C/C-SiC telescope structure for the laser communication terminal in TerraSAR-X

Abstract

In the frame of global networking, the modern information society is requiring faster and faster exchange of data. Thereby, conventional microwave technologies are touching their technological and economical limits. Laser communication terminals (LCT) based on optical high speed satellite links offer new possibilities for a technology leap in data exchange rates. Thereby, data links of 4 to 10 GBit/s have been in the focus of the development for the last 30 years. Due to the high frequency of light of about 500 THz (microwave < 10 GHz) and the very low wavelengths, highest data transmission rates at low power consumption can be build up at very large distances, using optical data transmission systems. Additionally, the divergence of the transmitting laser beam is significantly lower and the antenna gain is some orders of magnitude higher compared to microwave technology, leading to compact systems with low masses and volume. The current highlights in LCT development are the data links built up between the German satellite TerraSAR-X and the U.S. satellite NFIRE as well as between TerraSAR-X and the ground station at DLR in Oberpfaffen-hofen, both links built up in 2008 initially. Thereby, using a diode pumped laser beam with a wavelength of 1064 nm and a transmission power of only 1 W, bidirectional data links with a transmission rate of 5.5 GBit/s could be built up between the LEO satellites flying at an altitude of about 500 km, a distance of 5 000 km and a relative velocity of 25 000 km/h [1] as well as to the ground station at distances of 500 to 2 000 km. This equals a data transfer of about 200 000 DIN A4 pages/s or 400 DVD/hour, which is about 20 times faster compared to state of the art microwave technology. The technological challenge of LCT systems is the extremely high accuracy, requiring highly precise telescope structures (fig. 1and 2) for long term operation (> 5 years) in space environment. The main task of the telescope structure is the accurate positioning of the mirrors in order to minimise angular deviations of the receiving (RX) and transmitting (TX) laser beam, ensuring a reliable data transfer. Main sources for possible deviations are deformations of the telescope, caused by thermal expansion due to solar irradiation, or by vibrations, induced by different sources in the satellite. To obtain this extremely high accuracy of telescope structures, e.g. made of lightweight aluminium, active heating/cooling systems are needed in order to stabilize the temperature and finally the geometry of the structure. To avoid or reduce the high cost and masses of these tempering systems, passive telescope systems based on thermally stable materials are favoured. INVAR and Zerodur are typical representatives of materials, offering very low or zero CTE (table 1). However, the relatively high densities of these materials lead to high structural masses. Additionally, Zerodur is a very brittle ceramic material, leading to high machining costs. As a typical Weibull material, the reliability of Zerodur structures is decreasing with increasing volume or sur-face of the structure. Therefore, large structures require high wall thick-nesses, again leading to high masses. Another possible candidate is carbon fibre reinforced plastic (CFRP), offering very low density (CFRP 1.5 g/cm³) and high specific stiffness. However, due to the polymer matrix, the long term stability in space is critical. Additionally, swelling, caused by humidity, leads to changes in geometry during assembly and orbit release. Carbon fibre reinforced Carbon (C/C) offers low density and high stiffness as well as insensitivity to humidity. Main disadvantages are its generally high open porosity of about 10 %, making cleaning processes difficult and leading to particle release. At DLR, C/C-SiC materials, manufactured via the LSI (liquid silicon infiltra-tion) process originally have been developed for lightweight structures in thermal protection systems of reusable spacecraft [2]. Thereby, in the first process step, a preform made of carbon fibre reinforced plastics (CFRP) is manufactured via well known technologies like resin transfer moulding (RTM) autoclave technique or warm pressing. In the second step, the CFRP preform is prolysed at temperatures of up to 1650 °C in inert gas atmos-phere, leading to a highly porous carbon fibre reinforced carbon (C/C) preform. In the third and last process step, molten silicon is infiltrated in the C/C preform by capillary forces only. Parallel to the infiltration, the silicon reacts with a small amount of the C-matrix and C-fibres, forming SiC. The resulting C/C-SiC material is characterized by dense bundles of C-fibres embedded in C-matrix, again embedded in a dense SiC matrix with a small amount of not converted, free Si. In contrast to monolithic ceramics, C/C-SiC materials offer a high damage tolerance and a quasiductile fracture behaviour, comparable to grey cast iron. Thereby, large and thin walled, light weight structures can be realized at reasonable cost. Due to the high content of carbon fibres in the CFRP (F = 50 – 60 Vol.-%) and C/C perform, which are protected against Si attack very well, the properties of the C/C-SiC materials are generally dominated by the C-fibres. Thereby, C/C-SiC materials based on laminated C-fibre fabrics show an highly anisotropic material behaviour, offering relatively high strength and Young`s modulus in the in plane direction, parallel to the fibres. For the same reason, the CTE of the C/C-SiC XB material, based on high tenacity C-fibres (Tenax HTA), is generally very low in the in plane direction (-0.2 to +2.5 x 10-6 K-1 between 0 and 1500°C, depending on SiC content [3]), whereas significantly higher values of 2 to 7 x 10-6 K-1 are obtained in the direction perpendicular to the fibre orientation