Page 123 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

5.1 Vehicle design 111
(a)
FIGURE 5.3
(b)
Macroscopic (a) as well as microscopic (b) views of glass microspheres.
Syntactic foams have become widely accepted throughout the oceanographic and military com- munities, especially for the deeper diving vehicles, because of their reliability to perform over numerous excursions to their design depth. The desired density remains stable for many years of service. Syntactic foam typically exceeds a service life of 20 years. This section will address the understanding, care, and feeding of this high-performance material.
When glass microspheres (Figure 5.3) are used within the resin—some call them balloons while others call them bubbles—and the disbursement is controlled so the optimum packing factor is attained, then syntactic foam is created. This is referred to as “solid syntactic.” Solid syntactics range in density from 18 lb/ft3 (0.29 g/cc) to 42 lb/ft3 (0.67 g/cc.) depending upon the service depth
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requirements. Simply stated, the density of seawater (64 lb/ft , 1.0 g/cc), minus the air weight of
the syntactic foam, equals the amount of buoyancy in seawater. This simple equation is used whether it is syntactic foam, blown foam, ceramic spheres, glass balls, or balsa wood. Syntactics become the work horse when going beyond depths of 2000 ft (600 m). From there to 36,000 ft (11,000 m), in most instances, they are the buoyancy material of choice.
The theoretical maximum dry packing factor for a solid material is 68%. Using special tech- niques, the syntactic manufacturers are able to attain 80% packing of the spheres, thereby providing the maximum amount of buoyancy, or payload, to the vehicle. This method is called “using a binary,” a combination of glass spheres, typically at a 7:1 size differential, to provide the maximum amount of packing. Unfortunately, glass sphere manufacturers are not prone to manufacture these sizes as the market is limited and the costs are high. However, manufacturers that have a good R&D department can find ways around this shortcoming.
In most applications, solid syntactic foams are used due to their low moisture pickup at test depth. A well-constructed syntactic will have ,3% water absorption over an extended period at
(Courtesy Trelleborg.)