An afterbody for a turbojet engine is disclosed. The afterbody includes an afterburner casing, a thermal protection liner, and a diaphragm-forming assembly interposed between the liner and the casing. The diaphragm-forming assembly defines a flow area which is traversed by a ventilation stream. The diaphragm-forming assembly includes two annular plates overlapping one another, each being perforated with a plurality of apertures and being mounted on the liner and the casing respectively. The apertures jointly define the flow area of the assembly. Furthermore, the assembly is designed such that the expansion of the liner causes an angular displacement of the plate with respect to the plate, leading to an increase in the size of the flow area.