# Metal Additive Manufacturing for Satellites and Rockets
## 4 Thrusters
Another material class often used for aerospace applications is titanium aluminide alloys, containing the ordered \(\gamma\)-TiAl phase [61]. [PERSON] et al. used electron beam melting combined with laser metal deposition to produce a thruster nozzle from \(\gamma\)-TiAl for a cold-gas propulsion system, which can be used for navigation of small satellites to nanosatellites [62; 63]. The authors pointed out the necessity to use a material that could withstand temperatures of more than 700 \({}^{\circ}\)C, which are necessary to increase the propellant efficiency in cold-gas propulsion systems [64]. Furthermore, either investigations of the interaction between \(\gamma\)-TiAl and the often used propellant ammonia are necessary [65], or noble gases should be used as propellants instead [66]. [PERSON] et al. investigated laser powder bed fusion of a nickel alloy for the production of very small nozzles with diameters below 1 mm, as they are used in electrothermal or chemical thrusters of small satellites [67]. They found typical printing-derived surface features of the as-printed nozzles to cause shock wave reflections and other thrust losses in comparison with conventionally produced nozzles of similar surface roughness, and suggest postprocessing of the nozzle exit cone to avoid such shock-inducing protrusions. Powder bed laser processing was also used to prepare specimens from platinum rhodium alloy as well as tantalum and tungsten, finding that platinum rhodium additively manufactured parts could be used to build a combustion chamber and nozzle [68]. The authors suggest building the inner surface of a thrust chamber from noble metal alloys such as platinum rhodium or platinum iridium, while the outer wall could be prepared from tantalum and tungsten. [PERSON] et al. used selective laser melting to produce grids and keeper electrodes in ion thrusters from molybdenum and found the surface roughness to be problematic, but manageable by postprocessing, and the mechanical properties to reach about 80% of those gained with conventional production techniques [69]. Similarly, [PERSON] et al. described SLM used for the production of ion engine grids as well as for printing ion extraction systems and ion optics [70]. Miniaturized Hall thrusters were built by [PERSON] et al. using SmCo permanent magnets to create the necessary magnetic field even under high temperatures and a 3D printed anode from stainless steel 316, produced by selective laser melting of a more complex design than possible with conventional techniques [71]. As Figure 6 shows, the anode design significantly influences the propellant distribution, so that more sophisticated 3D printing enables tailoring the propellant distribution more homogeneously than that of equipment prepared by traditional manufacturing. Figure 6: Anode designs with corresponding propellant distributions. From [71], originally published under a CC-BY license. Another aspect, the reduction of manufacturing costs, was in the focus of a recent study of [PERSON] and [PERSON] who combined selective laser melting with off-the-shelf components to build a 20 cm-diameter microwave discharge ion thruster [72]. They found the ion production cost sufficient, while the mass utilization efficiency needed more future research to increase the thruster performance. In these and many other studies, investigating possibilities to use metal additive manufacturing for the preparation of thruster parts [73; 74; 75; 76; 77] report that the main problems are related to the reduced mechanical properties in comparison with wrought material and the necessity to postprocess the printed parts to reduce undesired surface roughness or printing artifacts. However, as these examples show, there are already well-working processes and several materials that can be used for this purpose, so that further implementations especially in CubeSats and other microsatellites and smaller satellites can be expected in the near future.