Highly capable Los Angeles and Seawolf class sub marines can handle blue-water missions, but the Navy needs a new modular-design submarine to meet littoral requirements within current budget constraints.
To meet all of the Navy’s submarine mission requirements, the Los Angeles (SSN- 688)-class and Seawolf (SSN-21)-class submarine designs relied on “optimum compromises.” The requirement for a 30-knot transit speed, for example, was met by freezing the design of the nuclear propulsion system for safety and then reducing the size of the submarine hull until drag was decreased sufficiently to allow a maximum transit speed of 30 knots. This tradeoff had an impact on the space for and design of sensor and launching systems, reserve buoyancy, and the total number of weapons in the weapon loadout.
Today, the mission requirements of the U.S. submarine fleet include operations in both blue-water and littoral zones. A top-down analysis of these requirements illustrates the need for a “new” design to be added to the mix of highly capable Los Angeles and Seawolf classes for operations in the littorals. The Navy currently is starting the Engineering and Manufacturing Development Phase for the New Attack Submarine (NSSN) and is again implementing the process of optimum compromises for the final design.
Because of shrinking budgets, the Navy must put more emphasis on ship construction and life-cycle costs than was observed in the 1970s and 1980s. The current “optimum compromise” design for the New Attack Submarine is projected to cost in excess of $1 billion per boat. To meet the littoral zone mission requirements, maintain the number of submarines necessary to meet all commitments, and stay within tighter budget requirements, the Navy needs to build a submarine with an average cost of $200- $400 million per boat.
A modular design similar to that used in the Spruance (DD-963)-class and Oliver Hazard Perry (FFG-7)-class surface combatants would allow the Navy to build sufficient submarines at $200-$400 million per boat. Already proved in the construction of the Netherlands Walrus and the Australian Collins classes, modular submarine shipbuilding would provide the flexibility for multiple hull configurations to accommodate different mission requirements. Separate propulsion system modules would allow for nuclear, diesel, or air independent propulsion (AIP) systems, to reduce construction and life-cycle costs. Separate weapon modules could be provided for different missions, including special forces insertion/extraction, mine laying, missiles for land attack or antishipping, and torpedoes for antisubmarine warfare or antishipping. Modular shipbuilding also could hold down construction costs and allow shipyards to compete in the world diesel and AIP submarine markets—which would help maintain construction and refit capability for the U.S. Navy.
Littoral Zone Mission Requirements
The first step in a top-down analysis of mission requirements is to identify precisely the littoral zones of the world. There is no real consensus on this issue, but an acceptable definition might be the shore within 50 miles of the high-water line and the ocean out to 100 nautical miles from the high-water line. In general, a maximum water depth of 100 fathoms complements the 100-mile limit. This definition includes all or part of the territorial waters of the world’s nations and is illustrated in Figure 1.
Coupled with the mission requirements—land attack, antisubmarine and anti-surface warfare, special forces insertion/extraction, minelaying, and intelligence gathering—this definition of the littoral zones provides the framework needed to develop the littoral zone submarine design requirements.
Transit time. Several studies have been done comparing diesel, AIP, and nuclear-powered submarines, all of which assume that the same operational requirements for the Los Angeles and Seawolf classes are needed to meet the littoral zone mission. Early intelligence gathering may require 30-knot-capable SSNs, but the U.S. Navy’s littoral zone fleet (amphibious ships) has a transit speed of 20 knots. Therefore, a 20-knot transit speed for the New Attack Submarine would be more than adequate. The USS Barbel (SS-580)-class diesel submarines built in the 1950s had transit speeds of 15+ knots; improvements in hull design and propulsion systems should easily allow 20 knots. This speed would make surface or snorkel transit of diesel and AIP submarines feasible. And, contrary to current thinking, a merchant vessel observation of a submarine transiting the ocean on the surface or on a snorkel is the same as the observation of a surface combatant transiting the ocean and is definitely not a breach of security.
