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A precision star tracker utilizing advanced techniques and materials
An image dissector star tracker has been developed which operates in the photon counting mode making it possible to utilize all digital electronics. A unique pulse processing circuit allows bright stars to be tracked as well as dim stars. Thermal mechanical stability has been greatly enhanced by fabricating a housing with graphite/epoxy composite material with a linear coefficient of thermal expansion near zero. Test results indicate the +10 Mv stars can be acquired and tracked, while position variation with star intensity is less than 2 arc seconds from 2.5 Mv to +10 Mv. The noise equivalent angle for a +8 Mv star is 3 arc seconds. Polynominal correction for remaining cross-coupling and nonlinearity reduced error over total 1 x 1 deg field to 1.5 arc seconds.
Structural requirements and technology needs of geostationary platforms
To identify structural requirements and technology needs of the geostationary platforms, two design alternatives are analyzed with respect to utilities accommodation, interface, and strength and stiffness requirements. Alternative 1 is comprised of 6,800 kg (15,000 lb) platform modules, each delivered to low earth orbit (LEO) with an attached orbit transfer vehicle (OTV) in a single Shuttle flight, deployed, and transferred to a geostationary constellation of platforms. Alternative 2 is made up of 16,800 kg (37,000 lb) platform modules, each delivered to LEO, deployed, mated to a 2 stage OTV (delivered to LEO in two additional Shuttle flights) and transferred to GEO for docking with other modules to form a single large platform. The top candidates for deployable structure elements are the expandable truss beam and the Astromast. The beam, fabricated of graphite/epoxy with a designed coefficient of thermal expansion of zero, provides good packaging density and high strength and stiffness per unit weight. The Astromast provides the best packaging density, but lacks the strength and stiffness required for the platform payloads. A graphite/epoxy mast with the Astromast packaging density is an advancement in technology needed for optimizing the geostationary platform designs.
Large furlable antenna study
The parametric study of the performance of large furlable antennas is described and the availability of various size antennas is discussed. Three types of unfurlable reflector designs are considered: the wrapped rib, the polyconic, and the maypole. On the basis of these approaches, a space shuttle launch capability, and state-of-the-art materials, it is possible to design unfurlable reflectors as large as 130 feet (40 meters) in diameter to operate at 10 GHz and 600 feet (183 meters) in diameter at 0.5 GHz. These figures can be increased if very low thermal coefficient of expansion materials can be developed over the next 2-5 years. It is recommended that a special effort be made to develop light weight materials that would provide nearly zero thermal coefficient of expansion and good thermal conductivity within the next 10 years. A conservative prediction of the kinds of unfurlable spacecraft antennas that will be available by 1985 with orbital performance predicted on the basis of test data and with developed manufacturing processes is summarized.
Optimal design of a thermally stable composite optical bench
The Lidar Atmospheric Sensing Experiment will be performed aboard an ER-2 aircraft; the lidar system used will be mounted on a lightweight, thermally stable graphite/epoxy optical bench whose design is presently subjected to analytical study and experimental validation. Attention is given to analytical methods for the selection of such expected laminate properties as the thermal expansion coefficient, the apparent in-plane moduli, and ultimate strength. For a symmetric laminate in which one of the lamina angles remains variable, an optimal lamina angle is selected to produce a design laminate with a near-zero coefficient of thermal expansion. Finite elements are used to model the structural concept of the design, with a view to the optical bench's thermal structural response as well as the determination of the degree of success in meeting the experiment's alignment tolerances.
Effects of additives on the processing and properties of LARC-TPI polyimide
The blending of LARC-TPI polyimide with the thermotropic liquid crystal polymer designated Xydar and with four different oligomeric imide materials has facilitated the resulting resin systems' processing into films while enhancing their mechanical properties and lowering their coefficient of thermal expansion to virtually zero. Two film-formation processes have been evaluated: (1) the casting of polyamic acid films followed by thermal imidization and biaxial stretching, and (2) the blown-film melt-extrusion of fully imidized LARC-TPI polymer. The best results have been obtained through the use of Xydar as a processing aid at levels in the 10-30 percent range.
