## 2 Materials and Methods
### Three-Dimensional Printing Technology Applications in Outpost Construction  
Any extraterrestrial outpost must satisfy two requirements: The first requirement is to provide a breathable environment (room) for inhabitants. Such a room contains breathable air with a pressure equal to the Earth's surface pressure (approximately equal to 100 kPa). In the planetary environment, this can be carried out using an inflatable module made of a light, resistant composite, such as ethylene-tetrafluoroethylene copolymer (ETFE). The inflatable structure is simply transported from the Earth to the remote planet; then, it will be filled with the breathable air under appropriate pressure. The second requirement is to shelter the inflatable module from outer environmental radiation, meteorites, temperature variations, etc. To address this requirement, 3D printing technology that is capable of manufacturing regolith-based building blocks can be utilized. Different 3D printing techniques can be used in extraterrestrial outpost construction [20]. At present, there are three applicable 3D printing techniques for printing building components on remote planets. These techniques are the Monolite machine (or the D-shape technology), contour crafting (CC), and the freeform construction technique [20]. These techniques were initially developed during the 1990s as full-scale manufacturing processes [85; 86; 87]. These three techniques are presently utilized in the manufacture of 3D components in terrestrial construction projects. Figure 7 shows a schematic representation of 3D printing processes, which is relatively similar for all three techniques. The differences between these three methods are derived from the applied materials and deposition methods. The main difference among these methods is how they work with materials. D-shape technology selectively activates or solidifies materials within a layer that has already been placed, similarly to a printer choosing specific areas to solidify. The other methods, on the other hand, use a process in which building materials are pushed out (extruded) together with a binder or catalyst. This mixture is similar to concrete and is used to build up the structure layer by layer [20]. Each of these techniques needs a raw material (regolith) and a binder liquid for the manufacturing process. The binder can be entirely or partially produced on Earth. The ratio of the liquid binder to the raw regolith is crucial as it has a direct relationship with transportation costs. So far, some large-scale projects pertinent to the application of 3D printing techniques in outpost construction have been conducted. The main examples are the works carried out by [PERSON] and [PERSON] (2012) [21], [PERSON] et al. (2014) [20], and NASA's 3D-Printed  
Figure 7: A schematic of the 3D printing process [87]. Habitat Challenge Competition (2019) [79]. In what follows, the above-mentioned projects and their achievements are briefly described. In a feasibility study supported by NASA, [PERSON] and [PERSON] utilized the CC technique to construct a lunar outpost [21]. In this research study, a dome-shape habitat composed of sulfur concrete and sintered regolith was constructed (Figure 8). Sulfur concrete can be produced by mixing pure sulfur and regolith [68]. Pure sulfur can be obtained via the decomposition of troilite minerals (FeS) found in lunar/Martian regolith [21], thereby dramatically reducing transportation costs. The researchers reported that the dome-shape structure constructed using the CC technique had a compressive strength close to 55 MPa, which is highly considerable. For comparison, the compressive strength of common terrestrial concrete varies from 17 MPa to 28 MPa [88]. In another feasibility study founded by the ESA, [PERSON] et al. used D-shape technology to fabricate regolith-based components for outpost construction on the lunar surface [20]. The outpost's structure included an inflatable module protected by 3D-printed components (Figure 9 (left)).