This chapter has presented a preliminary structural design of a spacecraft that carries many modular multifunctional tiles. A formation of these spacecraft is envisioned to capture solar power in space, and transmit it to a ground station on the Earth.
A design concept for these multifunctional 10 cm×10 cm tiles, each of which is capable of power generation and transmission, was described. These tiles are expected to be very lightweight; an initial mockup with a mass of 1.56 g was constructed, and this mass is expected to decrease by a factor of about 2. A structural framework was designed, using solar radiation pressure as a loading case and specific concentrated power as a performance metric, to hold these tiles in a planar configuration.
The final result is an ultra-lightweight spacecraft that measures 60 m×60 m, has a mass of 369 kg, and can be packaged into a cylinder with diameter of 0.92 m and a height of 1.50 m.
Chapter 5
Conclusions
At the outset, four challenges associated with the packaging and deployment of large planar struc- tures were posed: biaxial compaction, efficient packaging, packaging without permanent deformation, and deployment with low and predictable forces. This thesis presented two novel classes of methods for packaging large planar structures that simultaneously address each of these challenges.
These methods provide biaxial compaction by packaging structures with comparable in-plane dimensions and large area into cylindrical forms. In both of the classes of packaging methods, slipping folds are used to fold the structure; the first class of methods uses parallel slipping folds to z-fold the structure, and the second class uses folds arranged in concentric polygons to star-fold the structure. Once folded, the structure can be wrapped into a cylindrical package.
These methods are highly efficient at packaging; there are minimal voids in the packaged form, and the volume of these voids decreases in comparison to the volume of the packaged material for very large or very thin structures. At these scales, packaging efficiencies approach 100%. These packaging methods were demonstrated experimentally using meter-scale test articles. These tests demonstrated the feasibility of these packaging methods and validated the analytical tools that were developed to predict packaged shapes. Packaging efficiencies of up to 83% were obtained for these lab-scale models.
The maximum strains in the packaged form can be predicted analytically and thus packaging can be achieved such that the structure remains elastic throughout. This allows a structure to be packaged and deployed without damage or permanent deformation. These packaging methods scale according to the material thickness, and are applicable to a wide range of material thicknesses. This allows for the packaging of not only membrane structures, but also thicker structures with finite bending stiffness.
Methods for deploying these structures were also described. These methods were experimentally demonstrated, showing controlled and repeatable deployment. Deployment was conducted on both membrane test articles and test articles with thin-shell elements that provide structural bending stiffness.
Structural architectures were put forward that are compatible with these methods of packaging and deployment. One set of architectures pretension the structure to obtain out-of-plane stiffness;
these architectures are suitable for membrane structures. A second type of architecture uses the bending stiffness of strips with thin-shell elements to react out-of-plane loads. Preliminary analytical models were constructed to describe these structural architectures. Meter-scale physical models that typify these architectures were constructed; these models demonstrated feasibility of these structural architectures, and their compatibility with the packaging and deployment schemes.
These concepts, analytical tools, and methods enable the design of large space structures, such as solar power arrays, reflectarray antennas, solar sails, and drag sails. This thesis applied a particular packaging concept and structural architecture to the initial structural design of a 60 m×60 m space solar power satellite. The resulting spacecraft design concept is lightweight, with areal density of
≈100 g m−1, stiff enough to enable operation under dominant loading conditions, and packages into a cylindrical volume≈1 m in diameter and 1.5 m in height.
5.1 Open Questions
Over the course of this study, several questions were raised that are outside the scope of this thesis.
These questions warrant further investigation.
To eye, the physical models used throughout this thesis appeared undamaged after packaging and deployment, but the degree to which they recovered their shape and stiffness was not quanti- tatively measured. The as-deployed shape and stiffness of these structures must be measured, and comparisons must be made to the pre-packaged shap and stiffness.
The magnitude and distribution of forces required to hold the proposed structures in a wrapped state ought to be calculated, simulated, and measured. The proposed structures are elastically bent in the packaged configuration, storing strain energy, and forces must be applied to hold these structures contained. In order to design the containers for these structures, these forces must be known.
The packaging tests performed throughout this thesis were done, to a large extent, by hand.
This manual packaging leads to variability, uncertainty, and inefficiency. More systematic and deterministic packaging methods, e.g., using mechanical wrapping apparatuses or jigs, ought to be designed and used to ensure tight packaging.
The imperfection sensitivity of the packaging and deployment behavior of such structures should be studied. For instance, the effects of the rupture of a ligament during unfolding should be consid- ered. During deployment testing for this study, such ruptures were observed and their effects were minimal. The degree to which these structures are robust to imperfections should be quantified.
The structural analysis used in the design of the space solar power satellites should be further
advanced by considering non-linear effects e.g. the buckling of the TRAC longerons in bending, dynamic loading cases, and thermo-mechanical effects.