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At least 1,081 records · Page 60

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NASA NTRS1991

Energy stability of thermocapillary convection in a model of the float-zone crystal-growth process. II - Nonaxisymmetric disturbances

Energy-stability theory has been applied to investigate the stability properties of thermocapillary convection in a half-zone model of the float-zone crystal-growth process. An earlier axisymmetric model has been extended to permit nonaxisymmetric disturbances, thus determining sufficient conditions for stability to disturbances of arbitrary amplitude. The results for nonaxisymmetric disturbances are compared with earlier axisymmetric results, with linear-stability results for a geometry with an infinitely long aspect ratio and with stability boundaries from recent laboratory experiments.

Neitzel, G. P.↗

NASA NTRS1991

Evaluation of perspective displays on pilot spatial awareness in low visibility curved approaches

A series of part-task laboratory experiments are planned to study whether the Tunnel-in-the-Sky perspective display can provide a significant improvement in pilot spatial awareness over conventional flight deck displays. Pilot subjects will fly a series of curved approaches to landing using a computer-generated tunnel overlaid on a terrain database. Subjects' primary task will be to look at (localize) various targets in the simulated environment by rotating the head so that the direction indicated is along the line-of-sight as seen through the windshield. Comparisons will be made with subjects performing the same tasks, but using conventional glass cockpit instrumentation.

Dorighi, Nancy S.↗

NASA NTRS1991

Space dust and debris; Proceedings of the Topical Meeting of the Interdisciplinary Scientific Commission B (Meetings B2, B3, and B5) of the COSPAR 28th Plenary Meeting, The Hague, Netherlands, June 25-July 6, 1990

The present conference on space dust and debris encompasses orbital debris, in situ measurements and laboratory analysis of space-dust particles, comparative studies of comets, asteroids, and dust, the protection and maneuvering of spacecraft in space-debris environments, and the out-of-elliptic distribution of interplanetary dust derived from near-earth flux. Specific issues addressed include asteroid taxonomy, the optical properties of dust from cometary and interplanetary grains, light scattering by rough surfaces on asteroidal/lunar regoliths, and the first results of particulate impacts and foil perforations on the Long Duration Exposure Facility. Also addressed are collision probability and spacecraft disposition in the geostationary orbit, a flash on the moon caused by orbital debris, the limits of population growth in low earth orbit due to collisional cascading, and the simulation of cosmic man-made dust effects on space-vehicle elements in rocket and laboratory experiments.

Kessler, D. J.↗

NASA NTRS1992

Further analyses of human kidney cell populations separated on the Space Shuttle

Cultured human embryonic kidney cells were separated into electrophoretic subpopulations in laboratory experiments and in two separation experiments on the STS-8 (Challenger) Space Shuttle flight using the mid-deck Continuous Flow Electrophoretic Separator (CFES). Populations of cells from each fraction were cultured for the lifetime of the cells, and supernatant medium was withdrawn and replaced at 4-day intervals. Withdrawn medium was frozen at -120 C for subsequent analysis. Enzyme assays, antibodies and gel electrophoresis were used as analytical tools for the detection and quantization of plasminogen activators in these samples. These assays of frozen-culture supernatant fluids confirmed the electrophoretic separation of plasminogen-activator-producing cells from nonproducing cells, the isolation of cells capable of sustained production, and the separation of cells that produce different plasminogen activators from one other.

Stewart, Robin M.↗

NASA NTRS1992

Recent grazing impacts on the earth recorded in the Rio Cuarto crater field, Argentina

An anomalous alignment of oblong rimmed depressions has been observed on the otherwise featureless farmland of the Argentine Pampas. It is argued here, from sample analysis and by analogy with laboratory experiments, that the structure resulted from a low-angle impact and ricochet of a chondritic body originally 150-300 m in diameter.

Schultz, Peter H.↗

NASA NTRS1991

A comparison of controller designs for an experimental flexible structure

Control systems design and hardware testing are addressed for an experimental structure that displays the characteristics of a typical flexible spacecraft. The results of designing and implementing various control design methodologies are described. The design methodologies under investigation include linear quadratic Gaussian control, static and dynamic dissipative controls, and H-infinity optimal control. Among the three controllers considered, it is shown, through computer simulation and laboratory experiments on the evolutionary structure, that the dynamic dissipative controller gave the best results in terms of vibration suppression and robustness with respect to modeling errors.

