Deployment Dynamics of Thin Shell Space Structures
Researchers
Antonio Pedivellano
Andrew Lee
Alan Truong
Charles Sommer
Luke Chen
Eleftherios Gdoutos
Sergio Pellegrino
Description
Recent advances in flexible solar cells and electronics have offered the opportunity to develop novel lightweight and cost-effective structural concepts for deployable solar arrays and antennas. A promising path is to design continuous flexible strips, where 2 thin shell longerons, connected by transverse rods in a ladder-like structure, support a flexible functional film.
With this approach, the longerons would provide bending stiffness to the deployed structure, but also allow it to be elastically folded in a stowed configuration. The strain energy stored during packaging can also be used to self-deploy the structure in space. Engineering implementations of this concept require deep understanding of the dynamics of the structure during self-deployment, and of the behavior of its elastic folds, which are known to propagate during deployment of simpler shell structures, such as tape springs. Therefore, we have developed an experimental setup to study the dynamic deployment of 1 m-scale flexible strips, based on Triangular Rollable And Collapsible (TRAC) longerons. The strips are supported by cords at their ends, and are symmetrically folded at two locations.
A high speed stereo camera system, in combination with Digital Image Correlation (DIC), is used to track the longerons during deployment of the strip, identifying the location of the elastic folds and their angle.
We have performed deployment experiments on two strip prototypes, one of which covered by a thin membrane, to investigate the effect of a functional film on the deployment of the structure. Experiments in air and vacuum have shown the influence of air on the deployment during ground testing. The deployment process can be described by the evolution of the angle of the folds , which decreases until becoming zero in the deployed state.
Our study shows that the deployment of the structure is mostly symmetric, and the elastic folds do not propagate. The effects of air are significant for the strip with membrane, whose deployment is much slower than in vacuum. More details can be found in the references below.
We have also developed a finite element model of deployment, based on an explicit solver in Abaqus. Our model accounts for the effects of air using a combination of added mass (pre-computed from geometric arguments) and air drag (estimated from elementary aerodynamics).
While flexible strips can be used as standalone deployable structures, we are also using them as building blocks for a deployable spacecraft. Its structural architecture (shown below) consists of an assembly of trapezoidal strips arranged in a 4-fold symmetric square pattern. The Kapton film covering each strip is intended to support flexible solar cells and RF antennas, enabling the space solar power technology.
The deployment scheme for the space structure consists of two steps: first, uncoiling from a cylindrical configuration (E) to a star-shape (C); then, unfolding to the operational planar state (A). The uncoiling step is controlled by a deployment mechanism, whereas the unfolding process is driven by the strain energy in the structure.
To understand the unfolding dynamics of the space structure, we have performed deployment experiments on a 2 m-scale structural prototype. Based on a kinematic study of this architecture, a sequential deployment scheme was devised to unfold the structure in 2 steps (see video below), providing a more controlled dynamics. Each deployment step was initiated by removing a set of constraints from the structure. In early implementations, the constraints were applied by hairpins, removed by hand. Later on, we developed a Remotely Controlled Release System (RCRS), to make the concept of releasable constraints feasible in space, too. More recent experiments have shown that successful deployment of the 2 m-scale prototype can also be achieved with a single-step deployment.
Using the motion capture system in the Cast arena at Caltech, we have tracked the location of spherical targets placed at the 4 ends of the structure, during deployment. The average distance from the central shaft of the deployment mechanism is used to describe the deployment dynamics (see above).
Current efforts are focusing on the development of computational models of this process, capturing the complex contact conditions between multiple strips, as well their kinematic constraints, and the effects of gravity and air.
Publications:
Pedivellano, A., and Pellegrino, S. (2021). Deployment Dynamics of Foldable Thin Shell Space Structures. In AIAA Scitech 2021 Forum
Pedivellano, A., Gdoutos, E., and Pellegrino, S. (2020). Sequentially controlled dynamic deployment of ultra-thin shell structures. In AIAA Scitech 2020 Forum, 2020-0690
