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Test Engineering for Arc Jet Testing of Thermal Protection Systems: Design, Analysis, and Validation Methodologies

Jay H. Grinstead* and George A. Raiche† NASA Ames Research Center, Moffett Field, CA 94035

Tahir Gökçen‡ Eloret Corporation, Moffett Field, CA 94035

An integrated process of defining objectives and conducting arc jet tests in support of thermal protection systems (TPS) development at NASA Ames Research Center has been developed. Computational simulations of the nonequilibrium arc jet flows, validated by detailed measurements of flow field parameters using a variety of laser-spectroscopic and conventional techniques, can be used to characterize test conditions with high fidelity. Using simulations, test programs can be designed for a range of facility conditions that correlate critical aspects of the targeted flight conditions to the expected aerothermal, thermochemical, or thermostructural response mechanisms of a TPS material or subsystem. Basic research initiatives have supported the development of these simulation capabilities that enhance our efforts in meeting program-level requirements of NASA missions. Examples of our research efforts and mission-specific test programs will be described in the context of our testing methodology.

I. � Introduction ntry systems are a critical enabling technology for human and robotic space exploration missions. An important and necessary component of an entry system is the spacecraft thermal protection system (TPS). Mission

objectives and risk profile define the overall requirements for the TPS. These requirements, in turn, are both further refined and fulfilled through a development and qualification program coordinated across a variety of disciplines, including high temperature gas physics and chemistry, aerodynamics, materials science, and metrology. Central to the development effort is the use of high fidelity computational fluid dynamics (CFD) simulation. The CFD flow solvers incorporate real-gas energy transport models and have been developed to simulate the aerothermal environment for Earth and planetary atmospheric entry vehicles. Aerothermal simulation is combined with spacecraft trajectory simulation to predict the expected heating environment from entry interface to the lower atmosphere. Coupled with material thermal response codes using appropriate boundary conditions, these simulations become powerful and sophisticated tools for the design and sizing of a vehicle’s TPS. Supporting the development of the tools are ground tests in high enthalpy test facilities that simulate the flight environment over a range of conditions and at appropriate time and length scales. Ground testing in these facilities is used to validate the simulation tools; develop and select TPS materials; and qualify the final material for flight. No single high enthalpy ground test facility can replicate all transatmospheric flight conditions, however. Successful test planning strategies combine the advantages of different facilities with appropriate choices of test conditions to encompass, as thoroughly as possible, the critical features of the flight environment.1 A validation effort is broken down into discrete components, with each component designed to stimulate a specific aerothermal or material response process through parametric variation of test conditions. The capabilities and shortcomings of the facilities must be understood when formulating test objectives, interpreting test results, and integrating the results across the ground test envelope. Irreconcilable gaps, or extrapolations beyond the ground test envelope, would be bridged through flight tests with instrumented TPS.

Combining advanced experimental techniques and high fidelity simulation, researchers and engineers at NASA Ames have developed an approach to ground testing that leverages our knowledge of arc jet flow physics for test * Senior Research Scientist, Reacting Flow Environments Branch, MS 230-2, Senior Member AIAA. † Senior Research Scientist, Reacting Flow Environments Branch, MS 230-2, Member AIAA. ‡ Senior Research Scientist, Reacting Flow Environments Branch, MS 230-2, Senior Member AIAA.

E

25th AIAA Aerodynamic Measurement Technology and Ground Testing Conference

AIAA 2006-3290

25th AIAA Aerodynamic Measurement Technology and Ground Testing Conference5 - 8 June 2006, San Francisco, California

AIAA 2006-3290

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

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planning purposes. Considerable attention has been devoted in recent years to understanding the operating characteristics of the arc jet facilities through high fidelity CFD simulations. Applied to the test planning process, these simulations provide the means to rapidly explore potential test configurations. Several research activities, including code development, diagnostic technique development, nonequilibrium flow analyses, and spectral analyses, have contributed to our goal of characterizing arc jet flow fields and improving our simulation capabilities. In this paper, we will describe the arc jet facilities at Ames, discuss general considerations for high enthalpy testing, highlight our recent research in arc jet simulation and characterization, show how these efforts are applied to test design and analysis, and summarize results from recent arc jet tests of TPS materials and subsystems for NASA missions.

