Submarine Platform Technology
OVERVIEW OF FUTURE SUBMARINE PLATFORM TECHNOLOGY
Vision of Submarine Platforms for 2035
Over the next 40 years rapid proliferation of high-technology systems will render nonstealthy platforms and weapons systems increasingly vulnerable. The inexorable global spread of modern technology will allow hostile nations to increase their sea-denial capabilities through improved surveillance, enhanced reconnaissance, rapidly expanding information technology and precision weapons. This growing ability to inflict significant casualties on forces that can be detected and tracked easily places a premium on the value of stealth. U.S. forces, required to establish and maintain sea control when and wherever the national interest requires, will need maximum stealth capabilities. The increased value of, and emphasis on, stealth will likely result in increased reliance on submarines in future naval operations.
Submarine systems clearly fit into the definition of "sunrise systems," recently espoused by the Chief of Staff, U.S. Air Force: "… systems which incorporate stealth, high mobility, precise targeting, minimum logistical requirements, and operational autonomy."
Submarines can position early and covertly to strike key threat command-and-control nodes with precision missiles or to deploy ground forces and provide support. Submarines provide a stealthy platform with great range, mobility, endurance, payload potential, and survivability. In many hostile environments, the submarine may be the only survivable platform. Future submarines will offer
a significant degree of flexibility and reconfigurability, both internally and through the use of off-board vehicles, sensors, and weapons; they also will accommodate rapidly emerging technology to improve current capabilities and to enable new roles and missions. Advanced battle management systems that enable cooperative engagement with other naval forces will enhance the effectiveness of submarine participation in complex missions including antisubmarine warfare, strike operations, theater and national missile defense, and the deployment of ground forces for specialized warfare. The greater relative survivability (based on stealth, mobility, and endurance) of the submarine and the potential for expanding the range and depth of mission effectiveness suggest a greater role for submarines in the Navy of 2035.
In striving to attain this vision of future submarine platforms, a major objective must be to develop submarines and systems that can be acquired, operated, and maintained in the most cost-efficient manner possible. This drive for greater affordability must address the submarine's entire life cycle from design to disposal. Cost savings can be pursued aggressively through virtual design and prototyping, design for modular construction and technology insertion, system elimination and simplification, maintenance avoidance, and finally, ship disposal. Taken together, innovation in submarine design and the application of automation can result in a significant reduction in the manpower required to operate and maintain future submarines.
Warfighting Objectives Driving Technology
As naval warfare has evolved and matured, submarine mission areas have steadily broadened. Continued mission expansion will be driven by the ever-increasing value of stealth, endurance, and mobility. The following warfighting objectives serve to define important military capabilities desired by the year 2035; thus, they identify technologies to be pursued:
Sea control. The exercise of sea control and the certain denial of that control to adversaries are fundamental missions of the submarine. If a submarine is in an operating area, other platforms operate at its sufferance.
Precision strike. Covert on-station presence, early and for lengthy periods, is necessary in order to identify, observe over time, and destroy when directed potential threat command-and-control nodes and other vital targets with precision submarine-launched missiles.
Covert insertion. Deployment of ground forces of various numbers, configurations, and capabilities offers the advantage of determining optimum timing by covert and, if necessary, extended on-site observation of the tactical situation.
Coordinated fire support. Submarines must be able to launch strikes in support of forces both ashore and afloat, utilizing various weapons. In the near
future, the OHIO class Trident submarine could be configured to carry and launch between 100 and 200 tactical missiles.
Intelligence collection. The capability for tactical and national intelligence collection over an extended period is needed to provide forward covert surveillance both prior to and after onset of hostilities.
Theater antisubmarine warfare. This capability includes protection of sealift, both through constricted littoral areas and in the open ocean, as well as strategic ASW—antisubmarine warfare operations conducted against adversarial nuclear-powered ballistic missile submarines (SSBNs). Strategic ASW encompasses the ability to monitor the activities of potentially unfriendly SSBNs during peacetime, as well as to destroy them when so ordered.
Antisurface warfare. Attacks against traditional merchant and military targets must include the capability to destroy small, shallow-draft vessels. This capability also supports the submarine's effectiveness in conducting a blockade, either overt or covert, and in detecting, tracking and intercepting narcotics or arms control violators.
Strategic deterrence. The most broadly acknowledged submarine mission area provides the final line of direct defense for the U.S. homeland. As the nation's most survivable strategic deterrent force, carrying more than half of its strategic nuclear warheads, the Navy's force of SSBNs requires the continuous infusion of new technology to guarantee its strategic operational security and effectiveness over the decades ahead.
Missile defense. The future ability of submarine forces to participate as an integral element of the national missile defense (NMD) and theater missile defense (TMD) systems, especially as a missile platform forward-positioned off a hostile coast, will require further technology development. The potential for boost-phase intercept of enemy missiles and the potential for limited antiair capability for self-defense and forward-area air-denial operations are both areas of opportunity for further development.
Mine operations. Covert mine location, as well as possible disablement by submarines operating in hostile waters, is a prime element in thwarting an enemy's sea control-denial capability. In addition, the covert and remote placement of mines by submarines can deny an enemy the use of its own littoral waters and severely limit its naval surge potential.
Primary Technology Focus Areas
Six primary technology areas have been identified by the panel that couple to the military warfighting objectives noted above. The impact of emerging technology can be strengthened if developed and applied in the context of a systems engineering approach whereby the synergies afforded by technology development
on a broad front are applied in an integrated manner to the technology focus areas listed below.
Stealth is the fundamental enabler of submarine naval warfare, enhancing the ability to operate anywhere, at any time, covertly as the strategic and tactical deterrent. The technical challenges associated with enhancing the stealth of underwater vehicles, including both active and passive measures, are difficult and complex and necessitate an integrated development approach both to capture the synergies available and to ensure that the appropriate tradeoffs are made between different submarine signatures and other performance parameters.
Architecture, including hull structure, shaping, and materials, encompasses the use of innovative design, materials selection, and total systems integration to significantly improve submarine performance, payload capacity, and stealth while improving manufacturability and reducing costs. The goals of advances in architecture include greater speed for the same power input by reducing drag, greater stealth through the reduction of acoustic and nonacoustic signatures, and simplified fabrication using creative structural design and advanced materials.
