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At least 1,657 records · Page 92
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Process and design considerations for high-efficiency solar cells

This paper shows that oxide surface passivation coupled with optimum multilayer anti-reflective coating can provide approx. 3% (absolute) improvement in solar cell efficiency. Use of single-layer AR coating, without passivation, gives cell efficiencies in the range of 15 to 15.5% on high-quality, 4 ohm-cm as well as 0.1 to 0.2 ohm-cm float-zone silicon. Oxide surface passivation alone raises the cell efficiency to or = 17%. An optimum double-layer AR coating on oxide-passivated cells provides an additional approx. 5 to 10% improvement over a single-layer AR-coated cell, resulting in cell efficiencies in excess of 18%. Experimentally observed improvements are supported by model calculations and an approach to or = 20% efficient cells is discussed.

Rohati, A.↗
NASA NTRS1985

Considerations of large scale impact and the early Earth

Bodies which have preserved portions of their earliest crust indicate that large scale impact cratering was an important process in early surface and upper crustal evolution. Large impact basins form the basic topographic, tectonic, and stratigraphic framework of the Moon and impact was responsible for the characteristics of the second order gravity field and upper crustal seismic properties. The Earth's crustal evolution during the first 800 my of its history is conjectural. The lack of a very early crust may indicate that thermal and mechanical instabilities resulting from intense mantle convection and/or bombardment inhibited crustal preservation. Whatever the case, the potential effects of large scale impact have to be considered in models of early Earth evolution. Preliminary models of the evolution of a large terrestrial impact basin was derived and discussed in detail.

Grieve, R. A. F.↗
NASA NTRS1984

Considerations of and improvements to large-scale vegetation monitoring

The NOAA-7 polar orbiting sun-synchronous operational satellite carries the 5-channel advanced very high-resolution radiometer which acquires data globally at a spatial resolution of 4 km on a daily basis. These data provide a means for frequently monitoring global vegetation at continental scales. Techniques for compositing and cloud screening a green leaf density vegetation index product for Africa are presented for 9 sequential days beginning August 16, 1982 and are compared with a semi-operational vegetation index product produced by NOAA.

Gatlin, J. A.↗
NASA NTRS1984

Layered synthetic microstructure technology considerations for the extreme ultraviolet

It is demonstrated that layered synthetic microstructures (LSMs) can provide mirrors with enhanced normal incidence reflectance in the 300-A spectral region. Experimental results are presented for Pt-Si and Ir-Si structures fabricated by electron-beam deposition at pressures of about 10 to the -6th Torr, and it is shown that the enhancement is realized over a limited bandwidth of 75 A. One potential application of LSMs is their use as coatings on diffraction gratings to enhance normal incidence reflection efficiencies. Preliminary results for the Ir-Si combination on a diffraction grating show that LSMs enhance grating efficiencies in the EUV by a factor of about 3.

Keski-Kuha, R. A. M.↗
NASA NTRS1985

A consideration of atomic oxygen interactions with space station

A computer model is developed in order to calculate fluence for a generalized spacecraft in orbital flight based on mass spectrometer and incoherent scatter (MSIS) measurements of ambient density. The calculations are used to identify criteria for the selection of materials for the construction of an LEO Space Station. The model is generalized such that the surfaces for study can be oriented in any direction with respect to: a body coordinate system fixed to the spacecraft (E surfaces); and a solar inertial coordinate system. A parametric study was performed in order to evaluate the effects of altitude, inclination, and solar activity on atomic oxygen fluence. Values are obtained for samples of Mylar, Kevlar, Kapton, Tedlar, Teflon and sulfone, as well as for some ethylene polymers. The range of values for material reactivities is from 360 microns for ram-facing surfaces to 200 microns for solar power surfaces during each cycle of solar activity. The possibility of improving reactivity by employing certain chemical coatings is also discussed.

Leger, L. J.↗
NASA NTRS1984

Design and technology considerations for the far ultraviolet spectroscopic explorer (FUSE) telescope

An initial assessment of telescope designs satisfying the basic requirements derived for the FUSE mission is presented. A review of recent optical coating technology indicates that normal incidence telescope designs are viable only for wavelengths greater than about 600 A. It is noted that glancing incidence telescope designs, such as the Wolter Type II or thg Wolter-Schwarzschild, are the only designs with acceptable throughputs for the entire FUSE spectral range. Initial design trade studies have established the fundamental framework for the selection of an optimum Type II design satisfying the FUSE science and engineering requirements. It is also found that current optical fabrication technology, e.g., computer controlled polishing, can accommodate meter class glancing incidence components with figure errors commensurate with 1-2 arcsec imaging.

Davila, P. S. M.↗
NASA NTRS1983

Spectrum management considerations of adaptive power control in satellite networks

Adaptive power control concepts for the compensation of rain attenuation are considered for uplinks and downlinks. The performance of example power-controlled and fixed-EIRP uplinks is compared in terms of C/Ns and C/Is. Provisional conclusions are drawn with regard to the efficacy of uplink and downlink power control orbit/spectrum utilization efficiency.

Sawitz, P.↗
NASA NTRS1984

Space Station active thermal control technical considerations

A description of recent and planned thermal control technology developments at the Johnson Space Center and the other NASA Centers in support of Space Stations is presented. The program is centered around satisfying the needs of the users. Preliminary results of proof-of-concept high capacity heat pipes and two-phase devices are included which indicate that large amounts of energy (100 kW) can be transported long distances (50 m) with very small temperature differences. The presentation summarizes preparations for an 'evolutionary test bed' for advanced development of thermal technology which will provide data on components and systems for incorporation into the Space Station designs in the late 1980s. The results of the recently flown Heat Pipe Experiment aboard STS-8 are presented.

