Development of Economical Casting Process for NEMO-Type Acrylic Submersible Hulls

Abstract

This paper describes an improved process for fabricating spherical acrylic plastic pressure hulls within close dimensional tolerances. The process consists of casting acrylic plastic hemispheres in a precision mold assembly, machining their equatorial edge and cutting polar penetrations, bonding them together with a cast-in-place equatorial joint, polishing their inner and outer surfaces, and installing an aluminum hatch and penetration plate. The cost of the improved process is approximately 50 percent less than that of the standard process, which consists of bonding together 12 thermoformed and machined spherical pentagonal shell sections. In addition, 90 percent fewer bonded joints are required, resulting in an order-of-magnitude improvement in optical qualities. A full-scale prototype with an outside diameter of 66.500 in. (168.9 cm) and an inside diameter of 58.000 in. (147.3 cm) has been constructed and shown to be acceptable for manned service to a depth of 2500 ft by hydrostatic testing under sustained loading at pressures of 900, 1350, 1800, and 4000 lb/in.2 (6.2, 9.3, 12.4, 27.6 MPa). Implosion occurred after 13 min of sustained loading at 4000 lb/in.2 (27.6 MPa) and 75°F (23.9°C) simulated depth of 9000 ft (2743 m).