II. � NASA Ames Arc Jet Complex Motivated by the critical need for safe re-entry into Earth’s atmosphere, NASA Ames invested heavily in

developing arc jet test facilities for the study of thermal protection materials and systems. Much of this effort began in the early 1960s during the nascent era of NASA’s human spaceflight program. Expansion continued through 1970s to support the Space Shuttle program and planetary exploration missions. Today, Ames operates one of the most comprehensive networks of high enthalpy test facilities in the world and continues to support NASA programs for TPS development.

Three large-scale arc jet facilities are currently in use. A schematic of the arc heater, nozzle, and test cabin representative of the Ames arc jets is shown in Fig. 1a). Each facility is driven by a segmented, constricted-arc heater of similar design that can operate for long durations (up to 30 minutes). The Aerodynamic Heating Facility (AHF) uses conical nozzles of several expansion ratios with exit diameters from 30.5 cm to 91.5 cm. The AHF is capable of operating with air/argon and N2/argon as test gas mixtures. The 20 MW heater can produce enthalpies

a)

b) c)

Figure 1. a) Schematic of an Ames arc jet facility. b) Arc heater and nozzle of the Interaction Heating Facility (IHF). c) Test model in the IHF configured with the 33 cm exit diameter conical nozzle.

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from 11 to 32 MJ/kg at constrictor (reservoir) pressures from 1 to 10 bar. An optional mixing plenum downstream of the heater introduces air to the stream to reduce enthalpies and extend its operating envelope. The steam vacuum system holds the test cabin static pressure to 0.1 to 10 torr, depending on mass flow rates. A five-arm, programmable model support system provides flexibility and efficiency in testing multiple models during a run.

The Interaction Heating Facility (IHF), with a 60 MW arc heater, is the largest and most powerful arc jet facility operating within NASA. One of its design objectives was to enable development and validation of windward TPS materials for the space shuttle orbiter at peak heating conditions. The facility’s versatility in both power and size permits the study of aerothermal interaction effects that develop in flows over a maneuverable, lifting vehicle as well as simulation of peak heating conditions of super-orbital Earth entry. The IHF’s arc heater operates with air/argon test gas mixtures. The heater produces enthalpies from 7 to 46 MJ/kg at constrictor pressures from 1 to 10 bar. The facility can be configured with conical nozzles with exit diameters from 15 cm to 104 cm. The IHF heater and its operation are shown in Figs. 1b-c.

A unique capability of the IHF is its use of a semi-elliptic nozzle to generate wall-bounded shear flows. The semi-elliptical cross section, with an aspect ratio of 4:1, produces a relatively uniform heat flux and surface pressure distribution over a large, flat 80 x 80 cm test article. The nozzle operates at an approximate exit Mach number of 4.5. An uncooled boundary layer conditioner plate tailors the boundary layer before it reaches the test article so that it better simulates the boundary layer flow experienced on the windward side of a re-entry vehicle. The flow exits the nozzle above an articulated flat panel that houses the test article. Positive inclination angles raise the surface pressure and heat flux, while negative angles lower the pressure and heat flux. The 2.45 m nozzle length, coupled with the pressure and enthalpy range of the heater, permit the facility to operate with laminar and transitional boundary layers.2

The Panel Test Facility (PTF) also uses a semi-elliptic nozzle to produce a high-enthalpy boundary layer flow. The PTF’s 20 MW heater permits the facility to cover a similar operating envelope as the IHF with the semi-elliptic nozzle but at lower Reynolds numbers. This smaller facility has a 35 x 35 cm articulated test panel.

The arc jet complex houses two dc power supplies (20 MW and 75 MW), a five-stage, steam ejector vacuum system, a de-ionized cooling water system, and a high-pressure gas supply system. The magnitude and capacity of these support systems is what primarily distinguishes the Ames arc jet complex from other arc jet facilities in the U.S. and abroad.3

III. � TPS Validation and Verification The primary objectives of any test program for a TPS should be to lower the risk profile of and maximize the

performance for the mission the TPS supports. Simulations with various degrees of fidelity are used to predict the heat flux and total heat load to the TPS. Bondline temperature limits or other measures of structural integrity often define the performance requirement of the TPS based on the predicted heat flux and heat load. For the simulations to be used with confidence in design and risk analyses, they must be verified and validated (V&V). Verification addresses whether the numerics of the simulations are implemented correctly. Validation addresses whether the simulations model the physics accurately, and that is accomplished through comparison with ground test and flight data.

The first step in developing a sound test strategy is to identify where validation gaps exist. A diagram indicating some (but not all) elements of an analytical simulation that models TPS loads and performance is shown in Fig. 2. The analytical simulation in this context is a collection of specialized simulations supported by phenomenological models. The high-level output would be used as a performance and risk assessment metric for the subsystem-level TPS. Sizing margins, for example, are established based in part on uncertainties of various input parameters and conditions. Testing should focus on reducing uncertainties that lead to the greatest risk.