Sensor and connectivity improvements include hull designs that incorporate embedded sensors, which may allow for the elimination of the bow-mounted spherical array sonar systems, and enable advanced connectivity capabilities such as laser communications. For acoustic sensors, emerging technologies will expand the options for location of outboard sensors, as well as improve the performance of these sensors, including enhanced performance at higher submarine speeds, while reducing their cost and complexity. Enhanced connectivity in all aspects of command, control, communications, computers, and intelligence (C4I) should enhance the submarine's interoperability with all elements of naval, joint, and combined task forces. In the electromagnetic regime, the future submarine will require improved systems for intelligence collection, early warning, and robust connectivity without compromise of covertness.
Payload technologies include a wide-ranging menu of weapons and devices that the submarine can carry for offensive and defensive purposes when operating independently or as a component of a joint or combined force. Released from the constraints of conventional designs, the payload of a submarine can include a range of capabilities from torpedoes and missiles to unmanned off-board underwater or airborne vehicles, antisatellite weapons or satellites themselves, and various ground forces with their equipment.
Power-density improvements include the compression of the entire power plant in length, diameter, and weight to permit the design of more effective submarines with equal or greater payload and lower self-noise at high speed. The design flexibility gained from reducing power plant weight and size can be translated into reduced noise signature, optimal hydrodynamic shaping, and improved overall
performance. Improved power density coupled with an integrated electric power and propulsion system will yield an on-board energy source for advanced military applications such as directed energy beam weapons and hydraulic munitions (directed jets and high-energy vortices).
Off-board vehicles deployed by submarines will significantly extend the battle space and enhance sensing capability while reducing risk to the submarine and its crew. UUVs and UAVs will improve the effectiveness of forward-deployed submarine forces. Advanced technology and design are required for packaging energy, sensor, and handling requirements. Submarine wide-band high-data-rate (HDR) communication with off-board vehicles will be required to enable integrated force employment.
One conclusion of the 1988 Navy-21 study stated: ''… Submarines with increased capabilities could become major, multimission capital ships of the fleet … driven by the need to reduce vulnerability of forces … and by the opportunities offered by advanced missilery and quiet submarine technology …"1 This conclusion is as valid today as it was a decade ago.
The submarine is an indispensable platform in the U.S. Navy's tactical and strategic deterrence forces. The credible exercise of sea control and the certain denial of that control to adversaries hinge on a complete and up-to-date submarine naval warfare capability. The characteristics that endow the submarine with its unique warfighting capabilities are stealth, mobility, and endurance, coupled with flexible payload capacity and advanced information systems. Technology development for future submarine forces should be focused on building on and enhancing these characteristics.
The enabling technologies that support submarine naval warfighting objectives should be pursued in the context of total platform integration together with innovative cost management techniques, a process rapidly maturing in modern submarine design. The benefits of automation and other technological changes can lead to increased capability with reduced manning and maintenance while controlling affordability in both the short and long term.
The U.S. Navy should continue to design and build the best, most capable submarines possible, given the technology available and the long-term defense requirements of our maritime nation. This design should provide for the continuing introduction of rapidly evolving technology through either advances in software
or modular hardware replacement throughout the life of the platform. At the same time, the Navy can stimulate the development of submarine technology by espousing a broad and imaginative vision of future submarine naval warfare capabilities.
TECHNOLOGY FOCUS AREAS
Stealth is the fundamental attribute that enables a submarine to operate undetected for extended periods in forward areas and to execute multiple missions with great effectiveness and limited risk. Submarine stealth is neither absolute nor static, however, and it must be reevaluated and improved continually as the scope and effectiveness of opposing detection systems increase.
It is understood that stealth is a complex attribute not amenable to a single or point solution. An integrated systems approach to stealth enhancement is imperative; correcting only four of five stealth deficiencies is futile because the deficiency that remains can be sufficient to betray the submarine's presence. Thus, a number of technologies should be considered and applied in concert—and integrated closely with the submarine's overall architecture—to minimize all components of the submarine's signature. Stealth technologies can be divided into two broad categories: acoustic stealth and nonacoustic stealth.
Improving acoustic stealth requires both attacking the acoustic energy created by the submarine's machinery and its passage through the ocean and reducing the acoustic energy that the platform reflects from an opponent's active sonar transmissions. In addition to reducing the submarine's detectability, acoustic signature reduction also serves to improve the submarine's own passive sonar performance. Opportunities to enhance submarine acoustic stealth are available in the areas of radiated noise reduction and external flow control, including machinery noise suppression and advanced propulsor design; active sonar target strength reduction; acoustic transmission security; and own-ship signature monitoring.
Radiated Noise Reduction
The objective of radiated noise reduction is to realize a step reduction in ship-generated noise, including noise produced by the propulsion and internal machinery. Future efforts should emphasize low frequencies and the effect of higher speeds. Significant reductions may result from the use of active mounts, isolated structures, advanced hull treatments, and double-hull construction, as well as hull
and appendage shaping, vortex control, adaptive coatings, and the selective use of additives such as polymers and microbubbles. In addition, silencing weapons-launch transient noises can greatly reduce ship detectability in combat.
Active Mounts. Active mounts, which employ piezoelectric materials or other types of actuators to actively cancel mechanical vibration, can greatly attenuate major noise paths from the machinery to the hull. Such mounts can be incorporated into a system of shipwide active noise control techniques that will work together to maximize the effect of this technology at minimal cost. Successful implementation of this technology would reduce the need for specialized quiet machinery and individual mounting of machinery. Active mounts will be a component of specially designed isolated structures, such as the Modular Isolated Deck Structure (MIDS) designed for the New Attack Submarine, and another project involving active control of machinery platforms being developed under the auspices of the Office of Naval Research.
Isolated Structures. Isolated structures are being developed to work with and enhance the overall performance of active mounts. Novel structural concepts that channel noise transmission to the hull so as to result in optimized mount performance and minimized hull radiation hold considerable promise. New developments should focus on improved low-frequency performance.
