testing of spherical or cylindrical shells of these materials. When the 

 scanty experimental data was reviewed, only Pyroceram #9606 and 99-percent 

 alumina shells were found capable of withstanding, consistently, compres- 

 sive stresses above 200,000 psi level. Since both materials are about 

 equally strong in compression, the choice had to be made on some other 

 basis. Ninety- nine-percent alumina ceramic has qualities which make it 

 quite an attractive choice. It possesses modulus of elasticity of 50x10^ 

 psi; the raw material is inexpensive and can be easily produced in many 

 different shapes with very little investment in molds. The high modulus 

 of elasticity makes it an extremely desirable choice as it permits the 

 design and construction of rib-stiffened shells with shallow ribs and wide 

 rib spacing. Still, it suffers one very important disadvantage for use in 

 deep-diving vehicles when compared to Pyroceram #9606. Its specific 

 gravity is 3.85, while that of Pyroceram #9606 is 2.61. Since this amounts 

 to more than 20 percent in weight penalty, it hardly could be ignored in 

 the selection considerations for a vehicle that is to represent a cylindri- 

 cal structure with the lowest weight to displacement ratio that today's 

 underwater hull technology is capable of producing. 



Although the shell section joints can be made in different designs 

 and materials, only the already proven breech- lock joints and aluminum 

 and titanium alloys were considered. The only departure from the already 

 proven joint design was the incorporation of 0-ring seals and a decrease 

 in the size of the ceramic flanges (Figures 24-27;. Both modifications 

 are thought to be improvements over the original design (Figure 10) as 

 the axially compressed static 0-ring seals afford good sealing at both 

 launch and retrieval, while smaller ceramic flanges on the shell ends 

 permit the metallic joint to be further recessed into the shell wall, 

 and thus make the shell capable of receiving payloads with larger outside 

 diameter. 



The first choice for the joint material was high- strength aluminum 

 which had proven itself before during the experimentation with different 

 joint designs. Furthermore, aluminum joints would contribute less weight 

 to the structure than titanium joints. The aluminum alloy selected for 

 the joints was 7001-T6, which, after being forged into rings, was machined 

 to the desired dimensions. The compressive strength of this alloy in 

 excess of 90,000 psi is thought to be sufficient for most of the stresses 

 imposed upon the joint by the ceramic shell sections under hydrostatic or 

 launch loadings. The only apparent drawback of aluminum joints is their 

 corrodibility under repeated exposure to sea water environment. For this 

 reason, a second set of joints has been prepared for Benthos fabricated 

 from Tl-6Al-6V-2Sn titanium alloy. For repeated dives or long-term ocean 

 bottom submergence where the additional weight of titanium joints is com- 

 pensated for by their resistance to corrosion, they will find their employ- 

 ment. 



295 



