Prediction of aerodynamically-triggered condensation: Application to the Dragonfly rotorcraft in Titan's atmosphere

Expansion of the airflow over a wing or rotor causes adiabatic cooling, and in terrestrial aviation a localized condensation cloud can form in moist air. The cold methane-nitrogen atmosphere of Titan is, in relative terms, rather close to the condensation point and the possibility of analogous condensation on the NASA Dragonfly rotorcraft mission is examined using simple first-principles models and Computational Fluid Dynamics. It is found that nominal near-ground flight will not trigger condensation, but fog formation may be expected on rotor upper surfaces and in tip vortices in some extremes of the flight envelope, notably during high-altitude meteorological profiling flights. Evaluation of droplet re-evaporation and tip vortex decay timescales suggests, however, that obscuration of navigation sensors is not likely to occur from the vehicle body or rotor wake, even in cross-wind hover.