Speaker
Description
The PRODUCERS project, funded by ESA and led by Fluid Gravity Engineering, Ltd., aimed to experimentally investigate and computationally predict radiative markers associated with the destructive re-entry of spacecraft at the end of their life. The purpose of the research activity is to improve the analysis of spectroscopic and visual data obtained through dedicated remote observation campaigns of such events. The primary reference case under consideration are the Cluster-II satellites. The experimental aspect of this activity involves a thorough investigation of spectral signatures related to the demise of materials and components in the Plasma Wind Tunnel (PWT) facilities at the Institute of Space Systems (IRS) at the University of Stuttgart.
Two key phases of the experiment campaign are summarised. The first phase focused on determining the spectrographic signatures of individual materials demising under aerothermodynamic heating conditions relevant to the scheduled Cluster-II re-entries. A selection of five different aerospace material samples, including CFRP, aluminum, titanium, and stainless steel alloys was subjected to destructive testing in the PWT with an emphasis on emulating enthalpies and pressures of key trajectory points around the stagnation area of the probe, with the aim of producing and capturing emissions from highly excited liquid and gaseous outflows, i.e. droplets. The corresponding temporally resolved emission signatures were measured using two optical emission spectrometers and correlated with visual and emissivity-corrected thermographic temperature measurements of the front surface. The results imply that material emissions, specifically those from metallic alloys can provide the means to identify both the composition and the ongoing state of demise of the observed materials, however it must be noted that the energy accumulation in the observed metallic droplets appeared to insufficiently emulate wake flow conditions for demising spacecraft, to the effect that emission lines associated with the primary element of a given alloy was rarely identified in the reported experiments.
The second phase of testing focuses on similarly assessing typical emission signatures of key spacecraft components and structures extracted from recovered as debris from the original Cluster spacecraft debris. The composition of such test articles is heterogeneous, resulting in richer emission signatures during demise as compared to homogeneous material tests. The tests demonstrate the challenge of distinguishing between different polymers based on their emission spectra, as most yield similar carbonaceous and organic material emissions. However, flare-ups recorded in the spectra can be noted as being indicative of certain macroscopic failure events. While the testing further implies that the spectral signature of small scale equipment may in principle permit an identification of sub-components e.g. in the case of electronics equipment, it is important to note that this is likely impractical during remote observations due to length scale considerations.
The experimental campaign created a comprehensive database of spectroscopic markers for various materials, surface coatings, functional and structural components that are relevant for the re-entry of Cluster-II and other spacecraft. The impact of PWT testing conditions and the measurement setup designs on the prospect of recreating spectrographic markers representative of observable flight conditions, i.e. wake flows, are discussed.