Micro-mechanics of micro-composites
The Structural Dynamics branch at NASA LaRC is working on developing an active passive mount system for vibration control. Toward this end a system utilizing piezoelectric actuators is currently being utilized. There are limitations to the current system related to space applications under which it is desired to eliminate deformations in the actuators associated with thermal effects. In addition, a material that is readily formable into complex shapes and whose mechanical properties can be optimized with regards to vibration control would be highly desirable. Microcomposite material are currently under study to service these needs. Microcomposite materials are essentially materials in which particles on the scale of microns are bound together with a polyimide (LaRC Si) that has been developed at LaRC. In particular a micro-composite consisting of LaRC Si binder and piezoelectric ceramic particles shows promise in satisfying the needs of the active passive mount project. The LaRC/ Si microcomposite has a unique combination of piezoelectric properties combined with a near zero coefficient of thermal expansion and easy machinability. The goal of this ASEE project is to develop techniques to analytically determine important material properties necessary to characterize the dynamic properties of actuators and mounts made from the LaRC Si / ceramic microcomposite. In particular, a generalized method of cells micromechanics originally developed at NASA Lewis is employed to analyze the microstructural geometry of the microcomposites and predict the overall mechanical properties of the material. A testing program has been established to evaluate and refine the GMC approach to these materials. In addition, a theory of mixtures analysis is being developed that utilizes the GMC micromechanics information to analyze complex behavior of the microcomposite material which has a near zero CTE.
A concept for improving the dimensional stability of filamentary composites in one direction
Investigation of filamentary composites having high strength and stiffness and a zero thermal coefficient of expansion in one direction shows that advanced filament materials, such as boron, have substantial advantages over conventional filamentary composites. Various other results are discussed, including guidelines and analysis methods for further evaluation.
Thermally stable structural framework
Development of structural framework with zero or negligible thermal expansion is discussed. Combination of two types of metallic materials in binary trellis to compensate for thermal expansion is explained. Construction details are presented and illustration is included.
Design, fabrication and test of graphite/epoxy metering truss structure components, phase 3
The design, materials, tooling, manufacturing processes, quality control, test procedures, and results associated with the fabrication and test of graphite/epoxy metering truss structure components exhibiting a near zero coefficient of thermal expansion are described. Analytical methods were utilized, with the aid of a computer program, to define the most efficient laminate configurations in terms of thermal behavior and structural requirements. This was followed by an extensive material characterization and selection program, conducted for several graphite/graphite/hybrid laminate systems to obtain experimental data in support of the analytical predictions. Mechanical property tests as well as the coefficient of thermal expansion tests were run on each laminate under study, the results of which were used as the selection criteria for the single most promising laminate. Further coefficient of thermal expansion measurement was successfully performed on three subcomponent tubes utilizing the selected laminate.
Thermo/structural design considerations to achieve the Large Space Telescope line-of-sight requirements
The Large Space Telescope (LST) which is scheduled for launch in 1982, is a long-life, precision-pointing, earth-orbiting satellite requiring a structural system that provides high dimensional stability, minimum thermal distortion, and minimum response to onboard dynamic environments (e.g., reaction wheels). The results of a detailed thermostructural finite element computer analysis show that the telescope structure, even though fabricated from a material with a zero coefficient of thermal expansion, must be isolated from the external structure by a three-point support (flex joints or spherical bearings will accomplish this). Other thermo/structural analysis of the metering structure showed that second-order deformations have a significant effect on the alignment of the primary and secondary mirrors.
Secondary mirror subassembly in the 2.4 m NASA optical telescope
The secondary mirror is a circular disc provided with a shoulder. Three clamps located 120 deg apart support the mirror shoulder. On one side of the mirror shoulder, a coated graphite-epoxy ring connects the three clamps. Three axial flexures with a low radial compliance extend out from a support platform constructed of coated graphite-epoxy box structs and invar corners, and provide a nominal in-plane zero coefficient of thermal expansion. In each invar corner piece, a ball joint provides a connection to a expansion. In each invar corner piece, a ball joint provides a connection to a six-degree-of-freedom adjustment system that can move the secondary mirror with a high degree of precision in defocus, decenter, and tilt. Thermal controls that maintain structural-element temperature variations within specified limits insure that the assembly exceeds stability and figure control requirements on the secondary mirror.
