Folding and Deployment of Thin Shell Structures
Thin shells made of high modulus material are widely used as lightweight deployable space structures. The focus of this chapter is the most basic deployable thin shell structure, namely a straight, transversely curved strip known as a tape spring. Followi
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ntroduction and Background
Thin shells are widely used in deployable space structures. Open section cylindrical shells have been used on spacecraft for deployable booms for many years and, more recently, a wide range of singly- and doubly-curved this shells have been developed for deployable reflector antennas and radar structures. Some examples are shown in Figs 1, 2. This chapter presents a detailed study of the mechanics, design, realization and testing of a particular type of deployable thin-shell structures, known as tape springs. Tape springs are straight, thin-walled, elastic strips with curved crosssection. The simplest and most common form, shown in Fig. 3, is a cylindrical shell of uniform thickness t and transverse radius of curvature R, subtending an angle α. D. Bigoni (Ed.), Extremely Deformable Structures, CISM International Centre for Mechanical Sciences DOI 10.1007/ 978-3-7091-1877-1_5 © CISM Udine 2015
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Figure 1: Two spring back reflector antennas on MSAT-2 spacecraft. Courtesy: Communications Research Centre (CRC), Industry Canada.
(b) (a) Figure 2: Folding Large Antenna Tape Spring concept (Soykasap et al., 2008) (a) deployed and (b) folded.
Folding and Deployment of Thin Shell Structures
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Figure 3: Tape spring geometry.
The most common application of tape springs is in tape measures, also known as carpenter tapes, which were invented in the late 1920’s. Tape measures can be wound in a small case, and are strained elastically in this configuration, but they become essentially strain-free when they are extended. Typical geometric parameters of a tape measure are R ≈ 20 mm and α ≈ 50 deg, i.e. 0.87 rad. The values of α will be given in degrees, for clarity, but this value is converted to radians for the calculation of the arc-length of the tape spring cross section.
Figure 4: Early tape measure with cup-shaped case and 38 mm wide FatMax tape measure. A more specialized application of the same structural form is found in a type of deployable boom known as the STEM (Rimrott, 1966). In its simplest configuration this boom has α > 360 deg, to form a slit circular tube with an overlap region. Variants of this configuration include tape springs with α ≈ 360 deg and with interlocking tabs along the straight edges, that form closed-section tubes with higher torsional stiffness, and also booms formed by two or more STEM’s nested inside one another, as in the first set of solar arrays for the Hubble Space Telescope shown in Fig. 5. STEM’s have been used as low-gain antennas, to position scientific instruments away from spacecraft, to deploy solar arrays, etc. (Pellegrino,
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Figure 5: Four Bi-STEM booms were used in the Hubble Telescope first set of solar arrays (image courtesy of NASA).
1995). Both the tape measure and the STEM store elastic strain energy during folding and, in principle, they would both freely deploy into the straight, unstrained configuration when all constraints are released. However, free deployment of tape springs is uncommon: in t
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