MEMS Reaction Control and Maneuvering for Picosat Beyond LEO

The MEMS Reaction Control and Maneuvering for Picosat Beyond LEO project will further develop a multi-functional small satellite technology for low-power attitude control, or orientation, of picosatellites beyond low Earth orbit (LEO). The Film-Evaporation MEMS Tunable Array (FEMTA) concept initially developed in 2013, is a thermal valving system which utilizes capillary forces in a microchannel to offset internal pressures in a bulk fluid. The local vapor pressure is increased by resistive film heating until it exceeds meniscus strength in a nozzle which induces vacuum boiling and provides a stagnation pressure equal to vapor pressure at that point which is used for propulsion. Interplanetary CubeSats can utilize FEMTA for high slew rate attitude corrections in addition to desaturating reaction wheels. The FEMTA in cooling mode can be used for thermal control during high-power communication events, which are likely to accompany the attitude correction. Current small satellite propulsion options are limited to orbit correction whereas picosatellites are lacking attitude control thrusters. The available attitude control systems are either quickly saturated reaction wheels or movable high drag surfaces with long response times

Distributed Attitude Control and Maneuvering for Deep Space SmallSats

The aim of the Distributed Attitude Control and Maneuvering for Deep Space SmallSats project is to advance a multi-purpose, deep space mission-enabling technology for low-power attitude and thermal control of small satellites to a flight demonstration technology readiness level (TRL). The film-evaporation microelectromechanical systems tunable array (FEMTA) small satellite technology combines innovative microelectromechanical systems (MEMS) microfabrication and microscale effects in fluid surface tension to produce a thermally actuated capillary valve. Using water as the propellant, the FEMTA thruster can generate finely controllable thrust at a thrust to power ratio of about 200 microNewton per Watt (W)

Scaling law for direct current field emission-driven microscale gas breakdown

The effects of field emission on direct current breakdown in microscale gaps filled with an ambient neutral gas are studied numerically and analytically. Fundamental numerical experiments using the particle-in-cell/Monte Carlo collisions method are used to systematically quantify microscale ionization and space-charge enhancement of field emission. The numerical experiments are then used to validate a scaling law for the modified Paschen curve that bridges field emission-driven breakdown with the macroscale Paschen law. Analytical expressions are derived for the increase in cathode electric field, total steady state current density, and the ion-enhancement coefficient including a new breakdown criterion. It also includes the effect of all key parameters such as pressure, operating gas, and field-enhancement factor providing a better predictive capability than existing microscale breakdown models. The field-enhancement factor is shown to be the most sensitive parameter with its increase leading to a significant drop in the threshold breakdown electric field and also to a gradual merging with the Paschen law. The proposed scaling law is also shown to agree well with two independent sets of experimental data for microscale breakdown in air. The ability to accurately describe not just the breakdown voltage but the entire pre-breakdown process for given operating conditions makes the proposed model a suitable candidate for the design and analysis of electrostatic microscale devices

Simulations and Measurements of Gas-Droplet Flows in Supersonic Jets Expanding into Vacuum

Simulations and measurements of gas-droplet multiphase flows in application to supersonic expansions into vacuum have been considered and compared with each other. The experiments involved exposing a control surface to a supersonic plume from two different nozzles and measuring the size distribution of droplets at various locations. The simulations are based on the direct simulation Monte Carlo modeling of vapor-phase flow with a one-way coupling of droplet momentum and energy. The droplet trajectories are computed for the experimental conditions for droplets originating at the throat and lip of two different nozzles. The maximum droplet radius reaching the control surface and the variation of droplet size with angle predicted by the trajectory computations agree well with the measurements using Optical Microscopy

Microthruster Fabrication and Characterization: In Search of the Optimal Nozzle Geometry for Microscale Rocket Engines

A major consideration in microsatellite design is the engineering of micropropulsion systems that can deliver the required thrust efficiently with tight restrictions on space, weight, and power. Cold gas thrusters are one solution to the demand for smaller propulsion systems to accommodate the advancements in technology that have allowed for a reduction in the size and thus the cost of satellites. While much research has been done in understanding the flow regimes within these microthrusters, there is a need to understand how different nozzle designs affect microthruster performance. This requires that experimental data be collected on varying nozzles shapes (orifices, channels, and an annulus). Tests will be done in high vacuum conditions with varying thruster plenum pressures and with Nitrogen as the propellant. Temperature will be measured in both the thruster plenum and the vacuum chamber, while thrust will be measured using a micronewton torsional balance. The nozzles will be compared after calculating the specific impulse, thrust coefficient, discharge coefficient, and Knudsen number for each at the various plenum pressures. The plug array is expected to be the most efficient with a theoretical specific impulse that is higher than the other nozzles to be tested. The plug array design was found, during stochastic numerical simulations, to have enhanced performance through increased pressure thrust, a desirable attribute in low Reynolds number flows. The results from this research will be used to further develop the most efficient systems for attitude control on microsatellites

