SOLAR FURNACE. By Heiko Ritter JOURNEY TO THE INNER

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2 SOLAR FURNACE 88 Seite 1 By Heiko Ritter JOURNEY TO THE INNER THERMAL TESTS FOR

3 Seite 2 SOLAR SYSTEM B E P I C O L O M B O T he European Space Agency, ESA, is currently developing a mission to the planet Mercury under the name BepiColombo. BepiColombo was recommended by ESA s Space Science Advisory Committee (SSAC) in September 2000 as ESA s fifth cornerstone mission. T he European Space Agency, ESA, is currently developing a mission to the planet Mercury under the name BepiColombo. BepiColombo was recommended by ESA s Space Science Advisory Committee (SSAC) in September 2000 as ESA s fifth cornerstone mission. The aim of BepiColombo is to provide two Mercury orbiting spacecraft, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO). ESA will provide the MPO, which will be defined by a European industrial team, while ISAS/JAXA of Japan will provide the MMO segment of the mission, which will be defined by a Japanese study team. The current system baseline envisages the launch of the MPO and MMO in a common composite on a Soyuz/Fregat launch vehicle in Propulsion modules are used for the interplanetary cruise (lasting about four years) and the Mercury orbit injection. The BepiColombo mission to Mercury is subject of an on-going industrial Definition Study, carried out by two industrial teams in parallel lead by Astrium GmbH and AleniaSpazio, respectively. The implementation phase is planned to start towards the end of Fig. left: BepiColombo flying past Mercury Due to the proximity of Mercury to the Sun solar irradiation fluxes received in Mercury orbit are up to 10 times higher than received in an Earth orbit. In addition, due to the missing atmosphere, sunlit surfaces of the planet may reach temperatures of up to 700K causing significant infrared radiation. In its worst case conditions the MPO spacecraft will receive next to 14,500W/m2 of solar irradiation on sun pointing surfaces up to 5,000W/m2 of planetary infrared and up to 1,500W/m2 of albedo shine on surfaces facing the planet. In addition a pronounced flux of charged particles (protons and electrons) and X-rays have to be considered, which likewise may cause considerable degradation. Parallel to BepiColombo, studies are currently being conducted on a Solar Orbiter to explore our Sun in more detail. The ongoing definition studies assume a highly elliptic orbit with the closest point to the sun well inside the Mercury-orbit. The satellite would therefore be exposed to thermal fluxes of up to 25 solar constants or about 35,000 watts per square metre. All these loads have to be taken into account when defining the test programmes during the development of the mission. In the process, a distinction has to be made between development tests, which are intended to demonstrate the functioning of individual technologies under the condi- 89

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5 Seite 4 tions expected, and tests on complete instruments or an entire satellite, which confirm the integrity of the system and its reliability under extreme conditions. In the course of developing the BepiColombo mission and possibly the Solar Orbiter mission, together with the necessary technologies, a large number of thermal tests need to be performed. A major challenge here, compared to previous space missions, is the high thermal load of up to ten or even 25 solar constants. In comparison to this, the maximum amount of solar radiation encountered on a mission to Venus is only about two solar constants. DLR s solar furnace in Cologne offers the possibility of providing a largely homogeneous distribution of these high radiation loads on an area with about half a metre in diameter. The solar furnace was used at the beginning of 2003 to conduct a first test series on a Space Solar Array which is currently being developed for BepiColombo. Two arrays with different substrates and with the solar cells arranged in a variety of ways were tested. The solar cells themselves are based on GaAs technology and are being developed for use at temperatures up to and in excess of 250 degrees centigrade. In order to keep the array Fig. left: Solar array specimen surrounded by Multi-Layer Insulation (MLI) and mounted in the vacuum chamber. The blue areas are the solar cells, the silver-coloured parts are Optical Solar Reflectors (OSRs). Fig. top right: Mercury is the planet closest to the sun in our solar system. Fig. top middle: Mosaic image of Mercury, taken by Mariner 10. Distance from Mercury: 200,000 kilometres. Fig. top left: Solar research in DLR s solar furnace. below this temperature limit, Optical Solar Reflectors (OSRs) are mounted between the solar cells. The OSR reflects most of the incident solar irradiation and simultaneously emits a high amount of radiative energy in the infrared range. Thereby the temperature of the OSR is considerably lower than that of the solar cells leading to an overall cooling effect of the array. The aim of the test series was to create a data base for correlating a thermomechanical mathematical model, which shall enable further optimisation of the solar arrays. For this purpose, the arrays were exposed to different radiation intensities in a variety of orientations while the temperature behaviour was measured at critical points. In addition, a number of thermal shock tests were conducted simulating the moment at which the satellite emerges from Mercury s shadow. The solar array, which is very cold at that moment, is then suddenly exposed to the intensive solar radiation. The tests carried out have demonstrated the suitability of the solar furnace for conducting thermal tests for missions to the inner solar system. It is also worth mentioning that the test series was carried out during winter without causing any major difficulties or delays. One limiting factor is the available vacuum chamber. At present, it restricts the size of the test object to dimensions of about 20 centimetres. Also, reflections from the wall of the chamber led to problems. DLR is currently planning to install a larger vacuum chamber with cooled black walls, which will considerably expand the possibilities of the solar furnace for conducting thermal tests for missions to the inner solar system. Heiko Ritter, ESA, Noordwijk. 91