The numerical assessment of in-flight contamination is a global process, which needs consistent numerical processing of elementary ground tests and global modeling of in-orbit situations. If the traditional physical approach of Europe is followed, both sides must make use of consistent physical models, and upgrade them consistently. This article presents recent progress performed at ONERA, in collaboration with CNES, in this respect. For this physical approach, elementary material outgassing tests aim at characterizing each chemical species independently, based on TGA / MS coupling for in situ characterization. Processing large data sets of mass peaks versus time, and interpreting them as resulting from a few outgassed chemical species, each one with its own mass spectrum, requires heavy computations and smart algorithms. The first results shown here are very promising. QCM and mass spec data acquired during TGAs where fitted with very convincing models for deposit reemission and mass spectra for the reemitted species that were identified with database spectra. This makes us confident in the next step consisting in similarly interpreting outgassing QCM + MS measurements in term of discriminated species, although they are all outgassed simultaneously in that case.
Contamination modeling has struggled with the challenge of species separation. Without the capabiliity to physically identify the chemical nature of contaminants and their contributions, the realistic correspondence between a chosen model and its underlying physics is very difficult to demonstrate. With the development of TGA/MS coupling experimental techniques and specific data treatments, a species separation was achieved on the ScotchWeld EC2216 adhesive. After a detailed exposure of the species separation need, this paper presents the experimental facility and the numerical procedure to effectively get contaminants identification and differentiate their contribution in a mixture.
KEYWORDS: Diffusion, Molecules, Contamination, Physics, Data modeling, Mathematical modeling, Chemical species, Ultraviolet radiation, Chemical reactions, Mass spectrometry
Contamination modeling in Europe has long been based on physical mechanisms, such as desorption. However other physical mechanisms, such as diffusion, evaporation or mixing effects exist. These alternative mechanisms were experimentally evaluated and modelled. It was yet observed that, without an experimental capability to reliably separate the (re)emitted chemical species, it is very difficult to determine whether the modeling and its underlying physical mechanisms are representative of reality, or simply a mathematical fit of reality. This is the reason why in the last years emphasis was put on the experimental separation of species, mostly through TGA/MS coupling. This paper presents a review of these efforts and promising results on species separation to reach a really physical modeling of outgassing, deposition/reemission and UV synergy.
KEYWORDS: Contamination, Ultraviolet radiation, Optical properties, Reflectors, Global Positioning System, Space operations, Solar cells, Temperature metrology, Sun, Photolysis
Growing evidence was accumulated on the deleterious effects of the photofixation of contaminants on solar arrays power and on the optical properties of coatings. UV irradiation indeed promotes contamination accretion, even on surfaces on which condensation would not occur and strongly degrades the optical properties of contamination layers. Recent research conducted at ONERA enabled to implement a photofixation model in the numerical tool COMOVA. Present work aims at assessing the ability of this model to reproduce in-orbit cases and at estimating the sensitivity of the results to input parameters. Simulation results are reasonably close to the in-orbit degradations.
KEYWORDS: Contamination, Satellites, Ultraviolet radiation, Data modeling, Adaptive optics, Oxygen, Space operations, Thermal modeling, Solar processes, Control systems
The in-orbit aging of thermo-optical properties of thermal coatings critically impacts both spacecraft thermal balance and heating power consumption. Nevertheless, in-flight thermal coating aging is generally larger than the one measured on ground and the current knowledge does not allow making reliable predictions1. As a result, a large oversizing of thermal control systems is required. To address this issue, the Centre National d’Etudes Spatiales has developed a low-cost experiment, called THERME, which enables to monitor the in-flight time-evolution of the solar absorptivity of a large variety of coatings, including commonly used coatings and new materials by measuring their temperature. This experiment has been carried out on sunsynchronous spacecrafts for more than 27 years, allowing thus the generation of a very large set of telemetry measurements. The aim of this work was to develop a model able to semi-quantitatively reproduce these data with a restraint number of parameters. The underlying objectives were to better understand the contribution of the different involved phenomena and, later on, to predict the thermal coating aging at end of life. The physical processes modeled include contamination deposition, UV aging of both contamination layers and intrinsic material and atomic oxygen erosion. Efforts were particularly focused on the satellite leading wall as this face is exposed to the highest variations in environmental conditions during the solar cycle. The non-monotonous time-evolution of the solar absorptivity of thermal coatings is shown to be due to a succession of contamination and contaminant erosion by atomic oxygen phased with the solar cycle.
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