Backup Structure Research Articles

Ablative Heat Shield Design for Reentry Vehicle Using Numerical Analysis

At this paper, the thermal behavior of ablative materials as heat shields for reentry vehicles is investigated numerically. A one-dimensional finite difference solver is developed to simulate governing mass and energy equations. Four ablative materials; AVCO 5026-HCG, Carbon-Phenolic, Nylon-Phenolic, and Silica-Phenolic; are considered as a heat shield material for a reentry capsule with the diameter 2.8 meter. A heat flux profile from a simulated trajectory of a reentry capsule is used for investigation the performance and essential thickness of these four ablative materials. The only restriction for this simulation is the ultimate temperature of the backup structure which is beneath the ablative heat shield. At all simulations, the final thickness is defined by reaching the interface temperature, the temperature of the border between the ablative heat shield and solid shell, to 80±0.5 degrees Celsius. In addition, the sensitivity analysis is carried out to investigate the effect of properties variations on Carbon-Phenolic’s thickness for this specific heat flux profile.

Conceptual design studies of the 5 m terahertz antenna for Dome A, Antarctica

As the highest, coldest and driest place in Antarctica, Dome A provides exceptionally good observing conditions for ground-based observations over terahertz wavebands. The 5 m Dome A Terahertz Explorer (DATE5) has been proposed to explore new terahertz windows, primarily over wavelengths between 350 and 200 μm. DATE5 will be an open-air, fully-steerable telescope that can function by unmanned operation with remote control. The telescope will be able to endure the harsh polar environment, including high altitude, very low temperature and very low air pressure. The unique specifications, including high accuracies for surface shape and pointing and fully automatic year-around remote operation, along with a stringent limit on the periods of on-site assembly, testing and maintenance, bring a number of challenges to the design, construction, assembly and operation of this telescope. This paper introduces general concepts related to the design of the DATE5 antenna. Beginning from an overview of the environmental and operational limitations, the design specifications and requirements of the DATE5 antenna are listed. From these, major aspects on the conceptual design studies, including the antenna optics, the backup structure, the panels, the subreflector, the mounting and the antenna base structure, are explained. Some critical issues of performance are justified through analyses that use computational fluid dynamics, thermal analysis and de-icing studies, and the proposed approaches for test operation and on-site assembly. Based on these studies, we conclude that the specifications of the DATE5 antenna can generally be met by using enhanced technological approaches.

Fault Tolerance in Distributed Systems Using Fused Data Structures

Replication is the prevalent solution to tolerate faults in large data structures hosted on distributed servers. To tolerate f crash faults (dead/unresponsive data structures) among n distinct data structures, replication requires f + 1 replicas of each data structure, resulting in nf additional backups. We present a solution, referred to as fusion that uses a combination of erasure codes and selective replication to tolerate f crash faults using just f additional fused backups. We show that our solution achieves O(n) savings in space over replication. Further, we present a solution to tolerate f Byzantine faults (malicious data structures), that requires only nf + f backups as compared to the 2nf backups required by replication. We explore the theory of fused backups and provide a library of such backups for all the data structures in the Java Collection Framework. The theoretical and experimental evaluation confirms that the fused backups are space-efficient as compared to replication, while they cause very little overhead for normal operation. To illustrate the practical usefulness of fusion, we use fused backups for reliability in Amazon's highly available key-value store, Dynamo. While the current replication-based solution uses 300 backup structures, we present a solution that only requires 120 backup structures. This results in savings in space as well as other resources such as power.

Stiffness Considerations in Dimension Stone Anchorage Design

Abstract Modern building facade design concepts consider all cladding materials, including dimension stone cladding, as nonstructural elements designed to transmit localized gravity and lateral loads to the primary building structural elements. In practice, the structural backup behind dimension stone cladding can take a wide variety of forms, including cast-in-place concrete shear walls, brick and concrete masonry units, aluminum curtain wall system framing, hot and cold rolled steel subframes, and precast concrete panels and members. The flexural stiffness of these backup systems can vary widely, with cast-in-place concrete shear walls on one end of the stiffness spectrum and aluminum curtain framing on the other. The stiffness of dimension stone cladding relative to the backup system can have a significant effect both on the stresses induced in the cladding and the loads transmitted through the cladding anchors. Likewise, the stiffness of the anchor elements and the backup system can affect the loading and stress distribution within the cladding panels. This paper addresses some of the issues associated with the interaction of dimension stone cladding panels, panel anchors, and metal backup structures, and the effects of relative stiffness on load and stress distributions. Information is presented that was obtained from laboratory tests and analyses for new designs, as well as investigations of dimension stone cladding failures.