Advanced Hull Coatings. Advanced hull coatings offer several benefits: first, they serve as a barrier that attenuates radiated noise emanating from within the hull; second, they can reduce reflected acoustic energy, that is, acoustic target strength; finally, hull coatings can reduce flow-induced noise and drag. Again, new development should be focused on reducing low-frequency radiated noise. These treatments will require novel approaches to attack both specific low-frequency tones and wide-band low-frequency noise. A combination of special hull treatments covering various frequency regions, isolated structures, and active mounts is well suited but not limited to double-hull ship concepts.
Double-hull Construction. Double-hull construction, over all or part of the submarine's hull structure, can serve to streamline the shape of the hull hydrodynamically, and possibly enable an entirely new approach to submarine acoustic stealth. Rather than isolating equipment that radiates acoustic energy individually, on rafts, or both, the double-hull approach would attack acoustic quieting by working from the outside of the submarine in. The technological challenge here lies in developing hull coatings that could provide the required acoustic attenuation at a reasonable cost and would impose no significant maintenance penalty.
Weapons Launch Transient Noise Silencing. Weapons launch transient noise
silencing is a key to improving ship survivability. Advances in both weapons and launch systems are needed to achieve the necessary reduction of transient noises associated with current torpedo handling and launching systems. These new external systems should be developed with transient noise minimization as a fundamental objective.
External Flow Control
The objective of external flow control is to influence the flow field around the submarine to reduce noise, increase propulsion efficiency, enhance maneuverability, and reduce the hydrodynamic signature, especially when operating near the surface. A more complete understanding of the flow field around full-scale submarines is needed to facilitate development of efficient and effective methods of flow control. Potential applications of this understanding might include the techniques discussed below.
Pressure Field Modification. The pressure field around the submarine can be modified faborably by changing the shape of the hull and its appendages, optimizing the location of the propulsor, redistributing the pressure field through suction and blowing, and other innovative techniques such as riblets, vortex generators and annihilators, and adaptive surfaces. The challenge here is to integrate the ship configuration wisely with all functions that affect the pressure and velocity fields around the hull and through the propulsor.
Integrated Propulsor. Integrating the propulsor with the hull and the flow field around the hull could reduce drag and propulsion noise. For a propulsor to achieve these goals, it must be integrated with the entire flow field around the hull as well as the hull itself. Effective integration will require addressing a number of fluid flow problems, including issues involving the basic physics of fluid phenomena, that currently are not well understood.
Separation Control. Separation of flow from the hull or appendages results in turbulence, which in turn increases drag and flow noise. Although some sources of separation can be controlled or eliminated with good design practices, the challenge will be to adapt the flow field during maneuver or other conditions that might otherwise induce separation. Changes to the shape or the pressure field that are controlled properly offer the opportunity to totally eliminate separation of the flow field. Such changes to shape and material properties appear to be within the capabilities of emerging smart materials and structures (SMS) technologies.
Polymer Ejection. Full-scale testing has shown that polymer ejection not only will reduce the self-noise of a submarine, but also will decrease the drag of the hull and the radiated noise generated by the propulsor. Speed increases of 10 to
15 percent and reductions in self-noise exceeding 10 dBs (decibels) at certain frequencies for a given speed are possible. A current stumbling block is an appreciation of the amount of polymer that must be carried for particular roles—e.g., burst speed, tactical speed, and routine patrol—and how and where on the submarine it should be distributed. Employment of improved polymers and delivery systems, abetted by other technologies such as microbubbles, can address these issues. Polymer ejection can be deployed locally to improve sensor performance and reduce signal processing requirements.
Electromagnetic Turbulence Control. Electromagnetic turbulence control (EMTC) involves the interaction of magnetic and electric fields in a conducting medium (such as seawater) to generate forces. If the forces act inward toward the hull, they will dampen turbulence in the boundary layer, which will reduce both radiated noise and drag. If they act on only one side of the hull, they will produce maneuvering forces. EMTC is still in its infancy and presents major technical challenges such as power requirements, electromagnetic signature, weight, environmental compatibility, and practicality. Additional research is required before the feasibility and benefits of this potentially promising technology can be evaluated properly.
Active Sonar Target Strength Reduction
The objective of active target strength reduction is to minimize the submarine's reflectivity with respect to active sonar. Target strength can be reduced by special hull treatments (active and passive) and by geometric design and shaping.
Special Hull Treatments. Hull treatments have the potential to decrease reflectivity across the frequency range of future sonars. Passive treatments will generally have to be thick to gain significant performance improvements. Those treatment concepts that will minimize material compressibility are expected to have the least impact on ship buoyancy effects. Development of active treatments may address the difficult low-frequency range and minimize the need for very thick passive treatments. Both active and passive treatments should be integrated with hull treatments designed to reduce radiated noise (discussed above) and with structural design concepts. Double-hull concepts may be attractive in this regard.
Hull and Appendage Shaping. Shaping, along with structural design concepts, can be developed to minimize both reflection back to an active sonar source and radiation caused by an active sonar. Shapes can be designed to minimize reflections but must be coordinated with hydrodynamic and hydroacoustic performance. Structural designs must be developed to minimize reradiation of active
sonar transmissions. There will be some synergy of these designs with radiated noise reductions.
Acoustic Transmission Security
Although a basic objective in radiated noise reduction is to eliminate the transmission of acoustic energy into the ocean, some operations require deliberate acoustic transmissions. Thus, there is a need to minimize the detectability, classification, and locatability of such deliberate transmissions.
Acoustic Communications Security. High-data-rate acoustic communications between submarines and unmanned undersea vehicles, surface ships, or other platforms will be increasingly important to support submarine missions such as coordinated ASW, launch and recovery of special operations forces, and mine countermeasures. Mission security can be compromised unless the enemy's ability to detect, classify, and locate these transmissions is minimized. Providing security and the necessary data rate over tactically useful ranges offers a considerable technological challenge; mimicking the acoustic background is one possible technique.
Active Sonar Security
Active sonar at various frequencies is an essential sensor in many tactical situations (e.g., diesel-electric submarine prosecution and mine detection). Low probability of intercept (LPI) sonars have been developed in the past but have not achieved the degree of security desired. Further technology development in this area is required.
