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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1970

The far field diffraction pattern for corner reflectors with complex reflection coefficients

The far field diffraction pattern of a geometrically perfect corner reflector is examined analytically for normally incident monochromatic light. The states of polarization and the complex amplitudes of the emerging light are expressed through transformation matrices in terms of those of the original incident light for each sextant of the face in a single coordinate system. The analytic expression of the total diffraction pattern is obtained for a circular face. This expression consists of three component functions in addition to the basic Airy function. The coefficient of each function is expressed in terms of complex coefficients of reflectance of the reflecting surface. Some numerical results for different reflecting surfaces, including total internal reflection, are presented. The iso-intensity contours of the diffraction pattern evaluated from the analytical expressions for an uncoated solid corner reflector are also presented along with the photographs of the pattern.

Chang, R. F.↗
NASA NTRS1978

Synthesis of a laterally displaced cluster feed for a reflector antenna with application to multiple beams and contoured patterns

Two methods are described for efficiently synthesizing the excitation coefficients of a laterally displaced cluster feed in a reflector antenna subject to beam distortion. Applications are presented for rotationally symmetric paraboloids excited by an equilateral triangular array of feed elements. The basic cluster is a central element surrounded by a hexagonal ring. The first method - termed the sequential current method - determines a set of excitation coefficients which minimizes the phase distortion in the 'effective' aperture distribution of the reflector. The second method - termed the gradient optimization method - is such that the distortion in the secondary power pattern is directly minimized in a min-L2 form by a gradient optimization algorithm regarded as a systematic computer iteration procedure. Application to the synthesis of contour patterns is included.

Galindo-Israel, V.↗
NASA NTRS1978

A fixed tilt solar collector employing reversible vee-through reflectors and evaluated tube receivers for solar heating and cooling systems

The Vee-Trough/Evacuated Tube Collector (VTETC) was analyzed rigorously and various mathematical models were developed to calculate the optical performance of the vee-trough concentrators, and the quasi-steady state thermal performance of the evacuated tube receivers. Tests were run to verify the mathematical analyses. Back-silvered glass mirror, Alzak, Aluminized Teflon, and Kinglux (electropolished aluminum reflectors) were tested. Additional tests were run at temperatures ranging from 80 to 190 C (176-374 F). For the glass mirror reflectors, peak efficiencies, based on aperture area and operating temperatures of 125 C (257 F), were over 40%. Efficiencies of about 40% were observed at temperatures of 150 C (302 F) and 30% at 175 C (347 F). Test data for several days, predicted daily useful heats, and efficiency values are presented for a full year. These theoretical values were then compared with actual data points for the same temperature range.

Selcuk, M. K.↗
NASA NTRS1979

Aperture amplitude and phase control of offset dual reflectors

An approximate synthesis method is presented for solving the problem of offset dual-shaped reflectors. It is shown that the equations obtained by this method are partial differential equations that can be integrated numerically, except that these equations are generally not total and therefore do not have a 'smooth' solution. However, it is demonstrated that these equations form a nearly total differential that enables one to generate a smooth subreflector by integrating the differential equations and then synthesizing a main reflector which gives an exact solution for the specified aperture phase distribution. Several important illustrative examples are treated, which validate the adequacy of the proposed method.

Galindo-Israel, V.↗
NASA NTRS1979

Surface accuracy measurement system deployable reflector antennas

Conceptual optical sensor configurations for measuring the surface deformations of large, deployable space antennas are described. These antennas include precision deployable reflectors up to 30 meters diameter and 1000 GHz frequency and mesh deployable reflectors up to 100 meters diameter and 30 GHz frequency. For each representative antenna configuration, the surface deformation sensor provides continuous, real-time measurements at a sufficient number of sample points to be compatible with active surface control. Moreover, the sensor system does not interfere with the mechanical or microwave characteristics of either the antenna surface or the feed. For the applications considered, the sensor system consists of a central receiver ring containing six to ten long focal length, angle measuring instruments, each viewing a dedicated set of bright point targets at the antenna. The targets, either light emitting diodes or illuminated retroreflectors, are modulated to eliminate errors from spurious backgrounds. Very preliminary performance estimates indicate that the sensor system, using commerical grade components, can produce a 20th to a 30th wavelength accuracy (3 sigma).

Neiswander, R. S.↗
NASA NTRS1979

An efficient technique for the computation of vector secondary patterns of offset paraboloid reflectors

A series approach for the rapid computation of the vector secondary pattern of offset paraboloid reflectors wherein the feed is displaced is presented. We show that the Jacobi polynomial series method, which has been demonstrated to provide an efficient means for evaluating the radiation integral of symmetric paraboloid reflectors, can be extended to the case of an offset paraboloid without compromising the ease or speed of computation. The analysis leading to the series formula is also useful for deriving an analytic expression for the optimum scan plane for the displacement of the feed. Representative numerical results illustrating the application of the method and the properties of the offset paraboloid are presented.

Mittra, R.↗
NASA NTRS1979

Performance analysis for collector-side reflector systems

The F-Chart method of solar collector performance evaluation is extended to a flat-plate collector system augmented with planar side reflectors. The modified F-Chart evaluations show that the augmentation of the collector has approximately the effect of increasing the collector area by 60%, and the energy supplied by the collector better matches the load. The installed reflector cost of about 10% of the collector cost is shown to be well justified.

Espy, P. N.↗
NASA NTRS1980

Shaped reflector antenna analysis using the Jacobi-Bessel series

A vector radiation integral is derived for an offset shaped reflector illuminated by an arbitrarily located and oriented source. A procedure for expressing the integral in terms of a series of the Fourier transforms of an effective aperture distribution is discussed. The Jacobi-Bessel series is used to evaluate the Fourier transforms. Numerical results are presented for different reflector configurations and source locations.

