Impact of TPS Recession on the ORION CEV Aerodynamics

Abstract

The ORION Crew Exploration Vehicle (CEV) is being designed to take crew to and from both ISS and the Moon. The current baseline Thermal Protection System (TPS) for the CEV is Phenolic Impregnated Carbonaceous Ablator (PICA). Being an ablator, PICA will undergo recession during the reentry, gradually changing the shape of the Outer Mold Line (OML) and therefore changing the aerodynamics of the vehicle. Furthermore, due to the tailoring of the TPS thickness across the vehicle heat shield the actual shape of the OML will not be the ideal axisymmetric shape. This paper will begin by briefly describing the process used to analyze the CEV vehicle in terms of aerodynamics, aerothermodynamics, trajectory, TPS sizing, and the prediction of recession along the reference trajectories. The results of these analyses will be used to estimate the post-facto impact recession will have on the trim angle of attack and the trimmed lift to drag (L/D) ratio. II. Aerodynamics, Trajectory, Aerothermal, and TPS Analyses The aerodynamic database [4,5] for the ORION CEV is built upon a combination of CFD and experimental data. The aerodynamic database is used in the development of the CEV trajectory from entry interface to landing. The geometry of the CEV is assumed be axisymmetric. However it is known that the basic OML will be slightly asymmetric due to 1) the asymmetric tailored thickness of the TPS system, and 2) the asymmetric recession that occurs during reentry. Neither of these geometry effects is accounted for in the aerodynamic database, but instead is covered in a general fashion via uncertainty estimates. The CEV trajectories are flown through the CEV aerothermal database, described in references 2 and 3. The output of the aerothermal analysis is the time varying heating environments over the vehicle surface for any given trajectory. The time varying heating environments are supplied to the TPSSIZER tool [6] which performs the final TPS sizing analysis. The resultant TPS thickness distribution defines a Outer Mold Line (OML) which varies slightly from the purely axisymmetric shape assumed in the foregoing analysis. The time histories of the predicted recession provide an estimate of the overall shape change the CEV OML will undergo during reentry.