Experimental and theoretical data are presented for two interchangeable swirl brakes designed in connection with the Space Shuttle Main Engine (SSME) Alternate Turbopump Development (ATD) High-Pressure Fuel Turbopump (HPFTP) program. The experimental data includes rotordynamic data for a extensive variation of test variables. Comparison of the swirl brake performance revealed that a nonaerodynamic swirl brake design proved as efficient and at times better than an aerodynamic design. For this reason a theoretical investigation using computational fluid dynamics (CFD) was recently carried out. This modeling focused on predicting the seal inlet swirl ratio which is the primary swirl brake performance parameter. The nonaerodynamic swirl brake showed superior performance for a variety of test variable conditions. Strong separation vortices within the swirl vanes are the main reason for this finding.