sive strengths of 130,000 to 200,000 psi. Im- 
proved matrices, reinforcements, and composites 
offering higher compressive strength, modulus, 
shear strength, and environmental resistance can 
lead to greatly improved weight-to-displacement 
ratios and reliability. GRP materials now have 
demonstrated strengths of 100,000 psi; 150,000 
psi appears attainable in the next decade. 
Oxide ceramics are another potential material 
for construction of low W/D ratio pressure resist- 
ant enclosures. Alumina and beryllia appear the 
most likely candidates. Current technology does 
not permit precision manufacture of high strength 
ceramic parts larger than 18 inches. Spherical 
pentagonal ceramic plates imbedded in a metallic 
framework may offer a solution for spherical 
structures. 
b. Future Needs Much must be done to develop 
the nonmetallics into safe, reliable, producible, 
and fabricable engineering materials for deep 
submergence applications. Glass work should 
emphasize reliability, fabrication and penetration 
techniques, and joint design. Fiber-reinforced plas- 
tics need process and quality control improve- 
ment. 
Promising fibers such as carbon, boron, beryl- 
lium, alumina, and others should be developed 
further. Penetrations for manned hulls must be 
developed and evaluated. Oxide ceramics deserve 
extensive investigation with emphasis on mosaic 
structures to solve the scale-up problem. By the 
1980’s, nonmetals may be practical for manrated 
pressure hulls. 
4. Supplemental Buoyancy Material 
a. Current Situation Vehicle volume is an 
important criterion in maneuverability, which im- 
proves as volume decreases. Volume is greatly 
influenced by the buoyancy material employed. 
Combatant submarines have been of such limited 
depth capability that buoyancy has generally not 
been a problem. In fact, they carry lead for 
weight-growth margin and stability. Because of 
available pressure hull materials, deep submersibles 
ordinarily attain neutral buoyancy by carrying 
extra buoyancy material. 
Gasoline is used for supplemental buoyancy in 
the Trieste, which has descended 35,840 feet in 
the Pacific. But gasoline is inefficient as buoyancy 
VI-44 
material, causing the Trieste to be quite bulky and 
unmaneuverable. Its operations with a gasoline- 
filled buoyancy balloon are analogous to helium- 
filled balloon or blimp operations in the atmo- 
sphere. 
Titanium spheres are used on the Alvin to 
provide an effective net buoyancy. Radial fiber 
spheres, a variation on filament wound rocket 
motor case development, show great promise for 
supplemental buoyancy. Spheres with a weight-to- 
displacement ratio of 0.39 have withstood up to 
45,000-foot equivalent depth pressure; an 11-inch 
sphere with no surface resin coating has been held 
at 26,000 feet and tested to failure at 56,000-foot 
pressures. A 32-inch diameter sphere has been 
proof-tested to pressures equivalent to 22,000-foot 
depths. 
Most vehicles currently under construction will 
employ syntactic foam for supplemental buoy- 
ancy. This is a mixture of very light hollow glass 
microspheres in a resin matrix. Current technology 
has yielded syntactic foams with a weight-to- 
displacement ratio of 0.56 at 8,000-foot pressures 
and 0.69 at 20,000 feet. Thus, the achievable net 
buoyancy from each foam is approximately 28 
and 20 pounds per cubic foot of material respec- 
tively. (One cubic foot of water weighing 64 
pounds is displaced by a cubic foot of foam 
weighing 36 pounds yielding a net lift of 28 
pounds, etc.) 
The importance of relative density of syntactic 
foam is evident from analyzing its role in vehicle 
construction and operation. Every pound of vehi- 
cle negative buoyancy when submerged must be 
compensated by a pound of supplemental buoy- 
ancy. If each pound of buoyancy material contrib- 
uted only one-third pound of net buoyancy, then 
one pound of negative buoyancy would require 
the addition of three pounds of buoyancy mate- 
tial. 
The result would be that each pound added to a 
vehicle would compound to a total of four pounds 
of dry weight. Based on current costs for installed 
buoyancy material, each added pound of vehicle 
weight may cost an extra $200 to $300, a cost 
penalty approaching or exceeding that of excess 
weight on a jet aircraft. 
b. Future Needs Volume reductions, and perhaps 
very significant cost reductions, can result from 
improved syntactic foams or other supplemental 
