The upper part of Figure 1 shows a small-scale prototype of a developmental class of lightweight, deployable structures that would support panels in precise alignments. In this case, the panel is hexagonal and supports disks that represent segments of a primary mirror of a large telescope. The lower part of Figure 1 shows a complete conceptual structure containing multiple hexagonal panels that hold mirror segments.
The structures of this class are of the tensegrity type, which was invented five decades ago by artist Kenneth Snelson. A tensegrity structure consists of moment-free compression members (struts) and tension members (cables). The structures of this particular developmental class are intended primarily as means to erect large segmented primary mirrors of astronomical telescopes or large radio antennas in outer space. Other classes of tensegrity structures could also be designed for terrestrial use as towers, masts, and supports for general structural panels.
An important product of the present development effort is the engineering practice of building a lightweight, deployable structure as an assembly of tensegrity modules like the one shown in Figure 2. This module comprises two octahedral tensegrity subunits that are mirror images of each other joined at their plane of mirror symmetry. In this case, the plane of mirror symmetry is both the upper plane of the lower subunit and the lower plane of the upper subunit, and is delineated by the mid-height triangle in Figure 2. In the configuration assumed by the module to balance static forces under mild loading, the upper and lower planes of each subunit are rotated about 30°, relative to each other, about the long (vertical) axis of the structure. Larger structures can be assembled by joining multiple modules like this one at their sides or ends.
When the module is compressed axially (vertically), the first-order effect is an increase in the rotation angle, but by virtue of the mirror arrangement, the net first-order rotation between the uppermost and lowermost planes is zero. The need to have zero net rotation between these planes under all loading conditions in a typical practical structure is what prompts the use of the mirror configuration. Force and moment loadings other than simple axial compression produce only second-order deformations through strains in the struts and cables.
Simple algebraic expressions have been derived to describe the deformations, under load, of multimodule platelike and mast structures, thereby making it possible to design such structures without need for computers. Perhaps the most important rules for designing a tensegrity structure are that (1) the lengths of the struts and cables are critical and they determine the unloaded shape of the structure, but that (2) the preloads (discussed in the next paragraph) in the cables and struts determine the degree of rigidity under external load.
To make a module stowable, it is necessary to provide for disconnection of the ends of many of the struts and/or make the struts collapsible (e.g.,tele- scoping). To make a module deployable, one must provide means to reconnect the struts if disconnected and re-extend them if collapsed. The means of deployment must include means to apply the required preloads to the cables and struts. In cases of manual stowage and deployment, such means can include toggles and turnbuckles. For automated deployment, more sophisticated means are needed. In the structure of Figure 2, the struts are telescoping piston/cylinder units that are extended pneumatically and locked at full extension by spring-loaded mechanisms.