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  • Other Sources  (17)
  • 1995-1999
  • 1990-1994  (17)
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  • 1991  (17)
  • 1
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    Performance and lifetime assessment of magnetoplasmadynamic arc thruster technology (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: Journal of Propulsion and Power (ISSN 0748-4658); 7; 71-83
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  • 2
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    Second-epoch VLA observations of compact radio sources in NGC 253 (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Results are presented of second-epoch 6-cm VLA observations of the starburst galaxy NGC 253 in early 1989, approximately 18 months after the first observations identified more than 30 compact radio sources in the galaxy. The galaxy was also mapped at 3.6 cm. No new radio sources were found at 6 cm after the 18-month interval. Assuming that at least two-thirds of new type II supernovae would have been detectable radio emitters stronger than 3 mJy, the 95-percent confidence upper limit to the SN rate is 3.0/yr. None of the 13 sources with flux densities above 1 mJy showed significant decreases in strength over 18 months; the strongest source showed an apparent increase of 6.5 percent in flux density, which differs from that of Kronberg and Sramek (1985) for M82. The 3.6 map identified a number of new sources because of the reduction of confusion. The available spectral data show that a few of the resolved sources may be H II regions.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 102; 875-881
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  • 3
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    Perceptual and performance consequences of flight in virtual worlds (1991)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: There are two primary purposes for head-mounted systems in aeronautical settings. One is for helmet-mounted sights and teleoperated (head-slaved) weapons systems. Bennett, Johnson, Perrone, and Phatak (1988) evaluated head tracking performance during passive and controlled flight. In that study, comparisons were also made of head tracking performance in sterile and relatively complex virtual worlds. That study confirmed the robustness of head tracking performance across a wide variety of visual scenes. A second use of virtual world displays is for aircraft control. Aircraft controllability using head-mounted, panel-mounted, or simulated out-the-window scenes has been systematically examined. Those studies reported the range of rotorcraft flight tasks in which head-mounted virtual worlds provided some advantages. Two studies will be reported that examine the perceptual/performance effects of virtual worlds. The first examines head tracking performance with roll-stabilized versus non-roll stabilized virtual worlds. The purpose of the study was to (1) examine display strategies used in current display systems and (2) study the adaptability of observers to estimated glide slope angles using head-slaved versus head-stabilized imagery. The purpose of this study was to examine the usefulness of wide field-of-regards during final approaches to a runway.
    Keywords: MAN/SYSTEM TECHNOLOGY AND LIFE SUPPORT
    Type: Human Machine Interfaces for Teleoperators and Virtual Environments; p 114
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  • 4
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    Flexible fluoropolymer filled protective coatings (1991)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: Metal oxide films such as SiO2 are known to provide an effective barrier to the transport of moisture as well as gaseous species through polymeric films. Such thin film coatings have a tendency to crack upon flexure of the polymeric substrate. Sputter co-deposition of SiO2 with 4 to 15 percent fluoropolymers was demonstrated to produce thin films with glass-like barrier properties that have significant increases in strain to failure over pure glass films which improves their tolerance to flexure on polymeric substrates. Deposition techniques capable of producing these films on polymeric substrates are suitable for durable food packaging and oxidation/corrosion protection applications.
    Keywords: NONMETALLIC MATERIALS
    Type: NASA, Washington, Technology 2000, Volume 2; p 179-184
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  • 5
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    Magellan mission planning and orbital operations (1991)
    In:  CASI
    Details
    Publication Date: 2013-08-29
    Description: NASA's Magellan mission to understand the geological and geophysical processes that have shaped the planet Venus is reviewed. Through synthetic aperture radar, radiometric, altimetric, and gravity measurements, the scientific community hopes to improve its knowledge of the planet's topography and internal mass distribution. The spacecraft sequence software design, development, and test are described and the nominal operational flow of stored sequence design and generation are addressed. The ground processing of engineering telemetry and imaging data are described and the Magellan operational experience with nonroutine behavior of the spacecraft during the cruise and mapping portions of the prime mission is summarized.
