E. P. Muntz

The manipulation of capillary stream breakup using amplitude‐modulated disturbances: A pictorial and quantitative representation

Droplet formation from capillary streams of viscous low vapor pressure fluids in a vacuum has been studied. A new form of capillary stream breakup, which entails the use of an amplitude‐modulated sinusoidal disturbance, has been employed extensively in this work. The genesis of droplets formed due to an amplitude‐modulated disturbance on a stream is reviewed in some detail and illustrated pictorially. Because capillary stream breakup and subsequent droplet propagation took place in a vacuum there were no significant interactions with the surrounding atmosphere. Sensitive measurements of the relative speeds of each drop have been made by allowing them to travel 6 m in a vertical vacuum chamber, and measuring the time between drops using an optical method. Speed differences as low as 3.5×10−7 times the average stream speed have been measured. New information about capillary stream breakup is inferred by comparing the speed variations for droplet streams generated with both amplitude‐modulated disturbances a...

Experimental and Computational Studies of Temperature Gradient–Driven Molecular Transport in Gas Flows through Nano/Microscale Channels

Studies at the University of Southern California have shown that an unconventional solid-state device, the Knudsen compressor, can be operated as a microscale pump or compressor. The critical components of Knudsen compressors are gas transport membranes, which can be formed from porous materials or densely packed parallel arrays of channels. An applied temperature gradient across a transport membrane creates a thermal creep pumping action. Experimental and computational techniques that have been developed for the investigations will be discussed. Experimental studies of membranes formed from machined aerogels, activated by radiant heating, have been used to investigate thermal creep flows. In computational studies, several approaches have been employed: the direct simulation Monte Carlo (DSMC) method and discrete ordinate solutions of the ellipsoidal statistical (ES) and Bhatnagar-Gross-Krook (BGK) kinetic models. Beyond the study of Knudsen compressor performance, techniques discussed in this article could be used to characterize the properties of gas flows in nano/microscale channels.

Molecular velocity distribution functions in an argon normal shock wave at Mach number 7

The streamwise and normal moments of molecular velocity distribution functions throughout an argon normal shock wave have been measured in a Mach number 7.18 wind tunnel flow, and the experimental data have been compared with several analytical and numerical solutions. In the experiments, the molecular population distributions parallel and perpendicular to the direction of flow were derived using a Fabry–Perot etalon to resolve the Doppler shifts in electron‐beam‐stimulated fluorescence of the gas atoms. Predictions from Monte Carlo techniques give close agreement with the details of the experimental nonequilibrium distribution function contours. Bimodal Maxwellian and ellipsoidal functional forms show only general agreement with overall profile breadth.

The free molecule micro-resistojet - An interesting alternative to nozzle expansion

Abstract : The growing trend of using significant numbers of small spacecraft, thus enhancing the performance of communication and surveillance tasks previously done by a few much larger vehicles, has created a requirement for low power, highly efficient propulsion systems It is clear that this class of small satellites will require station keeping with low mass, low thrust, low impulse bit (I-bit) thrusters, orbital transfer and rendezvous capabilities, multiple orbital plane changes, and de-orbit provisions. Small satellites are not only mass limited but also seriously power limited. For the mass range below 50 kg, a figure of merit is 1 W/kg available for propulsion, although this could be increased during orbital transfer or plane change maneuvers. Small mass, power efficient thrusters (microthrusters) need to be developed. There is a growing realization that in many cases microthrusters will not be simply scaled down versions of present thrusters. For example, chemical thrusters have difficulty maintaining efficiency at small scales due to increases in frozen flow and viscous losses. For ion electric propulsion, the increase in surface to volume ratio as the size decreases makes the scaling of these thrusters to small size, while maintaining efficiency, extremely difficult. In this paper, an electric thruster that is scalable to spacecraft in the kilogram to hundreds of kilogram range is discussed. The Free Molecule Micro-Resistojet (FMMR) satisfies the various issues resulting from extreme size reduction and appears to be scalable to rather large size without penalty. The FMMR is intended primarily for use as an attitude control thruster. As shown by Muntz and Ketsdeve, there are only quite modest differences between the performance of a free molecule orifice expansion and limit isentropic expansion with respect to I(sub sp).

Characteristics, control, and uses of liquid streams in space

Assessing the possibilities for using liquid streams in space requires a quantitative knowledge of the response of a stream it is suddenly exposed to a high vacuum. The paper reviews what is known about the vacuum behaviour of low and finite vapour pressure liquid streams. A method for restricting evaporation from controlled droplet streams is suggested.

