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At least 883 records · Page 49

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NASA NTRS1982

Advanced Czochralski silicon growth technology for photovoltaic modules

Several economic analyses had indicated that large-diameter, multiple ingot growth using a single crucible with melt replenishment would be required for Cz growth to be economically viable. Based on the results of these analyses, two liquid and two solid feed melt replenishment approaches were initiated. The sequential solid feed melt replenishment approach, which demonstrated elements of technical feasibility is described in detail in this paper. Growth results of multiple ingots (10-cm-diameter, totaling 100 kg; and 15-cm-diameter, totaling 150 kg weight per crucible) are presented. Solar cells were fabricated and analyzed to evaluate the effects of structure and chemical purities as a result of multiple growth. The results indicate that, with semiconductor-grade silicon, feedstock impurity build-up does not seem to degrade cell performance. For polycrystalline cells, the average efficiencies are 15 to 25% lower than those of single crystalline cells. Concerns regarding single crystal yields, crucible quality and growth speed are indicated, and present status and future research thrusts are also discussed.

Daud, T.↗

NASA NTRS1982

LOX/hydrocarbon auxiliary propulsion system study

Liquid oxygen (LOX)/hydrocarbon propulsion concepts for a "second generation' orbiter auxiliary propulsion system was evaluated. The most attractive fuel and system design approach identified, and the technology advancements that are needed to provide high confidence for a subsequent system development were determined. The fuel candidates were ethanol, methane, propane, and ammonia. Even though ammonia is not a hydrocarbon, it was included for evaluation because it is clean burning and has a good technology base. The major system design options were pump versus pressure feed, cryogenic versus ambient temperature RCS propellant feed, and the degree of OMS-RCS integration. Ethanol was determined to be the best fuel candidate. It is an earth-storable fuel with a vapor pressure slightly higher than monomethyl hydrazine. A pump-fed OMS was recommended because of its high specific impulse, enabling greater velocity change and greater payload capability than a pressure fed system.

Orton, G. F.↗

NASA NTRS1983

Multimission advanced configuration

The proposed multimission configuration reduces the number of reflectors present on the spacecraft and their related supporting structure (masts, booms) and at the same time divides the feed system from the reflector system for possible maintenance and/or payload substitution. This implies: An antenna system that is split into two parts: (1) the reflector with the platform (2) the feed subsystem with the payload. Reuse of the same reflector at different frequencies (to achieve this, the reflector system will use additional components, i.e., frequency sensitive subreflectors (FSS)). In case of in-orbit refurbishment or maintenance, the reflector system may remain in orbit with the platform. The presence of the FSS allows separation of the focal position sufficiently for the multiple-contoured beam applications required by future missions. There are two of these reflectors (one for TX function, one for RV deployed) on the east and west sides of the spacecraft, and the solar panels are on the north and south sides. The payloads are aligned along the yaw axis. Apart from the size, this configuration looks like a conventional spacecraft.

Saitto, A.↗

NASA NTRS1983

Phased array-fed antenna configuration study

The scope of this contract entails a configuration study for a phased array fed transmit antenna operating in the frequency band of 17.7 to 20.2 GHz. This initial contract provides a basis for understanding the design limitations and advantages of advanced phased array and cluster feeds (both utilizing intergral MMIC modules) illuminating folded reflector optics (both near field and focused types). Design parametric analyses are performed utilizing as constraints the objective secondary performance requirements of the Advanced Communications Technology Satellite (Table 1.0). The output of the study provides design information which serves as a data base for future active phased array fed antenna studies such as detailed designs required to support the development of a ground tested breadboard. In general, this study is significant because it provides the antenna community with an understanding of the basic principles which govern near field phased scanned feed effects on secondary reflector system performance. Although several articles have been written on analysis procedures and results for these systems, the authors of this report have observed phenomenon of near field antenna systems not previously documented. Because the physical justification for the exhibited performance is provided herein, the findings of this study add a new dimension to the available knowledge of the subject matter.

Crosswell, W. F.↗

NASA NTRS1982

The 15-meter diameter mechanically scanned deployable antenna

A preliminary design with structural model data and thermal-performance estimates of a 15-meter mechanically scanned deployable antenna (MSDA) that could be launched onboard a Shuttle Orbiter to provide radiometric brightness temperature maps of the Earth and oceans in selected bands over a frequency range from 1.4 to 11 GHz is provided. The study objectives were met through the design of a unique, integrated, offset feed mast and reflector design that uses the deployable box-truss structure as a building block. The performance of this system is summarized. The all graphite-epoxy, 4.57-meter prototype cube that was completed in 1981 and is proposed for this reflector and feed mast design is presented.

