Robotic manufacturing of fibrous structures in space
Any further step for human exploration beyond the current low Earth orbit habitats, and especially habitats on the Moon, asteroids or Mars, will offer a new trade-off space for volume and mass of materials and products to be transported from Earth. Developing suitable technology in order to employ in-situ materials plays an important role in improving mission sustainability and providing new capabilities. In-Situ Resource Utilisation (ISRU) has the potential to significantly reduce launch mass, risk, and cost of space exploration; thus ISRU is considered as a key technology that enables long-term exploration, expansion of space activities, and settlement in space . Initial development of ISRU technology has focused mainly on life support consumables, such as water and oxygen, and on propellants for ascent [2,3]. Other resources available in the lunar and Martian regolith can be used for, for example, the production of surface habitats and infrastructure by various additive manufacturing technologies [4,5,6,7,8,9].
Additive construction can be accomplished by a variety of methods from slurry extrusion to sintering to melting techniques, with varying levels of difficulty, costs and technological readiness. Special challenges arise from additive construction for space applications such as construction in a vacuum or low atmosphere as well as under reduced or zero/ milli-gravity. While manufacturing of small elements using filament deposition modelling (FDM) process, such as crew tools and parts for life support systems, has been proven successful in zero-gravity conditions , the additive manufacturing technologies using powder or liquid resin could cause trouble due to cloud-forming of the regolith and spherical droplets of a binding liquid. Therefore, the powder-based methodologies would be difficult or even impossible due to the lack of atmosphere, and the low or lack of gravity would disqualify some layer deposition techniques . The most promising technologies for processing regolith into a structural material in space at the moment are solar, laser and microwave sintering [12,13,14,15,16,17]. However, it has been observed that 3D printing with raw regolith comes with issues related to thermal stresses and variation within the regolith feedstock . In addition, an important constraint of regolith structures produced by additive manufacturing is their low tensile strength due to layering of the material, which means that they either require some type of reinforcement material to account for these types of forces, or the structural form has to be designed to conform to compressive loads only. Therefore, when using pure regolith the number of shapes and sizes the structure can take is limited.
To bypass many of these issues related to 3D printing with regolith an alternative could be an in-situ production of fibres from regolith followed by the forming and assemblage into fibrous structures by an autonomous robotic manufacturing technique. The autonomous manufacturing of fibrous structures has a number of important advantages compared to additive manufacturing with regolith when constructing habitats or other surface structures:
- A fibre based process enables the possibility of orienting and locating each fibre in the structure to allow local differentiation of material properties
- Better control of the manufacturing process due to the use of continuous filament and absence of powders and liquids, therefore higher-fidelity process
- Fibres can be successfully used both in compression structures in the form of composites as well as tension structures, therefore larger variety of applications, for example pressure vessels which cannot be produced with additive manufacturing techniques
- Fibrous materials are highly formable which allows production of complex shapes in response to unique performance criteria or site conditions
- Fibre based or composite fabrication methods have demonstrated production of light-weight structures, important where large-scale, material efficiency, or mobility is needed
- Possibly better performance in response to thermal stresses
While significant research has been done to illustrate the possibilities that robots can add to construction and fabrication processes for material systems on Earth, an in-situ robotic fabrication process engages much different constraints. Construction systems on Earth were primarily designed for the condition of human production and assembly. However, in space, the fabrication process should be as much as possible autonomous. The robotic manufacturing process of the fibrous structure would therefore rely heavily on sensor feedback, as well as behavioural programming, in which the robot was programmed to act and react to its environment towards the execution of construction and assembly tasks .
The material feedstock for this process would be produced in-situ, in the form of fibres. The possibilities of producing fibres out of lunar regolith is a pre-condition for fibrous construction on the Moon. The abundance of silicates on the lunar surface, for example, allows for the production of fibreglass material on the Moon. Owing to the conditions of the lunar surface (high vacuum, lack of atmosphere and water), lunar fibreglass promises to be an excellent material and in many instances possessing properties superior to terrestrially produced equivalent material. To date, research carried out by NASA has proven the concept of lunar fibreglass and outlined a process to fabricate this material . Also metals could be extracted from regolith for the production of metal fibres through, for example, the process of biomining. Current knowledge is limited on the material and mechanical properties, as well as production techniques of such fibres, nor are the impacts of feedstock composition variation elucidated.
As part of the study, the Advanced Concepts Team and Spaceship EAC/DLR at the EAC-Cologne campus will investigate the production process of lunar glass and metal fibres, with a specific focus on acquiring characterisation data on the mechanical and thermal properties of produced fibres. Specific objectives to be addressed include:
- Impact of variation of regolith feedstock composition on the fibres, including varying simulant regolith used
- Impact of dopant material inclusion on the mechanical and thermal properties of the fibres
- Understanding of the critical process parameters on the production of fibres
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