Own-ship Noise Monitoring
Advances in the ability to predict and control own-ship radiated noise are necessary to continue to ensure stealth and mission effectiveness. The key to improvement is the development of both an instrumentation suite and the algorithms to provide real-time radiated noise assessments; these assessments should address radiated noise amplitude and directivity. Algorithm development is the most challenging aspect of this system, relying on sophisticated finite element and statistical energy analysis techniques.
A submarine's nonacoustic signature also must be managed in a coordinated and integrated fashion; each offending artifact should be addressed individually
and with regard for the ship's overall signature. This includes designing an inherently stealthy platform and then utilizing intelligent signature management techniques when conducting mission operations. Nonacoustic signature will become increasingly important as the role of the submarine is expanded in both littoral and open ocean areas. Some specific nonacoustic signatures that require the application of advanced technologies follow.
Radar can be used to detect hardbody (submarine or mast) and wake signatures. Recent investigations have shown that radar cross-section reduction technologies developed for aircraft can be applied affordably to the submarine platform. The integration of external flow control, discussed earlier, can lead to submarine and antenna configuration designs with reduced wake and other surface signatures.
Visual and Infrared Detection
Visual and infrared detection can be utilized to detect both the submarine itself and its wake. Camouflage technologies exist that can significantly reduce visual and IR detection ranges. These include passive paints and coatings and smart chameleon coatings that adapt to the background environment. These technologies have been proven for both air- and land-based applications. A major challenge in applying them to the submarine is addressing seawater-submarine environmental issues.
Low-frequency electric and magnetic field signatures are currently being reduced through the use of several control systems. At present, these systems run independently and with little active capability. In the future, they should be fully integrated with full, active closed-loop capabilities.
Stealth is the most salient feature of the submarine platform and one that must be maintained and enhanced if submarines are to continue to operate effectively in support of Navy and Marine Corps missions. The strategic and tactical value of submarine stealth is likely to grow in the future as surface ships become increasingly vulnerable to the combination of the widespread availability of visual satellite data and relatively low-cost GPS-based guidance systems. Improving
underwater stealth is a complex and technically challenging endeavor and one that will require an integrated, systems engineering approach if it is to achieve significant advances across the entire range of the signature spectrum.
Accordingly, the successful attainment of significantly improved submarine stealth will require an integrated approach across a broad front of technology development, characterized by stable, long-term funding and a high degree of coordination among seemingly disparate technology programs.
During the course of its study, the panel identified two significant bottlenecks that might impede the advancement of stealth technologies. The first of these is the lack of development of computational fluid dynamics tools specifically tailored for problems associated with acoustic and nonacoustic underwater stealth. CFD is a rapidly advancing field that offers important tools for increased understanding of the physical processes that contribute to the generation and propagation of detectable submarine signatures, but these developments in computational capability have not yet been systematically applied to Navy-specific problems. For this reason, there is a need to continuously develop powerful computational research tools, including CFD and nonacoustic modeling, specifically tailored to underwater stealth applications.
The second bottleneck is the uncertain support for naval test ranges and facilities. The complex problems associated with improving underwater stealth are not amenable to analytic solutions and therefore must rely on modeling and computational simulation. Making progress in such an environment is highly dependent on the ability to test accurately the designs indicated by the results of simulations and feed those test results back into the models. Near-final and final designs will require extensive real-world testing. To that end, both sub- and full-scale acoustic and nonacoustic test ranges and facilities—such as those at Lake Pend Oreille and Behm Canal—will be vital components of any stealth development program.
Submarine architecture encompasses the integrated analysis and design of hull structure, form, and materials. An integrated approach is required because changes to individual architectural components affect hydrodynamic and operational performance. The realization of electric drive, an integrated stern, and large lightweight conformal arrays is expected to confer significant performance benefits.
Hull Structure, Form, Materials
Active Signature Control
Advanced technology has seen a great reduction in component size as well as in the total displacement of the new nuclear-powered attack submarine (NSSN)
from the large space and weight requirements that underpinned the nuclear-powered attack submarine (SSN) 21 design. This technology will initially be incorporated into the new SSN and so will sustain Seawolf's substantial gains in noise quieting but will realize those gains with a smaller-volume ship. Follow-on advancements in this application will completely reverse the trend to larger systems and equipment in order to achieve the most demanding quieting goals. With the attributed relationships of displacement to cost, this has the potential to reduce the production cost of U.S. submarines.
Boundary Layer Control
A number of active boundary layer control technologies are currently in the research phase, including some that draw on the application of emerging MEMS and nanosensor technology developments. These have a potential for relatively high payoff but are currently in the earliest stages of development and will require further nurturing before they can be brought to fruition. Nearer-term benefits may be realized from selective boundary layer suction and advanced control techniques. Antifouling techniques and coatings may also be relevant to boundary layer control. The payoff for longer-term technologies, such as electromagnetic flow control, is more uncertain.
In addition, there are combinations of more conventional flow control techniques that can be exploited synergistically. The integrated benefits resulting from the multifaceted programmatic approaches to the varied contributors to disrupted smooth flow around the hull will continue to improve submarine architecture significantly. This disrupted flow is known to affect ship speed, stealth, and maneuverability. Recent advances in computational fluid dynamics may provide further opportunities to understand and ameliorate this problem.
Improved Sail Shaping
Advanced geometries will be of benefit to future-generation submarines. Alternative sail designs have been conceptualized that provide additional volume for maintenance access, external stowage, and mission payload configurability. Various geometries and materials have been identified that could provide improvements in hydrodynamic performance and reduced target strength and, in the long term, provide space and surface area for embedded sensors. Improved sail shaping could reduce life-cycle cost by facilitating maintenance.
Antifoulant surfaces are necessary to prevent the development of slimes (microfouling) and the attachment of natural marine organisms (macrofouling) on the hull, appendages, and propulsor. The challenge is to identify long-lasting
and environmentally acceptable biocides or bioresistant surfaces or other means to inhibit or remove micro- and macrofouling.
