UNDERSEA TECHNOLOGY 37 



Small undersea vehicles are usually composed of a single spherical shell 

 or a number of spherical shells either nested or connected by short stiffened 

 cylinders. Additional structure consists of reinforced openings for such 

 items as hatches, penetrating shafts, and electrical fittings. Submarines, 

 however, are basically structural cylinders. Submarines and small undersea 

 vehicles represent the only major engineering structures with small huU- 

 thickness-to-hull-diameter ratios that are subject to relatively high external 

 hydrostatic pressures. The submersible design must ensure structural sta- 

 bility, because buckling of the shell can occur at pressures considerably 

 less than those required to cause yielding of the hull. 



A submersible designed with a safety factor as high as required for most 

 commercial pressure tanks would be so overweight that it could not ac- 

 compUsh its mission. On the other hand, a serious responsibility rests with 

 the designer to assign a realistic margin of safety. The Navy has supported 

 theoretical analyses which provide upper and lower boundary predictions 

 for the strength of pressure hulls. These have been well publicized and dis- 

 cussed in various technical reports and publications. Certain strength- 

 reduction factors must be applied to the theories in the design of a sub- 

 mersible. This step is necessary because the strength is susceptible to such 

 manufacturing variables as unavoidable local deviations from circularity or 

 sphericity; residual stresses locked in the structure during fabrication due 

 to rolling, machining, and welding; and mismatches of structural elements 

 (such as frame hull connections, penetration reinforcements, etc.) which 

 do not satisfy conditions of simple or fixed support assumed by theory. 

 Additional factors influencing the design of a submarine include fatigue 

 life, variations in physical and mechanical properties of materials, and 

 various dynamic disturbances and local loadings. Because of these vari- 

 ables, design formulas cannot be derived entirely on the basis of theoret- 

 ical calculations. Suitable empirical relationships must be used where 

 manufacturing and service variables exist. To establish these empirical 

 relationships, the Navy has conducted and continues to support model tests 

 to simulate the actual conditions which are prevalent in pressure-hull fabri- 

 cation and which have an influence on service performance and on the ul- 

 timate collapse strength. 



HIGH-STRENGTH STEELS, TITANIUM, AND ALUMINA 



The prime requisite for achieving the capability of "going deep" is the 

 development of new materials. The current state of the art in material 



