The two-stage combustion hybrid rocket engine with a hybrid gas generator method offers numerous advantages over a solid-propellant gas generator and classical hybrid rocket engines. These advantages include a higher specific impulse in operation, lower lifecycle costs, and lower sensitivity to combustion pressure. This study was mainly designed to discover whether the fuel-rich effluent coming out from the hybrid gas generator was generated to meet the temperature requirement of less than 1300 K. By using parameters such as oxidizer-to-fuel ratios, oxidizer mass flow rates, grain dimensions, and fuel types, experimental tests were attempted the temperature change in the effluent. This study also examined the combustion sensitivity of each parameter to different equivalence ratios and the combustion behavior of each parameter to different temperatures. In the results, a combustion temperature as low as 1290 K was successfully achieved in the fuel-rich combustion of hydroxyl-terminated polybutadiene with a...