Hydraulically Smooth Surfaces
Surfaces with roughness elements on the order of 10 microns for full-scale submarines will reduce drag and associated flow noise by reducing energy losses. This is a first step toward flow control because, given smooth surfaces, other boundary-layer control techniques will become more efficient. Since the surface roughness of present submarines is about 200 microns, the challenge is to identify and develop materials with the desired smoothness that are inexpensive to produce, apply, and maintain. As part of the integrated approach to submarine stealth, research to develop this material should be coordinated with the development of advanced acoustic coatings.
Advanced nonmetals, including glass-reinforced plastics and new silicones, can be used for lightweight structures or coatings to enhance the durability, functionality, and producibility of structures that will be exposed to hostile operating environments.
Smart Materials and Structures
The smart materials and structures (SMS) currently under development in a DARPA program may eventually be used to control the flow field and to manage the state of the boundary layer of the hull and the propulsion inflow. Besides providing control of shed vorticity, it is postulated that these can simultaneously reduce both drag and the acoustic signature of the platform, as well as enhance the submarine's maneuverability. This program could incorporate the application of multipurpose coatings, that is, surface treatments that are anechoic, decoupling, antifouling, and drag reducing.
High-strength, Low-alloy Steel
High-strength, low-alloy steel (HSLA) is currently used in submarine applications. Additionally, the use of undermatched welding for HY 100 pressure hull applications has been approved. Future testing is still necessary before HSLA 100 can be approved for such applications. If it were approved as a logical lowrisk step, it has the potential to provide real fabrication and affordability benefits, eliminating preheating while the undermatched welding technology eliminates the need for the costly postweld heat treatments currently required.
Embedded sensing and actuation are possible in various applications throughout a ship for monitoring equipment and detecting unusual activity and potential problems in real time. Predicated on its ultimate uses and associated costs, MEMS technology could also be a primary contributor to manpower reduction on all Navy units.
As future submarine designs are conceived, evaluated, and implemented, the final product will be enhanced greatly by applying a broad and inclusive approach to submarine architecture, bringing together into a synergistic whole the myriad technologies (including those discussed above), innovative concepts, and processes that submarine design requires.
Sensor Performance and Connectivity
Significant improvements that will affect the design of future submarines can be expected in sensor performance and connectivity. Electronic technology will permit these advances and expanded submarine employment concepts will require them. The substantial relationship between sensor and connectivity improvements and the enhancement of submarine design requires an integrated approach to improvement. For instance, envisioned sonar sensor improvements will enable major changes in hull design, while new connectivity equipment may well have a notable effect on hydrodynamics due to potential requirements for topside devices, new antennas, and fairings.
Producibility and modernization also require integrated consideration. The wide disparity in the pace of technology development for electronic and information systems versus that for ship design means that technology insertion must be planned carefully.
Projected technology improvements such as the introduction of fiber optics and conformal arrays and the exploitation of research into phenomenology will result in major upgrades in capability.
Conformal Acoustic Velocity Sensing
Conformal acoustic velocity sensing (CAVES) devices will include an array of velocity field sensors used in place of pressure field sensors. They will enable a
much better noise decoupling, at greatly reduced weight, over previous conformal arrays, and can be placed over large areas of the hull using fiber optics. Further significant benefit will come from elimination of the spherical sonar array, freeing up weight and volume and permitting greater flexibility in arrangement.
Flow Noise Suppression
Flow noise suppression techniques can be used to simplify signal processing at high tactical speeds. These techniques include many that should be incorporated into a full-scale hydrodynamic approach to submarine design. Some specifics include biofoulants to reduce slimes and marine growth on the hull and propulsor, hydraulically smooth surfaces that avoid the drag-induced effects of roughness elements, the use of flow control to reduce the complexity of signal processing, conditioning the inflow to the propulsor, and the ejection of polymers and microbubbles to reduce drag and minimize acoustic and nonacoustic signatures.
Exploitation of active acoustics has the potential to be a major factor in countering the continuing reduction in passive ranges. Operationally, the most attractive approach lies in the use of off-board sources to avoid own-ship detection. Provisions for handling the necessary vehicles have to be considered, but the largest technology implications are for the high-power active sources themselves and for bistatic or multistatic processing. Research and development to meet this most important military requirement has begun, but the technology has to be resolutely pursued.
The dramatic development of information warfare technologies and exploitation of the attendant possibilities for greatly improved connectivity have been identified as prime drivers of naval force structure for the foreseeable future. It will be necessary for submarines to be fully capable of networking with other naval forces, including shore installations, surface ships, aircraft, and other submarines. Because of their typical far-forward deployment and their requirement for stealth, providing this networking capability will present a serious technical challenge. Nonetheless, high-data-rate, wide-band, secure communications are a must in the atmospheric regime and have to be accommodated to the preservation of submarine nondetectability.
Essentially, the same requirements for information transfer exist for underwater linkage between submarines; provisions for LPI transmission and reception
have to be included in advanced submarine design. Acoustic mimicking of the marine background is also possible. Foreign navies have developed a series of underwater communications systems that provide moderate data rates for ranges out to 100 km and mimic local marine life. There are limitations, but the covert aspect is obvious.
As an example of advanced technology to be incorporated in submarines, lasers have an obvious potential for vertical connectivity to and from submarines, with realistic possibilities on the horizontal plane as well.
Cooperative Engagement Battle Management Systems
Battle management systems that can enable cooperative engagement with other Navy and Marine Corps forces, as well as with other joint and combined forces, will be a required element in the total suite of systems for future submarines. Receiving data from a wide range of communications links and fusing them with information derived from its own on-scene sensors, including those that are off-board such as unmanned underwater vehicles (UUVs), unmanned aerial vehicles (UAVs), and fixed active or passive array fields, to generate the tactical picture and fire its weapons, will greatly expand the submarine's battle space and its mission effectiveness. Certain weapons will also evolve, providing signal-level data feedback and intersensor coherent processing with the submarine and its sensors to achieve greater accuracy and enable more rapid prosecution of offensive combat operations.
Other off-board systems for connectivity enhancement include both recoverable and expendable relay devices. These can permit covert communications to a safely remote transmitter that, in turn, could inhibit or completely deny the enemy an ability to track the transmitting unit even during necessary communications.
