Aerodynamic analysis of a flap-based deployable re-entry system in different flight conditions

Atmospheric re-entry is a crucial part of any space mission with recovery and landing requirements and for deep space missions involving the exploration of other celestial bodies in the solar system. Among the most innovative re-entry technologies developed over the years, deployable re-entry systems represent a research topic of increasing interest because of their ability to reduce the ballistic coefficient of the capsule, without exceeding the volumetric constraints. At the same time, they may raise the probability of collisions in low Earth orbit, thus its design needs to minimize the compound risk by substantially reducing the time to re-enter or enter. The current work perfectly fits in this scenario, focusing on a re-entry satellite based on a 16U CubeSat equipped with an umbrella-like, deployable heat shield, designed to be deployed before the re-entry phase. An aerodynamic control system based on eight flaps allows to control the descent trajectory and improves the precision targeting of the landing place. The aerodynamic behavior of the device in different conditions along the re-entry trajectory is investigated. A challenge is represented for those flight conditions corresponding to high values of altitude where Computational Fluid Dynamics (CFD) has several limitations. As a result, a molecular-based approach is mandatory for the aerodynamic treatment of the initial re-entry phase in the upper atmosphere due to gas rarefaction and thermal non-equilibrium. In the current work, the Direct Simulation Monte Carlo method is employed to carry out simulations using dsmcFoam+, developed by the University of Glasgow in collaboration with other researchers. The code is used to estimate aerodynamic coefficients and flow field evolution at those altitudes where CFD does not give satisfying results anymore. An analysis of an altitude condition of 100 km with both methodologies brought to light a comparable flow field evolution with very close values of stagnation pressure. Concerning the drag coefficient, the CFD simulation overestimated it by about 11 % with respect to DSMC. The work showed the reliability of CFD up to certain values of altitude and the necessity to use DSMC in the upper part of the atmosphere to get more realistic results.