Hull design. The safe operating depth for both the Los Angeles- and Seawolf-class submarines significantly exceeds the 100 fathoms (600 feet) required by our littoral zone definition. Reduction of the safe operating depth of the littoral zone submarine to 100 fathoms would permit the hull to be built using HY30 steel, which should reduce hull manufacturing costs by 50%. Welding HY30 steel is much easier because the impact of environmental factors on the process is negligible. This lower-grade steel also would reduce the costs to overhaul and upgrade the new submarine or to change out modules to allow an existing submarine to better meet changing mission requirements. The production of the modular submarine using automated welding techniques (as used by the Walrus-class and Collins-class programs) will result in the development of the jigs and computer programs needed to change out modules rapidly as mission requirements change.
Intelligence gathering and SEAL team missions are greatly enhanced by a submarine’s ability to position itself covertly close to the shore and to remain in this position for a certain period. To allow the submarine to sit in the silt/mud, all seawater intake and discharge openings need to be amidships or have optional amidships routings. Our Los Angeles and Seawolf SSNs do not meet this requirement, and current submarine hull designs—especially hull-mounted sensors and main propulsors—are extremely sensitive to groundings. To survive low-speed unintentional groundings and zero-speed intentional groundings, the New Attack Submarine design must include changes to hull and external tanks of the submarine; to the positioning and installation of the hull-mounted sensors; and to the main propulsor design.
Propulsion. The littoral zone missions require submarines to conduct operations in shallow waters, where a nuclear incident would have catastrophic environmental and political effects. An alternative to nuclear propulsion must be developed, and a modular hull design that provides a super-cooled conductor-type electric motor for propulsion and a separate power-generation module to generate the electrical power would meet all the requirements. The power-generation module may be nuclear, diesel-electric, or energy cells, with the propulsion system capable of supporting a 90-day patrol and maintaining the submarine submerged for at least 30 days. Current diesel-electric submarines can meet the 30-day submerged criteria if allowed to snorkel, and inclusion of the Italian-designed in-hull condensed gas system (to provide oxygen for running a diesel while submerged) would eliminate the requirement for snorkeling.
The possibility of detection of a submarine diesel or gas turbine can be minimized through the use of noise cancellation systems; however, other power-generation systems such as fuel cells may prove to be better than diesels. Modular design of the power-generation system will allow development of operational systems, testing, and back-fit of the multiple propulsion designs into the New Attack Submarine fleet.
Changes in the propulsion design should be revisited to optimize the submarine for use in the littoral zone. For example, placing externally mounted electric motor propulsors amidships would permit radical changes in the submarine sensors, launching system, and weapons loadout. The amidships propulsors would be less vulnerable to submarine groundings (see opening illustration on p. 39), and a complementary shift of the launching system to the stern would allow for multiple launching systems that are unaffected by submarine speed, depth, or attitude, and for weapon swim-out, which would eliminate launch noise transients. In addition, the launch and recovery of autonomous underwater vehicles (AUVs) and swimmer-delivery vehicles would be greatly simplified (Figure 2).
Sensors. The towed acoustic arrays used for blue-water antisubmarine warfare cannot be used effectively in water of 100 fathoms or less. Therefore, such arrays should be eliminated from the basic design and developed as a modular add-on to meet blue-water mission requirements. Hull- mounted sensors need to be designed and installed to survive inadvertent and deliberate groundings, with visual and electromagnetic sensors needing the most emphasis in the new design. The littoral zone submarine will need visual and electromagnetic sensors capable of operating at or just below the surface of the water. Sensors need to be developed that can be laid or positioned by the submarine or special forces then monitored through a fiber optic link. These sensors will enhance the submarine’s ability to collect intelligence covertly. The current development of AUVs may culminate in AUVs being the primary sensor platform for the littoral zone submarine.
Combat Systems. The Los Angeles-class and Seawolf-class submarine combat systems were designed primarily for blue-water warfare. They can meet the antishipping mission requirement of the littoral zone, but their antisubmarine capability in the littorals is limited. The Tomahawk capability of the Los Angeles and Seawolf classes fulfills the littoral zone land-attack mission, but the Tomahawk land-attack capability should be included in the New Attack Submarine only if inadequate numbers of SSNs are expected to be available for this mission. Modular design of the submarine should make this change, when needed, cost effective.
