Paul B. Willis

Shape memory self-deployable structures for solar sails

A cold-hibernated elastic memory (CHEM) structures technology is one of the most recent results of the quest for simple, reliable and low-cost self-deployable structures. The CHEM technology utilizes shape-memory polymers in open-cell foam structures or sandwich structures made of shape-memory-polymer foam cores and polymeric laminated-composite skins. It takes advantage of a polymers shape memory and the corresponding internal elastic recovery forces to self-deploy a compacted structure. This paper describes these structures and their major advantages over other expandable and deployable structures presently used. Previous preliminary investigations and experiments have confirmed the feasibility of certain CHEM structures for space applications. Further improvements in CHEM technology and structure design widen potential space applications, including advanced solar sail structural concepts that are revealed and described in this paper.

Open-Celled Rigid Foams for Self-Deploying Foam Antenna Structures

The overall goal of this program is the development of an open-celled urethane foam system for use in self-deploying antenna structures. Advantages of such a system relative to current inflatable or self-deploying systems include high volumetric efficiency of packing, inherent restoring force, low (or no) outgassing, low thermal conductivity, high dynamic damping, mechanical isotropy, infinite shelf life, and easy fabrication with methods amenable to construction of large structures (i.e., spraying). The performance of our rigid open-cell foam systems, their crush and recovery behavior, potential packing scenarios, RF performance, and modeling are discussed.

Application of Origami in the Starshade Spacecraft Blanket Design

A Starshade is a large deployable structure and sole payload of an external occulter. At 34m in diameter or more, starshades are designed to block most of the light from a nearby star so that a small orbiting space telescope can image and characterize the Earth-like exoplanets in orbit around it. The starshade resembles a sunflower with a circular central disk supporting petals that are arrayed around its periphery. The petal edges are precisely shaped to match an optical profile that prevents diffraction. The area circumscribed by the edges must be completely opaque, black, and non-reflective. The petals and ring structure are covered by specially designed deployable blankets that must remain completely opaque even if they become perforated by micrometeorites. The blankets must also not cause any significant on-orbit thermoelastic loads on the lightweight supporting ring and petal structures despite very large differential thermal strains that are developed between these Kapton blankets and the thermally stable composite ring and petal structures. There are two types of blankets: one for the deployable petals and one for the central support disc that is formed by a lightweight deployable ring truss structure.The starshade blankets cover such a large area that they must be unusually lightweight compared to conventional multi-layer insulated (MLI) spacecraft blankets. The blankets must also stow around the central hub of the spacecraft with the deployable ring and petal structures in a highly repeatable fashion. This makes them ideal candidates for origami folding schemes. Based on prior studies of large deployable rigid arrays, we began with variants on the origami flasher to fold the central ring blanket, which is a minimum of 20m in diameter. We looked at the simplest methods for integrating this large blanket with a mechanical ring truss while providing ample optical baffling and little to no thermally induced loads on the structure. Petal blankets were also developed using deployable softgoods with pseudo-mechanical and shingled designs with optically blocking folds for deployment. The design was developed iteratively utilizing a variety of prototypes to explore and demonstrate the interaction between the softgoods and rigid elements.© 2014 ASME

Materials Development For Precision Segmented Reflector Applications

Progress in the area of precision segmented reflectors (PSR) for space applications has been driven by panel development activities. A number of small to medium size panels have been fabricated to demonstrate feasibility. The primary emphasis to date has been on making the panels lightweight and with high precision surfaces. Because of this emphasis, composite materials, and in particular, graphite fiber reinforced epoxy materials, were an obvious choice for construction of the panels. Indeed, many of the panels in existence have been fabricated from materials of this type. In terms of space applications, however, where stability and durability are of concern, it appears that these materials may not possess the balance of properties required for long term missions. Testing of these materials in simulated use environments has shown that they are deficient in either their thermomechanical properties, their stability (dimensional and environmental) or their fabricability. Recognizing that the development of new or modified materials for panel construction will be imperative if PSR technology is to be utilized in long term space missions, a program has been initiated to achieve this end. An initial set of material requirements has been developed based on a variety of mission scenarios. It is clear that no one material will be applicable to all such missions. Some potential candidate materials or generic classes of materials have been identified. Because of the basic requirements of light weight and stiffness, attention has continued to focus on advance composites. In the near term, it is likely that modified fiber reinforced organic matrix composites have good applicability. In the longer term materials such as carbon/carbon composites, graphite/glass composites and metal matrix composites may ultimately provide the best mix of properties. In this paper, information on PSR panel material requirements, as well as, test data on state-of-the-art materials will be presented. In addition, potential candidate alternate materials and progress toward their development will be discussed.

Fabrication and Testing of Self-Deploying Foam Antenna Structures

A copper-coated, open-cell foam antenna structure was fabricated and demonstrated. The coated foams produced could be compacted upon heating and subsequently redeployed to their original dimensions with good retention of surface characteristics. RF testing demonstrated that the foam-based antenna was capable of some beam focusing, although the gain loss was rather high in comparison with a standard metal reflector of the same geometry. These losses are believed to result primarily from high surface error (710 micron) relative to the standard surface. This surface error was likely a result of foam warpage caused by internal stresses present in the relatively thin specimens. The results of orbital thermal analysis and modeling of the associated distortions indicate that a simple change in reflector geometry from constant thickness to bi-concave could correct most of the surface error observed in the foam antenna system. Overall, the results continue to indicate the promise of these systems for use in lightweight, low stowage volume communications systems.