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All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

In-Situ Monitoring of a Bismuth-Tin Alloy Solidification Using MEPHISTO

Experiments were carried out to study the morphological stability of Bi- 1 atomic % Sn alloys using the MEPHISTO directional solidification apparatus aboard Space Shuttle Columbia (STS-87, launched Nov. 19, 1997) and in ground-based studies. The Seebeck signal and temperature measurements indicate that convection was significant for ground-based studies. In the space-based experiments, interface breakdown was observed at growth velocities of 6.7, 27, and 40 microns/sec, but not at 1.8 and 3.3 microns/sec.

Abbaschian, Reza↗
NASA NTRS1999

Map Algorithms for Decoding Linear Block codes Based on Sectionalized Trellis Diagrams

The MAP algorithm is a trellis-based maximum a posteriori probability decoding algorithm. It is the heart of the turbo (or iterative) decoding which achieves an error performance near the Shannon limit. Unfortunately, the implementation of this algorithm requires large computation and storage. Furthermore, its forward and backward recursions result in long decoding delay. For practical applications, this decoding algorithm must be simplified and its decoding complexity and delay must be reduced. In this paper, the MAP algorithm and its variations, such as Log-MAP and Max-Log-MAP algorithms, are first applied to sectionalized trellises for linear block codes and carried out as two-stage decodings. Using the structural properties of properly sectionalized trellises, the decoding complexity and delay of the MAP algorithms can be reduced. Computation-wise optimum sectionalizations of a trellis for MAP algorithms are investigated. Also presented in this paper are bi-directional and parallel MAP decodings.

Lin, Shu↗
NASA NTRS1999

Main Chamber and Preburner Injector Technology

This document reports the experimental and analytical research carried out at the Penn State Propulsion Engineering Research Center in support of NASA's plan to develop advanced technologies for future single stage to orbit (SSTO) propulsion systems. The focus of the work is on understanding specific technical issues related to bi-propellant and tri-propellant thrusters. The experiments concentrate on both cold flow demonstrations and hot-fire uni-element tests to demonstrate concepts that can be incorporated into hardware design and development. The analysis is CFD-based and is intended to support the design and interpretation of the experiments and to extrapolate findings to full-scale designs. The research is divided into five main categories that impact various SSTO development scenarios. The first category focuses on RP-1/gaseous hydrogen (GH2)/gaseous oxygen (GO2) tri-propellant combustion with specific emphasis on understanding the benefits of hydrogen addition to RP-1/oxygen combustion and in developing innovative injector technology. The second category investigates liquid oxygen (LOX)/GH2 combustion at main chamber near stoichiometric conditions to improve understanding of existing LOX/GH2 rocket systems. The third and fourth categories investigate the technical issues related with oxidizer-rich and fuel-rich propulsive concepts, issues that are necessary for developing the full-flow engine cycle. Here, injector technology issues for both LOX/GH2 and LOX/RP-1 propellants are examined. The last category, also related to the full-flow engine cycle, examines injector technology needs for GO2/GH2 propellant combustion at near-stoichiometric conditions for main chamber application.

Santoro, Robert J.↗
NASA NTRS1999

Effects of G-Jitter on Directional Solidification of a Binary Alloy

A study of directional solidification of a weak binary alloy (specifically, Bi - 1 at% Sn) based on the fixed grid single domain approach is being undertaken. The enthalpy method is used to solve for the temperature field over the computational domain including both the solid and liquid phases; latent heat evolution is treated with the aid of an effective specific heat coefficient. A source term accounting for the release of solute into the liquid during solidification has been incorporated into the solute transport equation. The vorticity-stream function formulation is used to describe thermo-solutal convection in the liquid region. In this paper we present a numerical simulation of g-jitter. A background gravity of 1 microgram has been assumed, and new results for the effects of periodic disturbances over a range of amplitudes and frequencies on solute field and segregation have been presented.

