Abstract: The diverging part of the nozzle comprises an outer diverging part and at least one inner diverging part of outlet section that is smaller than that of the outer diverging part and designed to be separated therefrom and released at a predetermined instant in the operation of the combustion chamber corresponding to a predetermined altitude of the rocket engine. The inner diverging part comprises at least a first insert constituted by a cellular filling material associated with a stiffening structure and with a refractory material defining the inner wall of the first insert which is subjected to the hot gas produced in the combustion chamber associated with the nozzle, such that the mean specific gravity of the first insert lies in the range about 0.2 to about 0.4. Pressure-limiting means are interposed between the first insert and the outer diverging part and the first insert is fixed to the outer diverging part by rigid fixing means associated with selective cutting-off means.