The supporting phenomenological models represent simplifications of the physical processes according to reasoned assumptions and/or the necessity to limit their scope for tractability. High-temperature, hypersonic flow simulation employs models that compute chemistry, collisional energy transfer, radiation, transport properties, turbulence, and turbulent heat transfer. Material response simulation incorporates models for several different processes, depending on the behavior of the material. For reusable TPS materials, these would include gas-surface chemistry (catalysis) and conduction. Ablative materials would include additional models for surface chemistry, gas-surface chemistry (oxidation, nitridation), pyrolysis, recession, and mass transfer. Both types would account for thermally and aerodynamically induced loads and stresses. Each model would include many parameters determined experimentally or theoretically. Chemistry models, for example, may require tens or hundreds of rate constants for reactions between relevant species. Sensitivity analyses performed with a simulation can isolate those features that dominate overall uncertainty.4

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A. Facility Characteristics and Test Planning Methodologies High enthalpy ground testing can address uncertainties of several elements in Fig. 2. The approach depends on

the features of the aerothermal, thermochemical, or thermostructural processes to be stimulated through testing. In each case, the information obtained from a test program can contribute to the validation of the overall analysis process. Improving phenomenological model accuracy, understanding failure mechanisms, or characterizing variabilities in material performance are but three examples of the ways in which analytical tools can be validated. The validation gaps and uncertainties in a particular element will define test objectives. Fulfilling those test objectives raises issues concerning the appropriate test facility, test conditions, and the test article design.

The performance of a material or system should be evaluated in a ground test simulation that closely matches the significant flight environment parameters simultaneously. A well-designed ground test program would cover the range of expected conditions. However, various length scales present difficulties in matching relevant aerothermodynamic similarity parameters in a high enthalpy ground test simulation. Laminar stagnation point convective heat flux scales as R-1/2 while Reynolds number scales as R, where R is the effective radius of curvature of a blunt-body test article. This limits options for matching enthalpy and more than one other similarity parameter. The nonequilibrium characteristics of high enthalpy facilities also present challenges for simulation. Time scales become important due to finite reaction and diffusion rates. Time scales impact length scales in flows with very high velocities and further complicate efforts to achieve similarity.5 Additionally, the heat flux to a partially catalytic surface is sensitive to the thermodynamic state at the boundary layer edge which, in turn, can depend on the degree of chemical and thermal nonequilibrium in the stream.6 Some mission-specific aspects can be simulated only partially or not at all. Time-varying conditions of an entry profile typically cannot be achieved in most ground test facilities.

Arc plasma facilities provide the means for TPS materials to reach flight temperatures under conditions where surface chemistry effects approximate that of flight. Large-scale, low-pressure arc jet facilities in particular accommodate near flight-scale test articles, permitting simulations at appropriate enthalpies and laminar Reynolds numbers. For this reason they are the only test facilities that are used to flight-qualify TPS materials for entry vehicles with low ballistic coefficients. A survey of large-scale arc-heated facilities worldwide can be found in Ref. 7.

Arc heating of the gas stream produces a significant amount of dissociation and ionization. In the plenum region prior to the nozzle expansion, the gas remains in thermal and chemical equilibrium at temperatures on the order of 7000 K, depending on conditions. During the expansion through the nozzle, the flow accelerates, the density and collision rates fall rapidly, and the stream is left in a state of thermal and chemical nonequilibrium before it reaches the nozzle exit. For air flows, a significant portion of the total enthalpy remains in dissociated N2 and O2; the flow is

Figure 2. Elements of an analytical simulation that define the aerothermal environment and material response of a TPS. The output is used to assess performance and risk of a TPS subsystem.

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chemically frozen at temperatures for which much less dissociation would exist in chemical equilibrium. Almost all O2 remains dissociated in the free stream. This increase in the chemical portion of the total enthalpy reduces the velocity of the stream — energy locked in chemical nonequilibrium is unavailable as kinetic energy. The dissociated free stream reduces the Mach number due to both a lower velocity and higher sound speed. Dissociation also increases the density over what would result at equilibrium for the same total enthalpy. Subsequent shock and boundary layer dynamics with a nonequilibrium free stream would show differences relative to equilibrium, although the differences are mitigated somewhat by relaxation processes behind the shock. The shock standoff distance increases for a nonequilibrium free stream due to the smaller density ratio. Mass fractions at the boundary layer

edge are also affected, altering the amount of chemical energy available for recombination at the surface. Matching total enthalpy and surface pressure in flight and arc jet environments does not necessarily yield the same heat flux to a partially catalytic material.6 This potential difference in heat flux underscores one of the principal challenges in establishing ground-to-flight traceability in cases where gas-surface chemistry influences test results.

