Journal of Aerospace Engineering

The energy budget of the global atmosphere is dominated by tropical energy sources. To accurately estimate the energy balance requires the simultaneous measurement of its various components: sources (radiation budget and surface fluxes), conversions (convection and precipitations), and transports. These terms are related to the primary variables: temperature, pressure, water vapor, and wind, the observation of which is generally poor and irregularly distributed. Assimilation of available data in general circulation models should then present a more coherent space and time description to emerge. BEST (Bilan Energetique du Systeme Tropical—Tropical System Energy Budget) is a satellite project designed to provide the required simultaneous measurement of wind and precipitation fields in the tropical zone. The project‐sponsored satellite would be placed in a 400–600 km altitude 28.5° inclination orbit with a three‐year lifetime. It is scheduled to carry a rain radar, a wind Doppler lidar with four fixed telescopes, and a microwave scanning radiometer. The possible addition of a differential absorption lidar is also considered. The BEST project is currently under phase A study at Centre National d'Etudes Spatiales (CNES) in conjunction with several French scientific laboratories. This project is expected to involve international cooperation in view of a possible launch in 1998–2000 time frame. The BEST mission will contribute to the global energy and water cycle experiment (GEWEX) and global change international programs.

Some astronomers are considering the moon as an attractive location within the inner solar system for a variety of astronomical observatories, some of which could be operational early in the 21st century. This paper describes the computer‐aided structural design of a 122‐m diameter, fully steerable, parabolic radio telescope to be located on the moon. The loads acting on such a reflector differ substantially from those acting on a reflector that must operate in earth's environment. The moon has excellent advantages as a location for such an instrument. The absence of atmosphere completely eliminates the wind, snow, and ice loads. The gravity field is only one‐sixth that of earth's. The thermal changes from night to day are severe, but structural problems can be avoided by using a thermally stable composite material. Structural elements for the reflector dish have been analyzed and designed for static loads with a specially written interactive graphical computer program. The resulting structure has a mass nearly 40 times less than its earth's counterpart made of steel. The evaluation of the results of the design studies supports the possibility of building a large‐aperture parabolic radio telescope on the moon.

High‐resolution optical systems require support structures that isolate the optical elements from mechanical and thermal loadings. Because the requirements for precise fabrication of the support structure are generally much less rigorous than they are for the optical element surface, the optical quality of the system can be degraded if the optical element is rigidly attached to the support structure. For small optics that can be treated as rigid elements, kinematic (statically determinate, point support) mountings may be used. For larger optics, however, finite contact surfaces (pads) are required to reduce contact stresses so that a semikinematic support results. If the contact surfaces are not coplanar, the optical element can be distorted. To circumvent this latter problem, support pads with self‐aligning designs may be used. These designs may either be mechanisms or flexures. Flexures are used more generally because they are free of both friction and hysteresis. For large optics, the optical elements require numerous supports so that the support system is highly statically indeterminant. These systems require analysis using the finite element method to meet optical deflection design criteria.

Twelve model tests of chemical explosives (1 g and 0.25 g) of PETN were conducted at 1 g ($=6g$ [moon]) and 10 g ($=60g$ [moon]) using the geotechnical centrifuge to determine the effect of chemical explosives on cratering and loosening a compacted lunar simulant prior to excavation. Even small depths of burial of the explosives were found to increase markedly the volume and lip of resulting apparent craters; optimum depths of burial were not pinpointed. Volumes of craters from PETN charges of different weights but equal depths of burial, were related as $V={\mathrm{kW}}^{084}$ when charges were fully buried, but not for surface tangent charges. Tests conducted at 1 g and 10 g showed no detectable difference in crater volumes for the depths of burial tested. Constraints to extrapolating the research to the lunar surface (using simulant rather than actual lunar soil; working in earth's atmosphere rather than in a vacuum; and working at greater than 1 g [moon]) are acknowledged.

The utilization of explosives for excavation on the lunar surface is under serious consideration as a part of the design for construction of temporary and permanent bases. An excavation research program has shown that small‐scale explosives blasting in a lunar‐soil simulant will greatly reduce the digging forces required for scoop and dragline excavators. Some crater‐blasting parameters were determined for the lunar soil simulant at one Earth gravity and at 10 Earth gravities using a centrifuge. The size of the craters produced at 10 Earth gs matched those formed at one earth g by scaling according to the weight of the explosive. These data can be applied to explosive‐excavation problems such as habitat construction, burial of nuclear power sources, and the rapid construction of shelters remote from the main base to shield against solar‐flare activity.