Ellis, W. E.↗
NASA NTRS1985

The magnetic field of the earth - Performance considerations for space-based observing systems

Basic problems inherent in carrying out observations of the earth magnetic field from space are reviewed. It is shown that while useful observations of the core and crustal fields are possible at the peak of the solar cycle, the greatest useful data volume is obtained during solar minimum. During the last three solar cycles, the proportion of data with a planetary disturbance index of less than 2 at solar maximum was in the range 0.4-0.8 in comparison with solar minimum. It is found that current state of the art orbit determination techniques should eliminate orbit error as a problem in gravitational field measurements from space. The spatial resolution obtained for crustal field anomalies during the major satellite observation programs of the last 30 years are compared in a table. The relationship between observing altitude and the spatial resolution of magnetic field structures is discussed. Reference is made to data obtained using the Magsat, the Polar Orbiting Geophysical Observatory (POGO), and instruments on board the Space Shuttle.

Webster, W. J., Jr.↗
NASA NTRS1984

Design considerations for a 10-kW integrated hydrogen-oxygen regenerative fuel cell system

Integration of an alkaline fuel cell subsystem with an alkaline electrolysis subsystem to form a regenerative fuel cell (RFC) system for low earth orbit (LEO) applications characterized by relatively high overall round trip electrical efficiency, long life, and high reliability is possible with present state of the art technology. A hypothetical 10 kW system computer modeled and studied based on data from ongoing contractual efforts in both the alkaline fuel cell and alkaline water electrolysis areas. The alkaline fuel cell technology is under development utilizing advanced cell components and standard Shuttle Orbiter system hardware. The alkaline electrolysis technology uses a static water vapor feed technique and scaled up cell hardware is developed. The computer aided study of the performance, operating, and design parameters of the hypothetical system is addressed.

Hoberecht, M. A.↗
NASA NTRS1985

An experimental study of energy loss mechanisms and efficiency consideration in the low power dc arcjet

The potential utility of the low power dc arcjet in auxiliary propulsion was investigated. It was indicated that improvements in the areas of stability, energy efficiency, reliability, and electrode erosion are necessary to obtain a useful device. A water-cooled arcjet simulator was tested to investigate both the energy loss mechanisms at the electrodes and the stability of different conventional arcjet configurations in the presence of a vortex flow field. It is shown that in certain configurations only 25 to 30% of the input energy is lost to the electrodes. It is also shown that vortex stabilization is not difficult to obtain in many cases at the flow rates used and that a careful starting procedure is effective in minimizing electrode damage.

Curran, F. M.↗
NASA NTRS1985

Some key considerations in evolving a computer system and software engineering support environment for the space station program

The space station data management system involves networks of computing resources that must work cooperatively and reliably over an indefinite life span. This program requires a long schedule of modular growth and an even longer period of maintenance and operation. The development and operation of space station computing resources will involve a spectrum of systems and software life cycle activities distributed across a variety of hosts, an integration, verification, and validation host with test bed, and distributed targets. The requirement for the early establishment and use of an apporopriate Computer Systems and Software Engineering Support Environment is identified. This environment will support the Research and Development Productivity challenges presented by the space station computing system.

Mckay, C. W.↗
NASA NTRS1985

Aeroelastic considerations for torsionally soft rotors

A research study was initiated to systematically determine the impact of selected blade tip geometric parameters on conformable rotor performance and loads characteristics. The model articulated rotors included baseline and torsionally soft blades with interchangeable tips. Seven blade tip designs were evaluated on the baseline rotor and six tip designs were tested on the torsionally soft blades. The designs incorporated a systemmatic variation in geometric parameters including sweep, taper, and anhedral. The rotors were evaluated in the NASA Langley Transonic Dynamics Tunnel at several advance ratios, lift and propulsive force values, and tip Mach numbers. A track sensitivity study was also conducted at several advance ratios for both rotors. Based on the test results, tip parameter variations generated significant rotor performance and loads differences for both baseline and torsionally soft blades.

Mantay, W. R.↗
NASA NTRS1985

Consideration of radar target glint from ST during OMV rendezvous

The nature of radar target glint and the factors upon which it depends when using the Hubble Space Telescope as a radar target is discussed. An analysis of the glint problem using a 35 MHz or 94 MHz radar on the orbital maneuvering vehicle is explored. A strategy for overcoming glint is suggested.

Mcdonald, M. W.↗
NASA NTRS1985

Operational considerations in monitoring oxygen levels at the National Transonic Facility

Laboratory monitoring of the level of oxygen in sample gas mixtures is a process which can be performed with accurate and repeatable results. Operations at the National Transonic Facility require the storage and pumping of large volumes of liquid nitrogen. To protect against the possibility of a fault resulting in a localized oxygen deficient atmosphere, the facility is equipped with a monitoring system with an array of sensors. During the early operational stages, the system produced recurrent alarms, none of which could be traced to a true oxygen deficiency. A thorough analysis of the system was undertaken with primary emphasis placed on the sensor units. These units sense the partial pressure of oxygen which, after signal conditioning, is presented as a % by volume indication at the system output. It was determined that many of the problems experienced were due to a lack of proper accounting for the partial pressure/% by volume relationship, with a secondary cause being premature sensor failure. Procedures were established to consider atmospherically induced partial pressure variations. Sensor rebuilding techniques were examined, and those elements contributing to premature sensor failure were identified. The system now operates with a high degree of confidence and reliability.

Zalenski, M. A.↗