The thermal properties of the test article and the test model design also have an effect on test results. Surface temperature is typically the metric by which the performance of a TPS material is assessed. However, the temperature is controlled by the efficiency with which the incident heat flux is dissipated by other loss mechanisms at the surface: re-radiation, conduction, and, in the case of ablators, surface reactions, pyrolysis, and mass transfer (Fig. 3). Model holders designed for non-charring materials characterization often limit the conduction loss to attachments, thus permitting materials to approach their radiative equilibrium temperatures. Hot structures for flight vehicles may be designed to dissipate a larger faction of the incident heat flux through conduction and radiation gaps. The reinforced carbon-carbon panels of the space shuttle orbiter’s leading edge, for example, radiate to both the ambient and the interior of the leading edge cavity as a means for thermal energy management. Materials or component-level performance tests, for which temperature is the primary target variable, would be designed to achieve flight-defined heat fluxes by providing for similar dissipation mechanisms in their test models. Thermal analysis of arc jet model designs is therefore a crucial part of any test planning process to ensure that relevant thermal losses are incorporated.

In all cases, appropriate selection of test conditions requires detailed knowledge of a facility’s operating characteristics, with enthalpy being foremost among the relevant parameters. The confidence in any causal relationship between test conditions and test observables rests upon the accuracy to which those conditions are known. High fidelity simulation techniques for arc jet flows provide insight into a particular facility’s nonequilibrium thermochemistry typically unavailable with analytical engineering tools. Their use in pre- and post-test analyses is described in the following section. Efforts to validate their accuracy are discussed in section III.C.

B. CFD Simulation for Arc Jet Test Planning and Analysis Considerable attention has been devoted in recent years to understanding the operating characteristics of the arc

jet facilities. Arc jet flow simulation provides a high fidelity means to assess the state of the stream and its influence on test results. Simulation capabilities have been developed that model the flow in the arc heater,8,9 through the nozzle,6,10-12 and over a test article.6,13 Applied to the test planning process, simulation becomes a tool to rapidly explore potential test configurations and optimize the return on investment in testing resources to meet program objectives. The merits of a particular test design can be identified and understood early in the planning process. Most importantly, by using the same CFD flow solver for both arc jet and flight environment simulation,

Figure 3. Surface energy balance for a TPS material. Applied heat flux mechanisms (convection, radiation, catalysis) are dissipated by losses due to conduction, re-radiation, and ablation mechanisms.

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consistency between flight environment definition and test interpretation can be maintained — a critical need in resolving ground-to-flight traceability.

An end-to-end simulation of an arc jet test involves 1) a nozzle calculation that defines the arc jet free stream conditions; 2) a model calculation of flow over the test article to compute, using these free stream conditions, the heat flux and associated environment variables; and 3) a thermal analysis of the test article that includes a material’s thermal response. Facility data (such as flow rates) and test observables (such as heat flux, model surface and in-depth temperature measurements, and emissivity) are incorporated as boundary conditions and parameters where applicable. Coupling between the three components is incorporated where necessary — for example, gas-surface chemistry and mass transfer from an ablative material may influence the flow over the model. Trade studies become feasible with a virtual arc jet experiment. The influence of model size, for example, is explored as a means to tailor the heat flux distribution and velocity gradient across a material sample. Boundary layer interactions for which shear is a driving factor are investigated through simulation. Surface properties are varied parametrically to assess surface and in-depth temperature responses. Instrumentation requirements, such as measurement resolution or thermocouple placement, are also examined through sensitivity analyses as a means to maximize data utility.

