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With the intent of increasing the efficiency of laser-gas interactions, an external bidirectional multipass optical cavity has been developed for use with counterpropagating high energy pulsed lasers. By exploiting the closed-path recirculation of two counterpropagating laser pulses, obtained through polarization-dependent trapping of the pulses inside the cavity via a Pockels cell, such an optical cavity allows the counterpropagating pulses to repeatedly and concurrently interact within a specific region in space. This cavity allows the setup to exhibit an effective pulse repetition frequency much higher than the loading laser. This increased repetition frequency permits greater sensitivity in probing a medium or increased energy deposition, depending on application, over the single pass case. Pulses from a 12 mJ, frequency doubled, 5 ns FWHM, pulsed Nd:YAG laser were split and injected into opposing sides of a symmetric multipass 2.44 m (96 in) optical ring cavity. Using a Pockels cell, the counterpropagating pulses were locked into the cavity for ≥50 cavity round trips. This represents an 8 fold increase in the interacting laser energy over the single pass case. This cavity concept suggests a variety of potential applications ranging from established cavity processes, e.g. laser-based absorption spectroscopy studies and x-ray production, to new processes which can benefit from the counterpropagating feature of this design, such as non-resonant optical lattice gas heating and time-resolved coherent Rayleigh-Brillouin scattering diagnostic studies of a rapidly evolving flow., Journal Article for Applied Physics B. Prepared in collaboration with University of Colorado, Colorado Springs, CO; United States Air Force Academy, Colorado Springs, CO; and ERC Inc., Edwards AFB, CA. The original document contains color images.

Coherent Rayleigh-Brillouin scattering (CRBS) lineshapes generated from an all narrow-band pump experiment, a Direct Simulation Monte-Carlo (DSMC) approach, and published simplified models are presented for argon, molecular nitrogen, and methane at 300 & 500 K and 1 atm. The simplified models require uncertain gas properties, such as bulk viscosity, and assume linearization of the kinetic equations from low intensities (1 x 1015 W/sq m) operating in the perturbative regime. DSMC, a statistical approach to the Boltzmann equation, requires only basic gas parameters available in literature and simulates the forcing function from first principles, without assumptions on laser intensity. The narrow band experiments show similar results to broadband experiments and validate the use of DSMC for the prediction of CRBS lineshapes., For publication in Optics Express.

A propulsion system based on an array of radiometer vanes has been proposed for wind disturbance correction on near space vehicles. The radiometric force is produced by a vane due to a temperature gradient between a surface heated by insolation and a convectively cooled surface. Previous research has identified several key parameters that might affect the thrust produced by a radiometer array. These parameters include the heat transfer mechanisms on the individual vanes and between nearby vanes in an array, the proximity or spacing of radiometer vanes in an operational array, the effects of the vanes attached to a near space vehicle operating in the boundary layer near the surface of the vehicle, and the effects of attaching the vanes to a substrate on the vehicle itself. A combined numerical and experimental approach was used to investigate these effects. Numerical codes were used to assess the heat transfer mechanisms and the boundary layer flow. Experiments were designed and carried out to measure the force produced by a small array of radiometers. These results were combined to assess the performance of a notional array of radiometer vanes for the purpose of counteracting drag on a near space vehicle operating at an altitude of 60 km. The array performance was assessed based on the maximum force that can be produced as a function of array mass and area., The original document contains color images. Presented at the AIAA Thermophysics Conference, held New Orleans, LA, on 25-28 June 2012.

Several applications exist for transiting liquid droplets through the near-Earth space environment. Numerical results are presented for the charging of liquid droplets of trimethyl pentaphenyl siloxane (DC705) in three different plasma environments: ionosphere, auroral, and geosynchronous Earth orbit (GEO). Nominal and high geomagnetic activity cases are investigated. In general, high levels of droplet charging (100V) exist only in GEO during periods of high geomagnetic or solar activity. An experiment was conducted to assess the charging of silcon-oil droplets due to photoemission. The photoemission yield in the 120-200nm wavelength range was found to be approximately 0.06., For presentation at the International Symposium on Rarefied Gas Dynamics (27th), Monterey, CA, 10-15 July 2010. Prepared in collaboration with the Department of Astronautics, United States Air Force Academy, CO. The original document contains color images. All DTIC reproductions will be in black and white.

An overview of recent numerical and computational studies of radiometric phenomena is presented. Several important factors affecting radiometric forces have been analyzed, including the contribution of area, edge, and shear forces for different pressures and gases, chamber size effect, gas-surface accommodation, and multi-vane geometries., For presentation at the International Symposium on Rarefied Gas Dynamics (27th) to be held in Monterey, CA on 10-15 July 2010.

