http://hdl.handle.net/2268.2/1967 Tue, 21 Apr 2026 19:27:05 GMT 2026-04-21T19:27:05Z http://hdl.handle.net/2268.2/25233 Title: Development of Physical Models for Launch Vehicle Dynamics Simulation Abstract: This thesis presents the development and integration of a set of physical models for launch vehicle dynamics simulation within an existing simulation framework used at Spacebel. The work focuses on improving the representation of the physical environment and the associated forces acting on the vehicle, with particular emphasis on atmosphere, aerodynamics, and gravity. A modular modeling approach is adopted, in which new environment and force computation blocks are implemented and coupled with the vehicle dynamics. The developments include the implementation of an atmospheric model with stochastic turbulence based on the Dryden formulation, the computation of aerodynamic angles and loads, and improvements in the handling of kinematics, reference frames, attitude dynamics, and gravity modeling. The implemented models are verified through unit tests and consistency checks in order to ensure their physical validity and numerical robustness. In a second step, the impact of atmospheric disturbances on the vehicle response is investigated. Deterministic sensitivity analyses are first carried out to assess the influence of key parameters such as turbulence intensity and mean wind magnitude. Then, Monte Carlo simulations are performed to quantify the statistical dispersion of critical aerodynamic quantities, including the angle of attack, sideslip angle, dynamic pressure, and lateral aerodynamic force. The results show that the turbulence intensity mainly affects fluctuation-related metrics, while steady wind components and stochastic realizations can significantly modify the flight conditions at which peak loads occur. In particular, lateral aerodynamic quantities exhibit a very large variability across realizations, and rare but more severe cases are observed. These results highlight the strongly nonlinear nature of the coupled atmosphere--aerodynamics--dynamics system. Overall, this work provides a consistent extension of Spacebel's simulation framework and demonstrates its capability to support physically sound modeling and uncertainty-aware analyses for launch vehicle dynamics simulation. Thu, 22 Jan 2026 23:00:00 GMT http://hdl.handle.net/2268.2/25233 2026-01-22T23:00:00Z http://hdl.handle.net/2268.2/25225 Title: Thermal design of the characterisation test setup of improved radiative heat exchanger fins for the Einstein Telescope Abstract: This thesis focuses on the thermal design of a test setup to characterize future versions of improved fins with enhanced effective emissivity to improve cooling performance and reduce thermal noise for the radiative exchanger of the E-TEST prototype which demonstrated the feasibility of radiative cooling at cryogenic temperatures in the context of the Einstein Telescope. Thus, to validate such designs, a dedicated miniature exchanger was conceived, analyzed and dimensioned. The work began with a simplified thermal model to estimate how much heat the suspended fins that must be radiatively cooled down could tolerate while still reaching their target temperature of 25 [K], cooled against a sink at 10 [K]. This analysis showed that the system can accept about 3.5 [mW] of total heat for a first estimate of the emissivity of the fins of 0.8 with only a small fraction allowed from unwanted parasitic sources. Calculations of the error on the emissivity based on several parameters guided the design of key components such as tubes, cables and insulation. Then, a transient analysis examined how quickly the system cools down. Parameters change observation also showed how the objectives can vary. A more detailed 3D model using the ESATAN-TMS software is computed to compare these results and verify the assumptions. It provides a more realistic view of the temperature distribution across the miniature exchanger and statistically more accurate results. A transient analysis is also made for comparison. An improved model which reduces the complexity and the cost of such design is implemented as well as a multi-exchanger configuration to test the feasibility of the next phase. Overall, the study demonstrates that a compact test setup can be built to reliably measure the performance of improved fins in a cryogenic environment. Thus providing a solid foundation for future experimental campaigns. Thu, 22 Jan 2026 23:00:00 GMT http://hdl.handle.net/2268.2/25225 2026-01-22T23:00:00Z http://hdl.handle.net/2268.2/25224 Title: Automated single-mode fiber coupling at the focal plane of an optical ground station Abstract: The increasing need for secure, high-data-rate communications has motivated research in the field of free-space optical (FSO) communication technologies. FSO links offer significant advantages over conventional radio-frequency (RF) systems, including higher available bandwidth and inherently improved confidentiality due to their narrow beam divergence. In addition, the further development toward satellite-based FSO telecommunications provides an attractive framework for achieving fundamentally secure long-distance communications using Quantum Key Distribution (QKD). Nevertheless, several