Title: Launch and Recovery Improvements for the Littoral Combat Ship to Combat Wake Effects on the Remote Multi-Mission Vehicle
Author(s): Darryl Updegrove and Pedro Brancho
Abstract: The Remote Multi-Mission Vehicle (RMMV) is an unmanned, snorkeling, Diesel-powered, remotely operated subsurface vehicle that is part of the Mine Countermeasures (MCM) mission package (MP). The MCM MP is a modular system of systems that can be installed aboard the Littoral Combat Ship (LCS). The LCS is designed to launch and recover the RMMV in the stern wakefield while transiting at 5-6 knots. Testing to date has shown that the turbulent, aerated, waterjet driven wakefield of the LCS provides a challenging environment in which the RMMV has difficulty maintaining position and alignment to the launch and recovery system for capturing and lifting from the water, making this event problematic. Testing in early 2012 resulted in a high frequency of damage to the lifting structure on both the RMMV and launch and recovery system, even in relatively benign sea states. Outline of the key points to be made in the paper?Evaluation of the environment, RMMV handling characteristics, procedures, ship’s hardware and interface lifting structure led to the decision to redesign the lifting structure to accommodate misalignment between it and the RMMV. Although the lifting structure is uniquely designed for this function and was performing as the design intended, it was the lowest cost item that offered the highest potential gain through redesign. The concept of providing a stable lifting point that was suspended from the ship’s stern launch and recovery system was modified to providing a stable, but more compliant system that could roll with the movements of the RMMV when attached, so wakefield effects on the RMMV would not immediately result in snap loads in the lifting structure or lift lines. The modifications also improved the way snaps loads are transferred to the RMMV and the lifting structure. Testing demonstrated that these changes allowed up to 30 degrees of roll misalignment between the RMMV and the ship’s lifting structure while in the water before hard stops resulted in full structural loading. Additional analysis revealed that flat contact surfaces in the design were actually creating point and line loads due to RMMV to lifting structure relative motion. These loads resulted in permanent material deformations of the lifting surface, rendering them less than ideal, and in some cases unfit for use. The lifting member was redesigned to be compliant in five degrees of freedom. This redesign allowed lifting surfaces to stay properly mated and reduce damage to the lifting surfaces. Testing in 2013 demonstrated safe operations in sea state conditions where previous testing demonstrated damage to equipment would occur. Conclusions/Recommendations ?Having self-centering, compliant interface features on the lifting device allows for greater watercraft motion relative to the ship, providing a safer evolution. Operational risk is reduced, launch and recovery in higher sea states is viable, and operational performance is increased significantly. The concepts employed in this system are adaptable/suitable to other cylindrical shaped watercraft, and the possibility now exists for common launch and recovery features/capabilities between host ships and various watercraft.