High-speed Processors and High-data-rate Antennas
High-speed processors and HDR antennas are improving rapidly today in industry as well as in military applications. The submarine force could benefit from the incorporation of these technologies and the application of commercial equipment and programs wherever possible.
Sensors are the eyes and ears of the submarine, and improved connectivity
will greatly enhance the submarine's combat effectiveness. The panel thus concluded the following:
Improved sensor technology, especially the utilization of fiber optics and conformal arrays, will become an essential element of future submarine forces as potential targets become ever quieter and more difficult to detect.
To the degree that, from a C4I perspective, the submarine is indistinguishable from any other unit of the force, improved connectivity will be required for submarines to function as fully integrated units of joint task forces. Improved connectivity is the key to enabling a cooperative engagement capability.
Future submarine missions enabled by the submarine's stealth and relative invulnerability include national or theater missile defense; precision high-volume fire support of ground forces; launch, control, and possibly recovery of off-board air and undersea vehicles for remote sensing and weapons delivery; remote mine reconnaissance and offensive mining; delivery and control of multiple ground force elements; and deployment of off-board seabed sensors. Execution of these missions will require the submarine to carry and deliver new offensive and defensive weapons, including regenerative weapons, that are smaller and more lethal. Thus, the development of submarine payload technology must support development of the weapons, vehicles, and other payload deliverables, as well as the means to place weapons on target and, where appropriate, recover some of the devices. This development of payload systems must seek to achieve the greatest possible commonality, flexibility, and modularity so that the submarine's payload configuration or loadout can be modified readily as dictated by mission objectives.
Payload Technologies to Support Weapons Delivery
Missile size reduction that incorporates the use of precision guidance, more energetic propellants, and more effective warheads offers great potential to increase the rate and volume of firepower that the submarine can deliver.
Half-size (Length) or Smaller-diameter Torpedoes
Half-size (length) or smaller-diameter torpedoes can increase the number of missiles carried by submarines, thereby extending combat endurance without increasing volume. Size reduction in mobile mines, possibly the ability to launch several swimout or mobile mines simultaneously, can provide similar improvements in firepower. In addition to size reduction, advancements are needed that
provide a quick-reacting, high-speed weapon for close-aboard ASW engagements and a long-range standoff ASW weapon cued by off-board sensor targeting. Both missile-delivered standoff ASW weapons and quiet, long-endurance ASW torpedoes should be considered.
Technologies critical to high-speed weapons are power density, reduced drag hydrodynamics (e.g., supercavitation), and guidance and control. Longer-endurance weapons require increased energy-density propulsion, stealthy operation using closed combustion systems, and intelligent guidance and control with a communications link (e.g., fiber optics) to the firing platform to provide updates on targets and to provide tactical assistance in a severe countermeasures environment.
External Weapons Launchers
External weapons launchers could replace part or all of the submarine torpedo tubes and the torpedo room, thus eliminating up to 500 tons of displacement as well as a complex structure that is expensive to build and represents a major lifetime maintenance cost. The incorporation of external weapons also reduces the 21-in. limitation on weapon diameter so that weapons launchers or canisters of different sizes and shapes can be accommodated without costly pressure hull work. Development is required in weapons ejection technology (e.g., advanced gas generators), acoustic launch transient quieting, launch hydrodynamics, commonality with surface ship launchers, and readily interchangeable weapons modules.
Regenerative weapons technology offers a major opportunity for mission performance improvement and payload enhancement. Transforming the submarine's abundant electric power into directed energy of different forms has the potential to greatly improve the submarine's self-defense capabilities. Technologies such as pulsed energy, hydraulic vortices, and destructive jets should be investigated for their effectiveness against mines and incoming torpedoes.
Short-range Antiship or Antiair Weapons Systems
Short-range antiship or antiair weapons systems are needed for use against maritime patrol-type aircraft or ASW helicopters and against small shallow-draft surface craft that are unsuitable torpedo targets (e.g., fast-attack craft, drug runners, and blockade and arms control violators). Supporting technologies include encapsulation and launch, missile direction and guidance, and acoustic signature reduction.
Navy Tactical Missile Systems and Missile Defense Systems
Navy tactical missile systems and missile defense systems will be in the payload of future submarines. Technology support for these future missions includes launcher technology, smart warheads (e.g., ''brilliant" antitank weapons), deep-penetrating warheads, and CEC-like command-and-control systems.
Undersea Weapons Defense System
An undersea weapons defense system will be needed to mitigate the threat of advanced-capability torpedoes. An integrated system combining autodetection and alert, automated execution, jammers and deception devices (e.g., decoys), and hard-kill counterweapons should provide a robust solution. Such a system may also be effective against hostile submarines in close-in encounters or melees.
Payload Technologies to Support Unmanned and/or Autonomous Underwater Vehicles
Unmanned underwater vehicles (UUVs) and autonomous underwater vehicles (AUVs) will provide future submarines with a variety of new mission capabilities such as remote mine reconnaissance and location, environmental and operational intelligence collection, off-board sensing, seabed sensor deployment, and acoustic source positioning.
Platform technologies required to support these vehicles include automated launch and recovery systems, maintenance and energy replenishment techniques, mission planning and control systems, and secure underwater communications links for launch or recovery, control, data transmission, navigation, and rendezvous. Critical technologies for the vehicles themselves are advanced guidance and control and increased energy density (endurance), for example, advanced thermal and hybrid thermal-electrical concepts utilizing metal fuels. Technologies that enable submarines to support UUV or AUVs will generally apply to hosting manned vehicles such as the advanced SEAL delivery system and future manned minisubmarines. This subject is discussed further in the section on off-board vehicles.
Payload Technologies to Support Ground Forces
Marine Corps or special operations forces represent a payload that provides the submarine with a versatile and powerful capability for multiple missions: reconnaissance, sabotage, mine location and clearance, target designation, and so forth. Most of the technologies listed that support UUVs or AUVs are also applicable to supporting the missions of special operations forces. Additional requirements are for automated and quiet lock-out or lock-in of personnel and their vehicles, stealthy
surface launch and recovery of large numbers of troops, electromagnetic and acoustic LPI communications, and integrated mission planning and control.