An example of a virtual arc jet simulation is depicted in Fig. 4. The simulation is of a candidate TPS material tested in a stagnation configuration. Virtual stream conditions linked to a particular set of facility operating parameters are determined with a combined nozzle/model CFD simulation. Mass flow rate and total enthalpy are specified as input for the nozzle expansion calculation. As the nozzle calculation begins at the throat, appropriate boundary conditions are required. The simplest approximation is to assume uniform thermochemical equilibrium and sonic flow at the throat, which is sufficient to compute velocity, species densities, and temperature from the specified mass flow rate and enthalpy. The nozzle solution can be probed at the model location to supply upstream boundary conditions for a model calculation. The model calculation then computes the solution for the flow over the test article with prescribed surface boundary conditions and surface properties. For pre-test analyses, the goal would be to determine the required mass flow rate and enthalpy to generate the desired conditions on the model that best approximate the targeted flight simulation point. Boundary layer edge properties and boundary layer profiles can be examined. Environment parameters than cannot at present be measured (or measured routinely) such as shear, Reynolds numbers, or species mass fractions, can be extracted from solutions and compared to the simulation point. For post-test analyses, the input parameters to the nozzle calculation are determined by matching measured heat flux and pressure on a test model. The heat flux could be from a fully catalytic, water-cooled calorimeter. Alternatively, it could also be inferred from the measured surface temperature of a reference material with known emissivity and catalytic efficiencies. Conduction losses are accounted for in a coupled approach using a thermal simulation of the test model. The inlet conditions to the nozzle calculation are iterated until the computed pressure and heat flux match the measured values. Once the match to test observables has been achieved, the virtual arc jet simulation replicates the actual arc jet test environment. Features of the solution can be examined for sensitivity of input conditions to test observables; for uncertainty and error analyses; and for assessments of ground-to-flight traceability.

Figure 4. Virtual arc jet simulation. Facility operating parameters (enthalpy, flow rate) are inlet conditions for the CFD nozzle expansion calculation. Nonequilibrium free stream conditions from the nozzle solution become the upstream boundary condition for the CFD solution for flow over the test model. The thermal response of the model is then computed with a finite element simulation using boundary conditions from the model solution.

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Material response models, such as those for surface catalysis, ablation, and recession, developed through arc jet testing are sensitive to the degree of thermochemical nonequilibrium in the free stream and the state of the boundary layer flow over the surface. The accuracy of the response models may be strongly coupled to the accuracy (or resolution) of the boundary conditions with which they were developed, and these are typically obtained through simulation.14 Without validation of the arc jet CFD simulations, test observations may not be accurately — or correctly — captured in the material response models. Detailed chemical and physical measurements of arc jet flow properties and their spatial distribution could help reduce uncertainties in the simulations.

C. Arc Jet Characterization Validating the CFD flow solvers at arc jet conditions requires detailed measurements of thermochemical

properties. Several articles have addressed the requirements and approaches to arc jet characterization.15-17 Research activities at Ames, including code and diagnostic technique development, nonequilibrium flow analyses, and spectral analyses, have contributed to our goal of characterizing arc jet flow fields and improving our simulation capabilities.

Accounting for all contributions to the enthalpy at a particular point requires several measurements, with the minimum being velocity, temperature, and species densities. Recent progress in spectroscopic evaluation of gases in thermal and chemical nonequilibrium has produced several tools for performing nonintrusive optical measurements in arc jet flows. One of the more useful tools is laser-induced fluorescence (LIF), a spatially resolved, species-selective probe of individual atomic and molecular states. Two-photon LIF of atomic oxygen and nitrogen was first demonstrated in high enthalpy arc jet flows at Ames.18 Since then, LIF of atomic nitrogen has been applied to directly measure the velocity, translational temperature, and nitrogen number density in the free stream.19,20 The kinetic, thermal, and chemical modes of the total enthalpy can be quantified with the LIF data and other facility measurements.19 Comparisons with computed and measured centerline flow parameters have been reported.10,21

Optical diagnostics have also been developed for the shock layer region ahead of blunt body test articles. Emission spectra have been analyzed in an effort to characterize flow properties within the shock layer and their relation to the free stream. Spectra at multiple points along the stagnation streamline were compared to those generated from CFD and NEQAIR, a radiative transport code,22 to assess the degree of chemical or thermal nonequilibrium within the shock layer.23 Time resolved emission spectra and optical attenuation ahead of an ablating test model have been recorded with novel fiber optic- and laser-based instruments.24 Spallation from ablating test models has been quantified with a laser attenuation instrument.25 These diagnostics provide insight on the recession of ablative test models that can support the development of thermal response models.