Rahmat-Samii, Y.↗
NASA NTRS1982

Pyramidal-Reflector Solar Heater

Motor-driven reflector compensates for seasonal changes in Sun's altitude. System has flat-plate absorbers mounted on north side of attic interior. Skylight window on south-facing roof admits Sunlight into attic, lined with mirrors that reflect light to absorbers. Reflectors are inner surfaces of a pyramid lying on its side with window at its base and absorber plates in a cross-sectional plane near its apex.

Source record↗
NASA NTRS1982

Easily Assembled Reflector for Solar Concentrators

Reflectors for concentrating solar collectors are assembled quickly and inexpensively by method that employs precontoured supports, plastic film, and adhesive to form a segmented glass mirror. New method is self-focusing, and does not require skilled labor at any stage. Contoured ribs support film and mirror segments of reflector. Nine mirror segments are bonded to sheet. Combined mirror surface closely approximates a spherical surface with a radius of curvature of 36 inches (0.91 m).

Bouquet, F. L.↗
NASA NTRS1981

Design concepts for large antenna reflectors

A type of antenna reflector was studied in which a stiff structure is constructed to hold a membrane like reflector mesh in the correct position. An important basic restriction is that the mesh be controlled only by the structure and that no additional local shaping be employed. Furthermore, attention is confined to structures in which no adjustments would be made on assembly. Primary attention is given to the tetrahedral truss configuration because of its outstanding stiffness and dimensional stability.

Hedgepeth, J. M.↗
NASA NTRS1981

On the design of large space deployable modular antenna reflectors

The deployment kinematics, stowing philosophy, and deployment sequencing for large deployable antenna modules were verified. Mesh attachment methods compatible with full scale modules were devised. Parametric studies of large modular reflectors established size, mass, and aperture frequency capabilities for these assemblies. Preliminary studies were made devising means of delivering modules to orbit, and once there, of assembling the modules into complete modular antenna reflectors. The basic feasibility of creating mass efficient modules erectable into large structures in space was established.

Ribble, J. W.↗
NASA NTRS1981

Static and dynamic characteristics of large deployable space reflectors

A linear numerical model of the structural characteristics of deployable reflectors was developed. Due to cyclic symmetry of the reflector structure about its axis, only one of many segments is modeled using finite elements. The succeeding segments satisfy continuity of displacement and slope at the interface between consecutive segments. This process leads to (N/2+1) static or dynamic problems of smaller order and bandwidth where N is the number of segments. The solution of each reduced problem leads to motions having a distinct circumferential wave number. The dynamic coupling to the feed support structure is studied adopting modal synthesis.

El-Raheb, M.↗
NASA NTRS1982

Deployable Reflector for Solar Cells

Unfoldable-membrane-reflector concept leads to mobile photovoltaic generators. Hinged containers swing open for deployment, and counterbalance beam swings into position. Folded reflector membranes are unfolded as deployment mast is extended, until stretched out flat.

Johnson, K. L.↗
NASA NTRS1981

Test progress on the electrostatic membrane reflector

NASA is currently developing a low mass antenna which derives its reflector surface quality from the application of electrostatic forces to form a thin membrane into the desired concave reflector surface. The shuttle-deployed antenna would have a diameter of 100 m and an RMS surface smoothness of 10 to 1 mm for operation at 1 to 10 GHz. Surface quality measurements have been made on a highly deformable elastic membrane, pressurized by electrostatic forces. Included are the effects of the perimeter boundary, splicing of the membrane, the long-scale smoothness of commercial membranes, and the spatial controllability of the membrane using voltage adjustments to alter the electrostatic forces. The electrostatic membrane was found to operate well in an open-loop sense, showing a high degree of position stability and negligible power consumption in dry air. Visco-electric creep was not evident, but the polymer membrane did expand and contract considerably due to its hygroscopic expansion coefficient. A residual roughness of about 0.75 mm existed with the polymer used in these tests; this error is attributed to seams and membrane anisotropy where the material is stiffer in one direction.

Goslee, J. W.↗
NASA NTRS1982

Focal surfaces of offset dual-reflector antennas

An analytical technique is described for finding the best focal surfaces for offset-fed dual-reflector antennas. A ray tracing procedure traces the loci of rays incident on the main reflector onto a plane or 'screen' situated perpendicular to a central ray of the antenna system. Given, then, by computer graphics, the best feed locations for azimuth and elevation plane patterns, an aperture diffraction method is used which can compute the sidelobe levels and beamwidths resulting from aperture phase errors on scanned or multibeam patterns. High-magnification Cassegrain or Gregorian antennas, with tilt angles optimised according to Japanese criteria, produce excellent radiation diagrams many beamwidths from the central, unaberrated pattern direction.

Sletten, C. J.↗
NASA NTRS1984

Adjusting the Contour of Reflector Panels

Postfabrication adjustment of contour of panels for reflector, such as parabolic reflector for radio antennas, possible with simple mechanism consisting of threaded stud, two nuts, and flexure. Contours adjusted manually.

Palmer, W. B.↗
NASA NTRS1983

Large deployable reflector: An infrared and submillimeter orbiting observatory

The Large Deployable Reflector (LDR) is to be a dedicated astronomical observatory in space. It will operate in the 1 mm to 30 micron wavelength region where the Earth's atmospheric opacity makes ground-based observations nearly impossible. The primary mirror will be 20 m in diameter, made up of 37 individual segments. The reflector will be actively controlled to provide an overall surface accuracy of less than or approximately 2 microns. The LDR will be placed in orbit by the Space Shuttle and revisited at approximately 2 year intervals during its 10 year lifetime.

Swanson, P. N.↗