The United States submarine force consists of four operational boat classes, all of which are nuclear-powered. Operational vessels are divided into these classes as follows:

• 14 Ohio-class SSBNs (as well as an additional four Ohio-class SSGNs);
• 6 Virginia-class SSNs (with an additional six on order);
• 3 Seawolf SSNs; and
• 44 Los Angeles-class SSNs.

Sea-based deterrence is performed by 14 Ohio-class SSBNs that serve as the third leg of the U.S. strategic triad. An additional four Ohio-class submarines are configured as SSGNs that possess both strike and special force insertion capabilities. The three classes of U.S. attack submarines — Virginia, Seawolf and Los Angeles - are tasked with engaging and destroying enemy vessels, supporting on-shore operations and carrier groups, and carrying out surveillance.

Ohio-class

The sea-based leg of the U.S. strategic deterrent is performed by 14 Ohio-class SSBNs that are armed with the Trident II D5 submarine launched ballistic missile (SLBM). It is estimated that together these boats carry around 1,152 warheads (approximately 43%of the United States' operational arsenal). [1] Four of the vessels that previously carried the Trident C-4 missiles have been retrofitted with the longer-range and more accurate D5. Construction on the first Ohio-class SSBN, SSBN-726 Ohio, began in 1976 and the final boat of this class, SSBN-743 Louisiana, was commissioned in 1997. [2] Today, six SSBNs are based in the Atlantic at King's Bay, Georgia, and eight in the Pacific at Bangor, Washington State. [3,4] Since the end of the cold war there has been a strategic shift in deterrence patrols with 60% now taking place in the Pacific — due primarily to the change in threat perception. [5] This is reflected in the higher concentration of SSBNs that are now based at Bangor.

United States at-sea deterrence was previously performed by 18 Ohio-class SSBNs. However, the Clinton Administration's 1994 Nuclear Posture Review determined that 14 would be adequate to meet the country's strategic requirements. As a result, four vessels were reconfigured into SSGNs that carry up to 154 Tomahawk, or tactical Tomahawk, land-attack cruise missiles. It was the four oldest Ohio-class SSBNs — Ohio, Michigan, Florida and Georgia — that were selected for conversion and this process was carried out over a five year period between November 2002 and March 2008. Today, two of these vessels are based at King's Bay and two at Bangor. [6]

Los Angeles-class

The nuclear-powered Los Angeles-class SSN is armed with Tomahawk land-attack cruise missiles and MK-48 torpedoes. The boat was primarily developed for the purpose of anti-submarine warfare but is also capable of inserting Special Forces and laying mines. Today, 44 vessels are in operation, built between 1971 and 1996, and are located at five different bases. [7] Nineteen vessels are located at two bases in the Atlantic (Norfolk and Groton) and 25 vessels are located at three bases in the Pacific (Pearl Harbor, San Diego and Guam). [8] These deployments are further evidence of the increased emphasis that has been placed on the Pacific.

As a result of technical improvements being made over time, there are now three different variants of the Los Angeles-class. Beginning with the USS Providence in 1977, the vessels were equipped with 12 vertical launch tubes for Tomahawk missiles. Then, beginning with USS San Juan in 1982, the class was fitted with an advanced BSY-1 sonar system. [9]

Seawolf-class

The U.S. Navy also possesses three Seawolf-class vessels that are based at Bangor. This class of attack submarine is significantly faster and quieter than the Los Angeles-class and was originally developed to hunt Soviet SSBNs. [10] The Boat's stealthy capabilities make it well suited for the insertion of Special Forces. Although it does not possess a vertical launch capability, it can fire Tomahawk missiles through its torpedo tubes. [11] While the original plan was to produce as many as 29 submarines, the cost of constructing the Seawolf proved high and the end of the cold war meant that their primary function was no longer applicable. As a result, in 1995 Congress decided to terminate the program at three boats. [12]

Virginia-class

The Virginia-class, designed by the Electric Boat Corporation of Connecticut, represents the next generation of U.S. nuclear attack submarines and a more cost effective alternative to the Seawolf. With a number of vessels already in service, the Virginia-class will fulfill the same operational tasks that are currently being carried out by Los Angeles-class boats. But an added strength is the Virginia's ability to operate effectively in littoral waters — primarily due to its "fly-by-wire" control system that allows for better handling in shallow areas. [13] This provides it with enhanced intelligence gathering capabilities, something that is strengthened by the fitting of Photonics Masts with high resolution digital cameras. [14] Furthermore, unlike the Seawolf, the Virginia-class does possess vertical launch tubes for firing its land-attack Tomahawk missiles and also carries Unmanned Undersea Vehicles (UUV) and special force delivery vehicles. [15]

There are currently six Virginia-class submarines in service with an additional six under construction. Five of the operational vessels are based at Groton, Connecticut, and one at Pearl Harbor. [16] They are currently being built at an approximate rate of one per year but it remains unclear, at this stage, what the eventual force level will be. It is likely to be partly dependent on retirement rates from the older Los Angeles-class vessels.

Modernization

Current priorities for modernizing the submarine fleet include the construction of additional Virginia-class vessels and the undertaking of concept development studies for an eventual replacement for the Ohio-class SSBNs.

Sources:
[1] Robert S. Norris and Hans M. Kristensen, "Nuclear Notebook: U.S. Nuclear Forces, 2009," Bulletin of the Atomic Scientists, www.thebulletin.org, March/April 2009.
[2] "SSBN-726 Ohio-Class FBM Submarines," Federation of American Scientists, www.fas.org.
[3] "Fleet Ballistic Missile Submarines —; SSBN," United States Navy, www.navy.mil, 11 May 2009.
[4] Robert S. Norris and Hans M. Kristensen, "Nuclear Notebook: U.S. Nuclear Forces, 2009," Bulletin of the Atomic Scientists, www.thebulletin.org, March/April 2009.
[5] Robert S. Norris and Hans M. Kristensen, "Nuclear Notebook: U.S. Nuclear Forces, 2009," Bulletin of the Atomic Scientists, www.thebulletin.org, March/April 2009.
[6] "Guided Missile Submarines — SSGN," United States Navy, www.navy.mil, 13 January 2009.
[7] "SSN-688 Los Angeles-class," Federation of American Scientists, www.fas.org.
[8] "Attack Submarines — SSN," United States Navy, www.navy.mil, 17 March 2010.
[9] "Attack Submarines — SSN," United States Navy, www.navy.mil, 17 March 2010.
[10] "SSN-21 Seawolf-class," Federation of American Scientists, www.fas.org.
[11] "Attack Submarines — SSN," United States Navy, www.navy.mil, 17 March 2010.
[12] "SSN-21 Seawolf-class," Federation of American Scientists, www.fas.org.
[13] "Attack Submarines — SSN," United States Navy, www.navy.mil, 17 March 2010.
[14] "Attack Submarines — SSN," United States Navy, www.navy.mil, 17 March 2010.
[15] "SSN-774 Virginia-class NSSN New Attack Submarine," Federation of American Scientists, www.fas.org.
[16] "Attack Submarines — SSN," United States Navy, www.navy.mil, 17 March 2010.