A molding for immobilizing a cloth, cover or the similar in the groove of a supporting structure comprises a core from which at least two elastically deformable wings are extended. The rocket engine includes an injector, a fuel supply, an oxidizer supply, a pump for feeding fuel from the fuel supply to the injector, a pump for feeding oxidizer from the oxidizer supply to the injector, a combustor, and a nozzle, the combustor and nozzle forming a combustor and nozzle assembly. The apparatus is controlled in response to the defined markings. The second grate bars have at least one pin configured to be introduced into a pocket in the first grate bars and increase the larger width. The second component is derived from the difference between the first and second values by clipping this difference to eliminate, as appropriate, the part of the difference which is greater, in absolute value, than a predetermined value.