Cooperative-transportation dynamics of two helicopters transporting a rocket booster undergoing planar motions

Helicopters are widely used in the field of material transportation as flying cranes. There have been attempts to utilize helicopters to capture a rocket booster that has been slowed down in the air by a parachute. However, when the rocket booster exceeds the load limit of single helicopter, the cooperative use of two or more helicopters must be considered. The cooperative system becomes coupled, and complicated, as a result of the dynamic effects added by multiple helicopters and cables, as well as the remaining liquid fuel in the rocket booster. This paper establishes a fully-coupled dynamic model of two helicopters transporting a rocket booster, which can describe the coupled effects among the helicopter attitudes, load pendulum, and liquid sloshing. In order to improve the ability to resist external disturbances, and induce minimal attitude oscillations and load pendulum swing during flight, a cooperative controller consisting of a modified model-following controller and a hybrid piecewise smoother is proposed. To quantitatively prove the control performance and robustness, computational simulations were performed comparing the proposed controller with a previous combined controller.