In future space missions by NASA there will be a need for “Space Transfer Vehicles” to perform varying orbital transfers and descents. This requires engines capable of producing different levels of thrust. To accomplish this, the turbopumps employed in these engines should efficiently provide a wide range of flow outputs. However, current fuel and oxidizer turbopumps with vaned diffusers do not perform efficiently at off-design (low) flow rates mainly due to flow separation in the vaned diffuser.This paper evaluates the effectiveness of boundary layer control by fluid injection (blowing) for suppressing or eliminating the flow separation in a vaned diffuser. A 3-D flow model including vaneless and vaned diffusers of a liquid hydrogen (LH2) turbopump is studied using the CFD code FIDAP. The paper presents the results of the model at design and offdesign flow conditions.The model results showed that flow separation occurs at the top or suction surface of the vaneless diffuser and at the bottom or pressure surface of the vaned diffuser at off-design flow rates. When fluid injection was applied through the bottom surface of the vaned diffuser, the separated flow region was reduced almost entirely, resulting in an increase in pressure recovery of up to 21% with varying fluid injection rates. Results also showed that there is an optimum injection rate which is most effective in reducing or eliminating the region of flow separation.