Evaluation of Rarefaction Effects on a Winged, Hypersonic Re-entry Vehicle

An important aspect to be taken into account in the design of a re-entry vehicle is the evaluation of the effects of rarefaction on the aerodynamic coefficients and the heat flux. The evaluation of the lift and of the drag is important if a high aerodynamic efficiency re-entry is required. At high altitudes (or in rarefied regimes) a decrease of the lift and an increase of the drag occur. Also the moments can be different from those in continuum regime, implying different stability behavior. The availability of reliable aerodynamic coefficients during the re-entry at high altitudes is important also for defining the trajectory and for sizing the Reaction Control System. It is well known that the Navier-Stokes equations fail in rarefied regimes and a molecular approach such as the Direct Simulation Monte Carlo method (DSMC) is necessary. In the present paper two DSMC codes, DS3V and DS2V, have been used to analyze aerodynamics of the Unmanned Space Vehicle tests bed, named FTB-X (1.1.2 configuration) and the thermal load at the nose stagnation point, respectively. The FTB-X project is being developed by the Italian Aerospace Research Center (CIRA). The goal of the paper is also to verify the applicability of engineering methods such as bridging formulae and panel methods in a preliminary design of FTB-X. These methods estimate, in a prompt way, the aerodynamic coefficients between continuum and free molecule flow regimes. Computations have been performed on the whole vehicle in symmetric and side-slip flights, on the nose in axial-symmetric flow. The results verified the reliability of the bridging formulae to evaluate aerodynamics of the whole vehicle and, at the same time, the inadequacy of the engineering methods in evaluating the thermal load at high altitude.