Direct simulation Monte Carlo modeling of e-beam metal deposition

Three-dimensional direct simulation Monte Carlo (DSMC) method is applied here to model the electron-beam physical vapor deposition of copperthin films. Various molecular models for copper-copper interactions have been considered and a suitable molecular model has been determined based on comparisons of dimensional mass fluxes obtained from simulations and previous experiments. The variable hard sphere model that is determined for atomic copper vapor can be used in DSMC simulations for design and analysis of vacuum deposition systems, allowing for accurate prediction of growth rates, uniformity, and microstructure

Molecular Models for DSMC Simulations of Metal Vapor Deposition

The direct simulation Monte Carlo (DSMC) method is applied here to model the electron‐beam (e‐beam) physical vapor deposition of copper thin films. A suitable molecular model for copper‐copper interactions have been determined based on comparisons with experiments for a 2D slit source. The model for atomic copper vapor is then used in axi‐symmetric DSMC simulations for analysis of a typical e‐beam metal deposition system with a cup crucible. The dimensional and non‐dimensional mass fluxes obtained are compared for two different deposition configurations with non‐uniformity as high as 40% predicted from the simulations

Lyo Calculator – the Calculator of Primary Freeze-Drying

Freeze-drying (lyophilization) is an important process of a pharmaceutical solution state transformation for storage. Due to complexities of the process and costs related to experiments, numerical simulations of freeze-drying become more useful and cost-efficient for research and development of the process and the hardware. Many numerical models have been created to model separate steps of the drying process. However, these models are not available for any user. This work presents an open-source model of primary drying in a vial. Pseudo steady- state heat and mass transfer model was used to compute vapor pressure, drying time, product temperature, and percent of fluid dried as functions of time. To verify the numerical model, results were compared to experimental data of a mannitol (5%) solution and a numerical model created by M.J. Pikal. Results show accuracy within 20% at low chamber pressures and shelf temperatures. Simulations and analysis showed that the tool can be successfully used as a basic approximation of drying results for a single vial and constant chamber pressure and shelf temperature

PIC/MCC simulations of field emission driven microdischarges

The aerodynamic flow control using plasma has been shown to demonstrate great ability with potential applications in both subsonic and supersonic flow regimes [1]. In the past, both numerical and experimental work has been done on dielectric barrier discharges (DBD) as a plasma actuator of cm and mm scale [2]. The plasma, which is ionized gas with free electrons and ions, in cm–mm scale plasma actuators, is generated when sufficient electric filed passes through the gas to ionize it. Typically thousands of volts of potential is required to create high electric fields which then accelerate electrons to create ions by impact ionization. However, at the micron scale in addition to impact ionization, field emission at the electrode is also a source of electrons which in turn ionize the molecule. In addition to this, the breakdown potential is also lower at the micron scale for the same pressure as is observed from the modified Paschen curve. This study deals with the numerical modeling of the DC voltage driven DBD micro plasma actuator in Argon gas at 1 atm using particle in cell method with Monte–Carlo collisions [3] with field emission and the effect of the body force generated by this microdischarge on the macroscale flow using computational fluid dynamics. The DBD is modeled with kinetic approach because of the micron scale characteristic length of the device which leads to a Knudsen number, Kn ~ 0.4 which is rarefied. The body force per unit volume generated by the ions in the electric field is, fb = eE (ni – ne) [4], where e is the magnitude of electron charge, E is the electric field, ni is the ion number density, and ne is the electron number density. Preliminary simulations of the micronscale DBD with copper electrodes of each 100 µm length and 1 µm thick, separated by 1 µm thick and 200 µm long dielectric with relative permittivity of 10, indicated an average fb = 2 x 106 N/m3. The body force is introduced into the FLUENT simulation of 2D rectangular channel (0.5 mm x 5 mm) flow at multiple locations at the wall. The results for 250 V micron scale DBD show an increase in wall velocity of 2 m/s and an increase of 1.8% in exit mass flow rate, because of field emission alone. The results considering AC voltage and effect of variation of the gap between electrodes will also be presented. REFERENCES [1] Suzen, Y.B., Huang, P.G., Jacob, J.D. Numerical simulation of plasma based flow control applications, 35th Fluid Dynamics Conference and Exhibit, June 2005, Toronto, Ontario. [2] Corke, T.C., Post, M.L., Orlov, D.M. SDBD plasma enhanced aerodynamics: concepts, optimization and applications. Progress in Aerospace Sciences. 2007, 43, 193–217 [3] Verboncoeur, J.P., Langdon, A.B., Gladd, N.T. An object-oriented electromagnetic PIC code. Comp. Phys. Comm. 1995, 87(May 11), 199–211. [4] Macheret, S.O., Shneider, M.N., and Miles, R.B. Magnetohydrodynamic and electrohydrodynamic control of hypersonic flows of weakly ionized plasmas. AIAA Journal. 2004, 42(7)