Thermal The Space Shuttle design presented many thermal insulation challenges. The system not only had to perform well, it had to integrate Protection with other subsystems. The Orbite r’s surfaces were exposed to Systems exceedingly high temperatures and needed reusable, lightweight, low-cost thermal protection. The vehicle also required low vulnerability to orbital debris and minimal thermal conductivity. NASA decided to Introduction bond the Orbite r’s thermal protection directly to its aluminum skin, Gail Chapline which presented an additional challenge. Orbiter Thermal Protection System Alvaro Rodriguez The External Tank required insulation to maintain the cryogenic fuels, Cooper Snapp Geminesse Dorsey liquid hydrogen, and liquid oxygen as well as to provide additional Michael Fowler structural integrity through launch and after release from the Orbiter. Ben Greene William Schneider The challenge and solutions that NASA discovered through tests and Carl Scott flight experience represent innovations that will carry into the next External Tank Thermal Protection System generation of space programs. Myron Pessin Jim Butler J. Scott Sparks Solid Rocket Motor Joint—An Innovative Solution Paul Bauer Bruce Steinetz Ice Detection Prevents Catastrophic Problems Charles Stevenson Aerogel-based Insulation System Charles Stevenson 182 Engineering Innovations Orbiter Thermal most difficult problems one can materials would have to be developed, imagine. It is certainly a problem that as the technology from the previous Protection System constitutes a challenge to the best brains Mercury, Gemini, and Apollo flights working in these domains of modern were only single-mission capable. Throughout the design and development aerophysics.” He was referring to Engineers embraced this challenge by of the Space Shuttle Orbiter Thermal protecting the intercontinental ballistic developing rigid silica/alumina fibrous Protection System, NASA overcame missile nose cones. Fifteen years later, materials that could meet the majority many technical challenges to attain a the shuttle offered considerably greater of heating environments on windward reusable system that could withstand the difficulties. It was vastly larger. Its surfaces of the Orbiter. On the nose high-temperature environments of thermal protection had to be reusable, cap and wing leading edge, however, re-entry into Earth’s atmosphere. and this thermal shield demanded both the heating was even more extreme. Theodore von Karman, the dean of light weight and low cost. The In response, a coated carbon-carbon American aerodynamicists, wrote in requirement for a fully reusable system composite material was developed to 1956, “Re-entry is perhaps one of the meant that new thermal protection Thermal Protection System Could Take the Heat Orbiter remained protected during catalytic heating. While the re-entry surface heating of the an exothermic reaction. Since the surface Orbiter to reject a majority of the chemical Orbiter was predominantly convective, acted as a catalyst, it was important that the energy. Engineers performed precise arc sufficient energy in the shock layer interfacing material/coating have a low jet measurements to quantify this effect dissociated air molecules and provided the propensity to augment the reaction. Atomic over a range of surface temperatures for potential for additional heating. As the air recombination influenced NASA’s selection both oxygen and nitrogen recombination. molecules broke apart and collided with the of glass-type materials, which have low This resulted in improved confidence in the surface of the vehicle, they recombined in catalycity and allowed the surface of the Thermal Protection System. Flow NitrogenNitrogen and oxygen molecules areare dissociated in the shock layerlayer.r.. Atomsoms may rrecombineecombinee and form molecules on the vehicle surface. Boundary Layer Shock Layer Oxygen molecules Recombination of atoms on in the shock layer the surface of the vehicle separate into adds heat of dissociation to O+ and O- atoms. the Thermal ProtectionPrrotection System. Engineering Innovations 183 form the contours of these structural externally to the outer structural skin System to prevent a transition to components. NASA made an of the Orbiter to passively maintain the turbulent flow early in the flight when exhaustive effort to ensure these skin within acceptable temperatures, heating was at its highest. materials would operate over a large primarily during the re-entry phase Requirements for the Thermal spectrum of environments during of the mission. During this phase, the Protection System extended beyond launch, ascent, on-orbit operations, Thermal Protection System materials the nominal trajectories. For abort re-entry, and