Future space exploration past Earth orbit has a significant need for manufacturing in space beyond simple assembly of prefabricated parts. The next generation of very large aperture antennas will exceed the size achievable with conventional folding mesh technologies and new concepts are needed to support football-field-size structures. Technologies to address the problem have been developed using the formation of polyurethanes in a vacuum environment. Large inflatable structures can be stabilized by the formation of polyurethane foams of controlled density. For use in a vacuum environment, the availability of oligomeric precursors is important. Low-molecular-weight components would immediately evaporate, changing the stoichiometry of the reaction and potentially contaminate a space environment, but high-molecular-weight precursors have a much more limited range of properties.
Two technologies for ultra-light space structures and antennas were developed. The first, the Rigidization on Command™ (ROC™) concept, uses UV-curing resins to stabilize inflated structures. This removes the need for make-up gases inherent in typical inflatables. This technology is suited for extremely low-areal-density construction, but the thermal and mechanical dampening in these structures is low. A self-deploying foam antenna concept was developed at ATI for small aperture sizes, and a 0.6-meter antenna was fabricated and tested at NASA’s Jet Propulsion Laboratory.
These foam antennae have an areal density of approximately 5 kg/m2, which was achieved with a foam density of 48 kg/m3 (3 lb/ft3). While the areal density of these foams was as low as any state-of-the-art solution, it was still a factor of 5 above the threshold for very large aperture designs. The use of the foam technologies in combination with ROC™ systems was investigated to achieve the required low-areal-density antenna structures. The new foam materials would be used as a basis for an in-space foamed structure, using the ROC™ technology to provide the necessary shape control. The advantages of the new system include low areal density, high volumetric packing efficiency (with a target density of 10 kg/m3, the theoretical packing density is 100:1 from the liquid precursor to the foamed structure), and self-deploying characteristics.
The design fills a correctly shaped double-layer membrane with high-expansion polyurethane foam. The first chamber is inflated to give the antenna its desired shape. Then, the antenna reflector is “frozen” into the desired shape by UV rigidization to prevent possible distortion during the subsequent foam stabilization process. After rigidization, the second, smaller chamber is filled with foam to permanently stabilize the membrane shape and protect it from damage. This foam is made from open-cell material, allowing all gases used during expansion to rapidly diffuse out of the foam after deployment, greatly reducing the tendency to absorb heat under solar irradiation and thereby reducing the chance for thermal deformation due to changes in orbital solar exposure.
The critical part for this application was the controlled volume expansion to give a low-weight structural foam. Initial experiments showed that a completely unrestrained system foamed highly inconsistently, with volume variability of 500%, depending on the speed of skin formation. If the foaming took place in a perforated mold, however, the ratio of foam expansion became very controllable. Adding a third of a percent of water to the urethane mixture provided the desired foam density of 10 kg/m3 when expanded in vacuo.
To demonstrate the ability to produce a larger-scale antenna, both a 2-m ROC™-based antenna surface and a separate prototype foam backing unit were designed and fabricated. The antenna surface was measured via 3D laser position scanning. This allowed modeling of the antenna performance without extensive testing.