## 3 New Thermal Design Strategy for S-STEP
All these characteristics make the graphite sheet attractive to use for thermal control of small satellites with a limited mass budget. Table 3 lists the specifications of the graphite sheet used for the S-STEP satellite. In the S-STEP thermal design, the graphite sheet was used to form the heat transfer paths from the IAU, PCDU, and battery to the dedicated radiators, respectively (Figures 6 and 7a). Both ends of the graphite sheet were clamped on the unit and radiator, respectively, to implement a secure thermal contact. For PCDU, where a larger amount of heat rejection is required than others, two radiator assemblies with graphite sheets were applied to the unit. The thermal control solution using a graphite sheet could be more advantageous than the conventional heat pipe in terms of light weight and unnecessity to maintain the orientation to the horizontal plane of the pipe to avoid the gravitational effect during an on-ground test. A total of 15 graphite sheet layers, determined on the basis of a previous study [25], were applied with respect to each component. \begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Specification** \\ \hline Manufacturer & KANEKA Co. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Specifications of lightweight flexible graphite sheet [24]. Figure 6: Thermal design concept for high-power components. Figure 7: Examples of thermal paths for heat dissipation of high-power components: **(a)** IAU and **(b)** TRM. In the thermal design, the battery module is the only component requiring heater control for temperature management to maintain the electrical power storage capacity. This fact indicates that the power budget-saving can be expected based on the proposed thermal design strategy. A Kapton polyimide film heater was attached adjacent to the battery radiator. Through the preliminary thermal analysis, the on/off set-points of the heater are set as 15/16 \({}^{\circ}\)C, and the heater power was determined as 30 W to satisfy the requirement of heater duty to be lower than 80%. The effective heat dissipation of TRM is a crucial factor in achieving the long-term SAR imaging duration of 60 s in both left- and right-looking modes of SAR antenna in orbit. In the previous study [22], the concept of the multifunctional TRM assembly structure was introduced as the configuration shown in Figure 6. A single TRM assembly was integrated with two optical solar reflector (OSR) radiators at both ends to effectively dissipate the heat produced during SAR imaging operation (Figure 7b). In this study, the U-shaped configuration of each radiator further enhanced the heat dissipation capability by increasing the radiating area. In addition, the proposed design was intended to dissipate heat to the bent parts of the radiator, even if the incidence angle of heat flux varied with the attitude maneuvering of the satellite. The radiator design also provided mechanical interfaces to integrate the top and bottom honeycomb panels of the satellite. To minimize the thermal gradient over the TRM assembly, a graphite sheet was integrated along its sidereal surface. The TRM assembly was radiatively and conductively decoupled from the satellite using MLI and thermal washers to secure heat dissipation capability.