## 3 Passive Thermal Control Methods
### Thermal Straps Conductance Characterization  
The test unit consists of a copper thermal strap of the dimensions as described in Figure 3, a thin polyimide heater pad, and two thermocouples for temperature measurement. The entire unit was kept inside the thermal vacuum chamber (TVAC) which contains a temperature-controlled high emissivity shroud and baseplate. The main idea of this test is to find out the thermal conductance of a custom made thermal strap. Prior to the actual test, a preliminary start-up test was performed at a vacuum condition, at \(10^{-5}\) mbar, to analyze how conduction heat transfer plays a major role in transferring the heat from one end to another end. One end of the thermal strap was heated sufficiently using a thin polyimide heaterpad (from minco, HK6903) and a thermal interface material (TIM) is used between the heaterpad and the copper block to minimize the resistance as the entire unit is kept at complete vacuum condition where there is no medium. Two T-Type thermocouples were attached to both hot and cold ends of the thermal strap to measure the actual temperature. The custom designed flexible copper braid that is cold pressed at each of its ends into copper blocks is covered with MLI to block the heat from getting radiated out.  
Incremental heat loads were applied using an external power supply to the hot end, and the cold end is connected to an another copper block (cold sink) to generate a thermal gradient across the strap. Temperatures were measured from both the ends of the strap. It is assumed that the radiation loss is negligible due to MLI and the entire test sample is thermally isolated from the TVAC's baseplate (a bare printed circuit board and a PEEK sheet) using thermal insulators and thus conduction heat loss is also considered to be negligible.  
Figure 3: Thermal Strap—test sample.  
Figure 2: Thermal strap conductance.  
In the test setup, hot and cold end blocks of the copper strap were exposed to the test chamber due to the fact that the radiation from the copper blocks is assumed to be negligible as the surface emissivity of bare copper is 0.02 [27]. However, other measurement uncertainties, such as TVAC system thermocouple reading uncertainties of \(<\)0.5 \({}^{\circ}\)C, and external power supply error of 0.005 percent, are taken care of and ensured to be within the minimum allowable limits. The stray heat leaks from the sources such as wire harnesses (no harnesses used), and PC104 connectors (a standard interboard connector with 104 metal pins for board-to-board communication and it is made up of black high temperature, glass filled nylon) are neglected in this analysis. When the heater was turned ON with 0.5 W initially, the temperature started to increase and then stabilized after approximately 50 min. Temperature readings after stabilization were recorded from both ends and the same sequence was followed for the rest of the incremental heat loads. For varying heat loads from 0.5 W to 3 W, hot and cold end temperature from the TVAC system were recorded and used to compute the thermal conductance using Equation (7) and plotted as shown in Figure 4. With increasing heat load, the temperature difference between hot and cold ends is increased, but the strap conductance of the strap remains close to a constant value. Temperature gradient (\(\Delta T\)) for every incremental heat load (\(Q_{strap}\)) increases proportionally and this is the reason why the strap conductance is nearly constant. From the experiment, it is observed that the strap conductance varied between 5.17 W/K to 5.010 W/K for the given heat load, and thus an average value of 5 W/K is used for all the thermal simulations carried out in this study. This custom made thermal strap is expected to operate only under 3 W of heat load and hence, higher heat load (\(>\)5 W) test was considered to be unnecessary.