Journal of Aerospace Engineering

One of the first construction operations for building a stationary habitat on the moon or Mars will be excavation of soil. The reasons for this necessity are manyfold and range from the need to protect inflatable habitats against radiation to creating an underground space for building electrical power plants. Despite the tremendous amount of earth movement that has taken place on this globe, a sound theoretical basis for designing soil‐moving machines does not exist. This paper describes the results of experiments that were developed to evaluate empirically if and how soil could be excavated on the moon. No attempt has bee made to optimize or promote a particular method. The goal of the present study is rather to establish a sound knowledge base to use for the more‐detailed studies needed to design an operational system that will be successful on the moon.

Space Biospheres and Ventures, a private, for‐profit firm, has undertaken a major research and development project in the study of biospheres, with the objective of creating and producing biospheres. Biosphere II—scheduled for completion in March 1991—will be essentially isolated from the existing biosphere by a closed structure, composed of components derived from the existing biosphere. Like the biosphere of the Earth, Biosphere II will be essentially closed to exchanges of material or living organisms with the surrounding environment and open to energy and information exchanges. Also, like the biosphere of the Earth, Biosphere II will contain five kingdoms of life, a variety of ecosystems, plus humankind, culture, and technics. The system is designed to be complex, stable and evolving throughout its intended 100‐year lifespan, rather than static. Biosphere II will cover approximately 1.3 hectare and contain $200\text{,}000m^3$ in volume, with seven major biomes: tropical rainforest, tropical savannah, marsh, marine, desert, intensive agriculture, and human habitat. An interdisciplinary team of leading scientific, ecological, management, architectural, and engineering consultants have been contracted by Space Biospheres Ventures for the project. Potential applications for biospheric systems include scientific and ecological management research, refuges for endangered species, and life habitats for manned stations on spacecraft or other planets.

A concept for the artificial damping of free vibration by means of actively controlled initial distortions imposed on the structure is presented. Two formulations for active control are presented: The first simulates the natural damping properties of structures, while the second uses a more sophisticated modal strategy of control (but with a faster damping process). The general idea of damping by actively forced distortions is explained and followed by a simple example for a one‐degree‐of‐freedom system. Then, the simulation of natural damping (which is a particular case of active control) and the possibility of accelerating the damping process by the modal optimal strategy are discussed and demonstrated with some examples for a two‐degree‐of‐freedom system. Finally, the vibration control of a four‐degree‐of‐freedom system is presented to demonstrate the efficiency of the proposed method. The method of active damping is described for truss structures, but it can be easily generalized to include frame structures as well.

A structural analysis is performed for the rim support of a pressurized off‐axis paraboloidal membrane, serving as a space‐based solar concentrator. The function of the rim support is to take up the tensile forces created by the stretched membrane. This paper deals with the load analysis. The tensile forces transmitted by the membrane to the rim support are calculated, and are proportional to pb, where p is the inflation pressure and b is one‐half the major axis of the elliptical rim. Next, the internal forces and moments generated in the rim support by the membrane forces are calculated. The compression forces are considerably larger, at any point, than the shear forces; both are proportional to ${\mathrm{pb}}^2\text{.}$ The bending moments are proportional to ${\mathrm{pb}}^3\text{.}$ The critical point is found to be at the top of the rim, where both the bending moment and compression force are at their maximum.

A structural analysis is performed for the rim support of a pressurized off‐axis paraboloidal membrane, serving as a space‐based solar concentrator. The function of the rim support is to take up the tensile forces created by the stretched membrane. This paper analyzes the deformations in the rim support, based on an earlier evaluation of the internal forces and moments resulting from the load applied by the membrane. The deformations are calculated in the x‐ and y‐directions at all points along the rim. They are shown to depend on the rim geometry, the applied loads, and a dimensionless rigidity parameter comprising the inflation pressure, the major axis of the elliptical rim, the modulus of elasticity of the rim support material, and its moment of inertia. The angular deformation at each point is also evaluated. A simplified solution for small deformations shows them to be approximately proportional to the rigidity parameter.

Three materials, E‐glass, Kevlar, and S‐glass, are assessed in terms of their effectiveness as softening strips in the fail‐safe design of composite structures. The softening effect is assessed in terms of its influence on the failure behavior of AS/3501‐6 graphite‐epoxy laminates consisting of 16 plies in a lay‐up of $[\pm 45\text{,}{O}_2{]}_{\mathrm{2S}}\text{.}$ Softening strips are created in the basic material by locally replacing 0°‐ply fibers with the candidate softening materials. Initial damage in the panels is introduced by saw‐cutting' or as a result of an impact load in the geometric center of the specimen. In some cases, the specimens are fatigue‐cycled prior to the introduction of the initial damage; in other cases, the cyclic loads are applied subsequent to the saw‐cutting or impact loading. Moisture effects on strength degradation and the crack‐arrest capability of the softening strips are included. The experimental results indicate that the S‐glass softening material has a greater beneficial effect than Kevlar or E‐glass on the fail‐safe characteristics of the basic material. Finally, analytical correlations of some test results are presented.

This paper presents a general‐purpose mathematical formulation for the dynamic analysis of a rotorcraft consisting of flexible or rigid components, or both, that may undergo large rotations. In this formulation, two sets of coordinates are used, namely rigid‐body coordinates and elastic coordinates. The rigid‐body coordinates define the location and the orientation of a body frame with respect to an inertial frame. The rigid‐body rotational coordinates may be Euler angles, Euler‐like angles, or Euler parameters. The elastic coordinates define the elastic deformations with respect to the body frame. Nonlinear strain‐displacement relations are considered in order to be able to incorporate the effect of geometric stiffening. A systematic methodology that combines the traditional finite element and multibody approaches is developed to obtain a set of differential and algebraic equations governing the dynamics of the system. The resulting set of equations is highly nonlinear. Numerical schemes to solve this set of equations are also discussed. The formulation presented is general and allows the development of a modular code. The formulation also allows the code to be updated and grow without reformulation of the problem.

For brittle composite materials, a mixed experimental and structural model is developed to determine a consistent interlaminar shear fracture toughness $\left(G_{\mathrm{II}c}\right)$ in an end notched flexure (ENF) test. The ENF procedure presented here differs from the customary ENF test in that one specimen is used for multiple measurements of compliances and the corresponding critical loads and crack growths; this procedure reduces the number of specimens needed to determine $G_{\mathrm{II}c}\text{.}$ Using this test procedure, $G_{\mathrm{II}c}$ is difficult to compute from only the experimental results because the critical load is sensitive to the inhomogeneity of the material and the specimen variation; thus a model that integrates the experimental results with structural beam theory is developed. Using the experimental results, a functional relationship between the critical load and the important geometric and material properties is obtained. A structural model is developed from Castigliano's second theorem to compute the flexural and transverse shear deflection of the delaminated beam. Experimental results are presented for three 32‐ply multidirectional graphite‐epoxy laminates: $0{°}_{32};$ $0{°}_4\text{,}$ $90{°}_8\text{,}$ $0{°}_4;$ and $0{°}_4\text{,}$ $\pm 45{°}_2\text{,}$ $\mp 45{°}_2\text{,}$ ldquo;$0{°}_4\text{.}$ The structural model is compared with the experimental compliance and deflection results, and $G_{\mathrm{II}c}$ is determined using the experimental and structural model. $G_{\mathrm{II}c}$ obtained by this experimental procedure is compared to the $G_{\mathrm{II}c}$ determined using the customary ENF test procedure.