Benjapiyaporn, C.↗
NASA NTRS1997

The Tethered Satellite System: Scientific and Technological Results

The bi-national, US-Italian, Tethered Satellite System (TSS) program was designed to provide a unique opportunity to explore certain space plasma- electrodynamic processes and the orbital mechanics of a gravity-gradient stabilized system of two satellites linked by a long conducting tether. The second flight, TSS-LR, was launched February 22, 1996 on STS-75 and satellite deployment began at MET 3/00:27. A unique data set was obtained over the next five hours as the tether was deployed to a length of 19695 meters, which has allowed significant science to be accomplished. This presentation will focus on results from the TSS-LR mission that are most important to the future technological applications of electrodynamic tethers in space - in particular, the current collection process. Of particular significance is an apparent transition of the physics of current collection when the potential of the collecting body becomes greater than the ram energy of the ionospheric atomic oxygen ions. Previous theoretical models of current collection were electro- static - assuming that the orbital motion of the system, which is highly sub-sonic with respect to electron thermal motion, was unimportant. This may still be acceptable for the case of relatively slow-moving sounding rockets. However, the TSS-LR results show that motion relative to the plasma must be accounted for in orbiting systems.

Stone, Nobie H.↗
NASA NTRS1997

Collisionless Plasma Modeling in an Arbitrary Potential Energy Distribution

A new technique for calculating a collisionless plasma along a field line is presented. The primary feature of the new model is that it can handle an arbitrary (including nonmonotonic) potential energy distribution. This was one of the limiting constraints on the existing models in this class, and these constraints are generalized for an arbitrary potential energy composition. The formulation for relating current density to the field-aligned potential as well as formulas for density, temperature and energy flux calculations are presented for several distribution functions, ranging from a bi-Lorentzian with a loss cone to an isotropic Maxwellian. A comparison of these results with previous models shows that the formulation reduces.to the earlier models under similar assumptions.

Liemohn, M. W.↗
NASA NTRS1997

Real Time Characterization of Solid/Liquid Interfaces During Directional Solidification

A X-Ray Transmission Microscope (XTM) has been developed to observe in real time and in-situ solidification phenomenon at the solid/liquid interface. Recent improvements in the horizontal Bridgman furnace design provides real-time magnification (during solidification) up to 12OX. The increased magnification has enabled for the first time the XTM imaging of real-time growth of fibers and particles with diameters of 3-6 micrometers. Further, morphological transitions from planar to cellular interfaces have also been imaged. Results from recent XTM studies on Al-Bi monotectic system, Al-Au eutectic system and interaction of insoluble particles with s/I interfaces in composite materials will be presented. An important parameter during directional solidification of molten metal is the interfacial undercooling. This parameter controls the morphology and composition at the s/I interface. Conventional probes such as thermocouples, due to their large bead size, do not have sufficient resolution for measuring undercooling at the s/I interface. Further, the intrusive nature of the thermocouples also distorts the thermal field at the s/I interface. To overcome these inherent problems we have recently developed a compact furnace which utilizes a non-intrusive technique (Seebeck) to measure undercooling at the S/I interface. Recent interfacial undercooling measurements obtained for the Pb-Sn system will be presented. The Seebeck measurement furnace in the future will be integrated with the XTM to provide the most comprehensive tool for real time characterization of s/I interfaces during solidification.

Sen, S.↗
NASA NTRS1999

Development of Analysis Methods for Designing with Composites

The project involved the development of new analysis methods to achieve efficient design of composite structures. We developed a complex variational formulation to analyze the in-plane and bending coupling response of an unsymmetrically laminated plate with an elliptical cutout subjected to arbitrary edge loading as shown in Figure 1. This formulation utilizes four independent complex potentials that satisfy the coupled in-plane and bending equilibrium equations, thus eliminating the area integrals from the strain energy expression. The solution to a finite geometry laminate under arbitrary loading is obtained by minimizing the total potential energy function and solving for the unknown coefficients of the complex potentials. The validity of this approach is demonstrated by comparison with finite element analysis predictions for a laminate with an inclined elliptical cutout under bi-axial loading.The geometry and loading of this laminate with a lay-up of [-45/45] are shown in Figure 2. The deformed configuration shown in Figure 3 reflects the presence of bending-stretching coupling. The validity of the present method is established by comparing the out-of-plane deflections along the boundary of the elliptical cutout from the present approach with those of the finite element method. The comparison shown in Figure 4 indicates remarkable agreement. The details of this method are described in a manuscript by Madenci et al. (1998).

