: Distinguished Researcher Award at New Mexico Tech, Distinguished Alumnus Award from Banaras Hindu University, Eshbach Society Distinguished Visiting Scholar award at Northwestern University, Faculty Fellow award at Oak Ridge National Laboratory. Professor Chawla is the author or coauthor of various textbooks in the area of materials and serves on the editorial boards of a number of journals. He is editor of the journal International Materials Reviews.