Dual-bell nozzle (DBN) is a rocket nozzle concept that could provide substantial payload gain worth tens of millions of dollars. The control of transition, the reduction of side-loads, and the analysis of its stability have been the subject of numerous studies. The present paper focuses on the impact of radial secondary injection on DBN behaviour during its ascent and descent in the atmosphere. In the present study, the use of secondary injection allowed the transition nozzle pressure ratio to be increased by nearly 24% compared to a nozzle without any injection slot (smooth nozzle), and the lateral forces were reduced to less than 1% of the nozzle thrust. Finally, the jump in thrust during retransition was not found to be dependent on the secondary injection pressure, but was still reduced by over 61% compared to the smooth nozzle case.