Abstract: A rocket propellant or rocket motor is provided with an insulating layer ween the fuel proper and the inner wall of the housing forming the combustion chamber. The insulating layer is made of a foamed synthetic material. The foaming insulating material is introduced into the housing under pressure after the solid fuel body has been centered in the housing with a spacing between the inner housing wall and the fuel body the front face of which is covered with a synthetic material film for the subsequent removal of the layer of foamed insulating material from the front face of the fuel body. In an alternative method the inner surface of the combustion chamber wall may be coated with a layer of foamed insulating material prior to the introduction of the fuel into the combustion chamber, said layer of foamed insulating material forming a tubular member.