In addition to the arc jet free stream, optical diagnostic techniques have been employed to investigate the characteristics of arc heaters. Emission spectra have been acquired in the plenum of the AHF arc heater by collecting light from an optical fiber probe mounted to the plenum wall.26 Internal mode temperatures and radiative heat fluxes were deduced from the measured spectra. Recently, temperature measurement in the plenum of the IHF has been demonstrated using a diode laser absorption spectrometer.27 Excited state oxygen and nitrogen atoms have been probed, and temperatures were deduced from equilibrium population densities of these excited states. Correlation of optical data with facility inputs is being explored as a means to monitor bulk enthalpy. A better understanding of the state of the gas within the heater also improves the fidelity of boundary conditions used for nozzle expansion calculations.

IV. � Test Planning Methodologies in Practice The approach to test planning and analysis described in the previous sections has been applied to several arc jet

test programs involving the panel test facilities at Ames. The CFD simulation capabilities were critical factors in establishing appropriate test conditions that approximated the targeted flight environment. For example, the SHARP-B2 flight experiment was designed to evaluate ultra-high temperature ceramic TPS materials. Component testing of flight hardware was performed in the PTF arc jet, and CFD analyses of both the flight and the arc jet environments were used to optimize the heating profile during the test to approximate the predicted flight profile.28 Comparisons of calibration test data and CFD simulations for the IHF semi-elliptic nozzle were also performed in support of testing for the metallic TPS materials for the X-33 program.11 Flight and arc jet environments were examined to assess similarity for windward acreage testing. High fidelity simulations have been applied to test planning for subsystem-level TPS hardware for the recent Mars Exploration Rover (MER) missions. The design of the protective cover and its interface for the Transverse Impulse Rocket System (TIRS) of the entry vehicle was validated through arc jet testing in the PTF.29 More recently, our test methodologies have been applied to testing of candidate on-orbit repair techniques for the space shuttle Orbiter leading edge subsystem.30,31

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A more complicated and comprehensive example of our approach was in support of the X-37 reusable launch vehicle program. Three-dimensional arc jet CFD simulation was used to establish appropriate test conditions and test article design for a representative arc jet test simulating a wing leading edge.32 The primary objective of the test was to evaluate the thermostructural performance of a leading edge TPS assembly. The IHF arc jet with the semi-elliptic nozzle provides the boundary layer flow appropriate for reaching high Reynolds numbers with comparable boundary layer characteristics. A swept pylon test model was designed to simulate flow along the curved leading edge and its intersection with the incoming boundary layer. The arc jet operating parameters and model dimensions were chosen through the CFD analysis to match the flight predictions of heat flux, boundary layer edge Mach number, boundary layer thickness, and Reynolds number as closely as possible. Figure 5 shows the pylon model under test in the IHF arc jet. The thickness of the incoming boundary layer and the leading edge shock location can be inferred visually from the change in emission intensity of the stream. Figure 6a shows an example of the results obtained from a CFD solution of the nozzle flow over the model. The Mach contours are within the plane that

Figure 5. IHF arc jet test of a pylon test model used to simulate leading edge heating. The facility was configured with the semi-elliptical nozzle, and the pylon was attached to the flat test panel. The boundary layer thickness and leading edge shock location can be discerned by the change in the emission intensity of the free stream.

Temperature

a) b)

Figure 6. a) Computed Mach number contours in the plane that intersects the lead edge attachment line of the pylon test model. b) Finite element simulation used to analyze the thermostructural response of the test model. Heat flux and pressure distributions from the CFD solution were applied as boundary conditions for the finite element simulation.

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intersects the attachment line along the leading edge. The thickness of the incoming boundary layer and its intersection with the leading edge shock are also evident.

The simulation and test results were used in a finite-element analysis to assess the thermostructural response of a candidate leading edge system. The test model included features of a leading edge assembly that required performance validation. Figure 6b shows surface temperature contours obtained from the CFD solution. The heat flux and pressure distributions from the CFD solution were applied as boundary conditions for the finite element simulation. The CFD simulation was necessary to optimize the test design and facility operating conditions, confirm measured material surface temperatures, and define the boundary condition distributions for the finite element analysis.

V. � Conclusion An overview was presented of the significant issues involved in arc jet testing testing for TPS development and

qualification. Ground test simulation of relevant flight regimes requires consideration of facility capabilities (both size and power), appropriate test conditions, and design of test models. The role of high fidelity CFD simulation of arc jet facilities was described. A relationship between test results and flight expectations can be established by using the same CFD solver for analyses of both the flight and arc jet test environments. Simulation also provides greater insight into a particular facility’s operating characteristics and capabilities, and the merits of a particular test design can be explored through computational simulation early in the test planning process. One example of recent arc jet test programs for TPS subsystems was described. The unique capability of the semi-elliptic nozzle configuration for testing at large scale and high Reynolds numbers was demonstrated. Arc jet CFD simulation was shown to fulfill a critical role in the designs of these test programs.

