Non-Intrusive Multidimensional Investigation of Spallation Phenomena in Ablating Materials
Final Report Abstract
In this project spallation of ablative thermal protection material was investigated. The essential open question regarding spallation is that of the mechanisms that causes it. Proposed mechanisms from the literature, that lead to the failure of carbon fibers are shear forces, pyrolysis gas pressure or thermal stress. Extensive and comprehensive experimental test campaigns were conducted with different ablative materials in the arcjet facility PWK at IRS. Multiple non-intrusive diagnostic methods were used for the assessment of the main mechanisms which dominate spallation. High-speed imaging was used for the detection of particles and the study of 2D trajectories, an approach similar to the ones presented by other groups. The novel approach in this project was the study of spallation right on the ablator surface using photogrammetry (PGM) and two-color ratio pyrometry (TCRP). The 3D trajectories of the spalled particles were furthermore measured using plenoptic imaging (as developed earlier) which was originally not planned nor available. Three types of materials were investigated: the carbon preform Calcarb and the two carbon-phenolic ablators ZURAM and PICA-ETH. The influence of the plasma radiation on TCRP was proven to be negligible for the settings used in the present test campaign. The high spatial resolution of the TCRP data therefore allowed to study temperature differences between protruding carbon fibers and surrounding material. This means in situ investigations of differences in the range of 50 µm at temperatures of about 3000 K. The protruding fibers were found to be 100 K to 200 K hotter. The surface temperature was also measured using a thermal camera (TG). While the relative temperature distribution on the surface closely resembled that in the TCRP results, the TG temperatures were on average 200 K above those from TCRP. The photogrammetric surface measurements allowed for the first time to measure the size of protruding fiber bundles on the surface. Fiber bundles of ZURAM samples protrude up to 300 µm from the surface. The surfaces of Calcarb samples showed less protruding fiber bundles but showed a much less homogeneous recession on different parts of the surface. The recession of PICA-ETH samples was more homogeneous than on Calcarb, but similarly with few protruding fiber bundles. A high-speed camera was set up to image a region of approx. 100 x 100 at 5000 . The particle detection revealed that the carbon preform Calcarb experiences the most severe spallation with 2 to 4 times as many detected particle trajectories than the ZURAM samples under the same aerothermal condition. PICA-ETH, which unlike ZURAM is made from the carbon preform FiberForm instead of Calcarb, showed the least amount of spalled particles. The comparison of relative spallation rates between carbon preforms and carbon-phenolics under well characterized aerothermal conditions is a very important step for the understanding of spallation. The data has for the first time shown the role that phenolic resin plays in reducing the amount of spallation. In all tests, the spallation rate did not show an increasing or decreasing trend within the high-speed recording time (i.e. the last ∼13 s of the 30 s tests). The trajectories from the high-speed images also suggest, that the ejected particles from the ZURAM samples travel slightly further upstream than the Calcarb particles, which could be explained by the pyrolysis gas pressure pushing the particles outward. The plenoptic camera was used to measure the 3D trajectories of spalled particles in the flow field. This replaced the original plan to use the high-speed camera and a setup of 4 mirrors for tomographic imaging. This change was chosen as it allowed to use the high-speed imaging additionally in the same experiment at a higher resolution that it would have been possible with the tomographic setup. This is the first time that 3D particle trajectories could be measured in ablation tests. The measurements emphasize the important role of phenolic resin in the prevention of volume ablation. Without the presence of pyrolysis gases and char, the carbon fibers are weakened more severely below the surface, which makes them more susceptible to the shear forces in the flow and ultimately leads to more spallation.
Publications
- CaStaShape: Photogrammetric measurement of a camphor sample in the H3 test facility. Tech. rep. CaStaShape, TN-03, IRS-18-P14. Institut für Raumfahrtsysteme, 2018
F. Grigat and S. Loehle
- “Detection of Spallation Phenomena on Ablator Surfaces”. In: AIAA Scitech 2019 Forum. AIAA. Orlando, FL, Jan. 2020
F. Grigat et al.
(See online at https://doi.org/10.2514/6.2020-1706) - “Evaluation of Diagnostic Methods for Quantification of Spallation in Arcjet Facility Experiments”. In: AIAA Aviation Forum. 2021, p. 3130
F. Grigat et al.
(See online at https://doi.org/10.2514/6.2021-3130)
