# HERMES CubeSat Payload Thermal Balance Test and Comparison with Finite Volume Thermal Model
## 1 Introduction  
The thermal model of a spacecraft is normally exploited to verify that, once in orbit, the onboard equipment operates inside prescribed temperature ranges along each mission phase. On CubeSats, the most critical components are usually batteries and scientific instruments, as they operate in narrower temperature ranges compared to the rest of the hardware. The scientific payload (PL) on board the High Energy Rapid Modular Ensemble of Satellites (HERMES) 3U CubeSat [1, 2] is based on radiation-sensitive detectors that are required to operate within a relatively narrow temperature range, between \(-35\,^{\circ}\)C and \(-5\,^{\circ}\)C. It is the strictest thermal requirement among all the spacecraft components, and it constitutes the main driver for the thermal design. A detailed numerical thermal model is needed to properly assess the compliance of such requirement prior to the mission launch during detailed design activities. Nevertheless, the correct identification of the model parameters can be achieved only by means of a Thermal Balance Test (TBT) [3]. The testis performed on a PL Demonstration Model (DM), that is identical to the flight hardware except it has no functional electronics. To mimic the correct heat dissipation, the latter is simulated by electrical heaters strategically placed on locations upon the different boards. This paper presents the procedures adopted to set up and perform the test and discusses the obtained experimental results. Being a CubeSat mission, the full ECSS standard regarding thermal control and thermal testing ECSS-E-ST-31C [3] is partially applicable as reported in CubeSat tailored ECSS [4] but the HERMES team aims at reaching full ECSS standards to provide the best quality of the product. The currently available literature on CubeSat thermal balance test is very scarce [5; 6] and does not explain in a detailed manner the test setup and procedure. The goal of the paper is to provide the reader with all the technical information (e.g., requirements and technical equipment) needed to replicate the test with all the standards tailored from the ECSS. Furthermore, it is the first time the Finite Volume Method is applied for a CubeSat thermal analysis with the level of high geometrical detail reported in the following chapters. During the design activities of the HERMES mission, a Finite Volume Thermal Model (FVTM) was developed and actively maintained. The FVTM model is computed in OpenFOAM, a widely used open-source Computational Fluid Dynamics (CFD) solver which support also several heat transfer models. The model has a more accurate geometry with respect to the Lumped Parameter Approach method which is commonly used in space thermal analyses, e.g., when simulating with the ESATAN software, the European Space Agency Lumped Parameter Approach software used for thermal analyses; for more information on ESATAN the reader can refer to [7]. Indeed, the FVTM offers better insight into the heat fluxes within the model as shown in [7; 8; 9]. In this work, the post-processing of the experimental data is presented, and the test data is compared with the FVTM results, proving the application of the novel method to be reliable. Furthermore, the comparison creates a solid base upon which the method can be applied to other cases in space thermal analyses such as the generation of infrared images as reported in [10; 11; 12; 13]. Other results are presented in the article, such as the time needed to reach steady-state, and the temperature jumps caused by the simulated electronics heat dissipation are identified. Furthermore, it has been proven that the internal components of the PL are slightly influenced by the temperature changes at the mechanical interface with the Service Module. In the end, given the thermal behaviour of the PL, planned future updates of the thermal interfaces are presented.