Facility characterization continues to be a challenge and an opportunity. The confidence of analysis results that rely upon CFD simulation (or similar approaches) is limited by our understanding of the arc jet environment. Validation of arc jet CFD simulations requires detailed measurements of the thermochemical state of the gas stream. The value of advanced optical diagnostics, such as laser-induced fluorescence, was demonstrated through spatially resolved measurement of species densities in the free stream — information not directly obtainable from conventional diagnostics. Further development of validated CFD simulation capabilities will reduce uncertainties in test conditions which, in turn, improves boundary condition definition for aerothermal, thermochemical, and thermostructural response analyses.

Acknowledgments The work described in this paper was performed by several past and current members of the Space Technology

Division at NASA Ames. The authors would like to thank D.A. Stewart, C. Szalai, M. Loomis, T.H. Squire, B. Laub, D. Prabhu, R.D. McDaniel, D.M. Driver, M.J. Wright, J. Olejniczak, D.A. Kontinos, J.A. Balboni, I. Terrazzas-Salinas, G.J. Hartman, and C.A. Smith for their contributions and helpful discussions.

References 1B. Laub, J. Balboni, and H. Goldstein, “Ground Test Facilities for TPS Development”, NASA TM-2002-211400 (2002). 2W. Winovich and W.C.A Carlson, “The 60-MW Shuttle Interaction Heating Facility”, presented at the 25th International

Instrumentation Symposium, Anaheim, CA, ISBN 87664-434-5 (1979). 3I. Terrazas-Salina and C. Cornelison, “Test planning guide for ASF facilities”, NASA Ames Research Center, document

A029-9701-XM3 Rev. B, March 1999. 4D. Bose, M. Wright, and T. Gökçen, “Uncertainty and Sensitivity Analysis of Thermochemical Modeling for Titan

Atmospheric Entry”, AIAA-2004-2455, 37th AIAA Thermophysics Conference, Portland, OR (2004). 5G.R. Inger, “Scaling Nonequilibrium-Reacting Flows: The Legacy of Gerhard Damköhler”, J. Spacecraft Rockets 38, (2),

185-190 (2001). 6T. Gökçen, “Effects of flowfield nonequilibrium on convective heat transfer to a blunt body”, J. Thermophys. Heat Tr. 10,

(2), 234-241 (1996). 7R.K. Smith, D.A. Wagner, and J. Cunningham, “A Survey of Current and Future Plasma Arc-Heated Test Facilities for

Aerospace and Commercial Applications”, AIAA 98-0146, 36th AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV (1998).

8T. Sakai and J. Olejniczak, “Navier-Stokes Computations for Arcjet Flows”, AIAA 2001-3014, 35th AIAA Thermophysics Conference, Anaheim, CA (2001).

9T. Sakai and J. Olejniczak, “Improvements in a Navier-Stokes Code for Arc Heater Flows”, AIAA-2003-3782, 36th AIAA Thermophysics Conference, Orlando, FL (2003).

10J. Olejniczak and D.G. Fletcher, “An experimental and computational study of the freestream conditions in an arc-jet facility”, AIAA-2000-2567, 31st AIAA Plasmadynamics and Lasers Conference, Denver, CO (2000).

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11M.P. Loomis, F.C.L. Hui, S. Polsky, E. Venkatapathy, and D.K. Prabhu, “Arc-jet semi-elliptic nozzle simulations and validation in support of X-33 TPS testing”, AIAA-1998-864, 36th Aerospace Sciences Meeting and Exhibit, Reno, NV (1998).

12C. Park and S.H. Lee, “Validation of Multi-temperature Nozzle Flow Code”, J. Thermophys. Heat Tr. 9, (1), 9-16 (1995). 13T. Gökçen, C.S. Park, and M.E. Newfield, “Computational analysis of shock layer emission measurements in an arc-jet

facility”, AIAA-1998-891, 36th Aerospace Sciences Meeting and Exhibit, Reno, NV (1998). 14F.S. Milos and D.J. Rasky, “Review of Numerical Methods for Computational Surface Thermochemistry”, J. Thermophys.

