Possible Water Vapor Condensation in Rocket Exhaust Plumes

Abstract

In an attempt to explain the observed radiance in the exhaust plume of an Apollo engine, the possibilities of any condensation by homogeneous nucleation under the conditions in the plume flowfield are examined. It was found that only H2O could condense, the other species present in the exhaust, H2, remained unsaturated. Under the assumption of steady-state homogeneous nucleation, the accumulation of condensate as a function of the streamline coordinate has been computed from the Oswatitsch integral with empirical information gathered in previous experiments. Condensation is thus estimated to occur at about 45 m downstream of the nozzle exit. However, it has been found that the steady-state nucleation assumption may not be valid here, and the previous estimate may be too optimistic. Condensation under nonsteady homogeneous nucleation conditions would result in delayed condensation onset or none at all. A potentially useful alternative way of estimating condensation in plumes is outlined. Finally, keeping in mind that condensation may be induced by binary or heterogeneous nucleation, both of which favor earlier condensation onset, we conclude that condensation is not impossible in this exhaust plume.