Lim, K. B.↗

NASA NTRS1992

Nonlinear control via approximate input-output linearization - The ball and beam example

A study is made of approximate input-output linearization of nonlinear systems which fail to have a well defined relative degree. For such systems, a method is provided for constructing approximate systems that are input-output linearizable. The analysis presented in this note is motivated through its application to a common undergraduate control laboratory experiment, the ball and beam system, where it is shown to be more effective for trajectory tracking than the standard Jacobian linearization.

Hauser, John↗

NASA NTRS1991

Microgravity and the lung

Results are presented from studies of the effect of microgravity on the lungs of rats flown on the Cosmos 2044 mission, and from relevant laboratory experiments. The effects of microgravity fall into five categories: topographical structure and function, the lung volumes and mechanics, the intrathoracic blood pressures and volumes, the pulmonary deposition of aerosol, and denitrogenaton during EVA. The ultrastructure of the left lungs of rats flown for 14 days on the Cosmos 2044 spacecraft and that of some tail-suspended rats disclosed presence of red blood cells in the alveolar spaces, indicating that pulmonary hemorrhage and pulmonary edema occurred in these rats. Possible causes for this phenomenon are discussed.

West, John B.↗

NASA NTRS1992

Electrode performance parameters for a radioisotope-powered AMTEC for space power applications

The alkali metal thermoelastic converter (AMTEC) is a device for the direct conversion of heat to electricity. Recently a design of an AMTEC using a radioisotope heat source was described, but the optimum condenser temperature was hotter than the temperatures used in the laboratory to develop the electrode performance model. Now laboratory experiments have confirmed the dependence of two model parameters over a broader range of condenser and electrode temperatures for two candidate electrode compositions. One parameter, the electrochemical exchange current density at the reaction interface, is independent of the condenser temperature, and depends only upon the collision rate of sodium at the reaction zone. The second parameter, a morphological parameter, which measures the mass transport resistance through the electrode, is independent of condenser and electrode temperatures for molybdenum electrodes. For rhodium-tungsten electrodes, however, this parameter increases for decreasing electrode temperature, indicating an activated mass transport mechanism such as surface diffusion.

Underwood, M. L.↗

NASA NTRS1990

Nonlinear slew maneuver dynamics of large flexible spacecrafts

In this paper, the dynamics of three-dimensional, large-angle arbitrary slew maneuvers of a large flexible spacecraft are developed. The dynamical equations obtained allow maneuver specifications about any axis and are highly nonlinear. They also include coupling between the rigid orbiter and the flexible appendage and correction for motion stiffness. A decentralized control scheme is utilized for performing the maneuver of the rigidized body and for vibration suppression of the flexible appendage. The method developed in this paper is further applied to NASA Spacecraft Control Laboratory Experiment (SCOLE) test facility.

Kakad, Y. P.↗

NASA NTRS1991

Collisional effects in laser detection of tropospheric OH

The use of laser-induced fluorescence to measure OH in the troposphere is discussed. It is noted that quantitative laser detection of OH requires knowledge of collisional processes affecting the measurements and interference effects due to OH generated by laser photolytic processes. Collisions induce quenching of the excited A-state, as well as vibrational and rotational energy transfer in the excited and ground states. It is noted that the state-specific nature of these processes is important, especially the rotational level dependence of quenching and vibrational transfer in excited OH. Also important is the temperature dependence according to which quenching cross sections sharply increase with decreasing temperature. Laboratory experiments studying these collisional processes are described, and their implications for laser detection of tropospheric OH are examined.

Crosley, David R.↗

NASA NTRS1984

SCOLE dynamic analysis and frequency response using DISTEL software

The distributed element dynamic analysis package DISTEL is used to analyze the NASA/Institute of Electrical and Electronics Engineers' Spacecraft Control Laboratory Experiment (SCOLE). In this configuration, the Space Shuttle motion is coupled to the motion of a large dish antenna through a Shuttle-deployed flexible mast of 40 m long. Due to the high asymmetry of the system, the motions about the different axes (roll, pitch, yaw) are severely coupled. A general purpose software like DISTEL is especially suited for this kind of analysis. Modal frequencies of the complete spacecraft and impulse response (modal gains) to excitations at different locations are obtained. Mode-shape plots of the deformations of the entire system are given. Finally, results obtained at NASA and at Purdue University are compared to those found by the European space technology center, ESTEC.

