Microelectromechanical systems (MEMS) techniques offer great potential in satisfying the mission requirements for the next generation of miniaturized spacecraft being designed by NASA and Department of Defense agencies. More commonly referred to as 'nanosats', these spacecraft feature masses in the range of 10-100 kg and therefore have unique propulsion requirements. The propulsion systems must be capable of providing extremely low levels of thrust and impulse while also satisfying stringent demands on size, mass, power consumption and cost. We begin with an overview of micropropulsion requirements and some current MEMS-based strategies being developed to meet these needs. The remainder of the paper focuses on the progress being made at NASA Goddard Space Flight Center toward the development of a prototype monopropellant MEMS thruster which uses the catalyzed chemical decomposition of high-concentration hydrogen peroxide as a propulsion mechanism. The products of decomposition are delivered to a microscale converging/diverging supersonic nozzle, which produces the thrust vector; the targeted thrust level is approximately 500 µN with a specific impulse of 140-180 s. Macroscale hydrogen peroxide thrusters have been used for satellite propulsion for decades; however, the implementation of traditional thruster designs on the MEMS scale has uncovered new challenges in fabrication, materials compatibility, and combustion and hydrodynamic modeling. A summary of the achievements of the project to date is given, as is a discussion of remaining challenges and future prospects.
Previous experimental and computational studies have indicated that interfaces formed in steady, converging microchannel flows with similar liquids tend to be planar in nature under a variety of conditions relevant to micro-scale flows, including MEMS/microfluidic devices and even microcirculatory blood flows. Assuming a planar interface, we have developed an analytical framework to predict the fully developed interfacial location downstream of a convergence of identical microchannels. Results have been obtained for microchannels having rectangular, elliptical/circular and triangular cross-sections as a function of the inlet flow ratio. Two-dimensional results have also been obtained for fluids having unequal viscosities. Good agreement is found between this model and 3-D numerical simulations and experimental measurements provided that the flow inertia remains sufficiently small (Re≲10, typically). Where valid, application of this analytical, planar interface method represents a significant decrease in computational effort when compared to using CFD to determine interfacial positions.
Ruthenium oxide nanorods have been grown on Si wafer substrates under a variety of pre-existing surface conditions by reactive radio frequency sputtering in an electron cyclotron resonant plasma process. Nanorod formation by this method is fast relative to that observed in other processes reported in the literature, with nucleation being the rate determining step. Growth in the axial direction is limited by the availability of ruthenium precursors which competes with their consumption in the lateral growth of the nanorods. The availability of Ru precursors at the top of the nanorods can be controlled by surface diffusion and therefore substrate temperature. The ultimate length of the nanorods is determined by the mole fraction of oxygen used in the reactor ambient through the production of mobile Ru hyperoxide precursors. The results of this investigation show the way to develop a process for producing a high density field of nanorods with a specified length.
A thin liquid film on a horizontal solid surface undergoing radiative heat transfer with an external heat source and the surrounding environment is considered. Thermal gradients along the free surface give rise to a thermocapillary flow in the liquid that is opposed by a hydrostatic pressure gradient within the film. Transient and steady-state solutions are obtained for the interfacial shape and temperature and the velocity field. These results are compared with those from another model, in which a temperature distribution is imposed on the free surface of the film. At a critical value of the dynamic Bond number, a cusp in the form of a free-surface slope discontinuity appears in this fixed free-surface temperature model, but not in the radiation model. When the Bond number is less than this critical value, the time required to thin the film by a significant fraction of its original thickness is much larger with the radiation model. It is shown how the thermal boundary conditions used in the models directly cause these differences.
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