<p>The propulsion characteristics of a start-up large-swing-angle supersonic split line (SSSL) nozzle are numerically investigated. Focus is on the interpretations of its segmented thrust vectoring mechanisms and expressions using convenient theoretical methods. Within a nozzle pressure ratio (NPR) range of from 2.008 to 8.950, the present large-swing-angle SSSL nozzle typically exhibits three exhaust flow patterns and two operating modes. Correspondingly, the ensuing segmented vectoring performance is analyzed, and the underlying mechanisms are demonstrated. It is found that, despite the flow pattern variations in its two operating modes, a monotonic and segmented approximately linear dependency of the nozzle lateral thrust performance on NPR is generated. In calculating the nozzle mass flow rate, accurate results can be provided by applying the one-dimensional supersonic nozzle flow theory only if the effective throat area is well estimated. In formulating the nozzle lateral thrust, modified supersonic nozzle flow theories also demonstrate their potential. But in formulating the nozzle axial thrust, a convergent nozzle flow theory needs to be applied due to the present nozzle flow characteristics. These formulations provide preliminary and convenient thrust expressions for practical use.</p>