Motivation and Background Applications of pico and nano-sats include disposable short-term surveillance and communication missions in LEO, Lunar and planetary orbits, telemetry relays for orbit insertions and shadowed flybys, and upper atmospheric mapping. These smallsats can provide a cost effective solution where massive systems are not needed. There are however no mature technologies currently available for tunable propulsion and precise attitude control at this scale without sacrificing a substantial mass fraction of the vehicle.

Film-Evaporation MEMS Thruster Array for PicoSat Propulsion A.G. Cofer, W. O'Neill, S. Heister, and A. Alexeenko - School of Aeronautics and Astronautics, Purdue University; and E. Cardiff - NASA Goddard Space Flight Center

Power vs Thrust for smallsat propulsion.

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Power vs Thrust for Small Sat Propulsion

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FEMTA Max power–min thrust for 1U cubesat

Schematic of FEMTA operation. Meniscus position changes with the local heating of the capillary wall. A single array element is shown.

FEMTA Concept relies on exploitation of microscale effects of surface tension and its balance with stresses created by the vapor pressure, which is highly dependent on the liquid film temperature. A critical size of capillary for which the surface tension is being balanced by normal stresses due to the pressure drop across the boundary can be estimated from the Young-Laplace equation as 𝑑 =   where d is the gap size of the annular or slit capillary, 𝜏  is

the surface tension, pvap is vapor pressure which depends exponentially on the temperature of the liquid film. Specifically for water the critical gap size varies from d=60 μm to 10 μm for film temperatures from 20 to 50 °C.

• Theoretical Performance • Isp = 63.9 s • Power consumption = 300 mW • Max thrust = 65 microNewtons • Mass flow = 180 micrograms /s

12 FEMTA units positioned for 3-axis control on a Cubesat;

Force measurement on the microNewton thrust stand at Purdue’s  High  Vacuum  lab  – 75 milliWatts applied power – 18 microNewtons averaged thrust during power on

Power Applied

Fabrication FEMTA nozzles were fabricated at Birck Nanotechnology Center using combinations of wet etching and DRIE of a silicon substrate. Heaters are evaporated platinum with silicon oxide insulator

Modeling Modeling of the thruster is divided into three areas: heat and mass transfer simulation via COMSOL, Direct Simulation Monte Carlo analysis of nozzle flow, and a surface energy study to determine the location and shape of the meniscus. . The COMSOL analysis encompasses the heat transfer in the thruster, the generation of heat from the vanadium resistive elements and the flow of the liquid propellant to the meniscus. Direct Simulation Monte Carlo analysis is used to model the expansion out of the nozzle. An iterative bisection solving scheme is used to couple the multiple areas of study. This modeling is used as a baseline for performance and improvement of design.

Above: Heat transfer analysis of FEMTA with power off (left) and power on (right) Right: Direct Simulation Monte Carlo analysis of FEMTA nozzle with mass flow rates of 20 mg/hr (top) and 100 mg/hr (bottom)

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2-D Prototype single FEMTA nozzle, schematic (top), SEM top view (lower left) and cross section (lower right)