Theoretical, analytical, and experimental investigations of electrospray operation in vacuum facilities show that secondary species emission (SSE) plays a significant role in the behavior of electrospray thrusters during ground testing. A review of SSE mechanisms, along with an analysis of onset thresholds for electrospray thruster conditions, indicates that secondary species (e.g., electrons, anions, cations, etc.) must be carefully considered for accurate measurements and determination of performance and life. Presented models and experiments show that SSE-induced thruster-to-facility coupling can lead to considerable measurement uncertainty but can be effectively mitigated with an appropriate beam target design. The Electrospray SSE Control-volume Analysis for Resolving Ground Operation of Thrusters model is applied to experimental data to analyze SSE behavior. A heat and mass flux analysis of the Air Force Electrospray Thruster Series 2 (AFET-2) shows that SSE-induced Ohmic dissipation can cause performance limitations in ionic liquid ion source thrusters. The presented analytical models show that backstreaming current density contributing to less than 0.1% of measured emitter current density can cause substantial variation in propellant properties. Additionally, backstreaming current density contributing to less than 3% of emitted current can cause the 0.86 μg s−1 neutral loss rate estimated during AFET-2 testing. Arguments are presented to support the notion that glow discharges observed in electrospray thrusters during vacuum operation are a consequence of secondary species backstreaming to the emission site, rather than a process intrinsically caused by ion evaporation. Recommendations for general best practices to minimize the effects of SSE on electrospray thruster operation are provided.
Nomenclature α = function of Technology Readiness Level (TRL) β = complexity of each connection between pairs of components γ = 1/n Π i = expected profit for missions of type i A = Design Structural Matrix C AIT = assembly, integration, and test cost C D = development cost of each satlet variant C NR = non-recurring development costs C PMSE = aggregate system level PMSE estimate C i R = recurring costs (manufacturing costs, operations costs, and launch costs) C 1 = complexity due to number and flight readiness of components C 2 = complexity due to pair-wise component interactions Distribution Statement A: Approved for Public Release, Distribution Unlimited.2 C 3 = complexity due to topology of system architecture and complexity of integration E(A) = graph energy of the DSM m = number of interfaces m i = number of specific missions of type i resulting from market projection n = number of components Ps i = probability of success of architecture in providing capabilities for mission type i R i = nominal maximum revenue from mission type i AFS = Aurora Flight Sciences AIT = assembly, integration and test CER = cost-estimating relationship DSM = Design Structural Matrix IMCE = Integrated Model-Centric Engineering JPL = Jet Propulsion Laboratory PMSE = Project Management and Systems Engineering PODS = Payload Orbital Delivery System Satlet = cellularized satellite building blocks SSCM = Small Satellite Cost Model SysML = Systems Modeling Language TDRS = Tracking and Data Relay Satellites TRL = Technology Readiness LevelThis paper describes a model-based architectural design and analysis approach developed to support the initial design of cellularized spacecraft architectures, such as the DARPA Phoenix program. As one of its technical pillars, the Phoenix program is aiming to construct new "aggregate satellites" on-orbit by combining cellularized building blocks referred to as "satlets." A critical question that needs to be addressed is "should there be a single satlet type that provides all the required satellite functionality, or should there be multiple specialized types"? Our initial approach includes capture of satlet design and aggregated satellite design trade spaces using the Systems Modeling Language (SysML), specification of requirements as parametric constraints on the set of acceptable solutions, automated search of this trade space and generation of paretooptimal satlet architectures that satisfy mission requirements while maximizing a specified value metric. The initial results of our analysis suggest that a cellularized architecture should include sets of more specialized satlets: a central satlet type that includes components for computation and data processing, centralized attitude sensing and ground communication, a satlet type that provides actuation in the form of either reaction wheels or thrusters, a payload satlet type to provide any specialized functionality for a particular mission (e.g., an RF transceiver), and finally, "connector" satlet types that provide structural, mechanical and power inte...
Ionic liquid electrosprays can emit a polydisperse population of charged droplets, clusters, and molecular ions at high velocity. Secondary species emission (SSE) is a term that encompasses many concurrent impact and emission phenomena that occur when electrosprayed primary species strike a surface, resulting in a diverse population of secondary electrons, ions, clusters, and droplets. This Letter examines the spatial dependency of SSE behavior across an [EMI]Im electrospray beam using microscopy of the target surface and experimental quantification of SSE yields as a function of the plume angle. Microscopy of the beam target confirms our prediction of shock-induced desorption when operating at elevated beam voltages. SSE yield measurements show that, upon impact with a surface, incident primary species that consist of entirely positive charge will produce both positive and negative SSE. Furthermore, the results show that the SSE yields for an ionic liquid electrospray have strong spatial and energy dependencies. These findings have significant implications for understanding and predicting ionic liquid electrospray thruster lifetime and performance and focused ion beam applications.
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