Dimensional stability of Superinvar
Attention is called to new thermal expansion measurements on temporally stable Superinvar. The measurements indicate that its thermal expansivity vs temperature is more desirable than has been suggested by earlier measurements (Berthold et al., 1976). It is pointed out that at approximately 47 C, the thermal expansion coefficient is zero; however, the coefficient increases rapidly as the temperature departs from 47 C. A second zero-crossing of the expansivity is found at -21 C. The magnitude of negative expansivity between -21 C and 45 C remains bounded and small. No disruptive and reversible phase transition is found at temperatures down to -26 C.
Composite materials for space structures
Composite materials have been extensively used for spacecraft structures because of their unique combination of high specific strength and stiffness, low weight, good dimensional stability, and high specific damping capacity. High-modulus graphite reinforced epoxies have been the principal composite material system used. For early applications composites were selected primarily to reduce weight. However, the development of high precision structures such as The Space Telescope required high modulus composites with a coefficient of thermal expansion close to zero to meet design requirements. The use of advanced composites for space structures is reviewed in this paper. Barriers likely to limit further applications of composites are discussed and highlights of current research to improve composites are presented. New developments Jn composites technology which could impact future spacecraft systems are also reviewed.
Measurements of print-through in graphite fiber epoxy composites
High-reflectance accurate-contour mirrors are needed for solar dynamic space power systems. Graphite fiber epoxy composites are attractive candidates for such applications owing to their high modulus, near-zero coefficient of thermal expansion, and low mass. However, mirrors prepared from graphite fiber epoxy composite substrates often exhibit print-through, a distortion of the surface, which causes a loss in solar specular reflectance. Efforts to develop mirror substrates without print-through distortion require a means of quantifying print-through. Methods have been developed to quantify the degree of print-through in graphite fiber epoxy composite specimens using surface profilometry.
Composite sandwich structure and method for making same
A core for a sandwich structure which has multi-ply laminate ribs separated by voids is made as an integral unit in one single curing step. Tooling blocks corresponding to the voids are first wrapped by strips of prepreg layup equal to one half of each rib laminate so a continuous wall of prepreg material is formed around the tooling blocks. The wrapped tooling blocks are next pressed together laterally, like tiles, so adjoining walls from two tooling blocks are joined. The assembly is then cured by conventional methods, and afterwards the tooling blocks are removed so voids are formed. The ribs can be provided with integral tabs forming bonding areas for face sheets, and face sheets may be co-cured with the core ribs. The new core design is suitable for discrete ribcores used in space telescopes and reflector panels, where quasiisotropic properties and zero coefficient of thermal expansion are required.
Conceptual design of a self-deployable, high performance parabolic concentrator for advanced solar-dynamic power systems
NASA has initiated technology development programs to develop advanced solar dynamic power systems and components for space applications beyond 2000. Conceptual design work that was performed is described. The main efforts were the: (1) conceptual design of self-deploying, high-performance parabolic concentrator; and (2) materials selection for a lightweight, shape-stable concentrator. The deployment concept utilizes rigid gore-shaped reflective panels. The assembled concentrator takes an annular shape with a void in the center. This deployable concentrator concept is applicable to a range of solar dynamic power systems of 25 kW sub e to in excess of 75 kW sub e. The concept allows for a family of power system sizes all using the same packaging and deployment technique. The primary structural material selected for the concentrator is a polyethyl ethylketone/carbon fiber composite also referred to as APC-2 or Vitrex. This composite has a nearly neutral coefficient of thermal expansion which leads to shape stable characteristics under thermal gradient conditions. Substantial efforts were undertaken to produce a highly specular surface on the composite. The overall coefficient of thermal expansion of the composite laminate is near zero, but thermally induced stresses due to micro-movement of the fibers and matrix in relation to each other cause the surface to become nonspecular.
Moire interferometry for thermal expansion of composites
Moire interferometry by reflection is described and demonstrated for the case of a real reference grating of 1200 lines/mm. Extraneous beams can be displaced and stopped by using a wedge-shaped air gap between reference and specimen gratings. Double-order dominance, the use of diffraction sequences for reflection, the isolation of preferred sequences, and the use of two-beam interference are discussed. Experimental accuracy is enhanced significantly by using several data points to establish displacements along a line, and random errors characterized by one standard deviation can be as small as one microstrain. The method is well adapted to thermal environments, coefficients of thermal expansion of selected graphite-epoxy laminates being determined in the temperature range of 297-422 K. Very good precision was achieved for a wide range of thermal expansion coefficients, from approximately zero to 27 microstrains/K.