Abstract: Some experimental investigations have been done on a direct-connect test bed.The experimental investigation is aimed at exploring factors that affect unchoked ducted rocket combustor efficiency.Aluminum/magnesium-loaded fuel-rich HTPB (Hydroxyl Terminated Polybutadiene) propellant was used.The effect of factors, such as air/fuel ratio,dome height,angle of gas injector on performance of the combustor and the engine was examined.In a definite range of air-fuel ratio,with the increase of air-fuel ratio,combustion efficiency increases.After air-fuel ratio reaches some definite point,further increase of air-fuel ratio,a decrease trend of combustion efficiency is observed.Decreasing the dome height increases combustion efficiency.The impinging jet injection device improves combustion efficiency.If the fuel streams impinge with the airstreams directly at the exit of air-inlet,it makes negative effect on combustion efficiency.