Hydroxyl Tagging Velocimetry for Rocket Plumes

A non-intrusive method for measuring velocities in a rocket exhaust is proposed in a joint effort by MetroLaser and Vanderbilt University. Hydroxyl Tagging Velocimetry (HTV) uses an ultraviolet laser to tag a region of the flow with OH molecules, and interrogates them after a short delay with a second laser to obtain velocity from time-of-flight data. The method relies on the dissociation of H2O molecules naturally present in the flow, and thus requires no seeding. Being an all-optical technique, it is not adversely affected by high temperatures or high dynamic pressures. Analyses and experiments conducted during the Phase I effort demonstrated feasibility by showing that OH tag lifetimes in a simulated rocket exhaust environment were sufficiently long to enable accurate determination of tag displacements corresponding to typical rocket exhaust velocities. The method was demonstrated by measuring velocities in the exhaust gases of a laboratory scale burner at temperatures and chemical compositions representative of a H2/O2 rocket exhaust. Design tradeoff studies predicted that at measurement ranges of 20 to 50 feet the accuracies would be from two to five percent. This proposal outlines a plan to develop a prototype HTV instrument and demonstrate it in a rocket engine exhaust.