Design and optimization of a low-weight baffle for the laser altimeter BELA on board the ESA BepiColombo mission

This dissertation concerns the design and the optimization, from a thermal point of view, of a protective baffle for the transmitter part of the BepiColombo Laser Altimeter (BELA), which will operate in the Mercury’s orbit. The peculiar characteristics of the environment, in particular the short distance from the Sun and the presence of harmful agents, such as charged particles and strong UV and V-UV radiation, lead to the necessity of protecting the instrument from the environment. An optical filter has been implemented to reject the planetary fluxes, whereas a protective baffle must be designed to reject the solar radiation. Two different designs were candidates for this task: a cylindrical tube and a complex Stavroudis shape, which makes use of ellipsoids and hyperboloids. The properties of the environment have been analyzed, determining the maximal and minimal radiation for each position of Mercury in its orbit around the Sun and for each position of the satellite around the orbit. The peaks of radiation, and the maximal / minimal values averaged along one orbit, have been determined for the planetary IR radiation, for the albedo and for the direct solar fluxes, defining the thermal worst cases the s/c will experience. The possible treatments and coatings that can be applied to the two designs of the baffle have been investigated, focusing on the thermo-optical properties and the effect of their degradation under the expected environmental conditions. A campaign of tests has been carried out in situ to determine the effects of some thermal and mechanical treatments on titanium samples, representative of the cylindrical baffle. A brief introduction to the general thermal problem is then presented, describing the Lumped Parameter method and the software used for the radiative analysis and the solution of the thermal network: ESARAD and ESATAN. A method to implement wavelength-dependant thermo-optical properties has been introduced. Finally, the thermal models of the two versions of the baffle are illustrated, and the results from the thermal analyses presented. The two concepts have been analyzed comparing the thermal requirements of the unit and the indexes of performance. These indexes include the rejection efficiency, the heat fluxes dumped to the s/c and inside the instrument, the maximal temperatures of baffle and filter.