Editor’s Note: One of the toughest problems facing defense planners and programmers today lies in predicting the warfare requirement which will be faced far enough in the future to permit appropriate design of platforms with gestation periods measured in decades using technologies which may be at an embryonic stage in the development process. There are very few cases like the development of the nuclear submarine or the Submarine Launched Ballistic Missile where a crystal clear priority requirement existed in close proximity to perceived near-term technical attainment. They were classic cases of perfect coupling between requirement pull and technology push.
While most developments involve more reliance on the pushing from below rather than the pulling from above, there are several ways to approach the problem by considering both sides of the equation. One method is to wait for someone to have a great idea,· unfortunately, all too often it seems that the bosses, or the committees, do come up with a brain storm which turns out to be less than well-founded. Another approach, and the one which most successful programs follow, is to create a credible view of the future (a vision, if you will) on which to quantify a set of requirements which can be used to particularize the design of a weapons system within bounds of the technologically foreseeable. The success of those programs is usually dependent on the depth of effort put into examination of both military needs and industrial capabilities. The New SSN program looks to be a winner in that category on all counts.
When it comes to longer-range projection, however, it may be useful to fall back on the Jules Verne School of Prediction. When a novelist, such as Verne, has need of a futuristic view he usually learns all he can about his particular subject and also about the various sciences which act on that subject. He then proceeds to put the obvious trends together, treats the technical hurdles as presolved accomplishments and binds the package tightly with human nature. To give a submarine-specific example of such projection, THE SUBMARINE REVIEW asked a writer working on a submarine-related project to employ his novelist’s craft in a look at the world of undersea warfare in the mid-21st century.
Most members of the submarine community during World War Two could hardly have dreamed of nuclear propulsion, or titanium hulls, or supercomputer sonar signal processors. But apprentice torpedoman and squadron commander alike would have often yearned for the benefits such engineering marvels provide: longer submerged cruising endurance, greater test depths, and more powerful combat sensors. That was 50 years ago, and 50 years is a very long time, long enough to see both dramatic technological advances and major repositionings on the world geopolitical stage. What might naval submarines be like, and why may they be needed, if we project forward another 50 years?
This article will offer some suggestions and speculations, at once pragmatic and progressive, about the U.S. Navy’s nuclear powered submarines in the year 2050. Qualitative projections and suggestions will be offered as to future hardware capabilities, operational usage, and overall missions assigned, three factors that are always intimately related in naval submarine development and employment.
Hull Materials and Test Depth
The continuing trend for many years has been toward greater test depth. Recent advances in materials science may lead eventually to improvements dramatically beyond today’s roughly 1500 feet for steel (enough to stay below the Deep Scattering Layer) and 3000 feet for titanium (penetrating the upper reaches of the Deep Sound Channel).
Alumina ceramic composites, now being experimented with for
research minisubs, combine tremendous strength with densities low enough to approach neutral buoyancy. Utilizing such materials to build a fleet submarine, one might obtain a hull that is extremely thick yet avoids excessive displacement, permitting SSNs and SSBNs to achieve an order of magnitude increase in operational depth without sacrificing useable internal volume or machinery and payload weight capacity. Let us begin to examine what such subs could achieve.
First, there would be two potential sources of enhanced quieting just from the hull design itself:
1. A very thick hull may enhance acoustic isolation of the sub’s internal machinery.
2. The rigidity that comes with great thickness might prevent the hull popping sounds given off by conventional subs during rapid depth changes. (A thick and stiff hull might also avoid the need for internal ribbing, and might prevent hull resonances sometimes induced by internal machinery or external insonification, thus reducing active sonar cross section as well as passive signature.)
In addition, cavitation of the propulsion system at high speed would be reduced because the critical rotor RPM rate at which cavitation begins, everything else being equal, rises roughly with the square root of the depth. This would raise top quiet speed, and might raise .top maximum speed as well. That may become increasingly important in the future, not just for rapid transits to the battle area, but to achieve engagement supremacy (water superiority?) once there against surface craft with ever higher flank speeds of their own. SWATHs, pump-jet driven freighters, ASW hydrofoils, and perhaps other propulsion breakthroughs hard for us to imagine, will all make it harder and harder for an attack sub to intercept an enemy carrier battle group or merchant convoy and do useful work against it.
Within 50 years we may see both the need for and the available funding to permit constructing what we might label an FSSN, a future SSN, or FSSBN, a future SSBN. It is tempting to imagine a vessel able to dive routinely to, say, 15,000 feet, which is deeper than the average depth of all of the world’s oceans. Here are some of the advantages for both offense and defense that such a capability would bring:
1. Enhanced stealth, and thus survivability, relative to emerging ASW detection methods such as surface wake analysis, thermal plumes, magnetic anomalies, and blue-green laser scanning (lidar). (More sophisticated methods to reduce such signatures while at shallow depth can also be anticipated in the years to come.)
