Simulated Re-entry Environment Research Articles

Investigation of material response to atmospheric re-entry exposure of sub-structural Ti-6Al-4V components recovered from Space Shuttle Columbia

Aluminum and titanium char-layer deposits discovered on overhead windows recovered from the Space Shuttle Columbia accident revealed unexpected material response. It was observed that titanium melted and deposited before aluminum: behavior inconsistent with their relative melting temperatures. In close proximity to the windows, payload bay door (PLBD) latch rollers, containing Ti-6Al-4V, exhibited significant thermal damage that could not be explained by re-entry heating alone. The roller was considered a likely source of the deposition as damage was localized to the titanium region and the component was evaluated to identify features of ignition and/or combustion. A second examination was conducted on arc-jet samples obtained from a previous work that studied Ti-6Al-4V combustibility in a simulated re-entry environment. The samples were compared to the analysis of the PLBD roller and both exhibited features suggestive of titanium combustion. The PLBD roller possessed microstructural similarities to the arc-jet specimens that reacted more aggressively to the simulated environment.

Evaluation of Candidate Thermal Protection System Materials for Indian Manned Mission Gaganyaan in Plasma Wind Tunnel Facility

During atmospheric reentry of a manned crew module, the vehicle experiences severe thermal environments due to aerodynamic heating. Protection of crew module necessitates thermal protection systems (TPS) which shield the vehicle structure from the extreme thermal environments. Ground based testing in Plasma Wind Tunnels play a vital role in selection and qualification of TPS for re-entry missions. Three candidate TPS materials identified for the Gaganyaan mission of ISRO are evaluated in Plasma Wind Tunnel facility under simulated reentry environments. Tests are conducted at expected peak heat flux and total heat load conditions and performance of materials are compared based on thermal response and material surface recession characteristics.

Physical characterization and arcjet oxidation of hafnium-based ultra high temperature ceramics fabricated by hot pressing and field-assisted sintering

For this study, HfB 2-based ultra high temperature ceramic (UHTC) samples were prepared by hot pressing and field-assisted sintering (FAS) with 10–20 vol.% SiC (baseline), 5 vol.% TaSi 2, and 5 vol.% iridium. Dense billets were tested for hardness and mechanical strength. When compared, the FAS method consistently yielded materials with a grain size 1.5–2 times finer than samples processed via hot pressing. In general, room temperature flexural strengths of these materials were found to be lower (∼400 MPa) than similar fully dense HfB 2–SiC materials, with strengths between 500 and 700 MPa. Oxidation resistance testing of flat-face models was conducted in a simulated re-entry environment, at Q Cold Wall ∼250 W/cm 2 for 5 min. Samples processed by FAS had reduced oxide thickness and SiC depletion zones compared to the baseline HfB 2–20SiC material. In all cases oxide thickness was reduced by ∼3× and SiC depletion zone thickness was reduced ∼3× over the baseline.

High-temperature tensile properties and oxidation behavior of carbon fiber reinforced silicon carbide bolts in a simulated re-entry environment

The tensile properties and oxidation behavior of 2D C/SiC bolts prepared by chemical vapor infiltration were investigated in a simulated re-entry environment. The results showed that all the tensile strengths of 2D C/SiC bolts at test temperatures of 1300, 1600 and 1800 °C decreased, respectively, retaining 85%, 92%, and 94% of the virgin properties at room temperature. The tensile strengths and times to failure of 2D C/SiC bolts slightly increased with increase of test temperature from 1300 to 1800 °C. Microstructural observations indicated that the C/SiC bolts at lower temperature of 1300 °C were more susceptible to the reaction-controlled oxidation with the blunt fiber ends than those at high temperature of 1800 °C, which resulting in turn in great mechanical degradation and severe oxidation.

Ablation behavior of ZrB 2–SiC ultra high temperature ceramics under simulated atmospheric re-entry conditions

ZrB 2–20vol%SiC ultra high temperature ceramic (UHTC) was prepared by hot-pressing. Ablation tests of the flat-face models were conducted under ground simulated atmospheric re-entry conditions using arc-jet testing with heat fluxes of 1.7 MW/m 2 and 5.4 MW/m 2 under subsonic conditions, respectively. There was little weight or configuration change after ablation at a heat flux of 1.7 MW/m 2. However, ZrB 2–SiC composite underwent severe ablation and whose surface temperatures exceeded 2300 °C at a heat flux of 5.4 MW/m 2. Sharp-shape leading edge models were ablated under supersonic conditions with the stagnation pressure and Mach number of 1.2 atm and 2.7 M, respectively, and sharp-shaped leading edge C/SiC models were also ablated under the same condition for comparison. ZrB 2–SiC composite exhibited an excellent thermal-oxidative and configurational stability in the simulated re-entry environment compared with C/SiC material. Results indicate that ZrB 2–SiC ultra high temperature ceramics are the potential candidates for leading edges. The temperature limit for UHTC is controlled by the softening and degradation of the formed oxide scale.