The impact of the radiometer vane thickness and edge geometrical shape on the total radiometric force is examined numerically solving the ES BGK model kinetic equation. The flow of argon over a single vane and a multi-vane configurations is considered in the range of Knudsen numbers from 0.02 to 1. The shear force is found to reduce the total radiometric force for most vane configurations. It is shown that the change in the vane shape may offset the losses due to the shear force in the multi-vane geometry., For presentation at the International Symposium on Rarefied Gas Dynamics (27th), to be held in Monterey, CA on 10-15 July 2010.

This paper presents the results of research on a novel concept in satellite propulsion that relies on momentum transfer through projection of silicon oil droplet streams through space. The system is useful to satellites flying side-by-side in formation that require a constant distance between them in order to conduct certain missions such as interferometric synthetic aperture radar observations. The force required to maintain separation is beyond what electric thrusters can provide without using large amounts power and significant amounts of propellant. Rational for selection of the silicone oil DC705 as the best working fluid is presented. Droplet size, velocity, and spacing needed for station keeping of various satellite mass and separation distance combinations is evaluated. Droplet streams of diameters demonstrated in this study and speeds demonstrated in past research can satisfy propulsion needs of satellites with masses in excess of 1000 kg in any earth orbit. A droplet stream system requires an order of magnitude less mass than comparable electric propulsion systems and two orders of magnitude less power and is an enabling technology for side-by-side formation flying. Environmental impediments to collection of droplets transiting between satellites were analyzed and three significant obstacles identified; atmospheric drag, droplet freezing, and Lorentz forces due to droplet charging., For presentation at the JANNAF Joint Subcommittee Meeting (57th), to be held in Colorado Springs, CO on 3-7 May 2010.

A wide range of advanced launch concepts have been proposed in an effort to revolutionize space access through either a significant reduction in launch costs or significant improvements in launch performance. This paper briefly summarizes commonly proposed advanced launch concepts, including both concepts that employ propellant and propellantless concepts. Each concept is briefly described along with its potential in two generic mission classes: small satellite launch to LEO and large satellite launch to GEO. It is shown theoretically that there is significant room for improvement in the cost and performance of current launch systems. It is also shown, however, that historical predictions of launch costs reductions and/or performance improvements for new technologies have been highly optimistic with realized costs and performance leading to only incremental improvements instead of revolutionary advancements. All of the reviewed technologies still have significant technical challenges to overcome before yielding fully operational systems. The associated risk makes it difficult to justify the large investments required to develop such systems, indicating that a development path with useful products at points in between the current state-of-the-art and final goal is necessary., Prepared in collaboration with ERC Inc., Edwards AFB, CA. For presentation at the AIAA Joint Propulsion Conference (44 th), Hartford, CT, on 20-23 Jul 2008.

Advances in micro-technology manufacturing and capability have led to an increased interest in micro and nano-satellites. A propulsion system has been designed to meet the on-orbit attitude control requirements for nano-spacecraft. The Free Molecule Micro-Resistojet (FMMR), a low cost, low power, high propellant storage density, and green propulsion system, has been analyzed in this study to determine its ability to provide a slew maneuver for a typical 10 kg nanosatellite. Additionally, a FMMR technology demonstrator (TD) has been fabricated using traditional and Microelectromechanical Systems (MEMS) techniques. The TD has been analyzed and tested in this study to determine its performance characteristics while operating with water propellant. Experimental data shows that the FMMR, with a heated wall temperature of 580 K, can attain a specific impulse of 79.2 seconds with a thrust level of 129 micro-N. For a given mass flow, higher thrust levels can be achieved by increasing the temperature of the FMMR heater chip. The experimental results agree favorably with predicted values from kinetic theory. Applying the measured performance of the TD to an optimized setup, the FMMR system could provide a 45-degree slew of a typical nanosatellite in 60 seconds, which is acceptable for many nanosatellite applications., Submitted for publication in the Journal of Spacecraft and Rockets. Prepared in collaboration with University of Southern California, Los Angeles, CA.

Advances in micro-technology manufacturing and capability have led to an increased interest in micro and nanosatellites. A propulsion system has been designed to meet the on-orbit attitude control requirements for nanospacecraft. The Free Molecule Micro-Resistojet (FMMR), a low cost, low power, high propellant storage density, and green propulsion system, has been analyzed in this study to determine its ability to provide a slew maneuver for a typical 10 kg nanosatellite. Additionally, a FMMR technology demonstrator (TD) has been fabricated using traditional and Microelectromechanical Systems (MEMS) techniques. The TD has been analyzed and tested in this study to determine its performance characteristics while operating with water propellant. Experimental data shows that the FMMR, with a heated wall temperature of 580 K, can attain a specific impulse of 79.2 seconds with a thrust level of 129 micro-N. For a given mass flow, higher thrust levels can be achieved by increasing the temperature of the FMMR heater chip. The experimental results agree favorably with predicted values from kinetic theory. Applying the measured performance of the TD to an optimized setup, the FMMR system could provide a 45-degree slew of a typical nanosatellite in 60 seconds, which is acceptable for many nanosatellite applications., Presented at the AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit (43rd) in Cincinnati, OH on 8-11Jul 2007. Published in proceedings of the same as paper AIAA 2007-5185.

