Arc-jet Environment Research Articles

Equilibrium model for the ablation response of silicone-coated PICA

A novel equilibrium ablation and thermal response model for NASA’s Phenolic Impregnated Carbon Ablator (PICA) is presented. For the first time, the model accounts for the effect of NuSil, a silicone overcoat applied as contamination control during assembly, test, and launch operations for both the Mars Science Laboratory and the Mars 2020 heatshields. The protective effect of NuSil against atomic oxygen is discussed based on experimental results collected under arc jet environment. A detailed mass and heat transfer analysis for PICA-NuSil was implemented in the Porous material Analysis Toolbox based on OpenFOAM, PATO. The model includes specific boundary conditions for the coating removal and surface equilibrium processes using representative elements of the NuSil-environment system. The charred NuSil surface was modeled as SiO2 based on the observation that a silicon oxycarbide layer, formed during the decomposition of NuSil, transforms into domains of pure silica and charred PICA. Two-dimensional material response simulations were performed to compare PICA and PICA-NuSil using boundary conditions calibrated with arc jet data. The present work constitutes the first demonstration of a multi-dimensional ablation response of a silicone-coated carbon-phenolic ablator. It is shown that simulations agree with experiments, and the model reproduces measured temperature and recession. Differences between the PICA-NuSil response under Earth and Mars atmospheres are discussed.

Experimental analysis of spallation particle trajectories in an arc-jet environment

Spallation is a phenomenon in which solid particles are ejected off the surface of an ablative material in a high-enthalpy, high-shear flow field. The main contributor to this phenomenon in carbon-based heat shields is the mechanical erosion of carbon fibers weakened by oxidation decomposition. The dynamics of this phenomenon, which are poorly characterized in the literature, strongly affect the ablation rate of the material. In state-of-the-art codes, ablation by spallation is modeled using a “failure” ablation rate that is empirically determined. The present study aims at understanding the rate of ablation of low-density carbon materials. Results from a test campaign at the NASA Langley Hypersonic Materials Environmental Test System (HYMETS) arc jet facility are used to examine spallation. High-speed multi-camera imagery at 44,000 fps is used to generate velocity vectors of spalled particles emitted from carbon-fiber samples exposed to an arc jet airflow. The imagery recorded approximately 4×106 unique particles, indicating that spallation is a potentially non-trivial process. The velocities of the particles ejected from the surface were found to be between 10 m/s and 20 m/s, accelerating to velocities as high as 250 m/s further away from the sample surface. Although the particle diameters were not directly observable, estimates suggest anywhere from 0.06% to 5.6% of the mass loss from the sample occurred due to spallation.

Methodology for Flight-Relevant Arc-Jet Testing of Flexible Thermal Protection Systems

A methodology to correlate flight aeroheating environments to the arc-jet environment is presented. For a desired hot-wall flight heating rate, the methodology provides the arc-jet bulk enthalpy for the corresponding cold-wall heating rate. A series of analyses were conducted to examine the effects of the test sample model holder geometry to the overall performance of the test sample. The analyses were compared with arc-jet test samples, and challenges and issues are presented. The transient flight environment was calculated for the Hypersonic Inflatable Aerodynamic Decelerator Earth Atmospheric Reentry Test vehicle, which is a planned demonstration vehicle using a large inflatable, flexible thermal protection system to reenter the Earth’s atmosphere from the International Space Station. A series of correlations were developed to define the relevant arc-jet test environment to properly approximate the vehicle flight environment. The computed arc-jet environments were compared with the measured arc-jet values to define the uncertainty of the correlated environment. The results show that, for a given flight surface heat flux and a fully catalytic thermal protection system, the flight-relevant arc-jet heat flux increases with the arc-jet bulk enthalpy, while for a noncatalytic thermal protection system, the arc-jet heat flux decreases with the bulk enthalpy.

Two-Dimensional Ablation, Thermal Response, and Sizing Program for Pyrolyzing Ablators

The purpose of this paper is to describe and demonstrate new capabilities in the two-dimensional implicit thermal response and ablation program. These expanded capabilities include grid options for flight and arcjet geometries, a sizing algorithm for the flight-type geometry, and an orthotropic thermal conductivity model. Applications to analysis of an orthotropic low-density carbon-phenolic material in arcjet and flight environments relevant to the Orion crew module are presented. For the arcjet environment, multidimensional conduction effects strongly influence the in-depth thermal response. For a lunar return flight environment, in the shoulder region of the crew module (where the radius of curvature is smallest), the thermal response is influenced by multidimensional conduction and by the orientation of the orthotropic material.

