## 3 Embedded Battery Tile Design and Construction
### Embedded Battery Tile Prototype Construction  
The production cycle of the CFRP skins consists of a manual layup and autoclave curing. Although traditional and consolidated techniques have been employed, the reduced dimensions of the skins made the process technologically complex. An aluminum mold was obtained by milling a solid aluminum panel (Figure 8a). The manual layup of the prepregs was then performed, and the assembly of mold and prepregs was vacuum-bagged. An autoclave cure cycle with a polymerization phase of 90 min at 120 \({}^{\circ}\)C was set up, with thermal ramps of 3 \({}^{\circ}\)C per minute. The milling of the aluminum panels left circular traces on the mold, that have been also imparted to the CFRP skins (as shown within the red circle in Figure 8b). Dimensional checks have been performed on the skins. Their dimensions are within the tolerance of 0.1 mm, although some defects were noticed. The angle of the fibers has slight deviations of up to 1\({}^{\circ}\) from the nominal angle of \(\pm\)45\({}^{\circ}\). Figure 8c shows the alignment defect with red lines following the fabric. This defect can be caused by the distortion imposed on the prepreg with the manual layup operation. An undesired cylindrical curvature was noticed in the central section of the skins (Figure 8b) and has been verified with a laser displacement acquisition system. The skins' thickness was measured with  
Figure 8: CFRP skins manufacturing: (**a**) aluminum mold for inner and outer skins; (**b**) tool traces on the outer skins; (**c**) fiber alignment defect in the outer skins. Mold approximate dimensions: 220 \(\times\) 100 \(\times\) 15 mm. Skin sides length scale: 100 mm, thickness: 0.23 mm. an ultrasonic thickness gauge, and the mean thickness for all the skins was found to be 0.23 mm. The prototype assembly sequence is shown in Figure 9. First, the PCBs and the CFRP skins have been glued to form the inner and outer assemblies (Figure 6b). The inner assembly prototype is shown in Figure 9a. The gluing has been made with the Elan-tech(r) ADH 971.961 epoxy adhesive. The polymerizations were performed at room temperature. Then, the batteries terminals were soldered to the inner PCB connections pads (Figure 9b). The batteries' protection circuits, wrapped in the orange kapton tape in the figure, were not removed for electrical security reasons. The TermoPasty(r) thermal conductive paste H was spread on the surfaces of the batteries in contact with the Inner Skin (Figure 9c). Then, a commercial silicone adhesive was applied to the sides of the batteries (Figure 9d) as a structural joint. The same silicone adhesive was applied to the inner skin stiffeners to join the inner and outer assemblies. Finally, the subassemblies were mounted on the commercial CubeSat frame to ensure the mechanical compatibility of the whole spacecraft (Figure 9e). Figure 9: Embedded battery tile prototype assembly sequence: (**a**) bonding of the inner PCB to the inner skin with epoxy resin; (**b**) the soldering of the LiPo batteries terminals to the connection pads of the inner PCB; (**c**) application of the thermal conductive paste on the batteries; (**d**) gluing of the inner assembly to the outer assembly; and (**e**) polymerization of the siliconic adhesive with the subassemblies mounted on the commercial CubeSat frame. The final results of the assembly procedure are shown in Figure 10. The prototype is shown in Figure 10a. The measured thickness of the tile from the plane of the rails (Figure 1a) is 5.37 mm. This thickness does not include the solar cells. For the CAD model, the thickness is 5.24 mm, comparable to the real case. Accounting for the glued solar cells with their electrical connectors (shown in yellow in Figures 2c and 6b), the CAD model has a protrusion of 5.87 mm from the plane of the rails. The aforementioned CubeSat mechanical requirement 3.2.3 ([2] (p. 9)) on the maximum protrusion of the tile from the plane of the rails is thus met.