in curvature. The measurement of the spherical radius on the thermoformed 

 sector concluded the evaluation of the thermoforming process for fabrication 

 of 15-inch model acrylic plastic hulls. Once the selected forming process was 

 found to be feasible for forming the structural modules of the acrylic plastic 

 capsule models, the feasibility of forming structural modules for the 66-inch 

 prototype acrylic plastic capsule was explored. 



The method of forming large-scale pentagon-shaped structural modules 

 was similar, but not identical, to the method used for model structural modules. 

 The reason for not following the forming technique developed for the scale 

 models was the high cost of equipment (dies and presses) that this technique 

 would require for forming structural modules for the prototype 66-inch acrylic 

 plastic capsule. To decrease the costs of forming, it was decided to eliminate 

 the male die, and thus also the requirement for a very expensive press. 

 Although it was surmised that the elimination of the press-actuated male die 

 would increase the deviations from desired sphericity and thickness, it was 

 hoped that the increased deviation in sphericity from a nominal 33-inch- 

 radius would not be of such magnitude as to make the pentagon-shaped 

 spherical sectors unacceptable for pressure hull construction. 



Thus a single female die, henceforth referred to as the mold, was 

 designed and fabricated from aluminum. There was an important departure 

 from the design philosophy used in designing the dies for the 15-inch model. 

 In the previous case, the concave spherical radius of the female die was 

 exactly equal to the nominal external radius of the hull. The female mold 

 to be used in forming sectors for the 66-inch capsule had, however, a con- 

 cave spherical radius 0.187 inch larger than the nominal external radius of 

 33 inches. This intentional discrepancy between the nominal sector and 

 mold radii was based on the opinion of fabricating personnel that, because 

 of the elimination of the hydraulic-press-actuated male die, the formed 

 sector would not contact the whole surface of the mold, but just at the 

 apex and at the edges. Thus, in their opinion, if a 33-inch-radius mold 

 was used the curvature of the formed sector would be less than 33 inches 

 in most places on the sector. On the other hand, if a 33. 187-inch-radius 

 mold was used, the curvature of the formed modules would be about 

 33 inches. In absence of convincing arguments to the contrary, it was 

 decided to try this approach. 



The thermoforming blank was prepared in the same fashion as were 

 those for the 15-inch models. A circular 46.6 inch blank was first sawed on 

 a bandsaw from a 48 x 60 x 2.5-inch plate of acrylic plastic (Figure 26) and 

 then turned to a 46-inch disc with 200-rms edge finish in a vertical boring 

 mill (Figure 27). One corner of the periphery was chamfered 1/8 of an inch 

 for better support and sealing of the blank in the mold prior to forming. 



53 



