Abstract: 1. The method of casting a solid propellant in the casing of a rocket engine of spherical form having a continuous wall with a single opening therein which comprises, forming leaves of a material which melts at a temperature below the ignition temperature of the propellant and with curved edges concentric to the curvature of the spherical casing, inserting the leaves into the spherical casing through the opening therein, assembling the leaves to form a core having a greater width than the width of the single opening and with curved peripheral edges, casting the propellant around the core, curing the cast propellant to form a solid mass, and then supplying a heating medium in heat exchange relation with the core at the interior of the casting to melt the leaves and provide a central opening with flutes projecting radially therefrom.