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Since a heat exchanger used in a gas generator of an open-cycle liquid rocket engine was operated in a high-temperature environment, the coupled analysis for heat transfer characteristics and structural integrity should be performed simultaneously. For these reasons, a numerical analysis of the heat exchanger in a liquid rocket engine was performed to elucidate the effects of heat transfer and structural deformation simultaneously using conjugate heat transfer (CHT) analysis and open-source tools. For the baseline heat exchanger, which had an inner helically coiled tube with nine turns (Nc=9), the heat transfer characteristics were investigated and findings showed that the heat transfer performance was reduced from the sixth turn. Further analysis was performed to examine the effect of the number of turns in terms of heat flux and the corresponding pressure drop and the weight of the structure. The results indicated that the heat exchanger with Nc=3 had a significantly reduced outlet temperature due to an excessively shortened flow residence time. The heat exchanger with Nc=6 showed an outlet temperature similar to that of the baseline; it also presented advantages in terms of the pressure drop and structure weight. In addition, the thermal deformation and stress caused by temperature changes were numerically investigated to consider the structural integrity of the heat exchanger with Nc=3,6,9. Further numerical analyses were performed at various flow rates. As the flow rate of helium increased, the amount of heat received from the high-temperature exhaust gas from the gas generator increased but the outlet temperature of helium decreased gradually. Finally, the temperature difference between the outer and inner walls increased due to the high heat flux in the region around the inlet, resulting in an increase in thermal stress. Based on these results, the optimal shape and flow rate of the system were identified. Furthermore, the heat transfer performance was found to correlate with the flow characteristics of the coiled tube.