Due to the interaction between main oxidizer flow and the wall injected flow resulting from the regression process, a specific time characteristics identified in the frequency spectrum of streamwise velocity is generated in the hybrid rocket motor. In order to understand the response of the turbulent flow to two different types of external momentum forcing, LES analysis was conducted without considering the combustion. It turns out that both concentrated and distributed forcings do not lead to the disastrous resonance phenomenon. Energy contents are enhanced due to the added momentum but the peak frequency was not modified in the turbulent flow near the end of the rocket motor. Natural frequency of the flow system should be taken into account to further pursue the instability issue by using external forcing.