Tobias Hermann

Ablation radiation coupling investigation in earth re-entry using plasma wind tunnel experiments

An overview of a comprehensive experimental study to investigate ablative materials in a high enthalpy air plasma flows including radiation effects is given. Through the application of surface thermometry, in-situ recession measurements with photogrammetry, optical emission spectroscopy from vacuum-ultraviolet (120 nm) to near infrared wavelengths (950 nm), pyrometry and thermography, a complete set of data for the investigation of ablation radiation coupling has been acquired. The paper presents the background of this project, the suite of experimental setups and first results for a carbon preform sample and a cooled copper sample. All systems acquired data and first comparisons to chemical equilibrium calculations have been assessed.

Numerical Investigation of the Re-entry Flight of Hayabusa and Comparison to Flight Data

This paper presents a comparison of re-entry ight data measured by the NASA/JAXA Hayabusa Observation campaign with numerical calculations and ground testing in the NASA Ames Electric Arc Shock Tube (EAST) facility and the University of Stuttgart plasma wind tunnel, Plasmawindkanal 1 (PWK1). The numerical calculations were conducted using the University of Stuttgart URANUS code including thermal and chemical non-equilibrium as well as radiation transport modeled by the programs HERTA and PARADE. The ow eld calculations were extended by an ablation calculation using FABL (Fluid Gravity Engineering Ablation code). These simulations were performed for 17 trajectory points chosen from the nominal ight trajectory provided by NASA. The ground testing in the

Experimental setup for vacuum ultraviolet spectroscopy for earth re-entry testing

This paper presents the experimental system which has been designed to measure vacuum ultraviolet (VUV) optical emission spectra. It has been tested at the plasma wind tunnel PWK1 at the Institute of Space Systems. The setup, its calibration and qualification are reported. Wind tunnel experiments have been conducted with a local mass-specific enthalpy of 68.4MJ/kg and a stagnation pressure of 24.4 hPa. The radiation is collected through a magnesium fluoride window mounted close to the stagnation point of a sample. The received light is redirected to the spectrometer with a series of mirrors in an evacuated light path. The spectral range between 116-197 nm was investigated. The lower limit is due to the transmission of the window material. Measurements with a cooled copper and a carbon preform material sample are presented. The measured VUV spectra feature atomic nitrogen and oxygen lines for both the copper and the material sample. Atomic carbon lines are present in the case of the material sample.

Tomographic Optical Emission Spectroscopy for Plasma Wind Tunnel Testing

A tomographic imaging system for emission spectroscopic measurements is presented. The system is applied in the plasma wind tunnel PWK1 at the Institute of Space Systems at the University of Stuttgart. A plasma flow condition, corresponding to the Hayabusa re-entry trajectory point at 78.8km is analyzed using the tomographic setup. 3D emission distributions of non spectrally resolved radiation, as well as the 2D distribution of the spectrally resolved emission coecient are presented. The 3D reconstructions reveal that the transient uctuations of the plasma result in a substantial asymmetry which, however, decreases for longer times due to temporal averaging of the emission. 2D reconstructions show that a preferential arc-attachment location exists which results in higher atomic radiation and lower molecular radiation at this location.

Tomographic optical emission spectroscopy of a high enthalpy air plasma flow

A method is presented allowing for locally resolved emission spectroscopy using a tomographic setup. The approach presented in this work is applied to a high enthalpy air plasma flow. The resulting data sets allow for a three-dimensional (3D) representation of the non-symmetric flow field using photographs of the test section and 2D representation of the spectrally resolved radiance of the flow field. An analysis of different exposure times shows that transient fluctuations of the plasma can result in substantial asymmetry that approaches symmetry only for longer exposure times when the temporal averaging of the emission is significant. The spectral data allows the analysis of species selective excitation and emission. A non-equilibrium between atomic and molecular excitation temperatures is concluded for the investigated air plasma flow field. The spatial distribution of atomic electronic excitation temperatures are close to rotational symmetry while molecular rotational and vibrational temperatures exhibit asymmetric behavior.

Probing the use of spectroscopy to determine the meteoritic analogues of meteors

Context. Determining the source regions of meteorites is one of the major goals of current research in planetary science. Whereas asteroid observations are currently unable to pinpoint the source regions of most meteorite classes, observations of meteors with camera networks and the subsequent recovery of the meteorite may help make progress on this question. The main caveat of such an approach, however, is that the recovery rate of meteorite falls is low (<20%), implying that the meteoritic analogues of at least 80% of the observed falls remain unknown. Aims. Spectroscopic observations of incoming bolides may have the potential to mitigate this problem by classifying the incoming meteoritic material. Methods. To probe the use of spectroscopy to determine the meteoritic analogues of incoming bolides, we collected emission spectra in the visible range (320–880 nm) of five meteorite types (H, L, LL, CM, and eucrite) acquired in atmospheric entry-like conditions in a plasma wind tunnel at the Institute of Space Systems (IRS) at the University of Stuttgart (Germany). A detailed spectral analysis including a systematic line identification and mass ratio determinations (Mg/Fe, Na/Fe) was subsequently performed on all spectra. Results. It appears that spectroscopy, via a simple line identification, allows us to distinguish the three main meteorite classes (chondrites, achondrites and irons) but it does not have the potential to distinguish for example an H chondrite from a CM chondrite. Conclusions. The source location within the main belt of the different meteorite classes (H, L, LL, CM, CI, etc.) should continue to be investigated via fireball observation networks. Spectroscopy of incoming bolides only marginally helps precisely classify the incoming material (iron meteorites only). To reach a statistically significant sample of recovered meteorites along with accurate orbits (>100) within a reasonable time frame (10–20 years), the optimal solution may be the spatial extension of existing fireball observation networks.

The ESA ARC Project: Ablation Radiation Coupling for hypervelocity re-entry with low density type ablators

The ARC project is an ESA funded project aiming to evaluate critical issues of abla-tion radiation coupling for hypervelocity (> 10km/s) return missions of capsules equipped with carbon phenolic type heat shields. The plasma environment of such return trajecto-ries strongly radiates, while the ablation of the heat shield ejects species into the bound-ary layer. These species, which can include strong radiators, interact with the near-wall plasma resulting in further reactions, diffusion, and radiation, producing ablation-radiation coupling and blocking effects. The only recent flight data for such missions with carbon phenolic ablators comes from the Hayabusa and Stardust missions, which were observed during re-entry. However, the high energy radiation (vacuum ultraviolet and beyond) could not be measured due to atmospheric absorption. Theoretical estimations and numerical modelling predict that a considerable proportion of the radiation is emitted at these wave-lengths. Hence the goal of the ARC project is to use ground-based facilities to perform radiation measurements with vacuum ultraviolet capabilities over ablating samples and as-sess radiation-ablation coupling at conditions that are relevant to such fast return mission conditions. Calibrated measurements of vacuum ultraviolet radiation with an ablating sur-face in hypervelocity entry conditions are presented from facilities in Australia, Germany and France.