include life-support equipment, railing, ladders, or any other structural 

 components that have little influence on the stress distribution in the acrylic 

 plastic capsule. It is well to point out, however, that the actual t/R ratio 

 of the 1 5-inch models is somewhat smaller than that calculated previously 

 in the section "Selection of Capsule Dimensions." There was only one 

 reason for the t/R ratio of the 15-inch models to be in the 0.060 to 0.0685 

 range rather than at exactly 0.0685 as calculated: variation in thickness of 

 commercial acrylic plastic plate from the nominal 0.5-inch plate thickness. 

 Procurement of a nominal 0.625-inch-thick plate that subsequently could 

 be ground to the specified 0.51 3-inch thickness would have produced models 

 with the exact t/R = 0.0685 ratio, but only at the expense of 300% increase 

 in the fabrication cost of each capsule model. 



Although at a first glance the use of models with a somewhat thinner 

 wall than calculated appeared to be quite unacceptable, there was a positive 

 gain: the experimental data generated by destructive testing of models under 

 short-term, long-term, or cyclic pressure loading would be conservative. 



PHASE 2— FABRICATION PROCESS 



The selection of the proper fabrication process posed considerable 

 difficulties for two reasons: (1) it had to perform equally well in the fabri- 

 cation of large-scale and model capsules, and (2) economical fabrication 

 techniques for model capsules generally turn out to be uneconomical for 

 large-scale capsules, and vice versa. For reasons already briefly discussed, 

 the fabrication of the acrylic plastic capsule was chosen to conform closely 

 to Professor Piccard's concept except for the introduction of material grades 

 and shapes not commonly' available in his day. 



Acrylic Hull Fabrication 



Material Selection. The material selected for the fabrication of both 

 model capsules and any future full-scale capsules was grade G Plexiglas acrylic 

 plate stock. This material is the commercial grade acrylic plastic widely avail- 

 able as an off-the-shelf plate and sheet stock item in many different sizes and 

 thicknesses. 



Although the acrylic plastic under the trade name of grade G Plexiglas 

 was chosen for the NEMO program because of the proximity of a major 

 supplier that carries this material in stock, other commercially available acrylic 

 plastic plates with equivalent material properties sold under other trademarks 

 (Swedlow 310, Acrylite, and others) probably could have been used with 

 about the same success. 



38 



