Development and qualification of deployable membranes for space applications

Deployment systems for innovative space applications such as solar sails require technology for a controlled and autonomous deployment in space. Before employing such technology for a dedicated mission, it is necessary to demonstrate its reliability with a Technology Readiness Level (TRL) of six or higher. On the example of the design implemented in the Gossamer-1 project of the German Aerospace Center (DLR), a stowing and deployment process for large deployable membranes mainly considered for solar sailing is analyzed and tested. It is based on a combination of zig-zag folding and coiling of triangular sail segments spanned between crossed booms. Possible membrane materials are evaluated and a deployment technique is explored through theoretical analysis and tests in order to verify their functionality for large membrane space systems. The requirements for membranes that are exposed to the space environment are studied and the materials are analyzed regarding their resistance against atomic oxygen, radiation and their thermal properties. The folding geometry and force progressions are described mathematically. Load introduction aspects, the stress-strain state and the billowing of the deployed membrane are analyzed with finite element models. The folding lines were examined with microscopes, and their impact on thermal behavior is shown by analytical analysis. The membrane and deployment mechanisms were manufactured and integrated in an ISO 8 clean room environment, and the deployment process was verified in an extensive test campaign. It ranged from component level to system level and included mechanical vibration, static acceleration, fast decompression, thermal vacuum and laboratory deployment tests. It is shown that state-of-the-art aluminum-coated polyimide foils are sufficient for a demonstration of deployment technology in an Low Earth Orbit (LEO) and that coating systems based on a combination of aluminum, silicon oxide and titanium oxide enhance the membrane properties for solar sails. The model of the deployment force progression under zero gravity shows a tendency that the loads are transferred along the cathetus of the sail segments. The finite element models show generally low stresses in the deployed membrane and interface forces on the order of several Newtons for a 25m2 membrane. The analysis of the folding lines reveals that coatings in this region are damaged, and that hot spots can occur due to multiple reflections. The verification testing showed the general suitability of the membrane and of the deployment strategy itself. Materials, mechanisms, and a stowing and deployment strategy are presented that enable the controlled and autonomous membrane deployment for space sails. While the analysis presented is applied on a sail with an edge length of about 5 m, it allows an analysis of other configurations as well. This is of particular interest because currently-considered solar sails are about one order of magnitude bigger. With the environmental tests conducted, the membrane-related aspects of the deployment technology are on TRL six for a 25m2 LEO deployment demonstrator. The deployment strategy is scalable and materials are available that can be used for bigger solar sails as well. With respect to membrane-related aspects there is nothing to prevent the development of full-scale solar sails

Material Challenges (in VLEO)

Very Low Earth Orbits (VLEO) are increasingly seen as a new orbit regime for new small communication satellites that operate as part of a large constellation. Due to the high drag these orbits with altitudes between 100 and 400 km are considered "self-cleaning" but at the same time it poses significant technological challenges. This presentation addresses material challenges, specifically atomic oxygen erosion and material aging due to ultra violet radiation

Artificial Solar Wind for Space Material Developments

Investigating materials under space environment conditions requires the artificial recreation of both corpuscular and electromagnetic radiation under ultra-high vacuum conditions. DLR’s Complex Irradiation Facility combines a proton and an electron accelerator as well as electromagnetic radiation sources with broad wavelength range from ultraviolet radiation to infrared radiation in one facility. It is used to investigate the aging of materials in simulated space environment

Verification Testing of the Gossamer-1 Deployment Demonstrator

Gossamer structures for innovative space applications, such as solar sails, require a technology that allows their controlled and thereby safe deployment. Before employing such technology for a dedicated science mission, it is necessary, to demonstrate its reliability with a Technology Readiness Level of six or higher. The aim of the presented work is to provide a reliable technology that enables the controlled deployment and verification of its functionality with various laboratory tests to qualify the hardware for a first demonstration in low Earth orbit. The development was made in the Gossamer-1 project of the German Aerospace Center. This presentation provides an overview of the Gossamer-1 hardware development. The design is based on a crossed boom configuration with triangular sail segments. Employing engineering models, all aspects of the deployment were tested under ambient environment. Several components were also subjected to environmental qualification testing. An innovative stowing and deployment strategy for a controlled deployment and the required mechanisms are described. The tests conducted provide insight into the deployment process and allow a mechanical characterization of this process, in particular the measurement of the deployment forces. Deployment on system level could partially be demonstrated to be robust and controllable. The deployment technology is on Technology Readiness Level four approaching level five, with a qualification model for environmental testing currently being built

Development of New Solar Array Concepts for Space Applications

Solar arrays are the main power source in space. Conventionally they are composed of stiff backing structures and brittle PV cells. While the power demands of space missions are increasing, e.g. for electric propulsion, the increase of efficiency of the solar cells itself is limited. New developments make use of flexible and semi-flexible solar array designs in order to achieve higher power/mass and power/volume ratios. An example is a two-dimensional deployment of solar arrays in order to increase the deployed area as instigated in DLR’s GoSolAr project. Such deployment strategies can be combined either with conventional photovoltaics, also using thinned wafer technology, or with thin-film technologies that are truly flexible. Development of such technology involves also deployment testing and testing of materials under the specific radiation environment that is present in space. In this talk I would like to give an overview about the development aspects, showing how we try to go beyond conventional array designs but still using photovoltaic solar cells

Proton Spectra for the Interplanetary Space Derived From Different Environmental Models

