Orbiter Thermal Protection System Research Articles

Nondestructive Evaluation (NDE) for Characterizing Oxidation Damage in Cracked Reinforced Carbon–Carbon

In this study, coated reinforced carbon–carbon (RCC) samples of similar structure and composition as that from the NASA space shuttle orbiter's thermal protection system were fabricated with slots in their coating simulating craze cracks. These specimens were used to study oxidation damage detection and characterization using nondestructive evaluation (NDE) methods. These specimens were heat treated in air at 1143°C and 1200°C to create cavities in the carbon substrate underneath the coating as oxygen reacted with the carbon and resulted in its consumption. The cavities varied in diameter from approximately 1 to 3 mm. Single‐sided NDE methods were used because they might be practical for on‐wing inspection, while X‐ray micro‐computed tomography (CT) was used to measure cavity sizes in order to validate oxidation models under development for carbon–carbon materials. An RCC sample having a naturally cracked coating and subsequent oxidation damage was also studied with X‐ray micro‐CT. This effort is a follow‐on study to one that characterized NDE methods for assessing oxidation damage in an RCC sample with drilled holes in the coating.

The Trail to the Leading Edge

This article focuses on SwRI that had conducted impact experiments, experiments of the sort that would be needed now, on the orbiter's thermal protection system. Studies had included the effects of foam insulation, ablator, and ice striking both carbon-carbon materials similar to those that line the shuttle's leading edges and the silica thermal protective tiles that cover most of the rest of the craft. Concurrently, SwRI developed detailed analytic and numerical models of foam insulation's impact on thermal tiles. The models provided a curve for distinguishing between damage and no damage, based on speed and impact angles. Given an impact speed and angle for an incoming piece of foam insulation, the model determined whether or not tile material would sustain damage. The SwRI model agreed extremely well with previous tile impact tests and would agree with the five tests performed during the Columbia accident investigation of foam insulation impacting tiles.

Critically indexing for job/task improvement at the Kennedy space center

America's Space Exploration program is characterized by cutting edge technology in support of NASA's goals and research objectives. One principal component of the Space Shuttle System is the Orbiter Vehicle which undergoes necessary repairs and refurbishment at the Kennedy Space Center following each mission. The Orbiter's Thermal Protection System (TPS) is a mission critical component which protects the orbiter from the heat of re-entry from space. The University of Central Florida (UCF) research team was tasked with evaluating TPS processing requirements to reduce overall cycle times between operational mission. A job/task performance analysis was conducted to identify candidate TPS processes for evaluation and possible improvements. The research team developed a computer generated Improvement Potential Index (IPI) following a systematic data collection process. The IPI was then used to identify the top twenty processes which TPS engineers, quality assurance personnel and technicians identified as tasks which would most likely benefit from process improvement. The specified tasks were then analyzed for methods optimization. This paper documents the results of this effort to date, with emphasis on the Improvement Potential Index.

Effect of simulated mission loads on orbiter thermal protection system undensified tiles

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

Surface Step Induced Gap Heating in the Shuttle Thermal Protection System

An analytical study has been performed to investigate the excessive heating in the tile-to-tile gaps of the Shuttle Orbiter Thermal Protection System due to stepped tiles. The excessive heating was evidenced by visible discoloration and charring of the filler bar and strain isolation pad that is used in the attachment of tiles to the aluminum substrate. Two tile locations on the Shuttle Orbiter were considered: one on the lower surface of the fuselage and one on the lower surface of the wing. The gap heating analysis involved the calculation of external and internal gas pressures and temperatures, internal mass flow rates, and the transient thermal response of the Thermal Protection System. The results of the analysis are presented for the fuselage and wing location for several step heights. Nomenclature Ac = cross-sectional area for flow B = width of flow path h = tile step height k = thermal conductivity Kp = permeability constant m = mass flow rate M = Mach number P = pressure q = conduction heat flux Rec = Reynolds number for base pressure correlation s = straight line length between points T = temperature w = tile-to-tile gap width X, Y = Shuttle Orbiter coordinates AJf = longitudinal coordinate measured in the downstream direction from the point of separation kXff = longitudinal coordinate measured in the upstream direction from a forward-facing step <5 = velocity boundary-layer thickness 6* = boundary-layer displacement thickness 6eff = d for laminar flow, 1.56* for turbulent flow A = sweep angle of a tile with respect to the local flow \L = viscosity coefficient p = density

Shock-induced flow separation and the orbiter thermal protection system

The Space Shuttle orbiter's thermal protection system (TPS) is composed of reusable tiles separated by narrow gaps that accommodate the contraction and expansion of the aluminum structure that the tiles protect. When local pressure gradients exist, air flows through the tile gaps and releases heat energy by convection. The gaps represent a heat short to the structure, strain isolator pad (SIP), and filler bar. A typical problem is the pressure gradient created during entry by body flap deflection. After a brief description of how this problem affects the Space Shuttle orbiter, a theoretical and experimental review of the major parameters involved in gap heating are analyzed. Then, a review of well-known classical methods to resolve the gap aeroheating problem in the presence of a pressure gradient is presented, and a few solutions are illustrated to assess the sensitivity of each one. The following section starts with a basic relationship (called “eyeball” because of its simplicity) and follows the results up through the most modern engineering approach available in the literature. It shows that in all cases calculated significant areas of overtemperature were predicted. However, none of these methods could be correlated by experimental data. Lastly, the paper presents the solution obtained by using the most sophisticated method, based upon the Navier-Stokes equations. This approach shows excellent correlation with wind tunnel data. The application to four trajectory time points shows less severe results than the other methods. This can be explained by the introduction of a certain amount of conservatism to account for uncertainties inherent in the previous analyses. No correlation of this “exact solution” with the simple preestablished relationships has been found, indicating that more parameters than expected could be involved. However, an after-the-fact, semi-empirical engineering solution that fits the Navier-Stokes solution with good agreement was established.

