|Fig. 1: USS Virginia (SSN 774), the first submarine of the Virginia-class, returning to port at Groton, CN, 2004. (Source: Wikimedia Commons)|
At 11:00 in the morning on 17 January 1955, USS Nautilus became the world's first operational nuclear- powered submarine.  Since that day, the United States Navy has developed arguably the world's most capable nuclear-powered fleet, which includes all of its submarines and aircraft carriers. As the end of the 2014 fiscal year, the U.S. Navy operated 73 nuclear-powered submarines, which include 55 fast-attack SSNs, 14 ballistic missile SSBNs, and 4 guided missile SSGNs.  In order to maintain readiness for future conflicts around the world, the U.S. Navy must constantly develop newer, faster, quieter, more flexible, and more cost-effective submarines. This article seeks to describe these developments and lay out the path of submarine operations for the foreseeable future.
Fast-attack SSNs are the backbone of the U.S. submarine fleet, and they perform a variety of critical missions. From tracking enemy naval combatants, striking shore targets with cruise missiles, carrying out intelligence operations, and supporting special operations forces, fast-attack submarines provide versatile stealth and endurance capabilities around the world. Most operating SSNs are the Los Angeles-class from the 1970-80s, with 41 currently in service.  Towards the end of the Cold War, the Seawolf-class program aimed to build the most technologically advanced submarines to combat the Soviet Union. In 1995, with the Cold War threat vanished, the U.S. Congress decided that the costs of the program were unjustifiable and cut the Seawolf-class to only three submarines. 
Starting with the commissioning of USS Virginia (SSN 774) in October 2004, the new Virginia-class of submarines was designed to be the less expensive alternative to the Seawolf-class and to replace the aging Los Angeles-class. With twelve submarines commissioned and four under construction, the Virginia-class represents the next generation of the fast-attack fleet. The Navy's 2016 long-term shipbuilding plan calls for the construction of one or two Virginia-class submarines every year until 2045, with a planned total of 50.  This construction will take place in five distinct blocks, with each block incorporating technological advancements and minor design changes. The Block V submarines, for example, will include a Virginia Payload Module to potentially house more Tomahawk cruise missiles, which will enable these Virginia-class to replace the four Ohio-class SSGNs. These submarines are expected to have a service life of 33 years, which means that some will be in service until 2078. 
Virginia-class submarines feature many innovations that improve its capabilities, especially for operating in shallow water littoral regions. The most important feature for maneuvering in littoral waters is the fly-by-wire control system, whereby computers in the control center electronically adjust the submarine's control surfaces, a significant improvement from the hydraulic systems used in the Los Angeles-class. Also, two photonic masts, which house digital cameras for visible and infrared imaging, have replaced the traditional periscope found on older submarines.  The other innovations of the Virginia-class follow a theme of flexibility. They feature open architecture, modular construction, and the use of commercial-off-the-shelf components in order to adapt to new mission requirements, integrate more cost-effective construction methods, and accommodate new technologies as they become available. 
The nuclear reactor design onboard Virginia-class submarines also represents a major improvement over previous classes. The main advantage of this new reactor design is that it can operate for over 33 years, or the full lifetime of the submarine, whereas previous submarine classes required an expensive and time-consuming refueling overhaul about two-thirds of the way through their service life.  Life-of-the-ship reactors not only save billions of dollars in program costs, but also allow these submarines to perform more missions on deployments. 
|Fig. 3: An artist's rendering of an SSBN(X) firing a ballistic missile while submerged. (Source: Wikimedia Commons)|
Since the 1960s, nuclear ballistic missile submarines have been a crucial element of the United States' nuclear strategic triad. Deterrence strategy relies on a survivable and dependable second-strike capability, and SSBNs, or "boomers," are a virtually undetectable launch platform. The 14 Ohio-class SSBNs have been in service since the 1980s, and the oldest are scheduled for decommissioning in the late 2020s.  Consequently, the Navy has actively sought to plan for the next generation SSBN, dubbed the Ohio Replacement class, or SSBN(X). The Navy plans to build a total of 12 Ohio Replacement submarines, one per year from about 2030-2042, and they will have an expected service life of 42 years. 
Similar to the Virginia-class program, the Ohio Replacement submarines will be driven by the same motivation to design for flexibility and affordability, all while incorporating the latest in technological innovation. The SSBN(X) will feature many of the same systems as the Virginia-class, including a pump-jet propulsor instead of a traditional propeller, a nuclear reactor core that will last the lifetime of the submarine, and modular construction. If design and construction proceed according to the Navy's plans, the first SSBN(X) will be ready for strategic deterrence patrol in 2031. 
Before the success of USS Nautilus, U.S. submarines were powered by a combination of diesel engines and lead-acid batteries, both of which were dirty and hard to maintain. Submarines had to cruise at the surface long enough for the diesel engines to charge their battery packs, then they would switch to battery power to submerge. Underwater, the batteries only enabled the submarines to stay underwater for a limited time. Nuclear-powered submarines theoretically have an underwater endurance equal to the lifetime of the reactor, on the order of 30-40 years, which is why the U.S. Navy has relied exclusively on this technology for its submarine fleet.
With the advent of improved battery chemistries and continuing pressure to cut U.S. military spending, there is an increasing push for the U.S. to again build conventional diesel-electric submarines.  The benefits of a potential fleet of diesel-electric submarines, or SSKs, are clear. They would be much cheaper to build, and they would be safer to operate without the inherent dangers of a nuclear reactor with weapons-grade uranium. Also, an SSK running submerged on battery power has the potential to be even quieter than a Seawolf or Virginia-class.  As the U.S. submarine fleet slowly shrinks over the coming decades, there likely will be increasing calls for the reestablishment of SSKs to combat the rising submarine fleet strength of countries like China and Russia. For now though, the U.S. remains committed to an all nuclear-powered submarine force.
© Alexander Yachanin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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