Assessment of PWT Conditions for the STARDUST Post-Flight Evaluation

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Herdrich, Georg
Roser, Hans-Peter
Fertig, Markus
Winter, Michael
Wernitz, Ricarda
Lohle, Stefan
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On January 15th 2006, the STARDUST capsule re-entered Earth's atmosphere after its almost seven year journey with a re-entry speed of 12.8 km/s. Due to the nature of the mission, no diagnostics for the re-entry phase were installed on the capsule. Nevertheless, this was the fastest re-entry of a human made space vehicle ever and data on heat shield erosion and plasma characterization is considered to be very valuable for further missions since the entry conditions are typical for hyperbolic re-entries for sample return missions e.g. from Mars. Therefore, an observation mission was initiated by NASA aboard the NASA DC-8 airborne observatory [1]. Among experiments conducted by American and Japanese researchers, a contribution of Germany was made by the SLIT experiment. Here, a conventional spectrometer in a Cerny Turner configuration was fed by fiber optics with light collected by a small mirror telescope. Data was taken during 30 s around the point of maximum heating until the capsule left the accessible observation region. Due to difficulties in tracking not every spectrum contained data. Nevertheless, the emission of CN as a major erosion product of the PICA (Phenolic-Impregnated Carbon Ablator) heat shield material as well as N2 + and different atoms could be monitored successfully during that time. Due to the nature of the set up, no spatial resolution of the radiation data was achieved. Thus all measured values represent integration over both the visible part of the glowing heat shield and the plasma in the post shock region. The measured spectra were split up into continuum spectra, which represent a superposition of the heat shield radiation and the continuum radiation of potential dust particles in the plasma, and into line spectra of the plasma in the post shock layer. Planck temperatures and rotational and vibrational temperatures of CN and N2 + were extracted over the visible trajectory [2]. The paper provides a short summary of the obtained spectra for different flight altitudes of the STARDUST capsule. Furthermore a comparison of said spectra to the results of numerical simulations of the plasma emission with PARADE and URANUS radiation / flow field simulations will be presented. In order to further interpret the obtained data, ground testing at the IRS plasma wind tunnels (PWT) is required [3]. Among others, methods of measurement used in the IRS facilities include optical plasma diagnostics such as emission spectroscopy [4, 5], Fabry Perot interferometry, laser induced fluorescence and laser absorption spectroscopy. Thus, suitable measurement techniques for testing must be selected, essentially including emission spectroscopy in order to facilitate plasma characterization and consequently the comparability of PWK experimental results to the data obtained during STARDUST observation. Furthermore, testing conditions are identified and assessed. Here, constant surface heat flux is chosen as a general testing condition in a first approach. In addition, different ablative materials for testing are selected, starting with graphite as a basic material and including if possible the original PICA TPS or else, similar materials. Graphite ablation has previously been analyzed in the diffusion controlled temperature region (1600 K - 2300 K) at high flow velocities up to 1 km/s demanding for further experiments at higher temperatures. References: [1] Stardust Hypervelocity Entry Observing Campaign Support, NASA Engineering and Safety Center Report, RP-06-80, August 31, 2006 [2] Winter, M., Herdrich, G., "Heat Shield Temperatures and Plasma Radiation obtained from Spectroscopic Observation of the STARDUST Re-Entry in the Near UV", 46th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, Jan. 7-10, 2008, AIAA-2008-1212 [3] Herdrich, G., Auweter-Kurtz, M., Endlich, P., Kurtz, H., Laux, T., Löhle, S., Nazina, N., Pidan, S., Schreiber, E., Wegman, T. and Winter, M., "Atmospheric Entry Simulation Capabilities at IRS," 3rd International Symposium on Atmospheric Reentry vehicles and systems, Arcachon, France, March 2003 [4] Winter, M., "Emission Spectroscopic Investigation of the Flow Field around a Blunt Body in a High Enthalpy Flow", Dissertation (in German), Institut für Raumfahrtsysteme, Universität Stuttgart, Stuttgart, Germany, 2006 [5] Röck, W., "Simulation des Eintritts einer Sonde in die Atmosphäre des Saturnmondes Titan in einem Plasmawindkanal", Dissertation (in German), Institut für Raumfahrtsysteme, Universität Stuttgart, Stuttgart, Germany, 1999
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