Arc-jet measurements of low-density ablator spallation

The origin and behavior of particles resulting from the spallation of ablative thermal protection materials was investigated using an arc-jet wind tunnel. Particle tracking velocimetry was used to track particles produced through spallation by using high-speed images that determine their trajectories once entering the surrounding flow, which is the post-shock region of a Mach 5 free-stream. Analysis of particle trajectories revealed dependence on flow composition and sample geometry while indicating that the presence of impregnated phenolic resin did not affect spallation. Geometric effects could be attributed to changes in the aerodynamic shear stress at the sample surface, and oxidation was also found to play an important role in increasing the production of spalled particles. Spallation rates were found to be related to the mass loss rate due to ablation, although some geometric effects were also observed. To estimate the contribution of spallation towards overall mass loss, the acceleration of individual particles along their trajectories was analyzed to produce an estimate of spallation particle size distribution. Diameters in the 1 to 100μm range were found, suggesting that these particles result from the breaking of individual carbon fibers of the ablative material, and the overall contribution to mass loss was found to be approximately 6%.