2. Greater survivability through the thicker, stronger hull, which would be more resistant to enemy warheads both conventional and nuclear.
3. Increased flexibility to play hide-and-seek below the Deep Scattering Layer, and within and even well below the Deep Sound Channel itself.
4. Nap-of-Sea.floor maneuvering over much of the ocean bottom. Submerged terrain such as seamounts; mid-ocean ridges, and trenches can form an ultimate battleground with respect to: a) stealthy approach toward an enemy coastline or operational area, b) concealment laterally from enemy active and passive sonar using intervening bottom contours, c) concealment vertically by hiding in sonar ground clutter or by lurking beside an old wreck, and d) ability to lie in ambush with look up sensors watching for enemy submarines and surface craft. . In the submarine warfare of the year 2050, there could be real advantages to commanding the low ground. Additionally, the tactical need or desire to stay off the skyline, while coping with bottom topography in close proximity to the boat, gives nap-of-seafloor combat some of the flavor of submarine littoral warfare.
5. Availability of more horizontal seawater layers of varying density and reverberation characteristics, for enhanced concealment from enemy ASW forces and their weaponry. Deep ocean currents and marine life concentrations can create such layers well down in the bottom isothermal zone.
6. Reduced effectiveness of conventional enemy torpedo and depth charge warheads with greatly increased depth. (Of course this would apply to one’s own weapons directed against deep targets as well, suggesting the need for R&D on explosive charges and delivery platforms that would work well at pressures of three or four tons psi.)
7. Reduced cavitation of high-power active sonars. The critical wattage at which the water outside the dome begins to boil is higher with greater depth. This obviously improves effectiveness of the system.
8. Ability to exploit vertical temperature/density sonar terrain and weather features found around volcanic vents and black smokers as their super-hot exudations rise and then disperse. An inverted cone would result that, given apex temperatures of 500 or 800 degrees Fahrenheit, would have profound effects on sonar propagation.
9. Avoidance of the noise resulting from long-wavelength surface waves, which can penetrate down to 1000 feet and impair passive target detection.
10. Greater vertical separation, and hence greater passive (and also active) sonar signature transmission losses, relative to enemy ASW surface forces (or shallow-diving submarines) that may have localized the FSSN. Assuming spherical attenuation, ten times the depth implies one one hundredth the received signal strength.
Let us consider next some additional technological advances that may improve submarine quieting during the 21″ century.
1. Development of permanent or semi-permanent hull coatings (as opposed to continually discharging long-chain polymers from the nose of the vessel), to more effectively reduce water resistance and flow noise. This would benefit speed, quieting and sonar sensitivity.
2. Increasing use of hull coatings and/or tile coverings to reduce active and passive sonar cross sections.
3. Development of hull materials whose compressibility is equivalent to that of water, thus becoming almost transparent to sonar.
We can probably expect the competition between more capable sonars and quieter subs to continue indefinitely. More sensitive hydrophones, more sophisticated array designs, faster computers with bigger memories, and new signal processing algorithms, will all make it harder to hide when a sub wants to hide. Clearly, greater test depth provides an important advantage. Also, it seems likely that continuing research into marine biology, geology, and oceanography will have ever greater importance to national defense, if and when the deep ocean becomes (perhaps tragically) a theater of warfare. And what better platform to develop such vital data quickly and covertly, than an FSSN which can easily traverse the area in question?
Beyond these sonar considerations, other new and emerging sensor capabilities may become important. Consider three related ways a submarine might literally visualize the sea around it while well below periscope depth:
1. Active imaging through blue-green laser line scanners. Increasingly powerful lasers, charge coupled intensifying detectors, and image enhancement algorithms, may permit a sub’s CO and crew to see the ocean in their immediate vicinity. (Non-reflective coatings would be desirable to reduce one’s own detectability by such lidar emitters carried by enemy submarines or enemy surveillance satellites, aircraft, or surface ships, including lidar and lidarbuoys.)
2. Passive imaging through electronic intensification of natural bioluminescence. Many marine species emit such electro-magnetic energy, especially when disturbed by intruders or as a method of luring prey. Certain bacteria living near hot vents also emit weak bioluminescence. The natural lighting in the ocean depths could have important military uses some day.
3. As in 2., except, at relatively shallower depths like as 200 or 1000 feet, electronically amplifying and using for illumination whatever sunlight (or moonlight!) does manage to penetrate.
Since light is rapidly attenuated in seawater due to suspended particulates, these methodologies would apply only over relatively short ranges. However, since the density of marine life attenuates with depth, there may be areas of the ocean where visibility can be made better than near the surface. Great technical challenges would have to be overcome to create sensors capable of operating under ambient pressures of dozens or hundreds of atmospheres. Perhaps by the year 2050 it will be possible to “look around” to a range of 1000 feet or 1000 yards (ten boat lengths?). even more. What benefits might this bring?
