and casting process development was made. Other fabrication processes 

 besides casting could be used to fabricate hemispheres which subsequently 

 could be joined by bonding or by a mechanical lock. Casting is here again the 

 only process available for producing hemispheres with the required thickness 

 and out-of-roundness tolerances. Although the investment in hemispherical 

 molds, casting equipment, and casting process development is less than for 

 casting of monolithic spheres, it is still quite substantial. 



It is only when the capsule can be assembled from many small 

 structural modules that other fabrication processes can be effectively utilized 

 to fabricate economically a monocoque sphere. Thus, instead of an expensive 

 casting process, the much cheaper vacuum-assist mold forming can be applied 

 to produce spherical sectors with uniform thickness and sphericity. The only 

 limitation on this forming method is that the thickness to outside radius ratio 

 (t/R ) be less than 0.125 for spherical sectors up to a 75-degree spherical 

 angle (as otherwise the temperatures required in forming become excessively 

 high). (For larger sectors excessive variation in thickness also occurs due to 

 unequal stretching of the acrylic plastic.) If the acrylic plastic sphere is bro- 

 ken down into 12 structural modules of pentagonal shape, the vacuum-assist, 

 female mold forming technique becomes about the most economical fabrication 

 process for spheres up to 6 feet in diameter and 4 inches in wall thickness 

 (based on the 48 x 60-inch standard maximum size of acrylic plastic plates). 

 Once the spherical sectors have been thermoformed, very little additional cost 

 is required to machine them into pentagonal form. Spheres larger than 6 feet 

 in diameter can be assembled from spherical pentagonal structural modules 

 prepared by the vacuum-assist female mold form technique, but larger spheres 

 would require use of premium-priced, oversize acrylic plastic plates. If pro- 

 curement of oversize acrylic plastic plates is not feasible and 48 x 60-inch 

 standard sheets are used, the structural modules would not be spherical 

 pentagons of uniform size. The structural modules would either be spherical 

 triangles of uniform size or they would vary both in configuration and in size. 

 Thus, it can be seen that the spherical, pentagonal structural modules proposed 

 by Piccard are, from the viewpoint of initial cost for fabrication equipment, 

 fabrication process development, and fabrication process itself, the most 

 economical approach to assembling a 66-inch monocoque sphere if thermo- 

 forming of flat acrylic plastic plates is substituted for casting of pentagons. 



Since the objective of the NEMO concept was to provide a reliable 

 operational capsule for continental shelf depth, and not to set depth records, 

 no effort was made to design the capsule for the 6,000-foot depth originally 

 postulated by Piccard for his acrylic plastic capsule concept. Aside from the 

 four major design constraints, all design parameters were to be determined 

 during the course of the study. Thus, the resulting design would closely 

 match the operational requirements of the NEMO system. 



10 



