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Description
Ablative thermal protection systems (TPS) reduce the heat flux on a spacecraft during atmospheric entry through a number of effects. Modern materials consist of a porous carbon preform which is infiltrated with phenolic resin. During reentry, the resin inside the material pyrolyses which leads to the outflow of cold pyrolysis gases towards the surface and ultimately into the boundary layer and the flow field. This reduces the heat flux into the material. At the surface, the char remaining after the pyrolysis is decomposed through sublimation, oxidation and nitridation. This consumes heat and results in a recession of the ablator surface. In addition, the surface recession is increased through the mechanical erosion of the material, where solid particles are released from the surface. This undesired effect is summarized as spallation. Next to increasing the recession rate, spallation is also believed to alter the flow field and consequentially the radiation environment around the ablator. Yoshinaka et al. detected CN radiation in a supersonic plasma flow upstream of the shock, suggesting that the carbon was transported upstream trough spalled particles. Similar observations were made by Kihara et al. in a supersonic arc-heated flow. On the other hand, in similar ablation experiments by Raiche and Driver there was no significant emission of ablation products. However, in these experiments it is unclear, how much the severity of spallation differs between the tests. In this paper we present results from a test campaign where the number of spalled particles and the CN radiation and pyrolysis gas radiation in the flow field were measured simultaneously.
The test campaign was conducted at the plasma wind tunnel PWK1 at the Institute of Space Systems (IRS) at the University of Stuttgart and was targeted at the study of spallation of carbon preforms and carbon-phenolic ablators. The plasma condition was representative of the aerothermal loads that were experienced by the Hayabusa capsule at an altitude of 78 km during its reentry into Earth atmosphere. Before each test, the sample was placed outside of the flow. As soon as the desired plasma condition was set, the sample was moved to the center of the plasma flow, which defined the start time of the test. Each test had a duration of 30 s and ended with the shut-off of the plasma generator. The tested samples included the two carbon preforms Calcarb CBCF 18-2000 and Fiberform as well as the carbon-phenolic ablators Harlem and ZURAM. ZURAM is developed by the German Aerospace Center (DLR) and is produced using Calcarb as a carbon preform. The Harlem samples were produced at the IRS and both samples based on Calcarb as well as on Fiberform were tested.
The diagnostic setup allowed to study characteristic ablation performance parameters like the surface recession and surface temperature through photogrammetry and thermography respectively. The number of spalled particles were tracked via high-speed imaging. An Aryelle Echelle 150 spectrometer and a spectroscopic setup in Czerny-Turner configuration consisting of an Acton SpectraPro 2750 spectrometer coupled with an Andor Newton DU920N-OE CCD camera were used for measurements normal to the plasma flow.
The Acton SpectraPro spectrometer was located outside of the facility aiming at the sample. The plane-of-sight aligned with the entrance slit of the spectrometer was a horizontal slice in the flow field passing through the stagnation point. The measurement plane covered a width of approximately 66 mm along the stagnation line, comprising of 39 mm upstream of the sample and 27 mm on the sample. This was chosen so that, for the expected recession rate, the ablation layer was covered in the plane-of-sight for the whole duration
of the test. For a 300 lines per millimeter grating, the Andor camera can capture a wavelength range of 120 nm at a spectral resolution of 0.12 nm px1. The grating was centered at 380 nm aimed at studying the emission of CN in the flow field in the wavelength range of 320 nm to 440 nm.
The Echelle spectrometer was located on the opposite side of the test facility. Optical emission in the range from 250 nm to 880 nm were captured with this instrument. Its field of view was a circular spot with a diameter of 5mm recording the line-of-sight integrated radiance, that is aligned perpendicular to the flow and immediately upstream of the ablator surface. As the surface receded throughout the test, the measurement region moved upstream relative to the surface by up to 2 mm. The data allows to track the emission from pyrolysis products (e.g. H) over time and correlate it to the spallation frequency obtained through the high-speed images.
The final paper will contain an in-depth analysis of spallation rate, the radiation in the flow field and ablation performance characteristics like surface recession and temperature. Comparing the transient spallation rate with spatial and temporal profiles of CN emission will provide important contributions to the understanding of spallation mechanisms and the effect of spallation on the radiation in the flow field. This data can also serve as a validation for numerical models of spallation and the release of carbonaceous gases through spalled particles into the flow.
Summary
Results from an extensive ablation test campaign in the plasma wind tunnel PWK1 dedicated to the investigation of spallation are presented. The radiation of carbonaceous species and pyrolysis products in the flow field are analyzed and compared to the surface recession, the number of spalled particles and the surface temperature.
