# HERMES CubeSat Payload Thermal Balance Test and Comparison with Finite Volume Thermal Model
## 6 Discussion  
The thermal test performed on the payload DM produced sufficient data to move into the steady-state correlation with the FVTM. Apart from the steady-state temperatures, the test highlighted other important results, briefly summarised hereafter. First, the time to reach steady-state in a laboratory size TVAC has been established: this is important for future AIT/AIV activities adequate planning. Usually, the nominal approach is to create the numerical setup of the test and have an estimation of the transient time. In our case, having access to the TVAC and the DM in the design phase, it was faster to jump straight into testing, gathering all the data and then move into the numerical simulation for the FVTM. As a second result, the temperature increase in the boards caused by the heaters power dissipation has been analyzed: those are crucial data to refine the thermal control subsystem design for the HERMES satellite, as they give an actual measure of the temperature increase that the boards might experience whenever turned on. Furthermore, it allows to properly choose the set point temperatures in future tests in order to avoid the overshooting of the upper temperature limits of the boards whenever turned on. Third, and probably the most important, the auto-correlation matrices show that the TC at the payload interface with the TVAC (tc5) is weakly correlated with the thermocouples placed on the internal components of the payload. Figure 17 clearly shows this aspect: the red rectangle highlights the cells correlating tc5 with the TCs on the boards inside the PL (i.e., tc1-tc9). It means that temperature changes at the interface between the payload and the TVAC slightly affect the components inside the payload. This is good news in terms of thermal control complexity relaxation, as it means that once the flight segment is fully integrated, the SM temperature has little impact on the payload components leaving the PL and SM thermal behaviour substantially decoupled. Please note that Pearson's coefficient is influenced by the time delay between each temperature data but overall the qualitative results are not much influenced. Fourth, it is possible to observe from the auto-correlation matrices in Figure 17 how the correlation between tc5 (interface with TVAC) and the internal components increases with temperature. An explanation for such phenomena might be due to the fact that thermal contact resistance usually decreases as the temperature increases, thus the components are able to exchange bigger heat fluxes hence the correlation increases. Unfortunately, there is scarce literature that analyzes this phenomenon at ambient temperature but for higher temperatures and cryogenic ones such behaviour is confirmed [46; 47]. The behaviour might be confirmed from Table 5, especially on the PSU where the temperature jump \(\Delta T\) caused by the power on of the board is smaller in the hot cycle with respect to the cold ones, meaning that more heat has been dissipated towards other components due to the smaller contact resistance hence the lower temperature jump. For the PSU the temperature jump at \(-25\,^{\circ}\)C is around 47 degrees whilst at 30\(\,^{\circ}\)C is around 37 degrees, thus the phenomenon has a relevant impact on the temperature field. In any case, to fully confirm such phenomena, a dedicated test must be performed, an example is given in [44]. On the other hand, a slight, opposite behaviour is observed on all TCs placed on the structural components (i.e., tc8-tc15 in Figure 17). This behaviour still remains an open point. Data gathered with the presented set up do not suffice to further investigate the case,thus a dedicated test is needed to understand this phenomenon. In any case, since no major effect on the payload components has been observed due to this behavior, it is assumed not to represent a critical issue. As a final point, the FVTM is compared with the hot steady-state values.