Structural Design and Analysis of a 50m Fully Steerable Pulsar Radio Telescope

A radio telescope of 50m in diameter, basically, for pulsar observation, is to work at multi-wave bands among which the shortest wavelength is 13 cm. Proposed is a fully steerable exposed truss structure scheme of the telescope. The design is essentially a wheel-on-track style radio telescope with 6 rollers in three bodies for azimuth drive and support, while two sets of gear wheel for elevation drive. The main reflector is a mesh with crimped stainless steel wire spanned on a ``bowl-like'' back-up truss structure which is supported by 6 points on its bottom. After discussion of structural design and related dead-load analysis, wind and thermal disturbances are estimated. Particularly, response spectrum seismic hazard is performed with response spectra analysis. The results confirm that the 50m radio telescope can meet the observational requirements and survive seismic hazard of acceleration of 0.1 gravity with a light and cost-effective structure.

Calculated thermal behavior of ventilated high precision radio telescopes

Radio telescopes that operate at millimeter and sub-millimeter wavelengths need a reflector-surface precision of a few tens of microns and a pointing accuracy of a few arcseconds. When built in a conventional way from steel and aluminum, as in the case of larger-diameter telescopes, thermal control must be applied to reduce temperature-induced deformations, in particular of the reflector backup structure. We illustrate that it is possible to make model calculations - for instance, during the design phase - that simulate the thermal behavior and the operation of a telescope when servo-loop-controlled ventilation or climatization (air-conditioned ventilation) of the backup structure is applied. We explain the technique of model calculations, and present as an example the calculated thermal behavior of a ventilated 64-m-diameter telescope and of the climatized 30-m IRAM telescope. It is explained that the thermal control of a telescope mount is less demanding if frequent pointing corrections can be made

Heat transfer analysis of charring ablators under aerodynamic heating

PurposeThis work experimentally and numerically investigates the aerodynamic heating of the charring‐ablating materials.Design/methodology/approachThe experimental model is a stainless steel cone with an attached charring ablator, in which supersonic hot flow impinges. The initial numerical simulation is based on physical and mathematical models, including one‐dimensional, unsteady energy transport and mass conservation equations, coupled with calculations of aerodynamic heating, thermal degradation, heat transfer of the ablating surface and the ablation model. The problem is solved by an efficient numerical method.FindingsThe numerical calculations involve the time history of the temperature distribution inside the charring material and the backup structure. The results are consistent with the experimental data.Originality/valueThis study proposes an effective method to correlate one's own ablation rate equation, by a method of trial and error to find the correlation constants, and the corresponding time histories of the ablation rate or temperature that are closest to one's own experimental data. Then the correlation of the surface ablation rate can be applied with confidence in the numerical calculation of other cases.

The Effect of Misalignment on Rotor Vibrations

Rotors with three or more fluid-film bearings (or fluid seals) have “redundant” supports, and, therefore, interdependent bearing loads that are generally unknown both in magnitude and direction. The steady-state bearing eccentricities and the dynamic stiffness and damping coefficients of the bearings are, therefore, also unknown since both are functions of the bearing loads. Thus, the dynamic behavior of multibearing rotors generally cannot be predicted with good accuracy without access to a procedure for calculating the steady-state bearing loads and eccentricities. This paper outlines such a procedure in terms of both the influence coefficient method, the transfer matrix method, and the finite element method. Radial bearing misalignment and flexibility of the bearing back-up structures are accounted for. Once the eccentricities are available, the bearing stiffness and damping coefficients can be calculated in the usual way and used to predict critical speeds, instability threshold speed, and rotor response to imbalance. A numerical example is presented that illustrates some of the nonlinear effects of bearing support redundancy, notably the large variations in instability threshold speed with radial bearing misalignment. The example shows how the method can be used to determine the level of bearing misalignment that leads to optimum rotor stability. It is concluded that no simple guide lines exist by which optimum stability can be achieved. Neither perfect bearing alignment nor equal load sharing between bearings necessarily lead to optimum stability.

