This paper investigates the aerodynamics of combusted plume gases in RCS thruster nozzles with extensions in a typical crew module firing at three identified altitudes (at Z_a = 30km, 14km and 7km) using CFD. Two thruster configurations of different nozzle extension lengths (L_e = D_e and 3D_e) are examined for flow separation and effective thrust, and inferences are drawn based on flow dynamics. Whereas nozzles with L_e = D_e are able to generate over 95% of the expected thrust at 30 and 14km, for L_e = 3D_e, it drops to about 92% at 30km and 90% at 14km. At 7km, the thruster with L_e = De shows a thrust reduction of around 12%, and it drops by a remarkable 45% for Le = 3De. Such decrease is attributed to boundary layer separation within nozzle extensions due to overexpansion, which depends on the ratio of thruster combustion chamber pressure to ambient pressure, as well as scarfing of nozzle extensions. This paper documents the different flow features in RCS thruster nozzle extensions during their firing at different altitudes in a typical crew module configuration, and compares thrust estimates from using the RNG k-e and k-w SST turbulence models.