Yih-Kanq Chen

Ablation and Thermal Response Program for Spacecraft Heatshield Analysis

An implicit ablation and thermal response program is presented for simulation of one-dimensional transient thermal energy transport in a multilayer stack of isotropic materials and structure which can ablate from a front surface and decompose in-depth. The governing equations and numerical procedures for solution are summarized. Solutions are compared with those of an existing code, the Aerotherm Charring Material Thermal Response and Ablation Program, and also with arcjet data Numerical experiments show that the new code is numerically more stable and solves a much wider range of problems compared with the older code. To demonstrate its capability, applications for thermal analysis and sizing of aeroshell heatshields for planetary missions, such as Stardust, Mars Microprobe (Deep Space n), Saturn Entry Probe, and Mars 2001, using advanced light-weight ceramic ablators developed at NASA Ames Research Center, are presented and discussed.

Mars Pathfinder Entry Temperature Data, Aerothermal Heating, and Heatshield Material Response

The Mars Pathe nder probe contained instrumentation that measured heatshield temperatures during entry. A description of the experiment, the data, and an analysis of the entry environment and material response are presented. Navier ‐Stokes forebody heating calculations show a peak unblown radiative-equilibrium heat e ux of 118W/cm 2 at thestagnation point and120 W /cm 2 on theshoulderforturbulente ow. Theheatload is3.8 kJ /cm 2 on thenose,decreases along thefrustum,then increasesto 2.7 ‐3.1kJ/cm 2 on theshoulder depending on the onset time forturbulence. One-dimensional charringmaterialresponseiscalculated using threedifferentmodels.Stagnationpoint temperature data are consistent with about 85% of fully catalytic laminar heating. Shoulder temperature data are inconclusive, but are consistent with fully catalytic laminar heating or with 85% of fully catalytic heating with early onset of turbulence. Aft temperature data indicate a peak heat e ux and heat load of about 1.3 W /cm2 and 70 J/cm 2 , respectively. The aft heating proe le is about 20 s longer than the forebody heating proe le. Bondline temperaturedata, although not useful forquantitative analysis of aerothermal heating, clearly showtheheatshield had adequate thickness margins for the actual entry.

Navier-Stokes Solutions with Finite Rate Ablation for Planetary Mission Earth Reentries

A formulation of finite rate ablation surface boundary conditions, including oxidation, nitridation, and sublimation of carbonaceous material with pyrolysis gas injection, based on surface species mass conservation, has been developed. These surface boundary conditions are discretized and integrated with a Navier-Stokes solver. This numerical procedure can predict aerothermal heating, chemical species concentration, and carbonaceous material ablation rates over the heat-shield surface of reentry space vehicles. Two finite rate gas-surface interaction models, based on the work of Park and of Zhluktov and Abe, are considered. Three test cases are studied. The stream conditions of these test cases are typical for Earth reentry from a planetary mission with both oxygen and nitrogen fully or partially dissociated inside the shock layer. Predictions from both gas-surface interaction models are compared with those obtained by using chemical equilibrium ablation tables. Stagnation point convective heat fluxes predicted by using Parks finite rate model are usually below those obtained from chemical equilibrium tables and Zhluktov and Abes model. Recession predictions from Zhluktov and Abes model are usually lower than those obtained from Parks model and from chemical equilibrium tables. The effect of species mass diffusion on the predicted ablation rate is also examined.

Computational Analysis of Arc-Jet Stagnation Tests Including Ablation and Shape Change

Coupled fluid-material response analyses of arc-jet stagnation tests conducted in a NASA Ames Research Center arc-jet facility are considered. The fluid analysis includes computational Navier-Stokes simulations of the nonequilibrium flowfield in the facility nozzle and test box as well as the flowfield over the models. The material response analysis includes simulation of two-dimensional surface ablation and internal heat conduction, thermal decomposition, and pyrolysis gas flow. For ablating test articles including shape change, the material response and fluid analyses are coupled to take into account changes in surface heat flux and pressure distributions with shape. The ablating material used in these arc-jet tests was a phenolic impregnated carbon ablator. Computational predictions of surface recession, shape change, and material response are compared with the experimental measurements.

Development of a High-Fidelity Thermal/Ablation Response Model for SLA-561V

This paper describes the properties and parameters derived and/or developed to model the thermal/ablation response of SLA-561V. The model is based on arcjet data taken in the NASA Ames IHF and AHF arcjet facilities during 2004–2005. As such, this model does not account for potential ablation mechanisms associated with aerodynamic shear, as all of the aforementioned tests were conducted on 4-inch diameter flat-faced cylindrical samples. The model contains two interdependent elements: (1) a thermal response model to predict in-depth temperature response and (2), a surface ablation model to predict surface temperature and surface recession. As will be discussed, these two elements were developed separately and then integrated to comprise the high-fidelity response model (HFRM) for SLA561V. The properties and parameters included in the in-depth model will be described first, followed by a description of the surface ablation model, and, finally, the assumptions required to integrate the two.