Coyner, J. V.↗

NASA NTRS1981

LAMMR: A new generation satellite microwave radiometer - Its concepts and capabilities

Definition studies and baseline design are summarized for the proposed, and now discontinued, LAMMR. The instrument is an offset parabolic reflector with Cassegrain feeds. The three-meter aperture reflector, to be constructed using graphite-epoxy technology, rotates continuously at 0.833 rps. The scan drive subsystem includes momentum compensation for the rotating mass which includes the reflector, the support arm and Cassegrain subreflector, feed horns and radiometer. Two total power radiometers are recommended for each frequency, one each for horizontal and vertical polarizations. The selection plan, definition study specifications, LAMMR performance specifications, and predicted accuracies and resolutions after processing are shown.

Walton, W. T.↗

NASA NTRS1983

Interactive analysis of a large aperture Earth Observations Satellite

A system level design and analysis has been conducted on an Earth Observation Satellite (EOS) system using the Interactive Design and Evaluation of Advanced Spacecraft (IDEAS) computer-aided design and analysis program. The IDEAS program consists of about 40 user-friendly technical modules and an interactive graphics display. The reflector support system and feed mast of the EOS spacecraft are constructed with a box-truss structural concept, a lattice configuration which can be packaged for delivery in a single Shuttle flight and deployed in orbit. The deployed spacecraft consists of a 120-m by 60-m parabolic reflector, a 120-m-long support structure, and a 30-m feed arc beam on the focal axis. The spacecraft was modeled for structural, thermal, and control systems analysis and structural elements were designed. On-orbit dynamic and thermal loading analyses were conducted and spacecraft weights were determined.

Wright, R. L.↗

NASA NTRS1983

Beam forming networks for mm-wave satellite communications

Technology features of a beam forming network (BFN) employing ferrite devices to provide multiple beam antenna pattern control for satellites used in telecommunications are described. The BFN produces the phase and amplitude distribution for each horn in an antenna array, with the number of horns in the array being equal to the number of outputs in the BFN. One configuration involves microwave switches and permits illumination of a single feed horn at a time using ferrite latching circulators that function by reversing the circulation direction. A more flexible version, yielding a variable amplitude distribution across the feed horn array to accommodate changing traffic patterns or serving a TDMA system, includes the capability of forming nulls in the system with a variable phase shifter in the input ports. The antenna scan angles in phased arrays can be limited to 8 deg from center. Acceptable insertion losses have been demonstrated in BFN with hundreds of ports and switching rates as high as 10 kHz.

Sharon, T. E.↗

NASA NTRS1985

Ellipsoidal Balloon Antenna

Direct-broadcast ellipsoidal antenna 40 percent smaller than spherical reflector. Balloon antenna has feedpoint on surface of ellipsoid outside balloon, simplifying balloon deployment and feed support. Dielectric shrouds used locally to adjust shape of balloon and maintain feed distance at its optimum value.

Cramer, P. W.↗

NASA NTRS1983

Constant-output atomizer

A constant-output atomizer includes a body which has a generally frustoconical expansion nozzle for producing an air jet when a supply of pressurized air is connected to the nozzle upstream of the throat of the nozzle. A liquid feed line supplies liquid to be atomized by the air jet, and the body includes a groove which opens into the diffuser section of the nozzle downstream of the throat for conducting liquid from the feed line to the nozzle. The groove which extends in a direction perpendicular to the axis of the nozzle, and radially with respect to it, has a depth approximately equal to half the axial length of the nozzle. Liquid, conducted by capillary action in the groove to the nozzle, is atomized into a fine mist by the air jet in the nozzle; and the groove eliminates fluctuations in spray order.

Dea, J. Y.↗

NASA NTRS1984

Powder fed sheared dispersal particle generator

A particle generating system is described which is capable of breaking up agglomerations of particles and producing a cloud of uniform, submicron-sized particles at high pressure and high flow rates. This is achieved by utilizing a tubular structure which has injection microslits on is periphery to accept and disperse the desired particle feed. By suppling a carrying fluid at a pressure, of approximately twice the ambient pressure of the velocimeter's settling chamber, the microslits operate at choked flow conditions. The shearing action of this choked flow is sufficient to overcome interparticle bonding forces, thereby breaking up the agglomerates of the particles feed into individual particles.

Morrisette, E. L.↗

NASA NTRS1984

Construction concept for erecting an offset-fed antenna

A design concept for the construction of an offset-fed antenna is discussed. Antennas of this type are of interest for space applications because the configuration eliminates the effects of feed and feed-support blockage. The proposed construction concept is developed around the assembly of a stiff truss substructure by pressure-suited astronauts operating in extravehicular activity (EVA) assisted by a mobile platform that moves along the structure to position the astronauts at joint locations where they can latch members in place. Construction can be accomplished from the shuttle cargo bay in the course of a normal flight or from a space station platform. The concepts demonstrates the versatility of machine assisted manned assembly and is only one of many potential applications.

