Spallation of Carbon Ablators in Arcjet Facility Experiments

Spallation is the mostly undesirable phenomenon where solid particles are ejected from ablative materials leaving less mass to be decomposed at the ablator surface. The exact mechanisms and conditions that promote spallation as well as the extent of it are unclear to this date. This paper presents the results of an extensive test campaign targeted at measuring spallation of carbonaceous ablator materials in an arc heated air flow of & SIM;70 MJ.kg-1. The employed diagnostic methods are photogrammetric surface measurements, high-speed imaging, thermography, and two-color-ratio pyrometry using the raw digital single-lens reflex camera images from the photogrammetry setup. Data from the high-speed camera show a much higher spallation rate for the carbon preform Calcarb than for carbon-phenolic ablators. Temperature measurements show a difference of up to 300 K between carbon fibers and surrounding matrix. The pyrolysis gas from the phenolic resin is found to decrease the spallation rate of the carbon-phenolic ablator ZURAM compared to Calcarb, and the pyrolysis gas pressure slightly increases the distance that particles travel upstream after ejection from the front surface.