February 1, 2009  

A balancing act

The shortfall of nuclear-powered attack submarines can be mitigated with conventional subs

U.S. nuclear-powered attack submarines (SSNs) are key elements of the Navy’s striking power. They were critical during the Cold War and still play a vital role in the Navy’s ability to obtain, maintain and exercise sea control. Though their missions in peacetime have changed greatly since 1991, there are still potential threats in the various parts of the world’s oceans, and in the Pacific and the Indian oceans in particular, that might require combat employment of a significant force of the SSNs.

However, for all their capabilities, the SSNs are not best-suited for operations in the shallow waters of many enclosed and semi-enclosed seas. Moreover, the projected SSN force level is inadequate for wartime requirements. This situation can be changed for the better by balancing the SSN force with a small force of advanced conventional submarines (SSKs) optimized for shallow-water operations.

Determining the proper size and the composition of the Navy and each component of its battle force is not an exact science. Many assumptions regarding current and projected threats and the budgetary sources must be made many years out. In general, the broader framework set by national military and maritime strategy, and in the U.S. context its maritime theater strategies, should dictate the Navy’s size and composition, including its SSN force. However, the Navy first determines its SSN force level based essentially on the budgetary realities, then conducts SSN-force-level studies to validate that decision. This method of determining force level is deeply flawed. The budgetary restraints should come into play only after the desirable force level is determined from analysis of the current and projected military and maritime strategic situation. Budgetary realities almost always create a gap between the desired and affordable force levels. Any mismatch or disconnect should lead either to scaling down the ends or increasing the means; otherwise, the resultant SSN force level might be either too low (most likely) or too high relative to the Navy’s actual requirements.


The number and composition of the Navy’s submarine force has varied greatly over the past 25 years. The Reagan administration’s 600-ship Navy envisaged a force of about 100 SSNs. In the 1980s, the submarine force level peaked at 98 boats in 1987 and then began to decline. There were 85 to 88 boats during the early 1990s; 79 boats at the end of fiscal 1996; 65 at the end of 1998; 57 at the end of 1999; and 56 at the end of 2000.

By the end of 2007, the Navy’s SSN force consisted of 53 boats: 47 Los Angeles-class, three Seawolf-class and three Virginia-class. A total of 62 Los Angeles-class submarines entered service between 1976 and 1996. They carry four torpedo tubes and 26 torpedoes or Tomahawk missiles. The final 11 boats, known as 688I-class, were fitted with 12 Vertical Launching System tubes in their bow for 12 additional Tomahawk missiles. The final 23 boats of this class were extensively upgraded. By the end of fiscal 2007, 15 of 62 boats of this class were retired. Originally, the Navy planned to build 30 Seawolf-class SSNs. However, because of the changes in the security environment in the aftermath of the end of the Cold War, only three boats of this class were commissioned in 1997 and 1998. The third entered service in 2005. The Seawolf-class submarines were fitted with 30-inch torpedo tubes for firing 50 torpedoes or missiles. The Virginia class is slightly larger than the Los Angeles class and is equipped with new technologies. The first three boats of this class entered service between 2004 and 2007. In late December the Navy announced plans to build eight Virginia-class boats for about $2 billion each. The plans call for construction of one boat each in fiscal 2009 and 2010, and then two boats annually in 2011 through 2013. The Virginia class has operating costs that are more than 30 percent lower than the Seawolf class while being superior in stealth mine-warfare performance and its special warfare and battle group protection missions. Its open systems architecture enables faster and cheaper upgrades and means it can be configured for specific missions.


The main missions of the SSNs are intelligence, surveillance and reconnaissance (ISR), land attack with long-range cruise missiles, anti-submarine warfare (ASW), anti-surface warfare (ASUW), offensive and defensive mining, and covert insertion and recovery of special operations forces.

The SSNs provide a critical information-gathering capability that cannot be replicated. Using multiple sensors they can covertly observe air, surface or subsurface situations to an extent not possible using other platforms. They can operate in littoral areas inaccessible to other platforms. SSNs can perform ISR missions covertly, staying on station for days, weeks and even months. They also can observe activities off ports, naval bases and coasts at close range. They can collect more information on prospective targets than any other platform. However, the SSNs cannot observe the totality of the target or obtain imaging far from the coast the way satellites and aircraft can.