Simulation of Carbon-Carbon Crack Growth Due to Carbon Oxidation in High Temperatures

High-temperature gas-dynamic computational techniques are employed to study microflows in expanding crack channels caused by the oxidation of the channel carbon walls. Wall regression rates for three reinforced carbon- carbon samples that were tested in a high-enthalpy arcjet environment were modeled. The test geometries and flow conditions span flow regimes from the transitional through the continuum, but the same mechanism for wall material loss, atomic oxygen reaction with bare carbon, was used in all three cases. Kinetic (direct simulation Monte Carlo) and continuum (Navier-Stokes) gas-dynamic approaches were used. The predicted wall regression rates were found to agree with arcjet measurements, and the general specimen shape change was predicted. Local gas flowfield results were found to affect the oxidation rate in a manner that cannot be predicted by previous mass loss correlations. The method holds promise for future modeling of materials gas-dynamic interactions for hypersonic flight.

Solar Cell Degradation During the 26-Kilowatt Electric Propulsion Space Experiment Flight

The Electric Propulsion Space Experiment (ESEX) was launched and operated in early 1999 to demonstrate the compatibility and readiness of a 30-kW class ammonia arcjet for satellite propulsion applications. As part of thisflight, an array of onboard contamination sensors assessed the effects of the arcjet on the spacecraft. The sensors consisted of microbalances to measure material deposition, radiometers to assess material degradation due to thermal radiation, and solar cell segments to investigate solar array degradation. During firings, the solar cell segments showed decreasing open-circuit voltages, probably due to additional electrical load imposed by currents through the plume plasma. Over eight firings of the ESEX arcjet, and 33-min, 44-s operating time, the solar cells exhibited a 3% decrease in short-circuit current, attributable to decreased solar transmission of the cover glass. The spacecraft's main solar arrays, however, exhibited no degradation in performance. Contamination affected only the solar cell sensor segments placed near the thruster exhaust nozzle. In the backplane of the thruster, where the main arrays are located, no deleterious effects occurred, indicating that although engineering measures may be required for equipment in the immediate vicinity of the thruster, the arcjet environment is generally benign.

Mass Deposition Measurements from 26-Kilowatt Electric Propulsion Space Experiment Flight

The Electric Propulsion Space Experiment (ESEX) was launched and operated in early 1999 to demonstrate the compatibility and readiness of a 30-kW class ammonia arcjet for satellite propulsion applications. As part of the onboard sensor array, thermoelectric quartz crystal microbalances were used to measure material deposition at selected locations on the spacecraft surface. The sensors were held at a temperature that would allow them to condense metallic materials eroded from the electrodes, but that are insensitive to possible deposition of the propellant gas. The ESEX arcjet was fired eight times, constituting 33 min and 26 s of operating time. No material deposition was observed that could be attributed to nominal the arcjet firings, although during the first firing, significant deposition was observed near the arcjet nozzle. This deposition is attributed to contaminants within the arcjet body, collected during handling and storage, which were ejected during the first firing. For future programs, although engineering measures may be needed to protect spacecraft equipment in the immediate vicinity of the thruster body, the arcjet environment causes negligible deposition of electrode material.

Radiometric Analysis from the 26-Kilowatt Electric Propulsion Space Experiment Flight

The U.S. Air Force Research Laboratory's Electric Propulsion Space Experiment (ESEX) was launched and operated in early 1999 to demonstrate the compatibility and readiness of a 30-kW class ammonia arcjet for satellite propulsion applications. As part of this flight, an array of onboard contamination sensors was used to assess the effect of the arcjet and other environments on the spacecraft. The sensors consisted of microbalances to measure material deposition, radiometers to assess material degradation due to thermal radiation or contamination, and solar cell segments to investigate solar array degradation. Over eight firings of the ESEX arcjet, and 33 min, 26 s operating time, the radiometer near the thruster, viewing the arcjet plume and body, experiences a change in the thermal properties of its coating from the firings. Radiometers with no view of the arcjet, or a view of only the plume, show no change. In general, degradation effects are observed only on sensors near the thruster exhaust nozzle, a location unlikely to be used in an operational high-power electric propulsion system. No degradation effects are observed in the backplane of the thruster. For future programs, although engineering measures may be needed for spacecraft equipment in the immediate vicinity of the thruster body, the arcjet environment is generally benign.

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.

Transpiration cooling in an arc-jet environment.

Transpiration cooling in an arc-jet environment.