Structural Homology Design Using Equality Constraints

The concept of homology design has been devised for the application to large telescope structure by S.v.Hoerner. It is defined that the deformation of a structure shall be called homologous, if a given geometrical relation holds, for a given number of structural points, before, during, and after the deformation. Recently, the need of homology design in the structural design has been increase due to the required precision in the structure. Some researchers have utilized the theory on the structural design with finite element method in the late 1980s In the present investigation, a simple method using geometrical equality constraints is suggested to gain homologous deformation. The previous method is improved in that the decomposition of FEM eqation, which is very expensive, is not necessary. The basic formulations of the homology design with the optimization concept are described and several practical examples are solved to verify the usefulness and validity. Especially, a back-up structure of a satellite antenna is designed by the suggested method. The results are compared with those of existing researches.

The Submillimeter Telescope - A Status Report

SummaryThe Max-Planck-Institut für Radioastronomie, Bonn and the Steward Observatory of the University of Arizona, Tucson are collaborating on the construction and operation of a dedicated submillimeter facility. The Submillimeter Telescope (SMT) is a 10 m diameter reflector surrounded by a corotating enclosure. The instrument is an alt-azimuth mounted f/13.8 Cassegrain homology telescope with two Nasmyth and bent Cassegrain foci. The SMT will have diffraction limited performance at a wavelength of 300 μm. and an operating overall figure accuracy of 15 μm rms. The primary and secondary reflector surfaces are constructed out of aluminum-core, carbon-fiber-reinforced-plastic (CFRP) face sheet sandwich panels. The primary reflector backup structure and secondary support are fabricated from CFRP structural members. This modern technology provides both the means for reaching the required precision of the SMT for both night and day operation (basically because of the low coefficient of thermal expansion and high strength-to-weight ratio of CFRP) and a potential route for the realization of lightweight telescopes of even greater accuracy in the future. The SMT will be the highest accuracy radio telescope ever built.The SMT is located at an altitude of 3180 m on Emerald Peak (Mt. Graham) 120 km northeast of Tucson in southern Arizona. Measurements indicate that atmospheric conditions allow submillimeter observations during about 40% of the time in winter months. The telescope is placed in a co-rotating enclosure of novel design. The enclosure fits tightly around the telescope, with the focus flanges extending from the elevation bearings into the receiver rooms of the enclosure. A flat tertiary mirror is used to direct the beam through either of the two elevation bearings. Thus, we are able to mount receivers directly on the telescope while maintaining laboratory type conditions in the access area surrounding the receivers. On the “facility instrument” side, several receivers can be operated simultaneously.

Insights on the whirl-flutter phenomena of advanced turboprops and propfans

The new aircraft configurations that have two pusher propellers or propfans located at the rear fuselage are susceptible to a whirl-flutter-related instability characterized by a complex dynamic coupling involving not only the propeller-nacelle whirl modes but also the natural modes of the supporting backup structure. It is shown that the single-engine classical whirl-flutter formulation does not correctly predict these particular situations of instability, which were called whirl induced flutter. The problem is treated in this paper in the form of a parametric study using a 15-degree-of-freedom model. The main stiffness properties of both the supporting backup structure and the engine mounting system are systematically changed and the mechanism of the aforementioned phenomenon is explained on simple physical grounds. a b d h hy,hz Nomenclature = distance between fuselage and propeller centerlines = engine-propeller center-of-mas s distance from the pylon elastic axis = propeller distance from the pylon elastic axis = pylon elastic axis flatwise displacement = aft fuselage displacements in the lateral and vertical directions, respectively = engine polar and transverse moments of inertia, respectively = propeller polar moment of inertia = aft fuselage polar moment of inertia = nacelle polar and transverse moments of inertia, respectively = V/QR - shock mount system lateral and vertical stiffnesses relative to the nacelle, respectively = aft fuselage lateral, vertical, and torsional stiffnesses, respectively

