A supporting cupola is provided for the propellant of a base-bleed gas generator for a projectile, shell or the like. The supporting cupola is arranged in the discharge aperture of the gas generator and provides a reduced gas outlet area from the base-bleed combustion chamber during launch of the projectile. The supporting cupola is made of a material which is easily eroded by the hot combustion gases, such as magnesium. The reduced outlet area increases the pressure in the combustion chamber during the acceleration stage of the projectile so that an improved ignition of the propellant and reduced risk of extinguishment is obtained. Due to erosion of the cupola material the outlet opening area increases with time providing a regressive burning rate of the propellant which means an increased base-bleed effect with respect to the firing range of the projectiles.