Madenci, E.↗
NASA NTRS1998

NASA Advanced Fuels Program

NASA with the USAF Research Laboratory and it's industry partners, has been conducting planning and research into advanced fuels. This work is sponsored under the NASA Advanced Space Transportation Program (ASTP). The current research focus is on Alternative Hydrocarbon fuels, Monopropellants, and Solid Cryogens for storing atoms of Hydrogen, Boron, Carbon, and Aluminum. Alternative hydrocarbons that are under consideration are bi cyclo propylidene, spiro pentane, and tri propargyl amine. These three fuels have been identified as initial candidates to increase the specific impulse of hydrocarbon fueled rockets by 10-15 seconds over 02/RP-1. Formulation of these propellants is proceeding this year, and rocket engine testing is planned for the near future. Monopropellant investigations are focused on dinitramine based fuels, and potential collaborations with the US Navy. The dinitramine fuel work is being conducted under an Small Business Innovation research (SBIR) contract with the team of Orbital Technologies Corp. (Madison, WI) and SRI (Menlo Park, CA). This work may lead to a high density, high specific impulse monopropellants that can simplify the operations for launch vehicles and spacecraft. Solid Cryogens are being considered to store atoms of Hydrogen, Boron, Carbon, and Aluminum. Stored atom propellants are potentially the highest specific impulse chemical rockets that may be practical. These fuels are composed of atoms, stored in solid cryogenic particles, suspended in a cryogenic liquid or gel. The fuel would be fed to a rocket engine as a slurry or gelled cryogenic liquid with the suspended particles with the trapped atoms. Testing is planned to demonstrate the formation of the particles, and then characterize the slurry flows. Rocket propellant and propulsion technology improvements can be used to reduce the development time and operational costs of new space vehicle programs. Advanced propellant technologies can make the space vehicles safer, more operable, and better performing. Five technology areas are described: Monopropellants, Alternative Hydrocarbons, Gelled Hydrogen, Metallized Gelled Propellants, and High Energy Density Materials. The benefits of these propellants for future vehicles are outlined using mission study results and the technologies are briefly discussed.

Palaszewski, Bryan↗
NASA NTRS1999

Tropical Cirrus Physical and Radiation Characteristics from the NASA Aircraft Observations in Toga/Coare

During the Toga/Coare experiment in the tropical west Pacific, a cloud radiation experiment involving measurement from the NASA ER-2 high altitude aircraft and DC-8 airborne laboratory was performed. Observations include multispectral visible/IR and microwave radiometric imaging, active lidar profiling and radiation flux measurements. On a number of missions coordinated flights were made for simultaneous cloud observations with the ER-2 remote sensing and DC-8 below cloud and in cloud measurements. Since the field experiment there has been systematic analysis of the observations. Intercomparison of retrieved measurements will be presented. These include the relation between visible bi-directional reflectivity and infrared emissivity of tropical cirrus. Upper troposphere microwave water vapor retrievals and cirrus optical parameters have been correlated. The relation observed short wave flux divergence to retrieved cloud thickness and type has been studied. Calculations and statistics for the short and long wave heating profiles of observed clouds have been derived. A summary of key results will be presented.

