Technical Textiles 5/2018, December 2018
Warp-knitted textiles for space satellites
An expanding internet needs new types of reflectorsThe growing usage of the internet globally means that large volumes of data have to be transmitted. Data transfer involves transmitting digital information, e.g. music or video files, via electromagnetic waves between so-called earth stations on the ground and orbiting satellites. In this case, different frequency bands are used. Global research projects are involved in changing satellite communication into frequency bands that can transfer larger volumes of data, i.e. into so-called Ka, Q and V bands. Larger reflectors, which are suitable for higher energy frequencies, are needed for transferring large volumes of data .
Diagram of data transfer from e.g. an earth station, television pictures and digital radio
As the performance of the satellites increases, the diameter of the reflectors must also increase. The limit is 4.5 m, which is determined by the ability to transport them into orbit. However, modern telecommunications satellites have reflectors with diameters of up to 30 m and deployable surfaces. These are made from metallic, warp-knitted textiles.
The technical textile is tensioned on a support structure and kept in parabolic form with cables, so-called cable networks. Reflectors having warp-knitted surfaces are flexible and can be stored easily. Compared to other antenna systems, they also have a low surface density, which means that deployable reflectors have a high mass efficiency. Deployable, warp-knitted reflectors are the only large reflectors that are being used successfully in space satellites [3-5].
Textiles offer a viable solutionWarp-knitted reflector surfaces have been used in space since the 1960s. Their first known use was in November 1969 for transmitting television pictures of the moon landing back to earth during the Apollo 12 mission. The textile was produced on a 2-bar warp knitting machine from gold-plated metal yarns. In 1983, the centrally fed reflector of the TDRS-1 satellite from the Harris Corporation, Melbourne, FL/USA, was made from a warp-knitted metal textile and was deployable.
Since the 1980s, more and more missions have involved the use of deployable, warp-knitted reflectors having diameters of up to 20 m. Some of the examples include Inmarsat 4, Skyterra and Terrestar. The warp-knitted reflector surfaces are being improved all the time. One of the latest developments is AstroMesh from Northrop Grumman Astro Aerospace, Carpinteria, CA/USA (Fig. 2). All the products manufactured so far have been produced on machines from Karl Mayer Textilmaschinenfabrik GmbH, Obertshausen/Germany [6, 7].
Demanding applications require a high level of performanceHigh demands are made of textiles for use as reflector substrates. Above all, the yarns processed must be resistant to extreme temperatures (temperatures in orbit may range from -190 °C to +140 °C) and deliver a specific high-frequency reflection capacity. The reflection of electromagnetic waves depends on the surface roughness and the electrical conductivity of the material. Unlike a smooth surface, a rough surface disrupts the electromagnetic waves.
The requirements relating to roughness, electrical conductivity and temperature can be met by using gold-plated molybdenum or tungsten yarns. For this reason, both types of metal yarns are used for producing reflectors. Unlike molybdenum, tungsten can be drawn to a finer diameter. The advantage of this is that a narrower diameter causes less internal stress when the yarn is bent [8-10].
For use in reflector applications, the warp-knitted textile must, above all, be elastically pliable in every direction. This can be achieved by a construction having an isotropic structure. Thanks to the isotropic structure, the textile remains taut under specific thermal conditions, at the right level of pre-stretching, and creases are avoided. Atlas mesh and satin mesh are the constructions currently used for producing warp-knitted reflector substrates [11-12].
Over the last few decades, the USA has invested heavily in developing large deployable reflector antennas (LDRs). The funds have mainly come from the USA’s military budget.
Because of this high level of investment, the EU has come to depend on the USA’s duopoly of Harris and AstroMesh.
Textile reflectors from AachenThe Institute of Textile Technology at the RWTH Aachen University (ITA) has been researching into textile reflectors since 2014. The starting point was the project, “Ultralight Reflector Mesh Material for Very Large Reflector Antennas”, which ran from September 2014 to March 2017. The European production capacity for manufacturing a reflector having a diameter of 6 m, based on a single-layer, warp-knitted tungsten textile, was investigated during the project within the framework of the ESA-ESTEC Program. The ITA developed and produced a Ku band space reflector from gold-plated tungsten (Fig. 3). This was produced on a laboratory scale initially and subsequently in full production width. The process was based on 2D warp knitting technology. Work on the project ended at a Technology Readiness Level (TRL) of 5 for production at a sample level.
© Foto: © ITA
Warp-knitted textile made from gold-plated tungsten for use as a reflector surface, produced at the ITA
The feasibility of producing reflector surfaces in Europe is now established. Before the ITA became involved with the production of these types of warp-knitted reflector structures, expertise and knowledge in this field was concentrated in Russia and North America. The success of the ITA has, therefore, paved the way for European manufacturers to compete on the global aerospace market with innovative technology.
The ITA continues to be involved. The next project, “Development of a Versatile, Flexible, Deployable Reflector Surface Structure for Space Antennas”, started at the ITA in October 2018 [6, 13, 14].
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AstroMesh = registered trademark