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Standard radiator areas for Earth orbiting spacecraft have surface finishes with high emissivity and typically low solar absorptivity. This guarantees low temperatures even with the sun shining on these surfaces. The environmental conditions for Mercury orbits are dramatically different. The incident solar flux is up to ten times higher than close to Earth. Mercury surface temperatures reach 600K to 700K at the subsolar point. Due to the resulting enormous infrared heat loads, standard radiator coatings cannot be applied on radiators that have radiative heat exchange with the Mercury surface. A special kind of radiator, a directional radiator, has been developed for the BepiColombo Mercury Planetary Orbiter (MPO). Combined with a dedicated mission planning, that prevents direct incident solar flux on the radiator, the radiator is able to provide the required heat rejection capability. A special fin reflector construction shields the radiator from incident infrared (IR) radiation from Mercury but allows heat emission from the radiator at the same time. The combination of the thermal performance requirements with challenging system needs necessitates a sophisticated design. The structure must be able to withstand temperatures in the range from -150°C up to +400°C. In order to provide the needed thermal performance, the fin reflectors must thermally be decoupled from the main radiator structure. The concept must also be flexible enough to cover a radiator area of 1.8m by 3.6m with a number of interfering spacecraft components and instruments. Furthermore the fin construction must have an extremely low mass to meet the most stringent system aspect. The directional radiator has been developed over several years. In the frame of ESA Technology Development Activities, functional performance aspects of different concepts have been investigated at first. After successful demonstration of the required thermal performance of the selected concept, a detailed design for the MPO radiator has been developed and qualified. This paper explains the driving requirements arising from BepiColombo mission, summarizes the main results of the radiator Technology Development Activity and presents the final design, manufacturing and qualification steps, handling and integration aspects as well as first results of the system level verification tests. |
