| Summary: | 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 <inline-formula><math display="inline"><semantics><mrow><mn>1</mn><mo>.</mo><mn>6</mn></mrow></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mi mathvariant="normal">k</mi></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mi mathvariant="normal">V</mi></semantics></math></inline-formula>. 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 <inline-formula><math display="inline"><semantics><mrow><mo>∼</mo><mn>475</mn></mrow></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mi mathvariant="normal">s</mi></semantics></math></inline-formula>, with no observed unsteady modes. However, during voltage-induced thruster startup, unsteady electrohydrodynamic modes were observed.
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