Having an in-depth knowledge on the performance characteristics of space mechanisms in flight operation, with special attention to nominal vs. anomalous performance, is vital for mission success. On many unmanned spacecraft for Earth observation missions, reaction wheel assemblies are used in combination with magnetic torquers for their attitude control. Understanding the magnitude of potential electromagnetic interactions between both types of attitude control actuators is of particular interest for large spacecraft as they are usually equipped with strong magnetic torquers. In this frame, experimental investigations have been performed on simplified test set-ups with flight representative reaction wheel assemblies operated in external homogeneous magnetic fields as well as in close vicinity of magnetic torquers which create inhomogeneous magnetic fields. The test results have been successfully correlated with computer-based simulation output obtained from models with different levels of complexity. The impact of critical parameters like the location of magnetic torquers relative to reaction wheels and their material properties such as electrical conductivity and magnetic permeability have been particularly studied. It has been found that magnetic torquers pointing orthogonal to the reaction wheel spin axis cause the highest influence on the reaction wheel's performance characteristics. The material choice for the flywheel rotor, being either ferromagnetic or paramagnetic, has a strong influence when exposing the reaction wheel assembly to external magnetic fields. In general, the increase of loss torque noticed with all reaction wheels tested has been caused by eddy current effects. In this frame, the impact of using ferromagnetic materials has been surprisingly strong. Specifically, the local distortions and guidance of the magnetic field due to ferromagnetism has a highly amplifying effect on eddy currents. However, interestingly it has also been found that the impact of material choice is much more severe when considering homogeneous magnetic fields and strong magnetic torquers while being less important with relatively small magnetic torquers. The main reasons for this finding have been compensating effects of ferromagnetic vs. highly conductive materials. / Une connaissance approfondie des caractéristiques de performance des mécanismes spatiaux en vol, et plus particulièrement des performances nominales comparées aux performances anormales, est d’importance vitale pour la réussite d’une mission. Pour les missions d’observation de la Terre, la plupart des engins spatiaux non habités sont équipés d’ensembles de roues de réaction ainsi que de magnéto-coupleurs pour le contrôle d’attitude et la stabilisation. Comprendre l'ampleur des interactions électromagnétiques potentielles entre les deux types de capteurs de contrôle d'attitude est particulièrement pertinent pour les engins spatiaux de grande taille car ceux-ci sont généralement équipés de puissants magnéto-coupleurs. Dans ce cadre, des études expérimentales ont été réalisées sur des bancs d'essais simplifiés avec des ensembles de roues à réaction représentatifs du vol fonctionnant dans des champs magnétiques externes homogènes ainsi qu'à proximité immédiate de coupleurs magnétiques (champs magnétiques hétérogènes). Les résultats des tests ont été corrélés avec succès grâce à des simulations informatiques sur des modèles présentant différents niveaux de complexité. L'influence de paramètres critiques comme l'emplacement des magnéto-coupleurs par rapport aux roues de réaction et leurs propriétés matérielles telles que la conductivité électrique et la perméabilité relative ont été particulièrement étudiés. Il a été établi que les couples magnétiques pointant orthogonalement à l'axe de rotation de la roue de réaction ont le plus d'influence sur les caractéristiques de performance des roues de réaction. Le choix du matériau pour le rotor de volant, c’est à dire ferromagnétique ou paramagnétique, a une forte influence si l'ensemble de roue de réaction est exposé à des champs magnétiques externes. En général, l'augmentation de la perte de transfert de couple constatée avec toutes les roues de réaction testées a été causée par les effets de courants de Foucault.Dans ce cadre, l'influence des matériaux ferromagnétiques a été étonnamment forte. En effet, les distorsions qui en résultent et le guidage du champ magnétique amplifient fortement les courants de Foucault. Cependant, il a été constaté que l'effet du choix du matériau est beaucoup plus important si l'on considère des champs magnétiques homogènes et des grands coupleurs magnétiques. Toutefois, cet effet est moins important avec des petits coupleurs magnétiques.

Gravitational waves detection is a challenging scientific objective, faced by scientist in the last 100 years, when Einstein theorized their existence. Despite multiple attempts, it was only in 2016 that the first observation of a gravitational wave was officially announced. The observation, worth a Nobel Prize, was made possible thanks to a worldwide collaboration of three large ground-based detectors. When detecting gravitational waves from ground, the noisy environment limits the frequency bandwidth of the measurement. Thus, the type of cosmic events that are observable is also limited. For this reason, scientists are developing the first gravitational waves detector based in space, which is a much quieter environment, especially in the sub-Hertz bandwidth. The space-based detector is named laser interferometer space antenna (LISA) and its launch is planned for 2034. Due to the extreme complexity of the mission, involving several new technologies, a demonstrator of LISA was launched and operated between 2015 and 2017. The demonstrator mission, called LISA Pathfinder (LPF), had the objective to show the feasibility of the gravitational waves observation directly from space, by characterizing the noise affecting the relative acceleration of two free falling bodies in the milli-Hertz bandwidth. The mission was a success, proving the expected noise level is well below the minimum requirement. The free-falling bodies of LPF, called test masses (TMs), were hosted inside dedicated electrode housings (EH), located approximately 30 cm apart inside the spacecraft. When free falling, each TM stays approximately in the center of the EH, thus having milli-meter wide gaps within the housing walls. Due to the presence of such large gaps, the TMs were mechanically constrained by dedicated mechanisms (named CVM and GPRM) in order to avoid damaging the payload during the launch phase and were released into free fall once the spacecraft was in orbit. Prior to the start of the science phase, the injection procedure of the TMs into free-fall was started. Such a procedure brought each TM from being mechanically constrained to a state where it was electro-statically controlled in the center of the EH. Surprisingly, the mechanical separation of the release mechanism from the TM caused unexpected residual velocities, which were not controllable by the electrostatic control force responsible for capturing the TM once released. Therefore, both the TMs collided with either the surrounding housing walls or the release mechanism end effectors. It was possible to start the science phase by manually controlling the release mechanism adopting non-nominal injection strategies, which should not be applicable in LISA, due to the larger time lag. So, since any release mechanism malfunctioning may preclude the initialization of LISA science phase, the GPRM was extensively tested at the end of LPF, by means of a dedicated campaign of releases, involving several modifications to the nominal injection procedure. The data of the extended campaign are analyzed in this work and the main conclusion is that no optimal automated release strategy is found for the GPRM flight model as-built configuration that works reliably for both the TMs producing a nominal injection procedure. The analysis of the in-flight data is difficult since the gravitational referencesensor of LPF is not designed for such type of analysis. In particular, the low sampling frequency (i.e., 10 Hz) constitutes a limiting factor when detecting instantaneous events such as collisions of the TM. Despite the difficulties of extracting useful information on the TM residual velocity from the in-flight data, it is found that the main cause of the uncontrollable state of the released TM is the collision of the TM with the plunger, i.e., one of the end-effectors of the GPRM. It is shown that the impact is caused by the oscillation of the plunger or by the elastic relaxation of the initial preload force that holds the TM. At the end of the analysis, some improvements to the design of the release mechanism are brie y discussed, aimed at maximizing the probability of performing a successful injection procedure for the six TMs that will be used as sensing bodies in the LISA experiment.