Spinhirne, James D.↗
NASA NTRS1997

An Analysis of Closure Mechanisms in the Plasma Wake of the TSS-1R Satellite

Collisionless Plasma Expansion (CPE), also known as "plasma expansion into a vacuum," in its most fundamental form, is the process by which a plasma expands into a void, or region highly depleted of particles. In CPE, the expansion is driven by the highly mobile electron constituent of the plasma as it moves across the density gradient and into the void region. A bi-polar electric field is set up between this rapidly expanding electron front and the massive ions, which have a low thermal speed and lag behind in the region of the plasma-void interface. These ions are quickly accelerated by the expansion electric field and can, theoretically, approach the thermal speed of the electrons. CPE is a fundamental and a very robust process in that it only requires that a sharp density gradient exist in a plasma. It has already been observed to exist in various plasmas ranging over five orders of magnitude in density. The range of phenomena in which CPE has been observed includes the closure of plasma wakes created in simulated space plasmas in the laboratory; it was found to be the process controlling the refilling of the wake created by the Space Shuttle in the ionospheric plasma; and recent in situ data, obtained by the WIND spacecraft as it passed through the wake of the earth's moon last year, have shown CPE to be involved in the closure of the Lunar wake. CPE is also expected to influence the morphology and physics of the solar wind interactions with Mercury and Mars, and has been suggested as a potential factor in such wide ranging phenomena as the acceleration of plasma away from the coma of comets and the energization of ions that underlies the upward expansion of the polar wind into the earth's magnetosphere. The Tethered Satellite System (TSS)-1R data provides an opportunity to test this process in situ under semi-controlled conditions. The available data shows the intensity, drift energy and angle of inclination to the wake of the converging ion streams at a down stream distance of one satellite radius (80 CM).

Stone, N. H.↗
NASA NTRS1998

Contamination Observed on the Passive Optical Sample Assembly (POSA)-1 Experiment

The Passive Optical Sample Assembly-1 (POSA-1), part of the Mir Environmental Effects Payload (MEEP), was designed to study the combined effects of contamination, atomic oxygen, ultraviolet radiation, vacuum, thermal cycling, and other constituents of the space environment on spacecraft materials. The MEEP program is a Phase 1 International Space Station Risk Mitigation Experiment. The payload was attached by EVA to the exterior of the Mir docking module during the Space Shuttle mission STS-76. It was removed during STS-86 after exposure to the Mir environment for 18 months. POSA-1 consists of nearly 400 samples of thermal control paints, chemical conversion coatings, mirrors, optics, and other candidate materials for International Space Station Alpha (ISSA). POSA-1 also flew state-of-the-art materials, such as electrically conductive thermal control coatings developed under contract by IITRI and AZ Technology, and passive instruments for monitoring the atomic oxygen and ultraviolet radiation dose to the experiment. Contamination was detected on the POSA-1 experiment. On the side facing space (-Z(sub B) direction using the Mir coordinate system), visible contamination was observed. The contamination was uniform as would occur from a slow photodeposition process. A very definite film was deposited on the optical samples. The deposition appears to have directionality with definite shadowing effects. On the side facing the main Mr core (+ZB direction using the Mir coordinate system), no visible contamination was noted. Pre- and post-flight characterization of candidate spacecraft materials is discussed, particularly the effect of contamination on total hemispherical spectral reflectance (250 - 2500 nm), infrared emittance, the bi-directional reflectance distribution function (BRDF), and vacuum ultraviolet spectral reflectance. Extensive photography under normal and black light was performed. Results of electron spectroscopy for chemical analysis (ESCA) and Fourier Transform Infrared Analysis (FTIR) on selected samples are also discussed.

