## 2 Materials and Methods
### Three-Dimensional Printing Technology Applications in Outpost Construction
The last test was a compression test performed to check the resistance of both structures (Figure 10c,d). Finally, the AI space Factory won the competition, and currently, both teams are continuing their improvements of their proposed outpost design [79]. Apart from the type of 3D printing technique, the regolith sintering method also has a critical role in the success of the applied 3D printing process. As mentioned earlier, three methods, including laser sintering, solar sintering, and microwave sintering, can be utilized to sinter the regolith. These sintering methods use different types of energy sources, i.e., high-powered laser, solar concentrator, and microwave. The efficiency of these sintering methods was compared from different angles, such as the consumed energy, required time, heat penetration depth, the complexity of operations, regolith properties, and outpost location [64; 66; 92; 93; 94; 95]. According to these studies, the following results were obtained:  
Figure 9: The design of the habitat constructed using D-shape technology and internal inflatable structure (**left**). The 3D printing machine (**right**) [2]. * The laser sintering method needs an extremely high amount of energy for heating a small volume of regolith. Furthermore, the long time and high-temperature gradient during the printing operation can make the process problematic. * Regarding solar sintering, although the energy source is easily available and unlimited, the dependence of the energy supplement relative to the project's performance and location can be a significant challenge. For example, solar energy is not considerably available on the Moon's poles. * Microwave sintering requires an amount of energy that is nearly 33-44 times less than the laser sintering method [96]. * The heat penetration depth is also another challenge that should be considered. The heat penetration depth in the regolith is greater when using the microwave sintering method compared to the solar sintering method [97]. * The complexity of operations is another issue that is mostly critical in the solar sintering method. For example, the position maintenance of the focal spot, cleaning the lenses and mirrors from the dust, etc., are some of these challenges [97; 98]. * Regolith properties can also affect the performance of sintering methods. For instance, the efficiency of the solar sintering method is dependent on the regolith's optical properties [99]. On the contrary, it is believed that the complexity of the regolith's morphology may enhance microwave sintering efficiency [100]. The reason for this is regolith densification due to the presence of high portions of glass and ilmenite (FeTiO\({}_{3}\)) within the regolith's composition [101]. Figure 10: The 3D-printed outpost structures by the AI Space Factory team (**a**) and PENN State team (**b**). The performance of the outpost structures under compression tests (**c**,**d**) [79]. Based on the above-mentioned requirements, capabilities, and limitations, the microwave sintering method seems to be the most reliable option. Nevertheless, despite all its advantages, some challenges have not been addressed yet. For instance, at low frequencies, the heat penetration depth is greater, but less energy will be absorbed when sintering the regolith. In addition, in this method, energy absorption depends chiefly on the regolith's properties, such as the regolith's composition, grain size, density, temperature, moisture, and dielectric properties. Generally, self-insulation [98; 102], weak absorption at low frequencies, and thermal runaway are the most significant challenges of the microwave sintering method. Regardless of the 3D printing technique and regolith sintering method, it should be noted that the implementation of 3D printing operations in remote planets needs extra considerations.