The dynamic analysis and control system design of large space structures involve the solution of the large‐dimensional generalized matrix eigenvalue problem. The computational effort involved is proportional to the third power of the dimension of the matrices involved. To minimize the computational time a graph‐theory approach to reduce a matrix to lower‐ordered submatrices is proposed. The matrix‐reduction algorithm uses the Boolean matrices corresponding to the original numerical matrices and, thus, the computational effort to reduce the original matrix is nominal. The computational savings directly depend upon the number of submatrices into which the original matrix is reduced. A free‐free square plate is considered as an example to illustrate the technique. In this example a matrix of 16th order is reduced to three scalars corresponding to three rigid‐body modes, and three matrices of order three and one matrix of order four.

Parallel algorithms are presented for optimization of structures on shared‐memory multiprocessor computers. Structures subjected to multiple loading cases with limitations on nodal displacements, element stresses, and member sizes are optimized using the optimality‐criteria approach in a concurrent‐processing environment. Emphasis is directed toward parallelizing each computational step of the solution process. Parallel algorithms are developed to assure the best concurrent performance and speed‐up within each step. A substructuring algorithm is used to achieve the best work‐load balance during the solution of the generalized displacements. Concurrency is achieved through the use of the notion of cheap concurrency and the concept of threads. (A companion paper demonstrates the efficiency of the parallel algorithms through optimization of several truss and frame problems on an Encore Multimax shared‐memory computer for a variable number of processors.) The algorithms are particularly suitable for optimization of large structures such as space stations.

In a companion paper, we presented algorithms and procedures for concurrent optimization of framed structures on shared‐memory multiprocessor computers. Implementation of the algorithms and applications are presented in this paper. The efficiency and versatility of the algorithms are assessed by presenting four examples. Speed‐ups and work‐load‐balance issues at various steps of the optimization process are considered and discussed. The efficiency of the parallel‐processing algorithms increases with the size of the structure, thus making them particularly suitable for optimization of large structures such as space stations.

The problem investigated here concerns the simultaneous design and control of structures. The structure considered is a laminated stiffened composite plate for which an optimum control system is designed by the minimization of an appropriate performance index with respect to both the control forces and structural design variables consisting of stiffener areas and the number of plies of a given orientation. To ensure a physically realistic structure appropriate constraints on the stiffener sizes, total weight, and structural frequencies are imposed. Nonlinear mixed‐integer programming is used to force the number of plies to take on integral values. The entire problem is posed as a three‐level optimization problem. Using the well‐known independent modal space control method, effects of plate geometry, initial disturbance conditions, and control effort penalty parameters on the optimal design are considered. The minimization process requires derivatives of eigenvalues and eigenvectors with respect to the design variables. These derivatives can be computed by an involved analytical procedure or a relatively simple finite‐difference procedure. This paper also examines the computer cost‐effectiveness of these two procedures for the sensitivity‐derivative calculations.

The optimal layer thicknesses and optimal feedback control function are determined for a symmetric, cross‐ply laminate. The objectives of the optimization are to maximize the biaxial buckling load (design objective) and to minimize the dynamic response to external disturbances (control objective) subject to a constraint on the expenditure of control energy. The design/control problem is formulated as a multiobjective optimization problem by employing a performance index that combines the design and control objectives in a weighted sum. Numerical results are given for a laminate made of an advanced composite material. Comparisons of controlled and uncontrolled laminates as well as optimally designed and nonoptimal laminates indicate the benefits of treating the design and control problems in unified formulation. The implications of solving these two problems are discussed. The values of optimal design and control variables are given for a number of problem parameters.

This investigation considers the effect of transverse shear deformation on bending of the axisymmetrically loaded isotropic and orthotropic circular and annular plates undergoing large deflection. The analysis treats the nonlinear terms of lateral displacement as fictitious loads acting on the plate. The solution of a von Kármán‐type plate is, therefore, reduced to a plane problem in elasticity and a linear plate‐bending problem. Results are presented for simply supported and clamped plates and are in good agreement with the available solutions. For plates considered in this study, the influence of shear deformation on lateral displacement becomes more significant as the orthotropic parameter increases. The linear and nonlinear solutions for orthotropic plates deviate at a low value of the maximum deflection‐to‐thickness ratio $(w/h)\text{.}$ Consequently, the extent of $w/h$ within which the small‐deflection theory is applicable to orthotropic plates is much lower than the value of about 0.4 typically used for isotropic plates, and it depends, in general, on the degree of orthotropy. The technique employed in this study is well suited for the analysis of nonlinear plate problems.