Poelaert, D.↗

NASA NTRS1984

A mathematical formulation for the control of the SCOLE configuration

A mathematical formulation for the control of the Spacecraft Control Laboratory Experiment (SCOLE) configuration is given. Two equivalent approaches, one using a functional equation and the other an abstract wave equation, are illustrated. Such a formulation can help in digital computer simulation to evaluate control laws, providing insight, and generating control laws.

Balakrishnan, A. V.↗

NASA NTRS1984

Preliminary results of the dynamics and control of the SCOLE system

The Spacecraft Control Laboratory Experiment (SCOLE) system is assumed to be comprised of three main parts: the Space Shuttle Orbiter with its center of mass located at 0(sub 1); the mast, treated as a 130 ft long beam, connected to the Shuttle at 0(sub 2) and to the reflector at 0(sub 3); and the reflector, considered to be a flat plate with its center of mass at 0(sub 4). In the analysis given here, the angular momentum of the entire system is evaluated at point 0(sub 1) and the dynamics include the lateral displacements of the beam.

Bainum, Peter M.↗

NASA NTRS1984

Rapid torque-bounded line-of-sight pointing of SCOLE with a designer's choice of initial alignment

Researchers simplified the analytical expression of the line of sight (LOS) error, discovered and proved the independence of Euler angle Psi, calculated attitude angles corresponding to 0 degrees and 20 degrees LOS errors, determined choices of initial alignment, tailored the slew maneuvers for LOS pointing, simulated numerically the LOS pointing slew of the Spacecraft Control Laboratory Experiment (SCOLE), and evaluated the pointing performance.

Lin, Jiguan Gene↗

NASA NTRS1984

Rapid single-axis torque-bounded slewing of SCOLE configuration

The first general research objective was to address control design challenges of the Spacecraft Control Laboratory Experiment (SCOLE) via the two stage approach: (1) slew the whole as if it were a rigid body about one Space Shuttle body axis each time using the onboard Reaction Control System (RCS) thrusters; and (2) damp out excited vibrations. The second objective was to examine the feasibility of applying the approach to shuttle-attached flexible space structures. The following was accomplished: (1) a standard bang-bang control technique was adapted; (2) a slew rate limit was imposed in the design; and (3) slew acceleration deviation was defined as the index of slew performance degradation.

Lin, Jiguan Gene↗

NASA NTRS1984

Minimum time slew of the SCOLE configuration

The motivation was to develop a control design and analysis methodology directly applicable to design concepts of flexible spacecraft of interest the the U.S. Navy and to provide a testbed for the evaluation of large space structure control techniques developed at the Naval Research Laboratory. The topics covered include the following: (1) a list of key concepts; (2) evaluation of the Spacecraft Control Laboratory Experiment (SCOLE) model with DISCOS; (3) baseline results, line of sight error vs. time; (4) general formulation of optimization; (5) geometric interpretation, projected eigenaxis; (6) closed loop control law; and (7) future directions.

Fisher, S.↗

NASA NTRS1992

Impact-generated winds on Venus: Causes and effects

The pressure of the dense atmosphere of Venus significantly changes the appearance of ejecta deposits relative to craters on the Moon and Mercury. Conversely, specific styles and sequences of ejecta emplacement can be inferred to represent different intensities of atmospheric response winds acting over different timescales. Three characteristic timescales can be inferred from the geologic record: surface scouring and impactor-controlled (angle and direction) initiation of the long fluidized run-out flows; nonballistic emplacement of inner, radar-bright ejecta facies and radar-dark outer facies; and very late reworking of surface materials. These three timescales roughly correspond to processes observed in laboratory experiments that can be scaled to conditions on Venus (with appropriate assumptions): coupling between the atmosphere and earlytime vapor/melt (target and impactor) that produces an intense shock that subsequently evolves into blast/response winds; less energetic dynamic response of the atmosphere to the outward-moving ballistic ejecta curtain that generates nonthermal turbulent eddies; and late recovery of the atmosphere to impact-generated thermal and pressure gradients expressed as low-energy but long-lived winds. These different timescales and processes can be viewed as the atmosphere equivalent of shock melting, material motion, and far-field seismic response in the target. The three processes (early Processes, Atmospheric Processes, and Late Recovery Winds) are discussed at length.

Schultz, Pater H.↗