The direct-connect combustion tests and the numerical simulations of the rocket-ramjet combined cycle combustor, requiring large bases to install rocket chambers, were conducted to determine whether the combustor could be operated in ramjet mode. The rocket engines in the combustor were operated at fuel rich condition as fuel injectors for the freestream. For the rocket operation alone, the combustor was operated in scramjet mode. By the secondary fuel injection from the cowl, the supersonic freestream was decelerated to subsonic speed by the combustion-generated shock train in front of the secondary injection. The subsonic combustion of the secondary injected fuel resulted in higher combustion and thrust performances. The further upstream secondary injection was suitable for the ramjet operation. By the increase in fuel flow rate of the secondary injection, the thermal load on both the rocket chamber and throat was reduced without decrease in thrust. Gas sampling measurements at the exit of the combustor revealed that the merger of the secondary fuel with the rocket plumes resulted in the decrease of the combustion and thrust performances.