landing. protected the Orbiter’s outer skin from scenarios, the systems had to continue to exceeding temperatures of 176 °C perform in drastically different (350 °F). In addition, they were reusable Environments environments. These scenarios included: for 100 missions with refurbishment and Return-to-Launch Site; Abort Once During re-entry, the Orbiter’s external maintenance. These materials performed Around; Transatlantic Abort Landing; surface reached extreme temperatures— in temperatures that ranged from and others. Many of these abort up to 1,648°C (3,000°F). The Thermal -156°C (-250°F) in the cold soak of space scenarios increased heat load to the Protection System was designed to to re-entry temperatures that reached vehicle and pushed the capabilities of provide a smooth, aerodynamic surface nearly 1,648 °C (3, 000° F). The Thermal the materials to their limits. while protecting the underlying metal Protection System also withstood structure from excessive temperature. the forces induced by deflections The loads endured by the system of the Orbiter airframe as it responded Thermal Protection included launch acoustics, aerodynamic to various external environments. System Materials loading and associated structural At the vehicle surface, a boundary deflections, and on-orbit temperature Several types of Thermal Protection layer developed and was designed variations as well as natural System materials were used on the to be laminar—smooth, nonturbulent environments such as salt fog, wind, Orbiter. These materials included tiles, fluid flow. However, small gaps and and rain. In addition, the Thermal advanced flexible reusable surface discontinuities on the vehicle surface Protection System had to resist insulation, reinforced carbon-carbon, could cause the flow to transition from pyrotechnic shock loads as the Orbiter and flexible reusable surface laminar to turbulent, thus increasing separated from the External Tank (ET). insulation. All of these materials used the overall heating. Therefore, tight high-emissivity coatings to ensure The Thermal Protection System fabrication and assembly tolerances the maximum rejection of incoming consisted of various materials applied were required of the Thermal Protection convective heat through radiative heat Orbiter Tile Placement System Configuration Reinforced Carbon-Carbon Coating High-temperature Reusable Surface Insulation Tile Low-temperature Reusable Surface Insulation Tile Advanced Flexible Reusable Surface Insulation Blanket Flexible Reusable Surface Insulation Blanket 184 Engineering Innovations H Orbiter Tile Attachment System High-temperature Reusable Surface Insulation Reaction-cured Tile-to-Tile Gap Glass Coating Tile Densied Layer Filler Bar Koropon®-primed Structure Room-temperature Vulcanizing Adhesive Strain Isolation Pad transfer. Selection was based on the reusable surface insulation. Surface Fibrous Refractory Composite temperature on the vehicle. In areas coating constituted the primary Insulation tiles helped reduce the in which temperatures fell below difference between these two categories. overall weight and later replaced the approximately 1,260°C (2,300°F), High-temperature reusable surface LI-2200 tiles used around door NASA used rigid silica tiles or fibrous insulation tiles used a black borosilicate penetrations. Alumnia Enhanced insulation. At temperatures above glass coating that had an emittance Thermal Barrier was used in areas in that point, the agency used reinforced value greater than 0.8 and covered areas which small particles would carbon-carbon. of the vehicle in which temperatures damage fragile tiles. As part of the reached up to 1,260°C (2,300°F). post-Columbia Return to Flight Tiles Low-temperature reusable surface effort, engineers developed Boeing insulation tiles contained a white Rigidized Insulation. Overall, the The background to the shuttle’s tiles coating with the proper optical major improvements included lay in work dating to the early 1960s properties needed to maintain the reduced weight, decreased at Lockheed Missiles & Space appropriate on-orbit temperatures for vulnerability to orbital debris, and Company. A Lockheed patent vehicle thermal control purposes. minimal thermal conductivity. disclosure provided the first description The low-temperature reusable surface of a reusable insulation made of Orbiter tiles were bonded using strain insulation tiles covered areas of the ceramic fibers for use as a re-entry isolation pads and room-temperature vehicle in which temperatures reached vehicle heat shield. In other phased vulcanizing silicone adhesives. The up to 649°C ( 1,200°F). shuttle Thermal Protection System inner mold line of the tile was densified development efforts, ablatives and hot The Orbiter used several