A Dual-Rotor Horizontal Axis Wind Turbine In-House Code (DR_HAWT). This paper describes the DR_HAWT (Dual-Rotor Horizontal Axis Wind Turbine) in-house code developed by the present authors, the National Renewable Energy Laboratory (NREL) Phase VI validation test case, and a dual-rotor configuration simulation. DR_HAWT uses a combination of the Blade Element Momentum theory (BEM) and the vortex filament method to accurately predict the aerodynamic performance of horizontal axis wind turbines for a single or dual-rotor configuration. Using two-dimensional (2D) airfoil coefficients obtained from XFOIL, the aerodynamic power prediction produced by DR_HAWT resulted in an average error of less than 20 % when compared to the NREL Phase VI experiment. Upon correcting the 2D wind tunnel aerodynamic coefficient data for 3D flow effects, good agreement was obtained with the exception of the deep-stall region. In addition, a dual-rotor test case consisting of a combination of a half-scaled and full-sized NREL Phase VI rotor, is presented. The dual-rotor configuration resulted in a 16 % average increase in power generation as compared to the full-sized rotor alone.