We propose to build a high-power, room temperature compact continuous wave terahertz local oscillator for driving heterodyne receivers in the 1-5 THz frequency window. The local oscillator is based on the recently discovered terahertz emission of a high-power infra-red vertical external-cavity optically-pumped surface-emitting laser (VECSEL) when operated under dual-wavelength emission. The dual IR wavelength separation can be easily controlled by placing a thin etalon in the VECSEL cavity and the THz signal (corresponding to the beat frequency) is generated via a nonlinear periodically-poled lithium niobate crystal placed in the cavity. The VECSEL semiconductor gain element is designed and optimized to generate on the order of kilowatt internal circulating infra-red fields in the VECSEL cavity thereby generating tunable THz power in the milliwatt ranage. It is anticipated that this source will have wide applications within NASA and the broader commercial community.