Abstract: Solid-propellant rocket engine with a combustion chamber, with a thrust nozzle, with one or more grains in the combustion chamber, at least one of which comprises two parts, namely, an inner burner part and an outer burner part, wherein the latter is connected to a support structure, has a cross section changing over its length, is accommodated in the hollow space of identical shape of the inner burner part, and is axially displaceable. A thermal barrier is arranged at the nozzle-side end of the two-part grain. The outer burner part is fitted gap-free into the inner burner part. The contour formed by the contact surfaces of the two grain parts encloses a cross section expanding from the front to the rear. The outer burner part can be moved from a front, inactive position, which is positive-locking with the inner burner part, into a rear, active position with a defined flow gap between the contact surfaces of the grains.