# Towards Lunar In-Situ Resource Utilization Based Subtractive Manufacturing
## 2 Methods and Materials
### Abrasive Water Jet Cutting  
In all abrasive water jet (AWJ)) processes, a high-pressure water jet is used to accelerate added hard and sharp-edged abrasive particles. Garnet sand, corundum and SiC are primarily used for this purpose [68]. The particles impact the workpiece surface with high kinetic energy in a locally concentrated manner and remove material. The advantages of this process over other jet processes are the smallest jet diameters (<0.1 mm) [69] and low process forces [70], allowing thin-walled components to be manufactured with high material utilization. Another advantage is the high flexibility. This is demonstrated by the large number of manufacturing processes for which the abrasive water jet can be used in addition to cutting. For example, it can also be used for drilling, surface structuring, decoding, polishing, cleaning or for nonseparating processes, such as surface peening, which increases the relevance of the process for manufacturing in space. Further flexibility of the AWJ is demonstrated by the fact that a wide range of materials, for example, relevant for space applications (Section 1.4), can be machined. This includes soft materials, such as plastics [71], tough materials, such as metals [72], as well as thermal shock-sensitive and brittle materials, such as glasses [73] and technical ceramics [74]. In addition, composite materials, such as CFRP or sandwich materials, can also be processed [75]. However, according to the state of the art, two methods exist for generating an abrasive water jet. These two methods, the injection jet method and the suspension jet method, are shown in Figure 6.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Parameter** & **Terrestrial** & **Extraterrestrial** \\  & **Relevance** & **Relevance** \\ \hline Modularity and Plug and Play & o & + \\ Intelligent production plants/machines & + & o \\ Autonomous or semiautonomous processing & - & + \\ (low interaction with humans) & \(\circ\) & + \\ Energy efficiency & \(\circ\) & + \\ Weight and space optimized & - & ++ \\ \hline Legend: Importance: - low; o medium; + high; ++ very high. & \\ \end{tabular}
\end{table}
Table 9: Relevance of selected parameters for subtractive processes on their impact on the terrestrial and extraterrestrial applicability.  
Figure 6: Abrasive water jet cutting principles.  
In the first one, water is pressurized to 300-600 MPa and pressed through a nozzle, while accelerating to several 100 m/s. The jet formed entrains air corresponding to the Venturi effect and creates a negative pressure in the mixing chamber, drawing air into the mixing chamber through the abrasive feed. This gas flow is used to transport the abrasive into the mixing chamber. Furthermore, the three-phase jet is focused in the focusing tube. The suspension jet process, on the other hand, differs since a two-phase jet is formed. This is enabled by mixing the abrasive with the high-pressure water ahead of the nozzle, dispersing it and accelerating it together. Due to the absence of air in the jet, higher energy densities and higher particle loadings can be achieved in the water jet.