Until recently, the submarine's payload has been restrained by continued reliance on the 21-in. torpedo tube. Aggressive action should be undertaken to develop a flexible, modular submarine payload system that can accommodate a variety of weapons, such as the family of weapons discussed elsewhere in this report, as well as off-board vehicles and other deliverables.
The immediate goal of increasing power density is not necessarily reduction of submarine size, since displacement is an output, not an input, of the military requirements-based design process. Rather, the goal is enhancement of submarine stealth, affordability, and mobility, each of which will benefit from power plant space and weight reductions. A focused, integrated, and continuing program of both basic research and engineering development is required to achieve savings in weight and space for application to advances in payload, hull design, signature reduction, producibility, and maintenance.
Electric drive offers the promise of substantial improvement in submarine stealth. It also provides greater flexibility in propulsion machinery arrangement. Implementation of electric drive could facilitate a reduction in size of the submarine while maintaining or improving stealth and hydrodynamic shaping; if a reduction in size evolved, it could also effect a reduction in the propulsion power required. Reduction in the space necessary for propulsion equipment could also free space for improved operational capabilities. Electric drive should be pursued vigorously.
Superconducting Motors and Generators
Conservation of significant space and weight is the primary objective of applying superconductivity in propulsion or ship's power motors and generators. Although the practical application of superconductivity is not yet feasible, some experts believe that with applied effort, superconducting motors and generators can be developed for submarines before the midpoint of the period from 2000 to 2035. Technical issues that will affect the application of superconductivity to submarines include nonacoustic EMI detectability, reliability, cryogenic system acoustics, high-flux density, and quiet motor design.
Solid-state Power Electronics
Power electronics enable practical electric drive systems and provide improved system control. Ongoing power technology advancements and advances in metal oxide semiconductor technology, such as those that enable the development of power electronic building blocks, will allow more sophisticated power distribution systems and greatly improve the power density of practical electric drive systems.
Power Plant Simplification
Subsequent to the initial operation of Nautilus there have been concerted efforts to reduce the number of plant components, as well as the controls and instrumentation associated with both primary and secondary sections of submarine nuclear power plants. The NSSN design has realized significant reductions in the volume of the engineering plant compared to that of the 688I class or the Seawolf, while maintaining Seawolf levels of quieting. Further major progress may be possible with elimination of the diesel engine, snorkel system, and storage battery through introduction of high-power-density, air-independent alternative power sources. Simplifying auxiliary systems such as air conditioning, hydraulics, and high-pressure air can also serve to reduce the propulsion and ship service components' fraction of total weight and volume, as well as the maintenance required.
Advanced Technology Alternative Reactor Designs
Studies by the Navy Nuclear Propulsion Directorate have shown that an advanced technology reactor may be realized in the 2035 time frame with a continuing and focused R&D effort aimed at significant improvement in both the heat source and the energy transfer mechanics. A major technical challenge is providing materials of sufficient strength to be used at the high temperatures involved. The advantages include not only space and weight savings but also quieting and simplicity of power conversion.
Alternative Power Sources
The panel discussed both air-independent propulsion (AIP) and fuel cells with regard to the power requirements of submarines. Both have application to the auxiliary system of submarines, especially for off-board vehicles. On the other hand, the power available in the foreseeable future is not sufficient to provide the main propulsion power to support the speed, endurance, and multimission capabilities required of U.S. submarines. As an example, the fire control system power
required of the AIP-capable Swedish Gotland submarine is 75 kW; that of the 688I is 550 kW.
The study's Panel on Technology assessed various technologies for propulsion power using six attributes of merit: (1) functionality, (2) mobility, (3) supportability, (4) serviceability, (5) manpower requirements, and (6) overall cost. Nuclear power was rated better than the fuel cell in each category.2 The Panel on Platforms, like the Panel on Technology, decided that nonnuclear propulsion is not appropriate for U.S. Navy submarines.
While striving for improvement in the various aspects of power density, electric drive holds great promise to improve submarine stealth and design and should be pursued vigorously.
The potential benefits of submarine-borne unmanned underwater vehicles and unmanned aerial vehicles are significant. These vehicles and their embedded or leave-behind sensors will provide a cost-effective, clandestine force multiplier that greatly extends the sensor range of the submarine, allows for presence in high-risk regions or inaccessible littoral areas, and provides both timely and accurate knowledge of the battle space. At the same time, use of off-board vehicles can reduce risk to the submarine and its crew. Unmanned vehicles have the capacity and potential to develop and enhance mine reconnaissance and neutralization, tactical oceanography, bathymetry or survey, surveillance and intelligence collection, counterproliferation, tagging, decoys and remote jammers, and ASW through embedded sensors, deployed sensors, and trip wires, and to serve as an active acoustic source.
The technical issues that should be addressed to enable the development of capable low-cost vehicles include power density, duplex communications, guidance and control, geophysical and gravity field sensors, and stealth. Ultimate decisions will have to focus on expendability and reliability.
Unmanned Underwater Vehicles
Programs are extant today that will produce off-hull mine reconnaissance systems by about the year 2010. The systems will be either tethered or nontethered, will be preprogrammed, and will have search rates of 30 square nautical
miles per day and an endurance of days. As technology develops, modular reconfigurability may provide optimum flexibility for a submarine-borne UUV. Also, as new sail designs are introduced, a UUV can be housed in the sail; such an accommodation would remove many constraints, including both size and payload, now imposed by storage, launch, and recovery limitations of UUVs. Larger and more varied payloads would result, including more numerous and capable sensors, weapons, communications and navigation suites, countermeasures, and nonacoustic systems. Larger sails to accommodate UUVs may add to overall drag or have other adverse performance effects. It may be possible to both accommodate larger and more varied payloads and improve performance and maneuverability through the use of new hull forms.
Unmanned Aerial Vehicles
The basic ability to control UAVs from a submarine has been demonstrated, and this capability will be greatly improved in the future. The technological challenge will be to ensure tactical control and monitoring reliability of the UAV without compromising the host submarine's position. In addition, unless a vehicle can be developed with a cost-to-benefit ratio low enough to warrant expendability, a practical means of recovery without unacceptable compromise of submarine covertness is also required.