Monte Carlo Analysis for Spacecraft Thermal Protection System Design

This paper demonstrates a Monte Carlo analysis technique to establish margins on sizing a thermal protection system and identifies the chief sources of uncertainty in the material response modeling. Monte Carlo sensitivity and uncertainty studies are performed for the thermal protection systems of the Stardust Sample Return Capsule, Mars Exploration Rovers, and X-37 wing leading edge, using the Fully Implicit Ablation and Thermal response code. The computation results are presented and discussed in detail. It shows that a Monte Carlo approach provides more insight than the traditional Root-Sum-Square method into the relationship between the thickness margin of a thermal protection system and the probability of maintaining the temperature of the underlying material within specified requirements. Nomenclature erfi = inverse error function N = total number of samples R = random number x = input parameters y = output of interests σ = standard deviation * σ = mean x

Effect of Non-equilibrium Surface Thermochemistry in Simulation of Carbon Based Ablators

This study demonstrates that coupling of a material thermal-response code and a flow solver using a nonequilibrium gas/surface-interaction model provides time-accurate solutions for the multidimensional ablation of carbon-based charring ablators. The material thermal-response code used in this study is the two-dimensional implicit thermal response and ablation program, which predicts the charring-material thermal response and shape change on hypersonic space vehicles. Its governing equations include total energy balance, pyrolysis-gas mass conservation, and a three-component decomposition model. The flow code solves the reacting Navier–Stokes equations using the data-parallel-line-relaxation method. Loose coupling between the material-response and flow codes is performed by solving the surface mass balance in the flow code and the surface energy balance in the material-response code. Thus, the material surface recession is predicted by finite rate gas/surface-interaction boundary conditions implemented in...

Implicit Coupling Approach for Simulation of Charring Carbon Ablators

This study demonstrates that coupling of a material thermal response code and a flow solver with nonequilibrium gas–surface interaction for simulation of charring carbon ablators can be performed using an implicit approach. The material thermal response code used in this study is the three-dimensional version of fully implicit ablation and thermal response program, which predicts charring material thermal response and shape change on hypersonic space vehicles. The flow code solves the reacting Navier–Stokes equations using data-parallel line relaxation method. Coupling between the material response and flow codes is performed by solving the surface mass balance in the flow solver and the surface energy balance in the material response code. Thus, the material surface recession is predicted in the flow code, and the surface temperature and pyrolysis gas injection rate are computed in the material response code. It is demonstrated that the time-lagged explicit approach is sufficient for simulations at low s...

Validation of a Three-Dimensional Ablation and Thermal Response Simulation Code

The 3dFIAT code simulates pyrolysis, ablation, and shape change of thermal protection materials and systems in three dimensions. The governing equations, which include energy conservation, a three-component decomposition model, and a surface energy balance, are solved with a moving grid system to simulate the shape change due to surface recession. This work is the first part of a code validation study for new capabilities that were added to 3dFIAT. These expanded capabilities include a multi-block moving grid system and an orthotropic thermal conductivity model. This paper focuses on conditions with minimal shape change in which the fluid/solid coupling is not necessary. Two groups of test cases of 3dFIAT analyses of Phenolic Impregnated Carbon Ablator in an arc-jet are presented. In the first group, axisymmetric iso-q shaped models are studied to check the accuracy of three-dimensional multi-block grid system. In the second group, similar models with various through-the-thickness conductivity directions are examined. In this group, the material thermal response is three-dimensional, because of the carbon fiber orientation. Predictions from 3dFIAT are presented and compared with arcjet test data. The 3dFIAT predictions agree very well with thermocouple data for both groups of test cases.

Effects of Nonequilibrium Chemistry and Darcy—Forchheimer Pyrolysis Flow for Charring Ablator

The fully implicit ablation and thermal response code simulates pyrolysis and ablation of thermal protection materials and systems. The governing equations, which include energy conservation, a three-component decomposition model, and a surface energy balance, are solved with a moving grid. This work describes new modeling capabilities that are added to a special version of code. These capabilities include a time-dependent pyrolysis gas flow momentum equation with Darcy–Forchheimer terms and pyrolysis gas species conservation equations with finite-rate homogeneous chemical reactions. The total energy conservation equation is also enhanced for consistency with these new additions. Two groups of parametric studies of the phenolic impregnated carbon ablator are performed. In the first group, an Orion flight environment for a proposed lunar-return trajectory is considered. In the second group, various test conditions for arcjet models are examined. The central focus of these parametric studies is to understan...