An instrument is proposed for non-intrusive measurements of velocity in the plume of a large rocket engine of the type used in the first or second stage of a launch vehicle. The method is laser-based and has the potential for standoff distances in the tens of meters, so optical components can be a safe distance from the hot gases. The diagnostic does not require flow seeding, works over the full temperature range, and covers the full range of velocities of a typical rocket engine. The method, hydroxyl tagging velocimetry (HTV), has already been successfully demonstrated on a small rocket engine. The proposed effort will adapt this technique to large engines by minimizing the effects of beam attenuation and beam steering due to turbulence and developing a robust beam delivery and detection system. Because OH molecules survive at high temperatures for appreciable lifetimes, it is anticipated that the HTV technique will work in even the highest temperature rocket plumes. The proposed diagnostic will provide measurements not obtainable by current methods and will enable experimental data that can be used for validating computer models of rocket engine performance.