Development and Testing of the Dragonfly Geophysics and Meteorology (DraGMet) WIND Sensor

One of the science objectives for Dragonfly, NASA's 4th New Frontiers mission which will explore Titan with a rotorcraft lander, is to measure wind speed and direction on Titan's surface. This will help understand Titan's methane meteorological cycle, and the relationship of Titan's sand dunes to its climate. The Johns Hopkins University Applied Physics Lab (APL) has fabricated and tested a wind sensor patterned closely on the thermal anemometer flown on the Beagle 2 and Schiaparelli Mars missions.An array of 4 such WIND sensors form part of the Dragonfly Geophysics and Meteorology (DraGMet) package, each mounted at the end of one of the rotor arms such that one is always upstream of, and thus unperturbed by, the lander while it is stationary on the surface.This paper describes the design of the WIND sensor, a model for the expected response, the design of a test facility to measure the performance of the wind sensor in its relevant environment, as well as the test data.The challenge for this sensor, and the entire Dragonfly spacecraft, is the environment on the Titan surface, a Nitrogen-Methane atmosphere at a pressure of 1.5 atmospheres and a temperature of -180°C. The WIND sensor will only be used to measure wind speed and direction on the ground, not during powered flight.APL has built a Titan Pressure Environment Chamber (TPEC) that replicates both the pressure and temperature on Titan, albeit with pure nitrogen (the 5% methane on Titan has only a minimal effect on bulk transport properties such as heat capacity or conductivity). The WIND test enclosure for inside the TPEC creates a microenvironment free of turbulence and houses a motor-driven rotating platform that can accommodate 8 wind sensors for batch testing and can generate relative flow velocities from 0.05 to 5 m/s with a 360-degree directional control for each sensor.Seven generations of prototype sensors have been designed and built, exploring a number of deposition approaches, seed layers, and Rohacell coatings. Each sensor comprises custom platinum (Pt) film resistors that perform a combined heating and temperature sensing function. Three such resistors on a 1cm-long Kapton substrate are mounted on a lightweight Rohacell foam cylinder. The combination of readings from the three films allows retrieval of wind azimuth and speed. The current design, with a 70 nm Pt sputtered film showed the best performance with respect to overall uniformity and resistance variation. Tests have been performed to confirm the film is not significantly affected by erosion. An epoxy coating to mitigate contamination release from the foam has been added.Calibration measurements show that the response to the wind speed, expressed by the Nusselt number, follows the functional dependence derived in a sensor model. The direction dependence is expressed by an angular shear factor, that is the same for all sensors, but, for small Reynolds numbers, depends on the wind speed as well. The TPEC performance tests show that the sensors comfortably meet the measurement requirements outlined for the Dragonfly mission.