1. Greater ability to detect and stalk enemy submarines, in several ways:
a. Another submarine would in some sea conditions leave a trail of underwater bioluminescence that may persist long enough to be detected by electronic means. Analysis of this trail might yield data on course and speed as well.
b. Another submarine’s passage might also leave a trail of damaged or shredded marine life, which could also be detected by active or passive visual means. This would be true of both conventional screw-propeller and pump-jet powered vessels.
c. Nap-of-seafloor maneuvering might stir up bottom sediments, again leaving a spoor which innovative tacticians might exploit.
d. Persisting wake vortices left by the passage of enemy subs might reveal themselves through lidar doppler effects, in an analogy to how aircraft radar now detect wind shear.
e. At short ranges, using reflected light, a submarine might be able to directly observe by visual means another submarine, even when the latter fails to show up on passive (or even active) sonar because of intervening acoustic scattering and diffraction. Enemy submarines might also be detected passively by their obscuration or blocking of available light, which relates to the sonar hole in the ocean issue alluded to below.
2. New means to detect, avoid, and clear submerged or floating mines, using lidar with variable intensity and beam width. An FSSN with such imaging equipment would be better prepared to map or penetrate enemy minefields, which might sometimes have a more obvious visual signature that either an active or passive acoustic one. Unmanned (or rather, uninhabited) underwater vehicles (UUVs), or even robotic grapnels attached to the parent sub, might then be used to disarm the mines or move them aside.
3. Improved ability to detect and avoid deep-draft surface vessels. This is a significant hazard when a sub is operating shallow near a harbor or along coastal or mid-ocean shipping lanes.
The limited range of light underwater is not entirely a disadvantage, since it enhances the security of active visual scanning by an FSSN operating in the face of the enemy. Sometimes, as hinted above, an additional detection means that is only operative over short ranges can still be a powerful complement to existing methodologies (especially when it possess better inherent directivity). For instance, an FSSN which localizes an enemy boomer through a sound transient may then be able to track down that target, by proceeding to the original datum to pick up and follow the trail of effects the target’s passing had on the surrounding medium. · Complex tactics could evolve, including the intentional creation of a false trail, with doubling back to lie in ambush against one’s pursuer. Again, the basic characteristics of the ocean and its contents and boundaries become an important subject of measurement and analysis. Underwater meteorology, with its attendant understanding and prediction of both acoustic and visual conditions in different places and at different times, will remain a relevant topic for the submarine community in the future.
Next, speaking of the ambient noise environment of the sea, ambient sonar may eventually become a routine operating mode of acoustic surveillance. This technique uses the constant background noise of the oceans, resulting from surface waves, passing ships, marine life, and other sources, to illuminate targets and terrain features that may be surrounding one’s submarine. This is a hybrid of active and passive sonar: the listening submarine does not transmit, but it is listening for echoes off of targets rather than just their self-noise. Ambient sonar can also be thought of as a version of bi-static sonar, in which one vessel pings and another listens for the echoes.
The flip-side of ambient sonar is listening for holes in the ocean, obstructions to ambient sound resulting from enemy submarines in the vicinity. More powerful and subtle sonar equipment would permit detection in this manner at greater ranges with a lower false alarm rate. A very competent future submarine might defeat this mode of detection by actively transmitting a replica of local sea noises in the direction of a suspected listening enemy.
Other recent articles in THE SUBMARINE REVIEW have discussed approaches to the man/machine interface that cope with the potential information explosion resulting from new and multiple types of sensor data. Undoubtedly, we can look forward to ever more sophisticated virtual reality and/or holographic visual presentation modes that integrate optical and acoustic information (including three dimensional target motion analysis situation displays). This would be vital in high-speed nap-of-seafloor maneuvering, to avoid impact with bottom terrain or entry into canyon cul-de-sacs that leave one cornered by enemy SSNs or their torpedoes. The old concept of highway in the sea helm displays becomes relevant again. Accurate large scale (i.e., finely detailed) seafloor maps will become quite important too, as will high- precision submerged navigation systems, since crashing into a seamount can spoil your whole day, and a sub doing 60 knots (not impossible) advances 1000 feet every 10 seconds. On a more positive note, observe that deep diving subs, with proper maps and using acoustic and/or optical sensors, could obtain valuable pinpoint updates of their inertial navigation systems by referring to submerged terrain features for a kind of orienteering. This would be especially relevant for a futuristic boomer, whose survivability after launching would certainly be enhanced by an ultra-strong hull capable of diving to great depth.
Some of these thoughts suggest that submarine warfare may in the future become even more dynamic, three-dimensional, and fast paced. This will probably require an evolution beyond the traditional course log and bell book approach to conning the ship. Eventually, a closely-knit team might work under direction of the commanding officer to make continual changes to course, speed, and depth, striving to maintain the initiative in a complex underwater ballet not entirely unlike engagements between fighter aircraft or fighters and bombers. Simulations and wargaming could be used to get a better handle on this issue.
Part II will appear in the October issue of THE SUBMARINE REVIEW