Elevated Temperature Mechanical Characterization of Isogrid Booms

Structurally efficient isogrid booms, manufactured from rigidizable composite materials, are becoming an enabling technology for spacecraft structures because of their high packing efficiency. Selection of the materials used in the construction of rigidizable space structures is commonly driven by mechanical performance properties at elevated temperatures. Mechanical properties testing was performed on composite tow samples and on an isogrid boom at various temperatures. To characterize elevated temperature behavior, the isogrid booms, and its subelement composite tows were manufactured from ILC’s TP283E shape memory polymer (SMP) matrix resin and a carbon reinforcement. Both the flexural modulus and the tensile modulus of the composite tow samples were determined as a function of temperature. These values were compared to the calculated values for the composite based on rule of mixtures analysis. The predicted rule of mixtures composite modulus is used in ILC’s isogrid analytical code to predict the structural properties of the isogrid boom. A number of composite tow samples were fabricated by ILC and mechanically characterized by the Aerospace Corporation to gather independent performance data. An isogrid boom was fabricated by ILC and mechanically characterized at elevated temperatures by James Madison University (JMU). JMU tested this boom in tension, compression, and also performed preliminary creep testing at various temperatures. A similar isogrid boom was fabricated by ILC and tested by The Aerospace Corporation for composite CTE performance. This paper discusses the results of both the composite tow testing and the isogrid boom testing in preand post-packing conditions. A discussion of the correlation between the predicted values and the actual test values is also presented. Introduction NASA and DoD space missions in the near future will require much larger satellites, the sizes of which will be beyond the capabilities of current technologies. The types of Gossamer spacecraft that will be needed include antennas, solar arrays, sunshields, solar sails, and telescopes (Figs. 1-2). Some systems being considered are hundreds of meters in size to accomplish mission goals. Due to the increase in payload size required, innovative support structures, which can be packed into the faring of available launch vehicles, must be developed. In recent years, research and development work has been performed in this area. Of the available options, one of the most promising technological advancements is the rigidizable inflatable structure. A rigidizable inflatable structure is one that is fabricated on Earth, packed into the launch container, and inflated for deployment once on orbit. After deployment, the material is rigidized, or hardened, to form a stiff composite structure that no longer needs the inflation gas for support. This class of structures has unique benefits such as low packing volume, reduced mass, and in most cases, very high deployed structural efficiency. Several types of construction can be used in a rigidizable inflatable including monocoque, isogrid, IsoTruss, and truss-frame booms. Each composite structure can be fabricated into a varying geometric shapes utilizing any number of resin and fiber types. The fibrous reinforcement can be in tow or woven fabric form. In order to optimize the structure, the sizes of the tows and the weave styles of the fabrics can be varied. It is also possible to manufacture near-zero coefficient of thermal expansion (CTE) booms through the fiber and resin selection and by optimizing the volume fractions of each. However, key to all mechanical performance properties is the ability to fold and tightly pack the material. Member AIAA † Associate Fellow AIAA Undergraduate Research Assistant, Dept. of Int. Science and Tech. Associate Professor, Dept. of Int. Science and Tech. Senior Scientist, Materials Sciences Dept. Distinguished Scientist, Space Materials Lab Figure 2. ILC 3.2m Diameter TSU Hexapod Testbed Figure 1. 1⁄2 Scale Next Generation Space Telescope Sunshield