The need for low thrust propulsion systems for maneuvers on micro- and nano-spacecraft is growing. Low thrust characteristics generally lead to low Reynolds number flows from propulsive devices that utilize nozzle expansions. Low Reynolds number flows of helium and nitrogen through a small conical nozzle and a thin-walled orifice have been investigated both numerically, using the Direct Simulation Monte Carlo technique, and experimentally, using a nano-Newton thrust stand. For throat Reynolds number less than 100, the nozzle to orifice thrust ratio is less than unity; however, the corresponding ratio of specific impulse remains greater than one for the Reynolds number range from 0.02 to 200. Once the Direct Simulation Monte Carlo model results were verified using experimental thrust and mass flow data, the model was used to investigate the effects of geometrical variations on the conical nozzle's performance. At low Reynolds numbers, improvements to the specific impulse on the order of 4 to 8% were achieved through a combination of decreasing the nozzle length and increasing the nozzle expansion angle relative to the nominal experimental geometry., The original document contains color images.

A torsional impulse balance has been developed as a new diagnostic tool to study fundamental physical processes in micropropulsion systems and laser-surface interactions. The impulse balance has been designed and tested with a robust calibration system to measure impulsive forces with resolution as low as several nano-Newton-seconds. The behavior of the impulse balance was thoroughly studied and characterized. A simple analytical model of the balance s motion was developed from the general equation of motion of an underdamped, harmonically oscillating system. Also, two distinct methods of analyzing the experimental data from the nanoimpulse balance have been investigated. The first method resolves the total impulse as a function of the balance's maximum deflection. The second method enables the determination of the impulse and/or force applied as a function of time from the balance s time-resolved motion. A calibration scheme employing electrostatic actuation techniques is used to experimentally validate the model and impulse measurement techniques., The original document contains color images.

With the rapid progress of micro- and nano-scale fabrication technology, devices are continually being created which produce extremely small forces. This creates a distinct need for a measurement instrument and adequate calibration techniques which can resolve forces below 1 micro N. Two calibration methods for force balance measurements in the nano-Newton range are presented. These methods are based on a free molecule, gas dynamic expansion through a thin-walled orifice and the electrostatic actuation of a miniature comb drive. Due to the advantages and disadvantages of every calibration technique, multiple techniques are often required to validate performance results for micro-scale devices. Because these calibration techniques typically rely on completely different physical processes and can be described by different sets of analytical equations, the comparison of one technique to another is necessary when high accuracy is required. The gas dynamic and electrostatic force calibration techniques have been compared and were found to agree to within 8% for force levels between 35 nano-Newtons and 1 micro N.

The popularity of micropropulsion system development has led to renewed interest in the determination of propulsive properties of orifice flows since micronozzle expansions may suffer high viscous losses at low pressure operation. The mass flow and relative thrust for an under expanded orifice is measured as a function of orifice stagnation pressure from 0.1 to 3.5 Torr. Nitrogen, argon, and helium propellant gases are passed through a 1.0 mm diameter orifice with a wall thickness of 0.015 mm Near-free molecule, transitional and continuum flow regimes are studied. The relative thrust is determined by a novel thrust stand designed primarily for low operating pressure, micropropulsion systems. It is shown that the thrust indications obtained from the stand are a function of the facility background pressure, and corrections are made to determine the indicated thrust for a zero background pressure with nitrogen as propellant. Highly repeatable (within 1%) indicated thrust measurements are obtained in the thrust range from 5 to 500 Micro-N., Prepared in collaboration with Univ. of Southern California at Los Angeles.

The ability to measure extremely low thrust levels with unusual precision is becoming more critical as attempts are made to characterize the performance of emerging micropropulsion systems. Many new attitude control concepts for nanospacecraft involve the production of thrust below 1 micro-N. A simple, but uniquely successful thrust stand has been developed and used to measure thrust levels as low as 86.2 nano-Newtons with an estimated accuracy of +/- 11%. Thrust levels in the range of 712 nano-Newtons to 1 micro-N have been measured with an estimated accuracy of +/- 2%. Thrust is measured from an underexpanded orifice operating in the free molecule flow regime with helium, argon, and nitrogen propellants. The thrust stand is calibrated using results from Direct Simulation Monte Carlo numerical models and analytical solutions for free molecule orifice flow. The accuracy of the gas dynamic calibration technique, using free molecule orifice flow, has also been investigated. It is shown that thrust stand calibration using high Knudsen number helium flow can be accurate to within a few percent in the 80 nN to 1 micro-N thrust range for thin walled orifices when the stagnation pressure is accurately measured. The thrust stand and calibration technique exhibit significant improvement for accurate, low thrust measurements compared to currently published results., Prepared in collaboration with Univ of Southern California, Los Angeles.