Super-orbital re-entry in Australia - laboratory measurement, simulation and flight observation

There are large uncertainties in the aerothermodynamic modelling of super-orbital re-entry which impact the design of spacecraft thermal protection systems (TPS). Aspects of the thermal environment of super-orbital re-entry flows can be simulated in the laboratory using arc- and plasma jet facilities and these devices are regularly used for TPS certification work [5]. Another laboratory device which is capable of simulating certain critical features of both the aero and thermal environment of super-orbital re-entry is the expansion tube, and three such facilities have been operating at the University of Queensland in recent years[10]. Despite some success, wind tunnel tests do not achieve full simulation, however, a virtually complete physical simulation of particular re-entry conditions can be obtained from dedicated flight testing, and the Apollo era FIRE II flight experiment [2] is the premier example which still forms an important benchmark for modern simulations. Dedicated super-orbital flight testing is generally considered too expensive today, and there is a reluctance to incorporate substantial instrumentation for aerothermal diagnostics into existing missions since it may compromise primary mission objectives. An alternative approach to on-board flight measurements, with demonstrated success particularly in the ‘Stardust’ sample return mission, is remote observation of spectral emissions from the capsule and shock layer [8]. JAXA’s ‘Hayabusa’ sample return capsule provides a recent super-orbital reentry example through which we illustrate contributions in three areas: (1) physical simulation of super-orbital re-entry conditions in the laboratory; (2) computational simulation of such flows; and (3) remote acquisition of optical emissions from a super-orbital re entry event

Assessment of PWT Conditions for the STARDUST Post-Flight Evaluation

This presentation was part of the session : Extreme EnvironmentsSixth International Planetary Probe WorkshopOn 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, 199

Evolution of plasma species and heat – shield temperatures from Hayabusa Reentry Observation

Emission signatures of the Hayabusa capsule reentry, acquired by the Australian Ultraviolet Spectrograph instrument on the NASA DC-8 mission over the South Australian Woomera landing site on 13 June 2010, are analysed to identify thermal characteristics of the shock layer and the ablative heat-shield performance. The spectral range 330 to 470 nm of the Australian Ultraviolet Spectrograph instrument contains signatures of the air plasma species nitrogen and the ablation-related species cyanogen. The time-resolved evolution of averaged species number densities, vibrational and rotational plasma temperatures, and the heat-shield surface temperature are deduced through a multiparameter error-minimization analysis based on the European Space Agency plasma radiation database PARADE. The plasma temperatures indicate nonequilibrium conditions for cyanogen at altitudes above 70 km and equilibrium conditions after peak heating. Two separate thermal protection system activity periods are identified by the analysis. Early pyrolysis activity is indicated by a peak in cyanogen and nitrogen – hydrogen emissions at altitudes around 80 km. Broader peaks in the number of cyanogen molecules and reduced nitrogen – hydrogen emissions were identified around the time of peak heating at an altitude of about 57 km and elevated surface temperatures, above 3000 K

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

Temperature and heat flux measurement on hot models in short-duration facilities

An electrical preheating technique applied to a carbon-based model in an impulse facility has previously demonstrated surface temperatures around 2500 K, but the measurement of heat flux at such elevated surface temperatures was not previously achieved. A technique for fast-response surface temperature measurement of an electrically preheated carbon-phenolic model using an indium gallium arsenide detector with in situ calibration via a visible/near-infrared spectrometer is assessed through application in a short-duration cold-flow hypersonic wind tunnel. The method is reliable: the scatter in temperatures determined from successive acquisitions of the spectrometer data had a standard deviation of 3 K at a mean temperature of about 1500 K; the standard deviation of the Indium gallium arsenide detector results from the visible/near-infrared spectrometer data was 11 K at a temperature of about 1100 K after the termination of the hypersonic flow. The surface temperature history from the Indium gallium arsenide detector was analyzed using a one-dimensional transient heat conduction model to deduce the surface heat flux. Good agreement with an engineering correlation for stagnation point heat flux is demonstrated; however, uncertainties are large (±33%), as the thermal properties of the particular carbon-phenolic material were not available. The method is suitable for application in impulse facilities, but the effusivity (√ρ ck) for the heat shield material will need to be accurately defined for reliable deduction of surface heat flux

Quantitative emission spectroscopy for superorbital reentry in expansion tube X2

Asuperorbital reentry flow was realized in the X2 expansion tunnel of the Centre for Hypersonics of the University of Queensland, resulting in measurements of electronic excitation temperature, electron density, and particle densities of neutral and ionized atomic nitrogen and oxygen. A rectangular cold wall model was exposed to a flow corresponding to a 12.2 km/s flight equivalent velocity. Vacuum ultraviolet optical emission spectroscopy in the wavelength range between 116 and 185 nm was conducted through a window at the stagnation point. Spatially resolved optical emission spectroscopy of the stagnation streamline in the near-infrared wavelength range from 695 to 880 nm was conducted, analyzing the flow from the side. Population densities of excited atomic states, electronic excitation temperatures, and electron and ion densities were determined by analyzing the radiative transport in the flowfield. Additionally, the flowfield was numerically simulated using the code URANUS. Agreement in electron density (±6%) and electron temperature (±8%) in the equilibrium region is observed between the numerical simulations and the measurement. Significant differences between measurement and simulation in the distribution of the electron temperature at the shock are attributed to the modeling of the URANUS electron temperature

Influence of Ablation on Vacuum-Ultraviolet Radiation in a Plasma Wind Tunnel Flow

Plasma wind tunnel experiments have been performed simulating a Hayabusa reentry trajectory point at 78.8km altitude with a velocity of 11.7km/s corresponding to a local mass-specific enthalpy of 68.4MJ/kg and a stagnation pressure of 2.44kPa. Ablation-radiation coupling is investigated using a carbon preform sample, a lightweight carbon phenolic ablator sample, and cooled copper. Optical emission spectroscopic measurements in the vacuum ultraviolet (VUV) regime (116-197nm) have been conducted through a bore hole in the stagnation point of the different samples. Optical emission spectroscopic measurements in the UV/VIS spectral range (320-810nm) have been conducted viewing the plasma from the side. The stagnation point VUV radiation to the carbon preform sample is strongest, whereas it is weakest for the carbon phenolic sample. In the UV/VIS both carbon-based material samples lead to stronger plasma radiation than copper. Atomic number densities in front of the samples are largest for the carbon preform sample and lowest for the carbon phenolic sample. Rotational and vibrational temperatures are lower for both the carbon-based materials compared with copper. The measurements show for the first time experimentally the radiative heat flux mitigation by carbon phenolic materials