Knudsen Compressor Performance at Low Pressures

The Knudsen Compressor is a solid‐state micro/meso‐scale gas roughing pump based on the rarefied gas phenomena of thermal transpiration. Knudsen Compressors operate by imposing a temperature gradient across a high porosity, low thermal conductivity transpiration membrane, typically a silicon aerogel membrane. Previous optimization studies have concluded that significant reductions of both energy consumption and device volume per unit throughput and pressure difference can be achieved when each stage of the cascade operates with a Knudsen Number based on the mean pore radius of approximately one. Perforated aerogels (using the same bulk aerogel material, but with machined arrays of properly sized parallel capillaries) are appealing candidate low‐pressure transpiration membranes and are the focus of this investigation. Conventional drilling techniques using micro drills have successfully demonstrated perforated aerogel with pore diameters ranging from 210μm to 1mm. This range of pore sizes corresponds to efficient Knudsen Compressor operation between roughly 1 Torr and 10 mTorr. The other issue at low pressures is the larger Kn of the connector section which can introduce “reverse” thermal transpiration. Two conventionally perforated carbon doped aerogel membranes, with the mean pore diameters of 210 μm and 380 μm, have been tested at the operating pressure range of 2 Torr to 10 mTorr. Comparison with the predicated results showed the evidence of rarefied gas effects such as “reverse” thermal transpiration.The Knudsen Compressor is a solid‐state micro/meso‐scale gas roughing pump based on the rarefied gas phenomena of thermal transpiration. Knudsen Compressors operate by imposing a temperature gradient across a high porosity, low thermal conductivity transpiration membrane, typically a silicon aerogel membrane. Previous optimization studies have concluded that significant reductions of both energy consumption and device volume per unit throughput and pressure difference can be achieved when each stage of the cascade operates with a Knudsen Number based on the mean pore radius of approximately one. Perforated aerogels (using the same bulk aerogel material, but with machined arrays of properly sized parallel capillaries) are appealing candidate low‐pressure transpiration membranes and are the focus of this investigation. Conventional drilling techniques using micro drills have successfully demonstrated perforated aerogel with pore diameters ranging from 210μm to 1mm. This range of pore sizes corresponds to ef...

Gas-Surface Interaction Model Influence on Predicted Performance of Microelectromechanical System Resistojet

The free molecule micro-resistojet was designed as a micropropulsion system capable of performing attitude controlandprimarymaneuversfornanospacecraftwithmasslessthan10kg.Thedetailsofgas ‐surfaceinteractions between propellant molecules and surfaces held at elevated temperature are critical in predicting the propulsion system’ s performance and efe ciency. The aim is to assess parametrically the performance of a typical thruster geometry using a general Maxwell scattering model and two versions of the Cercignani ‐Lampis‐Lord model (Lord, R. G., “ Some Extensions of the Cercignani ‐Lampis Gas‐Surface Scattering Kernal,” Physics of Fluids, A , Vol.3,No.4,1991,pp.706 ‐710 andLord, R.G., “ SomeFurtherExtensionsoftheCercignani ‐LampisGas‐Surface Interaction Model,” Physics of Fluids, A , Vol. 7, No. 5, 1995, pp. 1159 ‐1161). The models are incorporated into a direct simulation Monte Carlo numerical code and are used to bound the predicted performance characteristics of the thruster. The total specie c impulse varies by approximately 20% over range of accommodation coefe cients from specular to diffuse surface scattering. However, there was only a maximum difference of about 5% between the models for a given accommodation coefe cient. Other more microscopic parameters, such as axial velocity distribution functions, appear to depend more on the scattering model assumed.

Thermal transpiration in microsphere membranes

Self‐assembled glass microsphere membranes as an alternative transpiration membrane for application in a Knudsen Compressor are discussed. A performance model is constructed and used to compare the performance of glass microsphere membranes to silicon aerogel membranes for this application. An initial experimental Knudsen Compressor stage based on glass microsphere membranes has been designed and experimentally tested. Preliminary performance results show a discrepancy between the predicted and observed pressure differences produced by the single stage. Several possible explanations for the discrepancy are discussed. Two variations of a proposed design for a Knudsen Compressor employing a microsphere transpiration membrane are discussed. It is concluded that beds of glass microspheres may be attractive candidates for transpiration membrane materials over the entire pressure range of operation for a micro‐scale vacuum pump, 10mTorr to 760 Torr.

Comparisons of Measured and Predicted Velocity Distribution Functions in a Shock Wave

Molecular velocity distribution functions measured in a normal shock wave in helium at M = 1.59 show deviations from equilibrium which are consistent in functional form, but not quantitatively, with the deviations predicted by the Chapman‐Enskog first iterate. The measurements do appear to be in substantial agreement with a Monte Carlo solution of the Boltzmann equation, as indicated by a comparison of the half‐widths of the parallel velocity distributions in the shock wave.

Free Molecule Micro-Resistojet: Nanosatellite Propulsion

Constellations and platoons of small satellites can offer an assortment of benefits over larger, single function spacecraft. The strict mass, volume, and power limitations of small satellites will require unique micro-technologies to help develop efficient propulsion systems for maneuvering. The Free Molecule Micro-Resistojet (FMMR) has been analyzed and tested in this study to determine its applicability for an upcoming Texas A&M (TAM) nanosatellite mission. The nanosatellite mission will demonstrate the performance and survivability of a water propelled FMMR for attitude control maneuvers and could mark the first meaningful operation of a Microelectromechanical Systems (MEMS) fabricated thruster in space. The Mark 3.1 design of the FMMR heater chip uses a deposited serpentine heater pattern to resistively heat a gaseous propellant expanding through long (13 mm), narrow (100 µm) slots. Experimental data shows that the FMMR, with a heated wall temperature of 575 K, can attain a specific impulse of 65 seconds with a thrust level of 1.2 mN for a nitrogen gas propellant with a mass flow of 100 SCCM. The expected specific impulse when run on a water vapor propellant is expected to be 80 sec at similar thrust levels. Higher thrust levels can be achieved by increasing the temperature of the FMMR heater chip and / or the propellant mass flow through the expansion slots. The measured performance of the FMMR in this study has proven to be adequate to perform the attitude control maneuver for the TAM nanosatellite.