# Metal Additive Manufacturing for Satellites and Rockets
## 4 Thrusters  
Thrusters can be categorized based on diverse working principles, such as Hall thrusters showing a high thrust efficiency and high ion beam density due to a moderate  
Figure 4: An additively manufactured one-piece solar panel. From [42], originally published under a CC-BY license. magnetic field applied to the acceleration zone [47], pulsed plasma thrusters [48], electrospray thrusters working with ionic liquid propellants [49; 50], etc. While Hall thrusters are most often used for satellites [51], applications requiring lower energies, such as the aforementioned CubeSats or other microsatellites, can be driven and navigated by microcathode arc or other plasma thrusters, field emission thrusters, electrospray thrusters, or different sorts of electric thrusters [52; 53; 54; 55]. One of the important differences between conventional production methods and additive manufacturing is the possibility to use more lightweight designs, but on the other hand to ensure the reliability of the 3D printed object in spite of possible differences in the mechanical or other physical properties, as compared to subtractive manufacturing methods. [PERSON] et al. and [PERSON] et al., e.g., discussed the redesign of a NASA thruster applied for spacecraft attitude control, as it is enabled by switching from conventional manufacturing to powder-bed fusion of the metal alloy Inconel(r)718 [56; 57]. They applied different specific designs for additive manufacturing and showed that manufacturing time and costs could be reduced by the new freedom of design, given by the additive manufacturing process. The same material was investigated by [PERSON] et al. who also tested stainless steel 316 L and CoCr for high-temperature applications in a liquid rocket engine injector, especially an injector head for an expander cycle and a gas generator setup, finding a large potential mass reduction of approximately 25% due to an optimized design and significantly reduced manufacturing costs for the selective laser melting process [58]. Different designs were also discussed by [PERSON] et al. who compared a design for a gridded ion thruster with full design freedom with a second one taking into account limitations of additive manufacturing and found that only the latter could be manufactured [59]. Figure 5 depicts the design for the maximized thruster performance (Figure 5a) and the more realistic design, taking into account material and geometry limitations, e.g., disregarding multimaterial structures. While the latter is less innovative, it can be manufactured according to recent technological methods. However, new developments may enable additive manufacturing of the first design, too, which would lead to higher perceived performance because of the rounder chamber shape. Besides the aforementioned materials, several metals and metal alloys were investigated as possible 3D printing materials for thrusters. Inconel(r)625 was chosen by [PERSON] et al. who developed a thruster for the use with hydrazine based on binder jetting 3D printing to reduce the surface roughness as compared to laser-based additive manufacturing techniques [60]. In this way, production time and complexity could be reduced; vibration tests showed an equivalent performance of the 3D printed parts in compari  
Figure 5: (**a**) Free design for maximized thruster performance; (**b**) design taking into account realistic additive manufacturing limitations. (**1**) Aluminum chamber, (**2**) connecting point importing the propellant, (**3**) channels guiding the gas, (**4**) gas exit, (**5/10**) hollow cathode for plasma generation, (**6**) electromagnet copper coil, (**7**) electromagnet iron core, (**8/9**) exit grids. From [59], originally published under a CC-BY-NC-ND license. son with flight-proven models, and hot-firing tests showed stable combustion, while the pressure roughness was slightly larger than in conventionally produced thrusters.