In land attacks SSNs can carry about 16 Tomahawks that are ready for launch plus up to 12 Tomahawks that can be reloaded and fired without resurfacing. In a carrier group, SSNs provide 20 percent of Tomahawk firepower. The SSNs are the principal and perhaps the most effective platforms for ASW and ASUW. Because of their long range and stealthiness they can lay mines at approaches to enemy naval bases and commercial ports, patrol areas of hostile submarines and surface ships as well as patrol merchant shipping routes. The SSNs can also conduct mine reconnaissance by using unmanned underwater vehicles to detect and report mine dangers. They are excellent platforms for clandestine insertion of special operations teams in the littorals.

In 1999, the Navy stated that on average 5.8 SSNs were required to keep one SSN continually on station in a distant operating area. This requirement changed little between 1999 and 2002. In July 2005, theater combatant commanders wanted 13.5 attack submarines to be on station continuously in six theaters. To maintain a force of 13.5 boats continuously on station would require 78 boats. The 54-boat force could generate only about nine SSNs deployed in forward areas. The current requirements for deployed SSNs are 10, or an average of 10 boats to be deployed on a day-to-day basis. The peak projected wartime demand is about 35 SSNs. This figure includes 10 SSNs at sea in the operating area and 25 additional SSNs to be surged from the continental U.S.

The Navy considers a force of 50 SSNs as an absolute minimum. To maintain that level two SSNs must be built per year over the next two decades to replace the retiring Los Angeles and 688I classes. However, even if the 30-year submarine construction plan of 2007 is fully implemented (two Virginia-class vessels per year starting in 2012), the number of SSNs will fall below 48 from 2020 through 2034. In fact, the Navy estimates that its SSN force will fall below 41 boats in 2028-2029. The Navy plans to decommission four SSGNs by 2028 and no replacements are currently planned. In 2006, the Navy initiated a study on the options available for mitigating the projected shortfall in SSNs. The study was completed in early 2007. By then the SSN force was projected to bottom out at 40 boats. Thus, the projected shortfall in the number of SSNs was eight boats.


The optimal solution to resolve the Navy’s 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 90B’s 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.

The Navy has three options for acquiring a force of SSKs: It can lease a small number of advanced SSKs from a friendly nation for a specific time; it can build SSKs at home based on its own specifications or under license; or it can order them to be built overseas.

Each option has advantages and disadvantages. Only a small number of submarines can be leased, so this option will not resolve the attack submarine force-level shortfall.

Building the AIP submarines in the U.S. would most likely result in a much higher cost than if they were ordered from a foreign shipyard. That’s because the Navy would likely require higher standards than those adopted by foreign yards, resulting in substantially higher procurement expenditures. This, in turn, would reduce the number of the AIP submarines built to two or three boats for each Virginia-class. Based on recent experiences, the build time for domestic AIP submarines would probably also be much longer than if they were built by foreign yards.

The prime candidates for procuring the AIP submarines are the Swedish Stirling AIP submarines and the German Howaldtswerke-Deutsche Werft Type 212A/214 fuel cell AIP. The Swedish design has advantages in its simplicity and lower price of about $365 million per boat. Type 212As, and especially the newest Type 214s, are extremely capable boats but are more expensive. The cost of a Type 212A is reportedly about $500 million.

A major and perhaps insurmountable problem for acquiring advanced conventional submarines is the Navy’s strong opposition to such a solution for the SSN force-level shortfall. The Navy opposed the construction of the SSKs for Taiwan because it feared that Congress might refuse to fund the procurement of the SSNs. These concerns are not unfounded. However, the Navy must first make the right decision that increases the effectiveness of its attack submarine force. Then it must start an intensive and sophisticated effort to educate Congress and the public why the attack submarine force must be balanced.

By acquiring a modest force of advanced conventional submarines the Navy would solve its perennial problem of having an inadequate number of attack submarines in its inventories. It would also not have to rely on submarines provided by other countries to enhance its own capabilities in shallow waters. The time is long past for the Navy to have its own modest force of advanced SSKs.

MILAN VEGO is a professor of operations at the Joint Military Operations Department at the Naval War College. The views expressed here are his own and do not necessarily reflect those of the Navy or Defense Department.