AbstractThe combustion characteristics of variable‐flow ducted rockets were evaluated in order to obtain optimum design parameters of the gas generators. The energetic solid fuels used for the gas generators were glycidyl azide polymer (GAP) with burning rate catalysts. The burning rate of GAP based energetic fuels was much higher than that of composite and double‐base based fuel‐rich propellants. The pressure exponent of the burning rate was high enough to obtain a wide range of variable‐flow rat. Two types of combustion test were conducted: a semi‐free jet and a direct connect flow. The pressure‐time response in the primary combustor obtained by the use of a throttable nozzle was evaluated.