Delta-development of a critical space mechanism based on on-ground and in-flight testing experience

For centuries, our understanding of the cosmos has relied on observations of electromagnetic radiation - from radio waves to optical light. Yet, a revolutionary new mean has emerged: gravitational waves. These weak ripples in the fabric of spacetime offer a new way to listen to the cosmos. Unlike traditional astronomy, gravitational-wave unlocks the observation of some of the most energetic processes in the Universe. The first direct detection occurred in September 2015, when two ground-based detectors (LIGO) observed the merger of two black holes. Since then, several other detections have been made, by LIGO and VIRGO, increasing the knowledge of the astrophysical processes generating gravitational waves. Although advances in technology have improved the sensitivity of ground-based detectors, now in their third generation, it is limited, at low frequencies, due to seismic noise and other environmental disturbances. With the present technology, space-based detectors are needed to overcome this limit. The Laser Interferometer Space Antenna (LISA) is a space mission, led by the European Space Agency (ESA), aiming to be the first space-based gravitational wave detector. It is composed of three spacecraft, forming an approximately equilateral triangular constellation, with sides of 2.5 million kilometers, following the Earth in its orbit. A laser interferometer measures the relative distance between two free-falling test masses (TMs), contained in two different spacecraft. The gravitational wave, passing through the constellation, will produce a tiny change in the TMs' relative distance, which is detected by the interferometer. The impressive sensitivity required for detecting gravitational waves relied on the development of new technologies. A demonstrator mission for LISA, called LISA Pathfinder (LPF), was launched in December 2015, advancing the technology readiness level (TRL) of the mission systems. In particular, a first version of a gravitational reference system (GRS) was flown demonstrating performance already aligned with the LISA needs. The main functions of the GRS are to enclose the TM and shield it from external disturbances, while allowing it to be in free fall. The GRS is composed by several subsystems, including the electrode housing (EH), the charge management system (CMS), and the TM caging and release mechanisms. The latter are necessary to hold the TM during the launch, and then release it once in orbit, fulfilling strict residual velocity requirements. Given the high preload required to withstand the launch vibrations, the TM caging is not able to perform also the delicate release operation. This is the task of the grabbing, positioning, and release mechanism (GPRM). The GPRM is a reusable device, able to grab, reposition, and release the TM multiple times during the mission. However, during LPF, high residual velocities were observed after the TM release by the GPRM. An extensive test campaign was run in the extended phase of LPF, collecting valuable data to understand and mitigate this issue. Additionally, on-ground testing of LPF engineering qualification model (EQM) highlighted non-idealities in the mechanism, that could have affected the performance. A new baseline GPRM has been defined for LISA, in order to mitigate the anomalies observed during LPF, but minimizing the impact on the GRS, maintaining the LPF heritage. A technology transfer program was performed from RUAG, supplier of the LPF GPRM, to OHB-Italia, responsible for the LISA GRS, including the GPRM. This program aimed at recovering the technological know-how, acquired during LPF, and to adapt the design to LISA requirements. The technology transfer included a breadboard model (BBM) phase, which results allows for setting the baseline for the LISA GPRM, embedded in an engineering model (EM). In this thesis, the activities, which enabled the definition of the GPRM baseline for LISA, are presented. These include the outcome from in-flight data analyses, GPRM improvements analyses, and on-ground testing on LPF EQM and LISA BBM. Finally, the testing activities planned for the LISA GPRM EM are described.