An innovative technical solution that overcomes the limitation induced by flow separation phenomena and allows increasing the vacuum-specific impulse of a main stage rocket engine is presented. After reviewing the open technical literature on flow separation and advance nozzles design, the newly proposed concept is extensively presented. Three-dimensional computational fluid dynamics analyses and experimental results (cold gas subscale models) demonstrate the effectiveness of a simple device, adaptable to any bell-shaped nozzle and which provides a solution to the flow separation issue, while increasing vacuum-specific impulse performances for booster and main stage engines. Moreover, this new concept gives rise to a wider engine-throttling range, even at low altitude, without incurring flow separation and associated side loads. Experimental data show a potential for favorable behavior even during transient phases, allowing significant reduction on transient side-loads activity. Finally, three-dimensional computational fluid dynamics computations have been extended to the hot firing case (chemically reactive hydrogen/ oxygen propellants), both in steady-state and transient conditions. The results show the suitability of the proposed concept for application to real rocket engines.