Tensile behavior and microstructural evolution of a carbon/silicon carbide composite in simulated re-entry environments

Carbon fiber reinforced silicon carbide matrix composites with a protective coating were prepared by chemical vapor infiltration. Their tensile strength was about 240–250 MPa at room temperature, and 330–390 MPa in vacuum. Tensile behavior in a simulated re-entry environment was studied and compared with that in vacuum. The microstructural evolution on tensile fracture surfaces were investigated by scanning electron microscopy, and an oxidation damage pattern of fibers on tensile fracture surface was presented and discussed. A significant strength reduction was observed in re-entry environments due to the interaction of load, high temperature, and oxidation. The non-uniform oxidation of fibers weakened the load-carrying ability due to decreasing effective loaded area and hence strength. Finally, a minimum diameter of total fiber cluster embedded in composites is suggested about 220 μm for use under high load in re-entry environments at 1800 °C.

Ultra-High Temperature Ceramics (UHTCs) via Reactive Sintering

Current high temperature ceramics, such as ZrO2, Si3N4 and SiC, cannot be used at temperatures over 1600°C due to their low melting temperature or dissociation temperature. For ultrahigh temperature applications over 1800°C, materials with high melting points, high phase composition stability, high thermal conductivity, good thermal shock and oxidation resistance are needed. The transition metal diborides, mainly include ZrB2 and HfB2, have melting temperatures of above 3000°C, and can basically meet the above demands. However, the oxidation resistance of diboride monolithic ceramics at ultra-high temperatures need to be improved for the applications in thermal protection systems for future aerospace vehicles and jet engines. On the other hand, processing science for making high performance UHTCs is another hot topic in the UHTC field. Densification of UHTCs at mild temperatures through reactive sintering is an attracting way due to the chemically stable phase composition and microstructure as well as clean grain boundaries in the obtained materials. Moreover, the stability studies of the materials in phase composition and microstructures at ultra high application temperatures is also critical for materials manufactured at relatively low temperature. Furthermore, the oxidation resistance in simulated reentry environments instead of in static or flowing air of ambient pressure should be evaluated. Here we will report the concept, advantages and some recent progress on the reactive sintering of diboride–based composites at mild temperatures.

Processing, properties and arc jet oxidation of hafnium diboride/silicon carbide ultra high temperature ceramics

The processing and properties of HfB2-20 vol%SiC ultra high temperature ceramics were examined. Dense billets were fabricated by hot-pressing raw powders in a graphite element furnace for 1 h at 2200°C. Specimens were then tested for hardness, mechanical strength, thermal properties and oxidation resistance in a simulated re-entry environment. Thermal conductivity of the current materials was found to be less than previous work had determined while the strength was greater. Oxidation testing of two flat-face models was conducted, at two conditions, for two 10-min durations each. It was concluded that passive oxidation of SiC plays a role in determining the steady-state surface temperatures below 1700°C. Above 1700°C, temperatures are controlled by the properties of a thick HfO2 layer and active oxidation of the SiC phase.

航空宇宙技術研究所のプラズマ風胴の特性と応用

A continuous arc-heated hypersonic wind tunnel has been developed at the National Aerospace Laboratory. The present investigation was undertaken to provide the characteristics of the tunnel for operation arc discharge power up to 60 kW. Application of the tunnel to the aerospace technolgy and science is discussed in relation to the characteristics. It is foreseen that its operational range can can cover a part of simulated re-entry environment for Lunar probes and vehicles as well as manned satellites and vehicles. Some of the experimental investigations concerned with aerodynamic heating which have been done so far are briefly explained as the application of the tunnel. Further application to the fundamental reseach on the interaction between flowing plasma and magnetic field or on the electrothermal propulsion by using hydrogen is also discussed.

Oxidation and sublimation of graphite in simulated re-entry environments.

Graphite thermochemical response tested in simulated environment for use as thermal shield for hypersonic reentry vehicles

プラズマ風胴の開発

The development of a 30kW continuous argon plasma wind tunnel has been made in National Aerospace Laboratory. Its operational range of stagnation pressure is up to 30mmHg and that of stagnation enthalpy is up to 1, 000cal/g. These ranges cover a part of simulated re-entry environment for manned satellites and vehicles.At the same time, a study about the high speed vehicle and a basic research of the flowing plasma are in progress.

Testing techniques for full-scale missile structures under simulated re-entry environment

The intricate pattern of stresses and strains induced in missile structures by the combined environments encountered during re-entry of the earth's atmosphere has necessitated the establishment of unique testing techniques and facilities for evaluating missile performance during this severe phase of flight.