Rapid In-Space Assembly and Manufacturing of Large Reticulated Truss Structures

Abstract

Modern deployable space structures have enabled spectacular missions like the James Webb Space Telescope, but they are constrained by the rocket fairing and a tradeoff between deployed size and structural precision that limits their use for future communications and astronomy applications. In-space assembly and manufacturing (ISAM), i.e., the construction of structures in the space environment, offers an approach to overcome these issues and enable novel missions both on orbit and on planetary surfaces. Structures constructed in space can be optimized for loads in space, achieve higher packaging ratios, and increase mission flexibility. Given these benefits and decreasing launch costs from modern reusable rockets, there is a resurgence of interest in ISAM from academic, commercial, and governmental entities. However, current ISAM concepts are hindered by inefficient construction processes with high size, weight, and power requirements and a lack of systems-level design of spacecraft and construction processes. This thesis aims to address these challenges to enable energy-efficient, rapid ISAM of large space structures. The first contribution is an analysis of the fabrication time of large truss structures, considering the constraints of spacecraft power, attitude control authority, and avoidance of flexural vibrations. The analysis shows that angular momentum storage of the spacecraft and flexibility of the structure are dominant constraints on fabrication time of gridshell geometries with diameters over 60 m, while the available power and control torque limit fabrication time for diameters under 60 m. This trade study provides quantitative estimates of the total fabrication time, e.g., five spacecraft constructing a 200 m diameter gridshell in five days, and highlights design tradeoffs to enable rapid ISAM, including using multiple spacecraft and varying the feedstock material based on the structure size. Motivated by the long fabrication timescales, the second contribution is an understanding of spacecraft attitude dynamics with changes in mass properties and environmental disturbances during construction. In particular, variable-mass rigid body dynamics are used to understand the feasibility of gravity gradient capture, a passive approach that exploits the gravity gradient disturbance during ISAM. The concept is illustrated with two case studies on the construction of truss structures, a 2D triangle unit cell and a 3D curved gridshell, by spacecraft in circular orbits. Based on the time reversibility of the equations of motion, initial conditions are computed that result in gravity gradient capture by solving the equations backward in time, considering the changes in mass properties from the prescribed construction sequence. The analysis highlights both the feasibility of passive gravity gradient capture and the sensitivity of initial conditions to small perturbations. It is found that deploying a gravity gradient boom before the start of the construction sequence can decrease this initial condition sensitivity by an order of magnitude, and more generally, designing an ISAM process to maintain the minimum principal inertia axis in the direction of the orbit radius vector can facilitate the robust passive gravity gradient capture of large structures. Finally, to aid the design of attitude control systems for ISAM spacecraft, ground experiments are presented to understand the attitude dynamics during Bend-Forming, a candidate deformation process for fabricating truss structures. The experimental results reveal the effect of metal springback during construction, which causes flexible vibrations and coupled motion of both the truss and spacecraft. Additionally, experiments with closed-loop, thruster-based position and attitude control highlight the possibility of control-structure interactions during construction, due to the decreasing natural frequency of the truss structure. Together, these contributions provide a framework for the efficient design of future ISAM spacecraft and construction processes to enable the next generation of space structures.