UC Davis

Small, inexpensive satellites called CubeSats are commonly used for conducting academic and commercial space research. Typically, there is not a robust thermal control system to dissipate heat from the CubeSat avionics or payloads, which limits onboard computing power and mission capabilities. This research proposes the use of a sublimator to reject waste heat from a CubeSat to enable more powerful computers to be flown along with payloads that require significant thermal cooling. During the sublimation process, the phase-change of ice to water vapor, heat is transferred away from the spacecraft and into the vacuum of space. In a water sublimator, liquid water from a supply line freezes within a porous component and sublimates to water vapor upon exposure to the vacuum of space. The CubeSat’s waste heat is channeled into the sublimator and transferred out of the spacecraft via the sublimation process through the porous component. The heat dissipation rate is determined by the sublimation rate, which depends on the mass flow rate of the water vapor released into space. The mass flow rate is limited by the physical properties of the porous component, and passively controlled by the amount of thermal energy available to cause the water ice to sublimate into vapor.

Even though sublimators have been used as spacecraft cooling technology for decades, they have not been used for CubeSat thermal control and the heat transfer and thermodynamic mechanisms in the sublimator are still not fully understood. The focus of this research is threefold. First, the design process for sizing a sublimator based on mission requirements and integrating it with a CubeSat thermal control system is explored. Second, a model is developed to examine the driving factors in sublimator performance. Most notably, this model incorporates both the conductive heat transfer through the sublimator, expressed as a thermal resistance network, and the rarefied water vapor diffusion through the porous component, expressed using the “Weber equation” to analytically model vapor behavior in multiple flow regimes. The resulting temperatures obtained from the thermal resistance network inform the parameters for the analytical Weber equation.

Lastly, an experiment is conducted specifically to validate the model for rarefied water vapor diffusion through a porous medium. The most notable result is the use of experimental data to validate the relationship between heat rejection, mass flow rate and pressure drop in the porous medium. These validated results are then used to inform sublimator design choices such as pore size and porosity. Throughout this thesis, shortcomings from literature are addressed, as are knowledge gaps from this current research to help identify future research directions. This thesis utilizes the sublimator model and experiment validation, combined with overall CubeSat and thermal control systems knowledge, to relate high-level mission requirements to detailed sublimator design choices.