Low Density Ablative Thermal Protection Systems for Recoverable Capsules : Mechanical and Thermo-physical Properties and Thermal Response under Atmospheric Conditions
- Paper number
IAC-08.C2.4.1
- Author
Dr. C. P. Reghunadhan Nair, Indian Space Research Organization (ISRO), VSSC, India
- Coauthor
Mr. Bibin John, Indian Space Research Organization (ISRO), VSSC, India
- Coauthor
Dr. Dona Mathew, Indian Space Research Organization (ISRO), VSSC, India
- Coauthor
Dr. George Joseph, Indian Space Research Organization (ISRO), VSSC, India
- Coauthor
Mr. Deependran B, Indian Space Research Organization (ISRO), VSSC, India
- Coauthor
Dr. Ninan Kovoor Ninan, Indian Space Research Organization (ISRO), VSSC, India
- Year
2008
- Abstract
Protection of space vehicles against high heat flux conditions as encountered during atmospheric re-entry warrants efficient thermal protection systems based on ablatives. When mass is a constraint, the preference is for a low mass fraction TPS like low density ablative. These function as a thermal protection system (TPS) against moderate aero-thermal environments during atmospheric re-entry while maintaining the TPS weight penalty at the minimum. They are usually provided in regions away from the stagnation zone but occupy fairly large areas of the reentry capsule. Their mechanical compaction is limited to the extent of achieving the minimum required structural strength, so that the density is maintained a minimum. This leaves a high porosity (usually of the order of 50-60 percentage by volume) resulting in a low thermal conductivity. Because of the low thermal conductivity, they are used both as a passive thermal insulation material as well as an ablative system and the functionality can be tailored to suit the requirements at each location and the maximum aero-heating without changing the TPS material from region to region. From this viewpoint, silica fiber reinforced, phenolic syntactic foam composites of varying specific gravities were developed, processed and evaluated for tensile, flexural and compressive properties and for thermo-physical parameters. The volume ratio of low-density filler and the phenolic resin was kept constant. Tensile strength increased with fiber composition and showed a maximum corresponding to 15 volume percentage of silica fiber and decreased on further addition. The specific strength properties also manifested a similar order. Morphological features corroborated the observed results of mechanical properties. The compositional dependency of the foam composites on their thermo-physical properties and thermal degradation behavior was also examined. The thermal response of the ablatives was studied by simulating a moderate atmospheric re-entry heat flux history on the specimen with maximum heat flux of about 40W/cm2. The thermal response was measured and the material surface behavior, mass loss and flammability were studied. For fiber fractions corresponding to various specific gravities the thermal simulation experiments were studied and it was observed that the char strength and its structural integrity was satisfactory for a specific gravity of 0.5. The maximum back wall temperature measured was 80 degree celsius and this meets the structural temperature constraint at the interface. Minor charring of the order of 5mm had occurred on the surface but the char strength was adequate to withstand the aerodynamic shear. It is observed that the system functions initially as an ablative and thereafter takes the role of a passive insulation material, the presence of char at the surface minimizes the thermal soak back effect. The thermal response was numerically modeled and fairly good comparison was obtained with the experimental results. This validates the accuracy of the measurement of the thermo-physical properties and delivers the low density ablative TPS system fully developed, characterized and functionally qualified for application in atmospheric re-entry. Subsequently, the low density ablative was used in a re-entry capsule and the module was successfully recovered.
- Abstract document
- Manuscript document
IAC-08.C2.4.1.pdf (🔒 authorized access only).
To get the manuscript, please contact IAF Secretariat.