Rhodes, M. D.↗

NASA NTRS1984

Secondary pattern computation of an arbitrarily shaped main reflector

The secondary pattern of a perfectly conducting offset main reflector being illuminated by a point feed at an arbitrary location was studied. The method of analysis is based upon the application of the Fast Fourier Transform (FFT) to the aperture fields obtained using geometrical optics (GO) and geometrical theory of diffraction (GTD). Key features of the reflector surface is completely arbitrary, the incident field from the feed is most general with arbitrary polarization and location, and the edge diffraction is calculated by either UAT or by UTD. Comparison of this technique for an offset parabolic reflector with the Jacobi-Bessel and Fourier-Bessel techniques shows good agreement. Near field, far field, and scan data of a large reflector are presented.

Lam, P. T. C.↗

NASA NTRS1984

Porosity and environment

Significant progress was achieved when it was realized that porosity could be analyzed successfully by considering not only heat flow, but also fluid flow within the solidifying casting. Sound castings may be produced by lowering pressure during melting (to allow dissolved gas to escape the melt) and increasing pressure during solidification (to force liquid metal into the mushy zone to feed shrinkage). Such techniques are especially effective if they are combined with chilling of parts of the casting to produce progressive solidification, which shortens the mushy zone and, hence, the distance that metal must travel to feed porosity.

Thomas S. Piwonka↗

NASA NTRS1983

Hyperfiltration wash water recovery subsystem - Design and test results

The Hyperfiltration Wash Water Recovery (HWWR) subsystem, designed to offer low-power high-volume wash water purification for extended mission spacecraft, is discussed in terms of preprototype design and configuration. Heated wash water collected from the shower, hand wash, and laundry flows into a temperature-controlled (374 K) waste storage tank. Two parallel 25 micron absolute filters at the tank outlet remove large particles from the feed stream. A positive displacement feed pump delivers wash water to the hyperfiltration module at a constant flow rate of 0.20 lpm with discharge pressure variations from 4181-7239 Kpa. The hyperfiltration membrane module is a single-pass design including 36 porous stainless steel tubes, and is designed to provide an approximate water recovery rate of 90 percent. Permeate and brine water flows are monitored by flow meters, and removal of urea and ammonia is achieved by adding 15 percent NaOCl solution to the permeate fluid stream. An alternate module design using two diameters of tubing (allowing a smaller pressure drop and a larger membrane area) gave a superior predicted performance over the first module with larger tubing throughout.

Reysa, R. P.↗

NASA NTRS1984

A comparison between GO/aperture-field and physical-optics methods for offset reflectors

Both geometrical optics (GO)/aperture-field and physical-optics (PO) methods are used extensively in the diffraction analysis of offset parabolic and dual reflectors. An analytical/numerical comparative study is performed to demonstrate the limitations of the GO/aperture-field method for accurately predicting the sidelobe and null positions and levels. In particular, it is shown that for offset parabolic reflectors and for feeds located at the focal point, the predicted far-field patterns (amplitude) by the GO/aperture-field method will always be symmetric even in the offset plane. This, of course, is inaccurate for the general case and it is shown that the physical-optics method can result in asymmetric patterns for cases in which the feed is located at the focal point. Representative numerical data are presented and a comparison is made with available measured data.

Rahmat-Samii, Y.↗

NASA NTRS1982

A decoupling approach to the control of large spaceborne antenna systems

A simple practical method for designing antenna-feed positioning control systems for large deployable spaceborne antenna systems with flexible booms is proposed. The approach is based on the mechanical decoupling of the antenna-feed from the boom so that the positioning control system can be designed without taking boom dynamics into consideration, thus avoiding a complex infinite dimensional control problem. The basic idea is illustrated by a simple angular positional control system attached to a flexible boom restricted to torsional motion only. The application of this approach to more complex situations is discussed briefly.

Wang, P. K. C.↗

NASA NTRS1984

The study of microstrip antenna arrays and related problems

The physical layout of the array elements and the proximity of the microstrip feed network makes the input impedance and radiation pattern values dependent upon the effects of mutual coupling, feedline discontinuities and feed point location. The extent of these dependences was assessed and a number of single patch and module structures were constructed and measured at an operating frequency of approximately 4.0 GHz. The empirical results were compared with the ones which were theoretically predicted by the cavity model of thin microstrip antennas. Each element was modelled as an independent radiating patch and each microstrip feedline as an independent, quasi-TEM transmission line. The effects of the feedline discontinuities are approximated by lumped L-C circuit models.

Lo, Y. T.↗