FUNDAMENTAL STUDIES OF MATERIAL RESPONSE IN ATMOSPHERIC ENTRY ENVIRONMENTS

Thermal protection systems (TPSs) for atmospheric entry environments react with boundary gases, with atomic oxygen being of particular interest. In addition to gas-surface interactions (often involving carbon), certain TPS... [ view full abstract ]

Thermal protection systems (TPSs) for atmospheric entry environments react with boundary gases, with atomic oxygen being of particular interest. In addition to gas-surface interactions (often involving carbon), certain TPS materials, such as phenolic impregnated carbon ablator (PICA), are pyrolyzed as they become very hot. This presentation will highlight two sets of laboratory experiments, one focused on atomic-oxygen interactions with model carbon surfaces and the other focused on the temperature-dependent yields of the pyrolysis products from PICA.

We have studied the interactions of ground-state atomic oxygen, O(3P), on sp² carbon surfaces with different structures at temperatures from 800 K to approximately 2200 K. Beams of 5 eV O atoms were directed at surfaces, and angular and translational energy distributions were obtained for inelastically and reactively scattered products using a rotatable mass spectrometer detector. Both CO and CO₂ are produced at the surface and in thermal equilibrium with it, with CO being the dominant product. The flux of CO reached a maximum at surface temperatures near 1200 K and decreased with increasing temperature. The increasing thermal desorption of O atoms with temperature limits the surface oxygen that is available for reaction at higher temperatures. With fewer reagent O atoms to react with carbon, the reactivity of the carbon surface decreases at high temperatures even though the surface is being constantly bombarded by highly reactive O atoms. The molecular-level scattering dynamics from the experiments have been used to formulate a finite-rate oxidation model including surface-coverage dependence, similar to existing finite-rate models used in computational fluid dynamics simulations.

The volatile chemical species evolved during the pyrolysis of PICA have been probed in situ by mass spectrometry during heating at controlled rates in the temperature range from 100 °C to 1200 °C. The molar and mass yields of the desorbing species as a function of temperature have been derived by fitting the mass spectra, and the observed trends are interpreted in light of the results of earlier mechanistic studies on the pyrolysis of phenolic resins. The main products observed were H2, CH4, H2O, CO, and CO₂. Several higher-mass, organic products were also observed. Heating-rate-dependent yields suggest that pyrolysis of PICA is a non-equilibrium process and that the relative importance of competing mechanisms depends on heating rate. The new data should enable the validation of non-equilibrium models whose aim is to simulate the response of TPS materials during atmospheric entry of spacecraft.