## 3 New Thermal Design Strategy for S-STEP
& \#2 & \(-40\) & \(85\) & - & - \\ \cline{3-6}  & & \#3 & \(-40\) & \(85\) & - & - \\ \cline{3-6}  & & \#4 & \(-40\) & \(85\) & - & - \\ \cline{3-6}  & ISL module & \(-25\) & \(65\) & - & - \\ \hline \multirow{9}{*}{Bus} & \multirow{3}{*}{Gyro} & \#1 & \(-40\) & \(65\) & \(-55\) & \(85\) \\ \cline{3-6}  & & \#2 & \(-40\) & \(65\) & \(-55\) & \(85\) \\ \cline{3-6}  & & \#3 & \(-40\) & \(65\) & \(-55\) & \(85\) \\ \cline{3-6}  & & \#1 & \(-20\) & \(40\) & \(-30\) & \(70\) \\ \cline{3-6}  & STA & \#2 & \(-20\) & \(40\) & \(-30\) & \(70\) \\ \cline{3-6}  & STCU & \(-20\) & \(50\) & \(-30\) & \(70\) \\ \cline{3-6}  & GPS Receiver & \(-20\) & \(60\) & \(-40\) & \(85\) \\ \cline{3-6}  & GPS ant. & \#1 & \(-25\) & \(55\) & \(-30\) & \(60\) \\ \cline{3-6}  & & \#2 & \(-25\) & \(55\) & \(-30\) & \(60\) \\ \cline{3-6}  & MAG & \(-25\) & \(70\) & - & - \\ \cline{3-6}  & & \#1 & \(-25\) & \(70\) & - & - \\ \cline{3-6}  & CSS (Coarse & \#2 & \(-25\) & \(70\) & - & - \\ \cline{3-6}  & Sun Sensor) & \#3 & \(-25\) & \(70\) & - & - \\ \cline{3-6}  & & \#4 & \(-25\) & \(70\) & - & - \\ \cline{3-6}  & & \#1 & \(-20\) & \(60\) & \(-30\) & \(70\) \\ \cline{3-6}  & RW & \#2 & \(-20\) & \(60\) & \(-30\) & \(70\) \\ \cline{3-6}  &Figure 4 shows the new thermal design concept for the proposed S-STEP system. The front SAR antenna surface (\(-z\) side) and side walls (\(x\) and \(y\) sides) are exposed to relatively lower environmental fluxes. Therefore, these surfaces can be used for dissipating satellite internal heat to outer space. Moreover, the incoming solar heat flux on the solar panel (\(+z\) side) can be used as a heat source to compensate for heat loss. The optimization of solar heat flux and heat dissipation of the satellite is achieved by applying various thermal coatings, MLIs, and radiators to control the heat exchange between structural elements and components, which results in minimizing the heater power budget, contributing to further satellite mass reduction due to decreases in solar panel and battery sizes. This thermal design concept is the most innovative and unique aspect of the proposed thermal design strategy for S-STEP that has never been attempted in the previous small SAR satellite programs. In the thermal design, an RF-transparent sunshield covers the external side of the SAR antenna for heat rejection. The primary satellite structures of aluminum-skin honeycomb panels are black anodized for minimizing the thermal gradient over the entire satellite structure by facilitating the internal heat exchange. The internal waste heats of payload and bus electronic units and solar panels are dissipated via mounting surfaces (\(-z\) side) to the SAR antenna. Thus, most honeycomb panel areas, including the mounting areas for electronic units, were cut-out for their effective heat dissipation (Figure 2c). These cut-outs were advantageous to reduce the satellite mass as well. The electronic unit mounting surfaces (\(+z\) side) were coated with Teflon-VDA tape with high emissivity. The rest of their surfaces were covered with MLI to avoid the absorption of heat radiated from the solar panel. For further heat dissipation, second surface mirror (SSM) foil radiators were integrated on the sidewalls (Figure 5). Figure 4: The proposed thermal design concept for S-STEP satellite. Figure 5: Allocation of SSM foil radiators on S-STEP satellite. For effective heat dissipation of PCDU, IAU, and battery producing relatively larger amount of waste heat than the others, this study focused on the unique characteristics of a lightweight flexible graphite sheet. The graphite sheet is a popular thermal interface material and has been used in various fields, such as portable devices, electronics, and space engineering [24]. It has many advantages, such as high thermal conductivity in the in-plane direction of the sheet (four times higher than copper), light weight, flexibility to bending, high heat-resistance, and availability to use in a vacuum.