Performance of the Dragonfly Lander’s Coaxial Rotor in Vortex Ring State

Dragonfly is a NASA New Frontiers mission with the goal of flying an autonomous relocatable rotorcraft lander to explore the surface of Saturn’s moon Titan in the mid-2030s. The Dragonfly lander is an RPM-controlled multirotor with four coaxial rotor pairs, each with two counter-rotating two-bladed fixed-pitch rotors. To support the lander’s development, the Dragonfly Team conducted a wind tunnel test campaign in September 2022 in the Transonic Dynamics Tunnel (TDT) at NASA’s Langley Research Center. Due to Dragonfly's concept of operations, especially its transition to powered flight after atmospheric entry, Dragonfly must transition through and operate near a potentially hazardous flight regime called Vortex Ring State (VRS). For this reason, achieving safe flight on Titan requires an investigation of Dragonfly's VRS regime. To that end, this paper uses TDT measurements in a Titan-surrogate environment (R-134a) and computational fluid dynamics to study the performance of a flight-like coaxial rotor system in VRS. The analysis suggests that Dragonfly's coaxial rotor system is potentially more robust to the onset of VRS than an isolated single rotor with the same design, i.e., VRS initiates at a higher descent rate, and that some of the characteristics of the subsequent VRS are different. Consequently, these results have important implications for the design and operation of Dragonfly, along with other eVTOL aircraft destined for both terrestrial and extraterrestrial applications.