# Towards Lunar In-Situ Resource Utilization Based Subtractive Manufacturing
## 2 Methods and Materials
### Subtractive Machining State of the Art and Trade-Off  
Subtractive processes are processes of cutting technology, which are designed for geometric shaping with high flexibility, quality and process reliability. The associated mechanical processes of milling, turning and drilling comprise the class of processes with a geometrically defined cutting edge, whereas the grinding process is classified as a process with a geometrically undefined cutting edge with bonded grain. A special type is represented by the processes with jet control, which operate with non-bonded grain and use a fluid, which can be either liquid or gaseous and, thus, generate an abrasive or separating effect. A central point for the applicability of subtractive processes in extraterrestrial conditions is their adaption to local constraints so that crucial challenges can be highlighted from the analysis of the environmental conditions of the Moon and Mars and the supply logistics compared to terrestrial processing. As the most important aspect, the availability of raw materials with high hardness can be mentioned, which can be used as cutting material and occasional going beyond the property profiles of terrestrially used raw materials in terms of performance or design due to environmental conditions [57]. The current research focus on OoE processing addresses the significantly subtractive processing of extraterrestrial rocks and raw materials using terrestrial technology. The technically implemented solutions are mainly suitable for the extraction of rock samples and drilling for analysis purposes. As part of the NASA-led Perseverance mission on Mars, a special core drilling tool was successfully used to extract samples from rock formations [58]. This mechanical acting tool is characterized by a specific robust design for reliable sample collection. However, the main challenges are the autonomous process control for the drilling process and the core removal from the tool. Either way, tool developments include technologies that, in addition to targeted chip/dust removal from the machining zone, utilize energy source coupling for greater efficiency to machine larger volumes of rock with greater energy efficiency. Consequently, pneumatic units for rock fragmentation are used, removing the fragmented raw material by expedient use of gas pressure [59]. All these technologies, however, require a holistic approach considering the aggregate control, the applied technology and the structural design of the tool system. The aspect of focused and flexible coupling of energy, in turn, is addressed by ultrasonic processes. Although complex process control is necessary, demonstrators proved the general feasibility and practicability [60; 61; 62; 63; 64]. In fact, drilling systems, considering the tool, the tool guidance and the machine technology, are available for surface area applications and as deep drilling units up to Technology Readiness Level (TRL 6) [59]. Either way, in contrast to terrestrially applied jet cutting with synthesized abrasive materials, the application of regolith as a cutting material is insufficiently addressed in the literature. That is counterintuitive because of the tremendous abrasive effect, requiring, for example, extensive protective safeguards for engineering units, as accelerated regolith particles can severely damage surrounding hardware due to rocket plumes from landing missiles [65]. This impact mechanism, in turn, can be made usable, controlled and managed by a liquid jet system, tailor-made for a continuous ISRU approach (Section 1). The advantages of using an abrasive material guided in a liquid are the high flexibility of the blasting, the negligible thermal influence on the material to be cut, and the high precision of the cut [66]. That extraterrestrial fluids can be conveyed, considered a prerequisite for technical feasibility in orbit is feasible and experimentally proven [67].