Reducing distortion and internal forces in truss structures by member exchanges

Manufacturing errors in the length of members or joint diameters of large truss reflector backup structures may result in unacceptable large distortion errors or member forces. However, it may be possible to accurately measure these length or diameter errors. The present work suggests that a member and joint placement strategy may be used to reduce distortion errors and internal member forces. A member and joint exchange algorithm is used to demonstrate the potential of this approach on several 102-member and 660-member truss reflector structures. It is shown that it is possible to simultaneously reduce the rms of the surface error and the rms of member forces by two orders of magnitude by member and joint exchanges.

Laser radar beam steering mirrors

The structural aspects of developing a flat laser radar beam steering mirror for optical discrimination of an array of spaceborne targets at 10.6 p.m are discussed. Results of trade studies to determine feasible design concepts capable of meeting the system performance goals during steering are included. Aperture diameters between 0.5 and 2 m were studied for three principal structures: (1) a thin glass sandwich coupled to adeep backup structure, (2) a full-depth beryllium sandwich, and (3) a full depth graphite epoxy sandwich. The results of computer simulations of the structural performance of each of these concepts are presented, and fabrication efforts in support of these concepts are described. These studies demonstrate that lightweight, dimensionally stable flat mirrors can be fabricated with present technology. Aluminum-clad graphite/epoxy mirrors were successfully manufactured within 0.5 pm rms of a flat surface, and initial test results indicate dimensional stability after several thermal cycles. A low cost, uncomplicated method to manufacture low thermal expansion aluminum-clad graphite/epoxy tubes as backup structural members for large space-based optics was also developed. Further investigations are warranted to determine the dynamic tensile strength of glass, to develop thermal distortion compensation methods for metal mirrors, and to improve the dimensional and temporal stability of clad graphite/ epoxy mirrors.

Whirl-flutter investigation on an advanced turboprop configuration

The whirl-flutter problem of an advanced turboprop configuration with two pusher propellers positioned at the aircraft fuselage cone is analyzed. Coupling between the two propellers and the flexible backup structure— pylons and aft fuselage cone—is allowed. Very interesting results lead to the conclusion that, for typical stiffness ratios between the backup structure and the engine suspension system, a special type of flutter involving mainly the backup structure may be dominant over traditional propeller-nacelle whirl flutter. This type of flutter is solely due to the propeller whirl and may be critical either in some modern configurations of aircrafts (as propfans) or in new conceptions of power plants installations employing additional vibration insulators at the pylon-fuselage attachments.

Integrated structural electromagnetic shape control of large space anatenna reflectors

The requirements for extremely precise and powerful large space antenna reflectors have motivated the development of a procedure for static shape control of the reflector surface. A mathematical optimization procedure has been developed that improves antenna performance while minimizing necessary shape-correction effort. The control procedure is based on regulating electromagnetic (EM) performance in contrast to previous work, which is based on controlling the rms distortion error of the surface, thereby indirectly improving antenna performance. The application of the control procedure to a radiometer design with a tetrahedral truss backup structure demonstrates the potential for significant improvement. The results indicate the benefit of including EM performance calculations in procedures for shape control of large space antenna reflectors.

Concept of inflatable elements supported by truss structure for reflector application

A concept of modularized inflatable space structure, which is composed of inflatable elements supported by a truss structure for high-precision reflector missions in relatively near future, is proposed to avoid some difficulties of conventional inflatable concept, such as the lack of inside hard points, and the precisely accurate manufacturing process. Some fundamental characteristics for surface accuracy of inflatable elements are studied, appropriate truss back-up structure is investigated, and the relevant prelimimary test results are described. Some aspects of the application of the concept to space VLBI antenna are also introduced.

