SSN VERSUS SSK
The optimal solution to resolve the Navys shortfall in its attack submarines in the next 15 to 20 years is to acquire a modest force of advanced conventional submarines (SSKs), specifically those fitted with an air-independent propulsion (AIP) system. Both SSNs and SSKs can be successfully employed in the littorals. However, and despite the claims of the SSN proponents, the SSKs have some obvious advantages in most enclosed and semi-enclosed seas (collectively called narrow seas.) The SSKs displace between 1,000 and 2,000 tons. For example, the Swedish AIP Gotland-class submarines displace about 1,240 tons on the surface and 1,500 tons submerged. The Scorpene Compact class has a submerged displacement of 1,450 tons and is optimized for operations in shallow waters.
In contrast, the SSNs are much larger. For example, the Los Angeles-class SSN displaces 6,900 tons, the Virginia class about 7,925-tons, and two of the three Seawolf-class boats about 9,100 tons. The third Seawolf class, the Jimmy Carter, displaces some 12,160 tons. The SSNs also have large crews, from about 130 on board Los Angeles-class vessels to 145 for the Seawolf class. Advanced conventional submarines are highly automated resulting in small crews. For example, the Gotlands and German-built Type 212As have a crew of about 25.
SSNs are much faster than any SSK. They are capable of high sustained speeds for a very long time. For example, the maximum submerged speed of the U.S. SSNs is listed as between 25 and 28 knots and as high as 32 knots. This is a distinct advantage in operations on the open ocean. In contrast, the Gotland/Sodermanland classes have a maximum speed of 10 knots on the surface and about 20 knots submerged. The corresponding figures for the Type 212A and French Agosta 90B classes are 12 and 20 knots. In the littorals, because of the much smaller maneuvering space and often rough topography of the sea bottom, all submarines must necessarily sail at low speed. Hence, the SSN advantages over SSKs are much reduced in such waters. However, because of their low speed and endurance, SSKs cannot be employed for operating with carrier and expeditionary strike groups.
SSN proponents claim that there is not much difference in the sizes of the SSN and SSK, but this is not true. The maximum length of the Los Angeles-class SSNs is about 360 feet, Virginia class about 380 feet, and the Seawolf more than 450 feet. In contrast, the length of the Gotlands and Type 212/Type 214 is about 200 feet. The height of the Type 212A and Type 214 is 38 feet and 43 feet, respectively. SSN advocates say the SSN is only 15 feet taller than the SSK. Yet this is not a negligible factor when a submarine has to operate in shallow waters.
Because of their smaller size and displacement, SSKs are more maneuverable. Proponents contend that modern SSNs such as the Virginia class fitted with a new computer-controlled autopilot and hovering system can maintain a specific depth to within 1/10 of a foot and remain at stable depth even in the roughest weather conditions. They can also penetrate close to shore whenever contours of the sea floor permit. However, the size and displacement of SSNs still greatly limit their maneuverability in shallow waters such as the Baltic Sea and many parts of the Persian Gulf. Advanced SSKs are also fitted with a number of features that greatly enhance their capabilities in shallow waters. For example, the Gotland-class is fitted with a rudder configuration that allows extreme maneuverability and operations very close to sea bottom. Its turning radius is very small. And Type 212As can reportedly sail in waters as shallow as 65 feet. Clearly, modern SSKs such as the Gotlands and Type 212A/Type 214 are far more capable and better suited for shallow-water operations than U.S. SSNs.
SSNs provide first-on-the-scene capability because of their stealthiness, high-sustained speed and mobility. Their covertness allows surprise attacks on the sea surface and on land. The very presence of an SSN in certain areas invariably has a great impact on enemy dispositions. SSNs also have virtually unlimited endurance submerged. However, in a typical enclosed or semi-enclosed sea their much smaller maneuvering area reduces that advantage. An AIP conventional submarine can transit rapidly on the surface and submerge for long, quiet patrols at low speed, preserving its batteries for high-speed bursts. The AIP allows a submarine to remain submerged up to three to four weeks. The Gotland/Sodermanland class reportedly can operate on AIP for two weeks submerged without the need for snorting and at a speed of five knots. The Type 212A has a range of 8,000 nautical miles at eight knots on the surface and has an endurance of about 12 weeks. The Agosta 90Bs range is 8,500 miles at nine knots snorting and 350 miles at 3.5 knots submerged. With the AIP, its range submerged is four times longer at a speed of about four knots. It can stay submerged for up to two weeks. The endurance of the Scorpene Basic AIP is about 50 days. In contrast, the non-AIP Russian Kilo-class and the German Type 209 class built in the 1970s and 1980s had an endurance submerged of about 72 hours.
One of the greatest advantages of SSNs over SSKs is that they are fitted with a large and diverse number of advanced sensors and weapons. They can carry larger numbers of heavyweight torpedoes or mines and long-range antiship and/or land attack cruise missiles. The SSNs also can defend themselves with stand-off weapons and, if necessary, withdraw into deep water.
The Virginia-class is also superbly quiet with an absolute minimum nonacoustic signature. It is optimized to operate in littoral waters. However, the great advantage of SSNs of stealthiness has been eroded by the significant technological improvements of diesel-electric submarines operating on AIP at low speed. AIP-fitted submarines are also much quieter than nuclear-powered submarines. The Gotlands have a very low noise, magnetic and infrared signature and because they are vibration-free they are extremely hard to detect. The Type 212A is perhaps the quietest conventional submarine at sea today. Its waterborne noise, magnetic, radar, infrared and pressure signature have been drastically reduced. Its pressure hull is built from a nonmagnetic material and its shaped hull has no straight lines.
One of the greatest disadvantages of the SSKs is their inability to deploy covertly and quickly from homeports many thousands miles away from their prospective operating areas. Hence, host nation support is critical.
SSKs should not be considered either as an expendable force or replacement for the SSNs. They should be deployed only in those littoral waters where the deployment of highly capable but also high-cost vessels is too risky. SSNs should not be deployed in the littorals to search and destroy enemy quiet conventional submarines. It does not make sense to use this $2 billion-plus platform against an enemy platform that costs between $200 million and $300 million. SSKs are excellent quiet platforms for attacking enemy surface combatants and merchant vessels, especially at the approaches to maritime trade choke points such as international straits and narrows and off enemy naval bases and ports. They can also be highly successful for secondary missions as covert offensive and defensive mining, mine reconnaissance, and covert insertion of special operations teams.
