Development of a Hyperelastic Membrane Computational Model for a Satellite Thruster

This work concerns the development and experimental verification of a computational model of a hyperelastic membrane, which will be used in the propulsion system of the PW-Sat3 satellite designed by the Students' Space Association at Warsaw University of Technology. PW-Sat3 is a 3U CubeSat whose main mission is to conduct orbital maneuvers by changing the orbit altitude with the use of a warm gas thruster. The system contains a heating chamber and a storage tank filled with two-phase butane and carbon dioxide. The substances are separated by a Viton membrane, with butane working as the propellant and CO2 serving as the pressurizing agent. The membrane is a key element of the system, allowing to minimize the volume occupied by butane and - as a result - maintain high pressure of the propellant and to ensure liquid phase of all the butane. This, in turn, provides greater accuracy in dosing the propellant. The motivation for the conducted research was to determine the required amount of the pressurizing agent in the tank for a given end-of-life propellant pressure, based on the properties of the membrane. This information is needed to ensure high efficiency of the propulsion system as well as to optimize the mass of the propellant on board. For this reason, an accurate computational model of the entire system - including the membrane - is neces-sary due to the difficulty in recreating microgravity conditions. The characteristics of hyperelastic materials are strongly non-linear; in addition, the material can be approximated as incom-pressible. Therefore, Mooney-Rivlin model was used to describe the parameters of the membrane. To calculate the coefficients used in the model, various modes of deformation were studied experimentally with the use of a universal testing machine avail-able at the University. Several interchangeable holders were used for different tests. The obtained results were imported into the Engineering Data module of ANSYS Workbench, whereby the necessary coefficients were calculated, and the stress-strain curve was generated. Afterwards, the behavior of the membrane was analyzed in ANSYS Mechanical under various load cases, including the end-of-life pressure, maximum operating pressure disregarding the tank wall (no restriction for the strain), and various other pressure values in this range. The computational model was then experimentally verified. The membrane was mounted in one half of the engineering model tank and pressed down using a flange-like part in place of the second half of the tank. The deformation was measured at various pressures and correlated with the FEA results. As the end result of the work, the correlation was evaluated and the optimal amount of CO2 in the tank was determined based on the demonstrated membrane deformation.