Zwiener, James M.↗
NASA NTRS1999

Space Adaptation of Active Mirror Segment Concepts

This report summarizes the results of a three year effort by Blue Line Engineering Co. to advance the state of segmented mirror systems in several separate but related areas. The initial set of tasks were designed to address the issues of system level architecture, digital processing system, cluster level support structures, and advanced mirror fabrication concepts. Later in the project new tasks were added to provide support to the existing segmented mirror testbed at Marshall Space Flight Center (MSFC) in the form of upgrades to the 36 subaperture wavefront sensor. Still later, tasks were added to build and install a new system processor based on the results of the new system architecture. The project was successful in achieving a number of important results. These include the following most notable accomplishments: 1) The creation of a new modular digital processing system that is extremely capable and may be applied to a wide range of segmented mirror systems as well as many classes of Multiple Input Multiple Output (MIMO) control systems such as active structures or industrial automation. 2) A new graphical user interface was created for operation of segmented mirror systems. 3) The development of a high bit rate serial data loop that permits bi-directional flow of data to and from as many as 39 segments daisy-chained to form a single cluster of segments. 4) Upgrade of the 36 subaperture Hartmann type Wave Front Sensor (WFS) of the Phased Array Mirror, Extendible Large Aperture (PAMELA) testbed at MSFC resulting in a 40 to 5OX improvement in SNR which in turn enabled NASA personnel to achieve many significant strides in improved closed-loop system operation in 1998. 5) A new system level processor was built and delivered to MSFC for use with the PAMELA testbed. This new system featured a new graphical user interface to replace the obsolete and non-supported menu system originally delivered with the PAMELA system. The hardware featured Blue Line's new stackable processing system which included fiber optic data links, a WFS digital interface, and a very compact and reliable electronics package. The project also resulted in substantial advances in the evolution of concepts for integrated structures to be used to support clusters of segments while also serving as the means to distribute power, timing, and data communications resources. A prototype cluster base was built and delivered that would support a small array of 7 cm mirror segments. Another conceptual design effort led to substantial progress in the area of laminated silicon mirror segments. While finished mirrors were never successfully produced in this exploratory effort, the basic feasibility of the concept was established through a significant amount of experimental development in microelectronics processing laboratories at the University of Colorado in Colorado Springs. Ultimately lightweighted aluminum mirrors with replicated front surfaces were produced and delivered as part of a separate contract to develop integrated segmented mirror assemblies. Overall the project was very successful in advancing segmented mirror system architectures on several fronts. In fact, the results of this work have already served as the basic foundation for the system architectures of several projects proposed by Blue Line for different missions and customers. These include the NMSD and AMSD procurements for NASA's Next Generation Space Telescope, the HET figure maintenance system, and the 1 meter FAST telescope project.

Ames, Gregory H.↗
NASA NTRS1999

Numerical Simulations of Crystal Growth of an Alloy Under Microgravity Conditions

The directional solidification of a dilute binary alloy (Bi-1.0 at.%Sn)is investigated. Results are obtained at a gravity level of I pg. Computations are performed in two dimensions with a fixed, non-uniform grid. The simulation involves a solution of the species concentration equation (modified to account for solute rejection at the interface) and energy equation (modified to account for phase-change) for both the solid and liquid phases, in addition to the constitutive equations for describing convective flow in the melt. The effects of conductive heat transfer in the ampoule and in a capillary tube in the sample are included. To gauge the effects of including this growth capillary tube in the apparatus, simulations both with and without the capillary tube are presented and compared. Fully transient simulations have been performed; no simplifying steady-state approximations are used, however, the influence of solute on the melting temperature at the interface is not included. Both thermal and solutal convective cells are seen to form. Convective velocities are significantly damped inside the capillary, causing less segregation due to convection. As solidification proceeds beyond the capillary tube, longitudinal segregation arises as a result of the change in cross-sectional area of solidifying material. The magnitudes of the velocities in this cell increase significantly once the solid/liquid front passes beyond the end of the capillary tube; this causes a corresponding increase in the level of radial solute segregation in the solidified material.

Simpson, James E.↗
NASA NTRS1998

The TSS-1R Electrodynamic Tether Experiment: Scientific and Technological Results

The bi-national, US-Italian, Tethered Satellite System (TSS) program was designed to provide a unique opportunity to explore certain space plasma- electrodynamic processes and the orbital mechanics of a gravity-gradient stabilized system of two satellites linked by a long conducting tether. The second flight, TSS-LR, was launched February 22, 1996 on STS-75 and satellite deployment began at MET 3/00:27. A unique data set was obtained over the next five hours, as the tether was deployed to a length of 19695 meters, which has allowed significant science to be accomplished. This presentation will focus on electrodynamic processes generated by the tether--in particular, the collection of electrical current from the ionospheric plasma. Of particular significance is an apparent transition of the physics of current collection when the potential of the collecting body becomes greater than the ram energy of the ionospheric atomic oxygen ions. Previous theoretical models of current collection were electrostatic--assuming that the orbital motion of the system, which is highly subsonic with respect to electron thermal motion, was un- important. This may still be acceptable for the case of relatively slow-moving sounding rockets. However, the TSS-LR results show that motion relative to the plasma must be accounted for in orbiting systems.