Off-board vehicles offer the potential to greatly expand the submarine's battle space. To date, the development of UUVs and UAVs has been inhibited by the decision to launch or recover them through the 21-in. torpedo tube. Technical measures to remove the 21-in. launch restriction in conjunction with the development of more flexible payload systems discussed above would enable more rapid development and deployment of off-board systems.
SUMMARY AND CONCLUSIONS
A conclusion of the Navy-21 study completed in 1988 states: "Submarines with increased capabilities could become major, multimission capital ships of the fleet … driven by the need to reduce vulnerability of forces … and by the opportunities offered by advanced missilery and quiet submarine technology. …"3 This statement by the Navy-21 panel was remarkably prescient and has been
largely confirmed by submarine tactical and technological development over the past decade.
While maintaining the submarine-unique surveillance, intelligence collection, and ASW missions of the Cold War, the utility of SSNs is recognized such that they are now integrated fully into Navy battle groups and their land-attack missiles are included in joint precision strike packages. The adaptation of the Army Tactical Missile System will permit SSNs to deploy tactical missiles for conventional deterrence and fire support to ground forces. This expansion of submarine missions, foreseen by the Navy-21 study, has been enabled by the steady insertion of new technology ranging from satellite navigation and HDR communications to advanced digital processing of acoustic and targeting data. At the same time, submarine operational availability has been increased and lifetime costs have been reduced by condition-based maintenance concepts and new reactor cores that last through the service life of the ship without refueling.
This retrospective view is important because it illustrates the fact that although there have been no dramatic breakthroughs in submarine technology since Navy-21, the steady infusion of new technology into nearly all aspects of submarine performance has resulted in a substantial increase in submarine warfighting effectiveness. Indeed, the dramatic improvement in operational capability over a period of 20 years, from the first submarine of the Los Angeles SSN 668 class to the final improved SSN 688 (688I) and then to the impressive Seawolf (SSN 21), could have been neither foreseen nor accomplished had a policy of waiting for a major technological breakthrough prevailed. This experience confirms the wisdom of maintaining a steady and stably funded submarine technology development program, sharply focused on mission effectiveness and prepared to promptly insert the products of that program into the submarine force through forward fit and/or backfit as appropriate. Thus, even while aggressively striving for break-throughs over the next 40 years, such as direct conversion of nuclear energy to electrical energy, the policy of actively refreshing and inserting new technology should be sustained.
In looking to the future, it appears certain that the expansion of submarine missions will continue, driven by the push of emerging technology, with the catalyst of the increasing vulnerability of nonstealthy naval forces operating anywhere including forward areas. Thus, freed from the primary Cold War focus on strategic deterrence and ASW, imaginative submarine design concepts will be required to support these expanded missions, new sensors, weapons, and off-board vehicles. Bringing new and expanded missions to operational reality will require the steady application of technology focused on military objectives and integrated across the submarine platform. Some specific enabling technology areas should be pursued vigorously to ensure the success of this vision:
Stealth technology. Acoustic and nonacoustic stealth will remain the fundamental characteristic that enables all submarine missions. Aggressive and
integrated development of all facets of submarine stealth technology is essential to maintaining the submarine's stealth advantage and its continued operational effectiveness.
Payload technology. As the scope and depth of submarine missions expand, the development of technology to carry, launch, recover, and deliver the submarine's varied payload gains in importance. Stealth, flexibility, modularity, commonality, and affordability are important characteristics to be sought in developing new payload technology.
Sensors and connectivity. The submarine's ability to sense, process, and fuse information in all operating environments and to network fully with the naval, joint, and national command structure is becoming increasingly vital to its operational effectiveness. The application and integration of fiber optics, acoustics, lasers, high-speed computers, and other new techniques will be required to achieve the necessary improvements in sensors and connectivity.
Power-density technology. Increasing the power density in submarine propulsion, auxiliary power, and in off-board vehicles offers substantial improvement in platform performance, system simplification, and affordability. The direct and indirect benefits of increased power density suggest a substantial investment in this area. Electric drive is an especially promising propulsion technology and is worthy of strong support.
Off-board vehicles. The employment of off-board vehicles to expand significantly the submarine's battle space holds great promise. A broad range of technology improvements is required to achieve the operational potential these vehicles offer; they include automated launch and recovery, vehicle guidance and control, submarine-to-vehicle data links, and vehicle endurance.
Submarine architecture. The broad technology category of submarine architecture brings together a wide range of science, technology, materials, and processes to integrate them into a submarine design that is efficient to produce, amenable to future improvement, affordably manned and maintained, and above all, operationally superior. By using an integrated product development approach and applying the power of advanced digital design systems, these diverse technologies and requirements can be combined, examined synergistically, and tested in virtual reality and/or reduced-scale prototypes to develop submarines that will best satisfy the expanding range of operational requirements.
Considering the vast changes that have evolved in the past 40 years and the rapid pace of technology in just the last few, the panel would be presumptuous to assume that it could predict, with more than a modicum of certainty, even the basic composition of the Navy of 2035. Obviously, knowledge of the nature and location of the most probable potential adversaries is an even more remote possibility. There are technologies about which we have not thought, yet they may be dominant 40 years from now. However, based on current trends and the opportunities
that technology seems to offer, a vision emerges of future naval forces that are dispersed, flexible, interconnected, and equipped with sensors and weapons to enable standoff engagements. The paramount characteristic of any future force will be stealth, the ability to dominate the battle space with minimum vulnerability.
It is certain then that submarines, as the original and most viable stealth platforms, will be indispensable elements of future naval forces, even though their precise size or design can be speculated on only in general terms. Concepts for future submarines can range from large mother submarines hosting multimission minisubmarines to undersea tugboats pushing or towing undersea barges with mission-tailored payloads of weapons or supplies. Regardless of the specific undersea concepts that emerge, the technologies examined in this study—and others yet to come—will be essential to support their development. Through these technologies, the United States can design and build the most capable submarines possible to satisfy the long-term security requirements of our maritime nation. To this end, it is important that the U.S. Navy actively stimulate the development of submarine technology by promoting a broad and imaginative vision of future submarine naval warfare capabilities.