Knowledge about the space radiation environment is crucial for the design and selection of materials and components used for space applications. This environment is characterized not only by the Sun’s electromagnetic radiation but also by charged particles categorized into solar wind, solar energetic particles (SEP) and galactic cosmic rays (GCR). Especially for material engineering and qualification testing, differential and integral spectra for particle energies ranging from keVs to GeVs are required. Up to now, a wide variety of models is available, whereas it is difficult to keep the overview. Although, e.g., the European Cooperation for Space Standardization (ECSS) standard includes instructions on how to investigate particle radiation, it does not provide an overall view. This paper shall support those in need of a comprehensive overview and provide comprehensive information about proton radiation spectra that can potentially be of use for space engineering tasks ranging from mission analysis to material and component design as well as qualification testing. The publicly accessible platforms OLTARIS, SPENVIS, and OMERE were examined for available proton spectra to be used. Exemplary, the particle radiation of solar cycle 23 is considered, which comprehends the years 1996–2008. A common drawback of the available models is their restriction to the MeV-range. Particularly when materials are directly exposed to the space environment, low energetic particles, specifically, the keV-range, are of high interest, since these particle transfer all their energy to the material. Therefore, additional data sources were used in order to include the usually neglected low energy protons into the derived spectrum. The data was transferred to common set of units and eventually could be compared and merged together. This includes a comparison of the most common models, incorporating data foundation, applicability, and accessibility. As a result, extensive and continues spectra are fitted that take all different models with its different energies and fluxes into account. Each covered year is represented by a fitted spectrum including confidence level as applicable. For solar active and quite times spectra are provided

Performance analysis and mission applications of a new solar sail concept based on crossed booms with tip-deployed membranes

For precursor solar sail activities a strategy for a controlled deployment of large membranes was developed based on a combination of zig-zag folding and coiling of triangular sail segments spanned between crossed booms. This strategy required four autonomous deployment units that were jettisoned after the deployment is completed. In order to reduce the complexity of the system an adaptation of that deployment strategy is investigated. A baseline design for the deployment mechanisms is established that allows the deployment actuation from a central bus system in order to reduce the complexity of the system. The mass of such a sail craft will be slightly increased but its performance is still be reasonable for first solar sail missions. The presented design will be demonstrated on breadboard level showing the feasibility of the deployment strategy. The characteristic acceleration will be evaluated and compared to the requirements of certain proposed solar sail missions

Controlled Deployment of Gossamer Spacecraft

Deployable gossamer structures for solar sails need to be deployed in a controlled way. Several strategies present have the disadvantage that the sail membrane cannot always be tensioned during the deployment process. In combination with a slow deployment, this involves the risk of an entanglement of the sail. Slow deployments of at least several minutes are desirable in order to keep inertial loads low and to implement Fault-Detection, Fault-Isolation and Recovery Techniques (FDIR). This might further require completely stopping and resuming the deployment process. For gossamer spacecraft based on crossed boom configurations with triangular sail segments, a deployment strategy is described that is assumed to allow such a controlled deployment process. With a combination of folding and coiling, it is ensured that the deployed sail area can be held taut between the partly deployed booms. During deployment, four deployment units with two spools each on which the sail is mounted (a half segment stowed on each) moves away from the central bus unit, the center of the deployed sail. The development was made in the Gossamer-1 project of the German Aerospace Center (DLR). The folding and coiling of the membrane is mathematically modelled. This allows an investigation of the deployment geometry. It provides the mathematical relation between the deployed boom length and the deployed sail membrane geometry. By modelling the coiled zig-zag folding lines it is possible to calculate the deployment force vector as function of the deployment time. The stowing and deployment strategy was verified by tests with an engineering qualification model of the Gossamer-1 deployment unit. According to a test-as-you-fly approach the tests included vibration tests, venting, thermal-vacuum tests and ambient deployment. In these tests the deployment strategy proved to be suitable for a controlled deployment of gossamer spacecraft. A deeper understanding of the deployment process is gained by analyzing the deployment strategy mathematically

Spacecraft Materials’ Reflectivity and Surface Morphology: Aging Caused by Proton Irradiation

The radiation environment in Low Earth Orbit (LEO) is dominated by protons captured by Earth’s magnetic field in the Inner Van-Allen belt. Defunct satellites and other space debris objects can be resident in this environment for several decades and even centuries. So far, there is little knowledge about the impact of long-duration proton exposure to the surface morphology and reflectivity in LEO environment. We report on a laboratory test campaign exposing typical spacecraft materials with protons of 100 keV and 2.5 keV kinetic energy and a fluence corresponding to an in-orbit duration of 100 years and 120 years, respectively, in an 800 km sun-synchronous orbit. Although we find microscopic changes in surface morphology, reflectivity changes of all tested materials were smaller than 15%. This result brings positive news for on-going efforts to use optical methods, e.g. lightcurve measurements or active polarimetry, for characterizing space objects, since it suggests that data can - to a good approximation - be analyzed without accounting for proton induced aging effects that might affect the materials’ optical properties over time

Special Testing and Test Strategies for Unique Space Hardware Developments

Hardware developments for new and innovative space applications require extensive testing in order to demonstrate the functionality under the expected environmental conditions. Within several projects the German Aerospace Center (DLR), Institute of Space Systems used its test capabilities for unique tests campaigns that went beyond standard qualification testing