During launching, a payload is submitted to large vibrations, which may damage it. To get rid of the problem, a solution would be to put an appropriate vibration isolator at the payload/launcher interface. Thus, a soft Isolating Payload Attach Fitting (IPAF) using Magneto-Rheological (MR) dampers is envisaged. In a pre-design phase for the launcher application, a preliminary study of the behaviour of a commercial MR damper (RD-1005-3) and its use in a 1-dof vibration isolator is carried out. In this paper, we report the MR damper behaviour analysis based on fluid and solid mechanics equations. In particular, we investigate chambers fluid compressibility and inertia effects. Then the damper model is used to evaluate the performance of a MR isolator in terms of equivalent transmissibility in passive mode and using skyhook control. The theoretical results will be soon compared to those from an experimental bench in construction.