Structural concepts for large solar concentrators

This paper deals with the types of structural arrangements that are appropriate for large solar concentrator systems for use in space. Past concepts originating in the 1960s are discussed. These include stiff sandwich panels and aluminum dishes as well as inflated and umbrella-type membrane configurations. The Sunflower concentrator, developed in the early 1970s, is described as a salient example of a high-efficiency collector. Existing concepts for high-efficiency reflector surfaces are examined with attention to accuracy needs for concentration ratios of 1000 to 3000. It is concluded that concepts using stiff reflector panels are most likely to exhibit the long-term consistent accuracy necessary for low-orbit operation, particularly for the higher concentration ratios. Quantitative results are shown relating the allowable error to the desired concentration and focal-length-diameter ratios. A new configuration is presented that addresses both of these problems. It consists of a deployable Pactruss backup structure with identical panels installed on the deployed structure after deployment in space. Analytical results show that with reasonable pointing errors, this new concept is capable of concentration ratios greater than 2000.

System Concept For A Moderate Cost Large Deployable Reflector (LDR)

A study was carried out at JPL during the first quarter of 1985 to develop a system concept for NASA's LDR. Major features of the concept are a four-mirror, two-stage optical system; a lightweight structural composite segmented primary reflector; and a deployable truss backup structure with integral thermal shield. The two-stage optics uses active figure control at the quaternary reflector located at the primary reflector exit pupil, allowing the large primary to be passive. The lightweight composite reflector panels limit the short-wavelength operation to approximately 30 microns but reduce the total primary reflector weight by a factor of 3 to 4 over competing technologies. On-orbit thermal analysis indicates a primary reflector equilibrium temperature of less than 200 K with a maximum gradient of about 5 C across the 20-m aperture. Weight and volume estimates are consistent with a single Shuttle launch, and are based on Space Station assembly and checkout.

Whg/Store Flutter with Nonlinear Pylon Stiffness

Recent wind tunnel tests and analytical studies show that a store mounted on a pylon with 'soft' pitch stiffness provides substantial increase in flutter speed of fighter aircraft and reduces dependency of flutter on mass and inertia of the store. This concept, termed the decoupler pylon, utilizes a low-frequency control system to maintain pitch alignment of the store during maneuvers and changing flight conditions. Under rapidly changing transient loads, however, the alignment control system may allow the store to momentarily bottom against a relatively stiff backup structure in which case the pylon stiffness acts as a hardening nonlinear spring. Such structural nonlinearities are known to affect not only the flutter speed but also the basic behavior of the instability. This paper examines the influence of pylon stiffness nonlinearities on the flutter characteristics of wing-mounted external stores.

Strong coma lobes from small gravitational deformations

Beam mapping at 1.3-cm wavelength has shown very strong sidelobes at the 140-ft telescope, up to four lobes in a row when pointing far west. Different types of observations and the theory of coma lobes lead in good agreement to the conclusion that the telescope suffers a large lateral defocusing, varying by 7.2 cm east-west (EW) and 2.8 cm north-south (NS) for pointing changes of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90\deg</tex> . Since the lateral deformation of the feed legs is only 0.5 cm, it must be mainly the optical axis which moves. This is achieved by a gliding rotation of the best-fit paraboloid, gliding along a slightly deformed surface, while rotating about the center of the average surface curvature. It is shown that the resulting lateral focal offset can be much larger than the surface deformation which causes it. The gliding rotation is also confirmed by a structural analysis. The observational and analytical methods described can also be applied to other telescopes. Polar mounts such as the 140-ft may show the effect in two directions: east-west and north-south, while alt-azimuth mounts can show it in only one direction: up-down. The effect of lateral defocusing is to be expected at other telescopes as well whenever the rim is more flexible than the center; and the resulting degradation of efficiency and beam shape will be significant whenever gravity is important in the error budget. In these cases the telescopes should be supplied with a variable lateral shift of the mount at the prime focus, computer-controlled as a function of the pointing, following the axial movements. This additional degree of freedom may improve the short-wavelength performance considerable for telescopes whose surface panels are more accurate than the gravitationally deformed backup structure.