Benchmark determination of Shuttle Orbiter entry aerodynamic heat-transfer data

Entry aerodynamic heat-transfer data were determined from the first flight of the Space Shuttle Orbiter. The convective heating rate data result from a rigorous mathematical analysis of one-dimension al, transient heat conduction within the orbiter thermal protection system (TPS) and reradiation from its surface during entry. Temperatures measured at the TPS surface during orbiter entry provide a surface boundary condition for the analysis. The rigorous mathematical treatment provides for a benchmark determination of in-flight convective heat-transfer rates. The source of the entry thermal data, the mathematical analysis technique, and the TPS thermal models are discussed. Typical convective heating rate data from the STS-1 mission are presented. cp GMT k NBS q #conv #rerad STS T Ts t x XI L e X p Nomenclature specific heat Greenwich mean time thermal conductivity National Bureau of Standards heating rate convective heating rate reradiated heating rate (eaT*) Space Transportation System temperature surface temperature = depth dimension - nondimensional body length = emissivity = thickness = density a . = Stephan-Boltzmann constant

Identifying the Aerothermodynamic Environment of the Space Shuttle Orbiter, Columbia

We consider the system identification problem associated with determining the aerothermodynamic heating environment of the Space Shuttle Orbiter Columbia during reentry. A mathematical characterization of this environment is required for mission capability assessment as well as operational flight planning. The identification problem is one of estimating certain parameters in a fairly complete nonlinear, nonstationary partial differential equation (diffusion) model of the heat transfer characteristics of the orbiter thermal protection system (tiles) interacting with the surface boundary phenomena of aerodynamic forced convection and radiation. Cur solution uses a maximum likelihood technique that incorporates discrete, depth-wise (thermocouple) temperature measurements and reentry trajectory data. Results of analyzing flight test data collected during the first flight of the Columbia are included for illustrative purposes.

Dynamic and static modeling of the Shuttle Orbiter's Thermal Protection System

Seventy percent of the surface of the Shuttle Orbiter is covered by a Thermal Protection System, consisting of ceramic tiles bonded to the Orbiter's aluminum skin through a felt pad. The structural evaluation of the tile/pad system is complicated by the nonlinear elastic properties of the pad. This paper describes the dynamic and static analysis methods utilized to model this nonlinear behavior. Comparisons between test and analysis are shown. Vibration tests of individual tiles identified tile response which results from the nonlinearity of the pad. These phenomena include a highly distorted response waveform when the tile/pad system is base-driven dinusoidally and resonant frequencies which are dependent on both steady pre-load and amplitude of the base drive input. The dynamic analysis accounts for the nonlinear stiffening, hysteresis and viscous behavior of the pad material. Material characterization tests of the pad revealed a load displacement nonlinearity which is agravated with repeated load cycles. A nonlinear stress analysis procedure was developed and programmed to permit the calculation of stresses at the tile/pad bondline for an arbitrary pad contiguration.

Reacting Shock Layers with Slip and Catalytic Boundary Conditions

The influence of wall slip and catalytic atom-recombination on the flowfield and wall heat flux are calculated for high-altitude flight and arcjet-flow conditions. Boundary equations, which include velocity slip, temperature jump, and wall catalytic atom-recombination, are coupled to the viscous reacting multicomponent NavierStokes equation. These equations are solved using a time-dependent finite difference technique applied to spheres in an arcjet flow (Reynolds number of 550) and a high-altitude flight case representative of the space shuttle orbiter (Reynolds number of 450). The results indicate that catalysis strongly influences the temperature jump, but not the velocity slip. Slip increases the atom fraction and temperature at both the wall and in the flowfield. Likewise, the shock stand-off distance, the wall heat flux, and friction coefficient are increased over the nonslip cases. The reacting gas calculations confirm the chemically frozen nature of the shock layer in arcjet flows. design the reusable space shuttle orbiter for highaltitude, low Reynolds number, atmospheric entry necessitates a more comprehensive treatment and understanding of the interaction between a high enthalpy gas flow and a relatively cold surface. At low densities the continuum model of the gas breaks down in regions of large gradients such as those near a cold body. Corrections to the equations for the boundary conditions are then required for the flow. These influences are reflected in the calculations of surface heat-transfer rate and chemical composition of the flow near the wall. The aim of this paper is to quantify how these low density phenomena interact, how they influence interpretation of test data obtained on thermal protection systems, and how they alter the predictions of heating rates and performance of the space shuttle orbiter thermal protection system (TPS) during its long high-altitude entry. The approach taken here is to obtain finite difference solutions to the reacting Navier-Stokes equations for the flow around spheres in both space shuttle flight and arcjet environments. The wall boundary conditions for these solutions are obtained from slip and jump relations for a nonequilibrium multicomponent gas mixture. They include the effects of catalytic atom recombination. From the solutions one can assess the effects. of the boundary conditions on the flow properties, the heat flux, etc. At low densities, the continuum-flow equations are no longer adequate close to the wall because the mean free path becomes long compared to characteristi c lengths associated with significant changes in macroscopic-flow parameters. The flow in a region next to the wall having a thickness on the order of a mean free path (the Knudsen layer) cannot be described by the Navier-Stokes description (Kogan1). To determine the flow properties within the Knudsen layer requires the direct solution of the Boltzmann equation matched to the solutions for the outer flow (Navier-Stokes equation) and the wall boundary condition. This is most conveniently done through the use of a slip model in which slip and jump properties are used for the boundary conditions for the Navier-Stokes equations.