The littoral zone missions of intelligence gathering and special forces insertion/extraction require combat systems with open system architecture that is modular in design and allows for quick reconfiguration and upgrades prior to deployment. One mission may require emphasis on visual data collection and analysis while another may require electromagnetic data collection and analysis. The "fly by wire” concept of controlling propulsion and helm have been incorporated into the new Navy surface combatant designs, but the submarine force has been reluctant to incorporate the concept. The Australian Collins-class uses a standard console not only for the combat system but also for control of the ballasting, depth, and helm. Combining these functions will reduce the space, weight, and personnel requirements dramatically.
Launch Systems. The launch systems in U.S. Navy submarines have seen few changes since World War II. The number of torpedo tubes has varied, but size and operation have seen very little change. Several radical new launch designs currently are being studied by the Navy, and the littoral zone submarine launch system needs to accommodate at least a 36-inch launch vehicle, which can be designed as a swimmer-delivery vehicle. A minimum of four weapon launching systems (tubes) will be needed to accommodate the littoral zone missions.
The current submarine weapon storage design requires the loading or offloading of horizontally stored and launched weapons through a round hatch. This process is slow and labor intensive and includes a high possibility of damage to the weapons. The French-designed rectangular loading hatch (or plug) should be evaluated for its ability to allow rapid loadout, change of loadout, or reload of weapons. This design requires additional structural support that can be achieved easily with the reduction of the submarine safe operating depth to 100 fathoms.
Weapons. The weapons needed for the littoral zone missions will include swimmer-delivery vehicles, torpedoes for antisubmarine and antisurface warfare. Harpoon and Tomahawk missiles for surface warfare, mines for blockading, special vehicles with sensors for mine and obstacle detection, antiair-warfare missiles and acoustic countermeasures, and special-delivery vehicles for sensors that can be directed to areas too dangerous for the submarine. Design of the weapons must be revisited to optimize their integration with the submarine storage and launching systems. A detailed analysis of each mission requirement will be needed to determine the optimum loadout of weapons for each deployment (e.g., covert mining to blockade a port will require at least 20 mines). The submarine also must carry some minimum number of torpedoes and/or missiles for self-protection during all missions.
Manning. A significant part of the total life-cycle cost of the submarine, manning for the New Attack Submarine will be based upon the “optimized design” of the boat that is determined during the top-down mission requirements analysis. The use of automated AIP propulsion systems would reduce propulsion system manning and training costs by 50%. The automation of the submarine ballasting, depth control, helm, and propulsion control will permit further reductions in manning by 10%-20%. Automation of the weapons loading, handling, and launching is needed to maximize the number of weapons and AUVs that can be carried. These reductions in manning will be crucial in providing the space needed for missions such as special forces insertion/extraction.
Conclusion
Our current planned mix of Los Angeles- and Seawolf-class submarines will not fulfill the littoral zone missions. The Navy needs a new-design submarine that will meet both littoral zone and budget requirements. Shifting to AIP or diesel propulsion; reducing the safe operating depth and using HY30 steel for the hull; and the use of modular design and increased automation will reduce costs. All of these factors can be implemented, through a modular design New Attack Submarine, to meet new or changing mission requirements at a cost acceptable to Congress.
Commander Wright retired in 1986 with 14 years of sea duty, including tours as weapons officer/supply officer and missile systems officer. He was an exchange officer with the Royal Australian Navy, where he participated in the development, testing, and delivery of three major upgrades to the RAN guided-missile destroyer combat systems. His last tour was at Naval Sea Systems Command, where he was the 1LS and Surface Ship Weapon System Integration Manager for the Mk 50 torpedo program. Since his retirement, he has provided program acquisition support to various NavSea offices, including Trident and Seawolf. Commander Wright holds a 1988 personal patent for an explosive net concept, used by the Army, Navy, and Marine Corps for development of shallow-water and land mine-clearing systems.