A rocket engine includes a generally cylindrical annular combustion chamber, an injector attached to an injector end of the combustion chamber, a generally cylindrical annular step collar fitting within the combustion chamber at the injector end thereof, and an attachment of the combustion chamber, the injector, and the step collar. The maximum outer diameter of the step collar is from about 0.020 inches to about 0.024 inches smaller than the inner diameter of the combustion chamber, and has a length of from about 21 to about 31 percent of the length of the combustion chamber. The step collar is made of a material having a melting point greater than that of the combustion chamber. The attachment preferably includes a clip structure joining the combustion chamber and the step collar. The clip structure includes a C-shaped annular clip having a first leg extending parallel to the chamber length and with an inner diameter of about that of the outer diameter of the combustion chamber and affixed thereto, a second leg extending parallel to the chamber length and with an outer diameter of no greater than that of the outer diameter of the step collar and affixed thereto, and a web extending between the first leg and the second leg.