Abstract
Electrospray thruster life and mission performance are strongly influenced by grid impingement, the extent of which can be correlated with emission modes that occur at steady-state extraction voltages, and thruster command transients. Most notably, we experimentally observed skewed cone-jet emission during steady-state electrospray thruster operation, which leads to the definition of an additional grid impingement mechanism that we termed “tilted emission”. Long distance microscopy was used in conjunction with high speed videography to observe the emission site of an electrospray thruster operating with an ionic liquid propellant (EMI-Im). During steady-state thruster operation, no unsteady electrohydrodynamic emission modes were observed, though the conical meniscus exhibited steady off-axis tilt of up to 15°. Cone tilt angle was independent over a wide range of flow rates but proved strongly dependent on extraction voltage. For the geometry and propellant used, the optimal extraction voltage was near 1.6 kV. A second experiment characterized transient emission behavior by observing startup and shutdown of the thruster via flow or voltage. Three of the four possible startup and shutdown procedures transition to quiescence within ∼475 μs, with no observed unsteady modes. However, during voltage-induced thruster startup, unsteady electrohydrodynamic modes were observed.
Highlights
Introduction and BackgroundColloid electrospray thrusters are a class of electric propulsion device that produce thrust through electrostatic acceleration of charged liquid droplets
A map of the steady-state emission behavior was obtained by taking high-speed videos of the emission site while parametrically sweeping the emitter bias voltage and propellant flow rate
The flow rate range was chosen to span nominal operating setpoints, and the voltage range was chosen between the observed minimum extraction voltage and the maximum voltage output of the power supply
Summary
Colloid electrospray thrusters are a class of electric propulsion device that produce thrust through electrostatic acceleration of charged liquid droplets. Application of an electrostatic field to a liquid results in electrohydrodynamic (EHD) phenomena classified into different modes based on emission behavior. Electrospray emission modes affect the performance and lifetime of colloid thrusters, and largely depend on bias voltage, propellant flow rates, and fluid properties. Thrust produced by an individual electrospray emitter is on the order of tens of nanonewtons to a single micronewton [1]. The high precision thrust offered by colloid thrusters makes them attractive for missions that require ultra-fine pointing and stability control, such as the Laser Interferometer Space Antenna (LISA) and Habitable Exoplanets (HabEx) Observatory [2,3]. The Colloid MicroNewton Thruster (CMNT) was Aerospace 2020, 7, 141; doi:10.3390/aerospace7100141 www.mdpi.com/journal/aerospace
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