Stone, Nobie H.↗
NASA NTRS1997

Multiangle Observations of Directional Reflectances of Snow Fields

Accurate measurements of snow areas and surface albedo are crucial to advancing our understanding of the global climate system. This is because of the highly reflective nature of snow combined with its large surface coverage (snow can cover up to 40 % of the Earth's land surface during the Northern Hemisphere winter). The reflectance of snow varies with both solar incidence angle and the viewing angle. Visible sensors with different spatial resolutions have been used to infer the snow parameters. Currently, only nadir-viewing directional reflectance data are available from satellite observations. Observations at multiple angles are needed to infer the hemispheric reflectance albedo of snow fields. We propose to study the directional reflectance of snow fields using POLDER data, which contains information from different viewing angles and polarization. POLDER was successfully launched an the ADEOS-1 satellite in August, 1996, however, because POLDER data are not yet available, data from ASAS, a pointable, airborne spectroradiometer, were used in this study. Data collected over Glacier National Park of Montana show strong angular dependence. Preliminary results confirm the anisotropic nature of the snow reflectance. Knowledge of the bi-directional reflectance function(BDRF) of snow -covered surfaces is the key to developing a true albedo model in the future.

Chang, A.T.C.↗
NASA NTRS2000

Ion Transmission to the Focal Plane of the Chandra X-Ray Observatory

The severe degradation in spectrometric performance observed by the AXAF CCD Imaging Spectrometer (ACIS) instrument on-board the Chandra X-ray Observatory (CXO) was associated with a sharp increase in charge transfer inefficiency combined with relatively constant dark current. It was also observed that damage occurred only during passage through the earth's radiation belts, and only when ACIS remained in the focal plane during the passage. Subsequent measurements and analyses support the conjecture that the damaging radiation entered through the Observatory's High-Resolution Mirror Assembly (HRMA) aperture. A mechanism whereby low energy magnetospheric protons and/or heavier ions are scattered through the HRMA and reach the focal plane with just enough energy to stop in the CCD's charge transfer region currently provides a reasonably consistent explanation of all observed phenomena. In this paper, we will describe analyses which support this conclusion. Simulations using standard ion transmission codes were used to generate a bi-directional response function (BDRF) for the mirror surfaces and transmission through the various path elements. The BDRF was convolved with the geometry via ray-trace. Damage estimates using measured proton fluence and ground measurements of ACIS- type CCD damage vs. proton energy will be presented and compared with observed on- orbit damage.

Kolodziejczak, J. J.↗
NASA NTRS2000

Towards Development of Microcalorimeter Arrays of Mo/Au Transition-Edge Sensors with Bismuth Absorbers

We report on our progress towards the development of arrays of X-ray microcalorimeters as candidates for the high resolution x-ray spectrometer on the Constellation-X mission. The microcalorimeter arrays (30 x 30) with appropriate pixel sizes (0.25 mm. x 0.25 mm) and high packing fractions (greater than 96%) are being developed. Each individual pixel has a 10 micron thick Bi X-ray absorber that is shaped like a mushroom to increase the packing fraction, and a Mo/Au proximity effect superconducting transition edge sensor (TES). These are deposited on a 0.25 or 0.5 micron thick silicon nitride membrane with slits to provide a controllable weak thermal link to the sink temperature. Studies are underway to model, test and optimize the TES pixel uniformity, critical current, heat capacity and the membrane thermal conductance in the array structure. Fabrication issues and procedures, and results of our efforts based on these optimizations will be provided.

Tralshawala, Nilesh↗