A numerical approach for the aeroelastical stability of an over-expanded rocket engine is proposed in this paper. The main idea is to offer a better understanding of the repercussions likely to appear from the aeroelastic coupling in terms of side loads that may be responsible for damage effects on the engine. After a brief description of the stability model issued from previous works (Pekkari's team) and details upon a numerical fluid–structure coupling code, comparative calculations are conducted. The stability model is then called into question and a finer analysis is proposed to explain its major tendency to over-predict the aeroelastic frequency shift in comparison with numerical coupling results.