Heat Tr. 8, (1), 24-34 (1994). 15C.D. Scott, “Survey of Measurements of Flow Properties in Arcjets”, J. Thermophys. Heat Tr. 7, (1), 9-24 (1993). 16S.P. Sharma, C.C. Park, C.D. Scott, S. Arepalli, and J. Taunk, “Arc Jet Characterization”, AIAA 96-0612, 34th AIAA

Aerospace Sciences Meeting and Exhibit, Reno, NV (1996). 17D.G. Fletcher, “Nonintrusive diagnostic strategies for arcjet stream characterization”, Measurement techniques for high

enthalpy and plasma flows, NATO Research and Technology Organization proceedings, RTO-EN-8, 3B-1–3B-37, Neuilly-Sur-Seine Cedex, France (2000).

18D.J. Bamford, A O’keefe, D.S. Babikian, D.A Stewart, and A.W. Strawa, “Characterization of Arc-Jet Flows Using Laser-Induced Fluorescence”, J. Thermophys. Heat Tr. 9, (1), 26-33 (1995).

19D.G. Fletcher, “Arcjet flow properties determined from laser-induced fluorescence of atomic nitrogen”, Appl. Opt. 38, (9), 1850-1858 (1999).

20J. Grinstead, D. Driver and G. Raiche, “Radial Profiles of Arcjet Flow Properties Measured with Laser-Induced Fluorescence of Atomic Nitrogen”, AIAA-2003-400, 41st Aerospace Sciences Meeting and Exhibit, Reno, NV (2003).

21J.M. Sheeley, M. Holt, and M.P. Loomis, “The effect of entrance conditions on an expanding arcjet flow”, AIAA-1999-0311, 37th Aerospace Sciences Meeting and Exhibit, Reno, NV (1999).

22E.E. Whiting, C. Park, Y. Liu, J.O. Arnold, and J.A. Paterson, “NEQAIR96, Nonequilibrium and Equilibrium Radiative Transport and Spectra Program: User’s Manual”, NASA Reference Publication 1389 (1996).

23D.G. Fletcher, G.A. Raiche, and D.K. Prabhu, “A method for determining species mass fractions from spectrally resolved emission measurements”, AIAA-2000-2699, 31st AIAA Plasmadynamics and Lasers Conference, Denver, CO (2000).

24G. Raiche and D. Driver, “Shock Layer Optical Attenuation and Emission Spectroscopy Measurements During Arc Jet Testing with Ablating Models”, AIAA-2004-825, 42nd AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV (2004).

25C. Park, G.A. Raiche, and D.M. Driver, “Radiation of Spalled Particles in Shock Layers”, AIAA-2004-1349, 42nd AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV (2004).

26J.M. Donohue, D.G. Fletcher, and C.S. Park, “Emission spectral measurements in the plenum of an arc-jet wind tunnel”, AIAA-1998-2946, 7th AIAA/ASME Joint Thermophysics and Heat Transfer Conference, Albuquerque, NM (1998).

27S. Kim, J.B. Jeffries, R.K. Hanson, and G.A. Raiche, “Measurements of Gas Temperature in the Arc-heater of a Large Scale Arcjet Facility using Tunable Diode Laser Absorption, “ AIAA-2005-900, 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV (2005).

28M. Loomis and G. Palmer, “Pre-flight CFD analysis of arc jet and flight environments for the SHARP-B2 flight experiment”, AIAA-2001-982, 39th Aerospace Sciences Meeting and Exhibit, Reno, NV (2001).

29C. Szalai, Y. Chen, M. Loomis, F. Hui, B. Thomas, and S. Buck, “Mars Exploration Rover TIRS Cover Thermal Protection System Design Verification”, AIAA-2003-3767, 36th AIAA Thermophysics Conference, Orlando, FL (2003).

30T. Gökçen, G.A. Raiche, D.M Driver, and J.A. Balboni, “Applications of CFD Analysis in Arc Jet Testing of RCC Plug Repairs”, AIAA-2006-3291, 25th AIAA Aerodynamic Measurement Technology and Ground Testing Conference, San Francisco, CA (2006).

31D.M. Driver, F. Hui, J.A. Balboni, and I. Terrazas-Salinas, T. Gökçen, and G.A. Raiche, “Aeroheating Testing of Shuttle Wing Leading Edge Repair Concepts for Return to Flight”, AIAA-2006-3297, 25th AIAA Aerodynamic Measurement Technology and Ground Testing Conference, San Francisco, CA (2006).

32T. Gökçen and D.A. Stewart, “Computational Analysis of Semi-elliptical Nozzle Arc-jet Experiments”, AIAA-2005-4887, 38th AIAA Thermophysics Conference, Toronto, ON (2005).