Using an afterburner in turbojet engines is an effective method for momentary thrust augmentation; however, it increases fuel consumption, exhaust-gas temperature, and the need for precise nozzle control. Therefore, simultaneous analysis of afterburner thermodynamic behavior and the role of the variable nozzle is important for maintaining stable engine performance. In this study, the effect of afterburner activation on turbojet engine performance is investigated under steady-state conditions at the design point, and afterburner fuel calculation, geometric design, and nozzle-area control are considered in a unified framework. First, by calculating the remaining oxygen at the combustor outlet, the maximum combustible afterburner fuel mass flow rate was determined as 0.208 kg/s. Then, afterburner geometric dimensions were extracted using AEDsys, and flow pressure loss was calculated using a blockage ratio of 0.314. Subsequently, the afterburner and variable nozzle models were added to the base engine model, and performance simulation was performed in MATLAB/Simulink. Results showed that, while the operating point of upstream turbomachinery remained unchanged, engine thrust increased from 3697 N to 4724N, indicating a 27.77% increase. The total fuel mass flow rate in the afterburner-on condition also reached about 2.6 times the initial value. Compressor map analysis showed that effective nozzle-area control is necessary to prevent the compressor operating point from moving toward surge. These results can provide a basis for preliminary design, performance analysis, and integrated controller development in afterburner-equipped engines.