    Keywords: ASTRONAUTICS (GENERAL)
    Type: ESA, Spacecraft Flight Dynamics; p 457-464
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  • 6
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    Outline for a theory of intelligence (1991)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: Intelligence is defined as that which produces successful behavior. Intelligence is assumed to result from natural selection. A model is proposed that integrates knowledge from research in both natural and artificial systems. The model consists of a hierarchical system architecture wherein: (1) control bandwidth decreases about an order of magnitude at each higher level, (2) perceptual resolution of spatial and temporal patterns contracts about an order-of-magnitude at each higher level, (3) goals expand in scope and planning horizons expand in space and time about an order-of-magnitude at each higher level, and (4) models of the world and memories of events expand their range in space and time by about an order-of-magnitude at each higher level. At each level, functional modules perform behavior generation (task decomposition planning and execution), world modeling, sensory processing, and value judgment. Sensory feedback control loops are closed at every level.
    Keywords: CYBERNETICS
    Type: IEEE Transactions on Systems, Man, and Cybernetics (ISSN 0018-9472); 21; 473-509
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  • 7
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    Life cycle cost based program decisions (1991)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The following subject areas are covered: background (space propulsion facility assessment team final report); changes (Advanced Launch System, National Aerospace Plane, and space exploration initiative); life cycle cost analysis rationale; and recommendation to panel.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: NASA, Washington, Space Transportation Propulsion Technology Symposium. Volume 3: Panel Session Summaries and Presentations; p 989-1023
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  • 8
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    Ion beam sputtering in electric propulsion facilities (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Experiments were undertaken to determine sputter yields of potential ion beam target materials, to assess the impact of charge exchange on beam diagnostics in large facilities, and to examine material erosion and deposition after a 957-hour test of a 5 kW-class ion thruster. The xenon ion sputter yield of flexible graphite was lower than other graphite forms especialy at high angles of incidence. Ion beam charge exchange effects were found to hamper beam probe current collection diagnostics even at pressures from 0.7 to 1.7 mPa. Estimates of the xenon ion beam envelope were made and predictions of the thickness of sputter deposited coatings in the facility were compared with measurements.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: AIAA PAPER 91-2117
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  • 9
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    Electric propulsion - An evolutionary technology (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The NASA Lewis Research Center conducts and directs an electric propulsion research and technology program aimed at providing high-performance electric propulsion system options for a broad range of near- and far-term missions. This evolutionary program emphasizes the development of propulsion systems for three classes of missions: (1) near-term auxiliary propulsion applications such as North-South Stationkeeping for next generation communications satellites and orbit maintenance for orbiting platforms such as Space Station Freedom; (2) advanced solar electric propulsion and SP-100-class nuclear electric propulsion (NEP) for earth-space orbit transfer and robotic planetary missions; and (3) very high power systems to support major space missions including the Space Exploration Initiative. To cover widely disparate mission requirements, the program includes research on electrothermal, electrostatic, and electromagnetic systems. This paper provides an overview of the program with a focus on recent progress.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: IAF PAPER 91-241
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  • 10
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    Precise tracking of the Magellan and Pioneer Venus orbiters by same-beam interferometry (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Results of a demonstration of same-beam interferometry, the simultaneous tracking of two spacecraft in orbit about a distant planet by widely separated earth-based radio antennas, are presented with regard to the Magellan and Pioneer 12 orbiters at Venus. Signals from both spacecraft fall within the same beamwidth of the tracking antennas. The plane-of-sky position difference between spacecraft is precisely determined by double-differenced phase measurements. This data type complements line-of-sight Doppler and improves orbital accuracy by up to an order of magnitude. The predicted error is dominated by solar plasma fluctuations, and is 15.1 psec for a 5-min integration. The rms of the residuals is less than this by about 25 percent. The shape of the spectrum computed from residuals is consistent with that derived from a model of solar plasma fluctuations. Same-beam interferometry is expected to enhance tracking accuracy and efficiency for proposed multiple missions to Mars.
    Keywords: SPACE COMMUNICATIONS, SPACECRAFT COMMUNICATIONS, COMMAND AND TRACKING
    Type: AAS PAPER 91-191 , In: Spaceflight mechanics 1991; Proceedings of the 1st AAS(AIAA Annual Spaceflight Mechanics Meeting, Houston, TX, Feb. 11-13, 1991. Pt. 2 (A93-17901 05-13); p. 1269-1288.
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