Fold and Zip: A Highly Stiff Rollable Arm with Cross-Sectional Shape Change

This study presents a shape-transition-based deployable boom that combines compact storage with increased flexural stiffness after deployment. The structure consists of three commercial tape springs connected by flexible tapes. In the stored state, the tape springs maintain a flexible open cross-section and are coiled inside a compact carrier. During deployment, they pass through a passive transition passage that continuously transforms the cross-section into a triangular closed shape. An interlocking unit maintains the deployed geometry without a separate active locking mechanism. Three-point bending tests were conducted on U-shaped, N-shaped, O-shaped, and the proposed Δ-shaped specimens. The Δ-shaped structure exhibited an average flexural stiffness of 20.55 N/mm, approximately 14.4, 5.4, and 3.9 times greater than those of the U-shaped, N-shaped, and O-shaped specimens, respectively. Cantilever loading tests using a repeatedly deployable and retractable prototype documented tip-deflection responses at selected deployment lengths and tip loads. The results indicate that cross-sectional shape transition can provide a compact stored configuration and increased measured flexural stiffness after deployment in the present prototype.