upon the end plates by the internally or externally located equipment when 

 the capsule was lowered on the sea surface. The spring assemblies (Figure 23) 

 on the tie rods were of such design that when the capsule was submerged to 

 its design depth of 1,000 feet and remained on station for 100 hours, the 

 diametral decrease of the hull would match the original extension of the 

 springs and thus the total tie-rod preload would decrease to zero. Upon 

 depressurization of the capsule, the dilation of the hull would contract the 

 springs again and the tie-rod system would again be under the specified pre- 

 load of 100 pounds. 



Some models were not equipped with internal tie rods in order to 

 (1) simulate NEMO systems that would not utilize tie rods and (2) to 

 compare experimentally the effect of preloaded tie rods on the critical 

 pressure of the acrylic hulls. 



All models equipped with operational polar plates were ballasted 

 with properly distributed weights simulating the weight distribution on 

 the 120-inch-diameter NEMO. A portion of the weights rested on the 

 bottom plate simulating the weight of the crew, controls, and life support, 

 while a sufficiently large weight was suspended from the bottom plate to 

 give the whole model negative buoyancy (Figure 57). 



2. Simulated end plates were structurally similar to the operational 

 end plates but had none of the structural details or working parts found in 

 the operational end plates (Figure 63). The simulated end plates were used 

 on the majority of models (Figure 64) for economy. Since, however, they 

 like the operational end plates were designed to fail in the 1,300-to-1,600-psi 

 pressure range as were the acrylic plastic hulls, they were generally not 

 employed in the model tests at low temperatures where the implosions 

 were predicted to take place at pressures in the 2,000-psi range. 



3. Heavy-duty end plates were simply metallic end plugs of the same 

 thickness and curvature as the acrylic plastic hull closing off the polar openings 

 in the acrylic plastic hull (Figure 65). The heavy-duty end plates were fabri- 

 cated from 6AL-4Va titanium alloy to give them high strength while at the 

 same time keeping their rigidity low so that the mismatch between the 

 rigidities of the end plate and the hull did not generate undue stress concen- 

 trations in the acrylic plastic. The heavy-duty end plates were used only in 

 those capsule models that were to be tested at low ambient temperatures 



that could increase the critical pressure of the acrylic plastic hulls above the 

 design pressure of the operational or the simulated hatches of 316 stainless 

 steel. 



