Investigation of Preliminary Operations and Test Mass Injection into Geodesic Motion of a Critical Space Mechanism

The Laser Interferometer Space Antenna (LISA) mission, planned for launch in 2035, represents a major step toward advancing our understanding of the Universe through the detection of gravitational waves in space, building on the groundbreaking achievements of ground-based detectors. In space-based gravitational wave detection and particularly in LISA, an inertial body, a Test Mass (TM), acting as a mirror for interferometric measurements, must be released into geodesic free fall to initiate scientific operations. This function is performed by the Grabbing, Positioning, and Release Mechanism (GPRM), which injects a 1.96 kg TM into a geodesic trajectory with residual velocities, relative to the hosting spacecraft, below 15 μm/s (linear) and 500 μrad/s (angular). During the LISA Pathfinder (LPF) mission, a technology demonstrator flown by ESA in 2015, several TM injections exceeded these requirements due to unintended dynamic re-contacts between the mechanism’s primary end-effector (plunger) and the released TM. These events were attributed to the erosion of a micrometric gap between the TM and the plungers during preliminary maneuvers preceding the nominal TM injection, which is performed by secondary end-effectors known as release tips. Analysis of in-flight telemetry collected during TM repositioning tests revealed a bistable equilibrium in the TM rotation about the y-axis during motion reversal of the plunger, identified as a primary contributor to gap erosion. The bistable behavior is attributed to non-idealities in the GPRM guiding system significantly degraded the performance of the repositioning function and TM handover from plunger to release tip prior to its release into free fall. This thesis presents breadboard-level experimental investigations aimed at evaluating the actual LPF flight configuration of the GPRM guiding system (slider–roller) and assessing the performance of a potential design improvement based on a symmetric roller–roller configuration. Our finding highlights, the slider–roller solution used in LPF exhibited better overall lateral motion behavior, however ground testing demonstrated that the roller–roller configuration significantly reduces the hysteretic lateral motion observed in the LPF design during short-stroke tests. This result is particularly relevant, as the plunger reverse motion during handover occurs over a short stroke. Finite Element Method (FEM) and multibody modeling are employed to extrapolate ground-based results to in-flight behavior, supporting design decisions for the LISA GPRM. With additional investigation focusing on release procedures, a newly identified disturbing effect, premature TM release, observed in LPF telemetry is addressed. The premature TM release is characterized by TM displacements and rotations incompatible with the expected holding action of the mechanism and occurring before the nominal release instant, i.e., prior to the separation of the release tips from the TM. Concurrently, an interfering fictitious force signal generated during release-tip extension can mimic contact between the tips and the TM, producing misleading diagnostic signals such as false confirmation of a held TM. Ground testing and FEM analyses demonstrate that this interference originates from plunger deformation near the force-sensing element. By incorporating this effect into a multibody model, the in-flight release signals are reproduced, allowing the root cause of premature releases to be identified. These findings highlight the benefit of integrating force sensing into the control loop during the TM handover from plungers to release tips and define the mechanism stiffness required to suppress fictitious force generation in the LISA GPRM. Improvements to the release procedure are also proposed in this thesis to enhance its robustness for the upcoming LISA mission. In preparation for LISA, delta design of the GPRM and improvement on the release procedure are expected to avoid anomalous observed in LPF and enhance the probability of compliant release conditions. In this context, investigation of the impulses imparted to the TM under nominal release conditions (TM release with out recontact with plungers) through ground-based characterization in essential to estimate expected in-flight TM velocity. In this thesis, individual impulse contributions arising from tip–TM adhesion, inter-tip retraction delay, as expected for symmetrically penetrating tips on the TM, and differential retraction velocity are identified and experimentally quantified under varying preload conditions. For selected LPF release tests agreement between ground-based predictions and in-flight impulses are assessed quantitatively using Monte Carlo simulations and statistical correlation analyses.