obstacles, such as atmospheric turbulence and imperfect mechanical alignment, severely impact the performance of these communication methods, as they degrade the single-mode fiber (SMF) coupling efficiency at the optical ground station. This research focuses on the correction of the non-common path aberrations (NCPA) caused mainly by imperfect mechanical alignment in a fiber-coupled optical system. These aberrations appear downstream of the wavefront sensor (WFS) and must be corrected through measurements at the focal plane, located at the optical fiber entry. The measurements, performed using a power meter, guide a wavefront sensorless adaptive optics (WSAO) system, which includes a fast-steering mirror (FSM) and a deformable mirror (DM). The division of tasks between the FSM and the DM is necessary to avoid exceeding the stroke limit of the DM, but constitutes a major challenge for the design of the correction procedure. Moreover, the maximum power that can be coupled into the optical fiber is not known, which also complicates the calibration of the system. To address the different challenges, this work first proposes a review of the theoretical principles of optical fibers and optical aberrations. An in-depth investigation of SMF coupling is then conducted and applied to the studied optical system. This is followed by a sensitivity analysis of the fiber coupling efficiency (FCE) with respect to variations in the considered optical system. Based on this study, an automated calibration procedure is proposed and implemented through the Multi-Stage M-SPGD (MSM-SPGD) algorithm, a novel adaptation of the Modal Stochastic Parallel Gradient Descent (M-SPGD) introduced in this work. The design of this calibration algorithm is validated through simulations under various perturbation scenarios. Within its operational limits, the algorithm systematically converges to the nominal state of the system, recovering near-maximum FCE. Experimental results further confirm its effectiveness, showing that the algorithm significantly increases the power coupled into both multimode fibers (MMF) and SMF, and effectively compensates system aberrations. These results demonstrate that the MSM-SPGD algorithm provides a reliable and practical solution for maximizing fiber coupling in adaptive optical systems. Thu, 22 Jan 2026 23:00:00 GMT http://hdl.handle.net/2268.2/25224 2026-01-22T23:00:00Z http://hdl.handle.net/2268.2/25209 Title: Design and Evaluation of Plasma Neutralization Techniques to Produce Hyperthermal Flows Representative of VLEO Conditions Abstract: The very low Earth orbits (VLEO) offers major advantages over more traditional low Earth orbit (LEO) altitudes in terms of spatial resolution and signal latency. However, operating at these lower altitudes implies interaction with a rarefied atmosphere dominated by atomic oxygen, at orbital velocities of the order of 8 km/s. These interactions lead to aerodynamic drag and material erosion over prolonged periods. Air-breathing electric propulsion (ABEP), is one of the most promising concepts to enable sustained operation in VLEO by collecting the residual atmosphere and using it as propellant to counteract drag forces. To support ABEP development, the von Karman Institute operates the DRAG-ON facility, which produces a fast particle flow representative of VLEO energies. The main limitation of DRAG-ON is that the particle flow is composed of ion species, whereas the orbital environment is essentially neutral. The present work therefore aims to improve the representativeness of DRAG-ON by developing and experimentally validating a practical ion neutralization strategy. Several neutralization approaches compatible with DRAG-ON constraints are compared, and the perforated plate concept is selected as the most suitable compromise between efficiency, integration simplicity, and preservation of a globally axial flow. To evaluate the performance of this concept, an apparent neutralization efficiency metric is defined from empirical data. A series of experimental campaigns is conducted to map the influence of operating conditions and plate geometry. The results show that perforated plates can strongly reduce the transmitted ion current and achieve high apparent neutralization efficiencies, with clear trends indicating improved performance for higher aspect ratio plates. However, measurements also reveal an unexpected increase in ion energy downstream of the plate, suggesting an additional acceleration mechanism likely linked to plasma potential effects. Finally, a novel polyimide erosion experiment is performed as an indirect neutral diagnostic, providing evidence of a significant neutral contribution downstream of the neutralizer. Overall, this work demonstrates that perforated plate neutralization is a promising solution to reduce the ionic fraction of the DRAG-ON particle flow. At the same time, it highlights the need for further investigation of the underlying physics and for improved neutral diagnostics to fully quantify the neutral flux properties. These developments are necessary steps toward achieving a truly VLEO-representative ground facility for ABEP intake testing and material degradation studies. Thu, 22 Jan 2026 23:00:00 GMT http://hdl.handle.net/2268.2/25209 2026-01-22T23:00:00Z