AbstractDucted rockets operate with the combustion of fuel‐rich gas generated in a primary combustor and the ram‐air induced from the atmosphere to a secondary combustor. Thus, the combustion performance depends on the mixing and reaction processes of the fuel‐rich gas and the air flow in the secondary combustor. The energetic solid fuels examined in this study consisted of ammonium perchlorate based composite propellants containing boron particles. The combustion efficiency of boron particles in the secondary combustor was determined as a function of air‐mixing process and air/fuel ratio. Two types of air‐inlet were used to evaluate the combustion of boron particles. Combustion tests conducted by direct connect flow indicate that the combustion efficiency increased significantly when a multi‐port consisting of two forward‐ports and two rear‐ports was used.