In the course of establishing a thermal mathematical model of the pressurized volumes of the HERMES Spaceplane, it was found that the flow field in these volume strongly influences the overall thermal behaviour of the ACS (Atmosphere Control Section).The analysis of the flow field was carried out to get more detailed information about the heat transfer between the walls and the air inside the Spaceplane. Furthermore, the effects of gravity and mixing of return air from other volumes were studied.The analysis was carried out with the PHOENICS Software Package, specially developed for fluiddynamic simulations.The results gained by use of a two dimensional flow field indicate that gravity effects are neglectible due to the relatively high air velocities of the forced convection prevailing during all mission phases. Free convection does not contribute to the air movement.With a three dimensional flow field model the mixing effects were studied. It will be shown that the return air from the capsule does not seriously affect the flow field in the cabin, if air outlets and diffusore are placed correctly.A slightly varied 3-d model was used to determine the heat transfer coefficient between the walls and the air. Exact values will be determined when the final configuration of the cabin interior will be fixed.