There's a Case for Diesels

By Ensigns Michael Walker and Austin Krusz, U.S. Navy

There are two reasons to make a ship nuclear-powered: mission and cost. First, the mission capabilities that the Navy asks of its submarine force are demanding. Because the United States has oceans on either border, having a strong submarine force is important not only in coastal defense, but also for protecting major sea lanes by controlling the open ocean. Nuclear attack submarines exercise sea control to support surface strike groups, shadow ballistic-missile submarines, and deny enemy ships access to areas of interest. 1 In this sense, the submarine force is a “blue water” force, operating throughout the world. Nuclear power enables a submarine to meet this requirement, as it allows for nearly unlimited endurance, with the nuclear reactor never needing to be refueled in a 25-year lifespan. 2 This means that a U.S. submarine’s limitations are only consumables, such as food.

A submarine’s main capability is stealth: A clandestine weapon, it carries few defensive measures on board, instead relying on concealment for safety. Because of this, a submarine is most vulnerable when snorkeling. Diesel-electric submarines snorkel frequently, to clear the exhaust from running their diesel generators to charge their batteries. They must slow down when snorkeling, because of the fragile nature of their masts and to prevent exposing themselves. Because nuclear propulsion is independent of air, nuclear submarines have no need to snorkel; when operating on station, they can maintain maximum stealth by staying completely submerged. 

The nuclear reactor on board a submarine allows it to operate at high speed for long periods of time with unlimited range. In comparison, diesel submarines operate using electric batteries and can only remain submerged for a few days at slow speed, or a few hours at top speed. Speed is a significant tactical factor, as it determines maneuverability and the ability to quickly change depth with flow over the hydroplanes. Nuclear power provides attack submarines with a sustained submerged speed of more than 30 knots, considerably greater than any contemporary diesel submarine. 3 Superior speed, range, stealth, and endurance make the nuclear submarine a very effective offensive weapon, capable of projecting power and taking the fight to the enemy. Still, on several accounts, the nuclear advantage is eroding.

The high cost of nuclear technology means that relatively few nations have fielded nuclear submarines, making a nuclear fleet an important technological and economic statement. This is why the reasoning that nuclear power is cheaper, as is taught to future submarine officers at the U.S. Naval Academy, is not fiscally rational. 

In 2012, the Congressional Budget Office (CBO) studied the viability of nuclear-powered surface combatants, including future destroyers, amphibious assault ships, and dock landing ships. 4 The study concluded that, depending on the price of oil, there is a certain price where a specific ship would break even—where it would cost the same to use nuclear power as conventional power. The amphibious assault ship, the largest ship, has the lowest break-even price. Even so, the price per barrel of oil would need to be nearly triple today’s values (roughly $65) and adjusted for inflation.

While the CBO report did not study submarines, a submarine would require less fuel than any of these larger surface combatants and could be expected to have a much higher break-even rate. The price of oil is not expected to skyrocket to anything near a break-even rate in the next 25 years. Thus, the cost of nuclear power cannot compete with the cost of diesel power. Meanwhile, building at the current rate of two Virginia (SSN-774) - class submarines per year, at a cost of roughly $2.7 billion per unit, puts a huge strain on the Navy’s shipbuilding budget and teeters on being fiscally unsustainable with the emergence of the Columbia (SSBN-826)-class ballistic-missile submarine program. 5

Such a high cost for a nuclear-powered fleet makes it impossible to keep pace with near-peer competitors in producing large numbers of capable submarines. Currently, Russia and China have large numbers of ingoing submarines, and the United States must keep up with both peer competitors. The U.S. industrial base is maxed out at building two Virginia -class fast attack submarines per year. Previous plans even dropped as low as one per year with the start of the Columbia -class program. Even with current talk of ramping up to three per year, the Navy anticipates a shortfall in fast-attack submarines spanning the years 2025 to 2041, as the Los Angeles class reaches the end of service. 

Meanwhile the People’s Republic of China is building the world’s largest submarine factory, a facility to build four subs at a time. 6 Russia has built 13 submarines in the past decade. 7 The need for a strong and robust submarine force is increasing. The U.S. Navy must counter other nations’ expansions while simultaneously maintaining the current force structure. Investing so heavily in a small number of nuclear-powered ships puts the United States at greater risk in a shooting war. A nuclear submarine is arguably a good investment if it lasts the entire expected lifespan; however, if war breaks out and submarines are lost, the U.S. Navy could not keep pace with its adversaries, economically or industrially. To this end, diesel submarines offer a solution: for the price of one Virginia- class sub, the Navy could buy six or seven conventional submarines of the German Type 212 class.

Allies play an important role in countering adversary submarine threats. In other warfighting domains, the United States uses its industrial power to trade, selling military equipment from firearms to fighter jets. When it comes to submarines, though, the United States does not export nuclear technology because of security concerns, with information on the program classified at the top secret level. These restrictions are removed with diesel submarines, however. In addition, the market for conventional submarines is robust, with many Asian nations looking to establish and modernize their submarine fleets to check Chinese aggression. During the George W. Bush administration, Taiwan was offered eight diesel submarines, but the deal languished when no foreign or U.S. shipbuilder was willing to construct an independent class without also selling it to the U.S. Navy. 9 With a domestic diesel program, the Navy would bolster its forces as well as those of its allies.

While nuclear-powered submarines are able to maintain operational excellence and meet mission requirements, diesel technology is catching up. The diesel submarines of today are not those of the days of the USS Nautilus . They operate much more quietly on new-generation diesel engines with advanced batteries. AIP technology has improved significantly the stealth performance of a new generation of submarines at a fraction of the cost of a nuclear-powered boat. When operating on batteries, AIP-equipped submarines are almost silent, with the only noise coming from the shaft bearings, propeller, and flow around the hull. 10 Nuclear submarines require large reduction gears and a robust cooling system to maintain safe operation of the reactor. Noisy pumps circulate cooling water around the reactor core at all times, then pump the same cooling water back into the ocean, leaving nuclear submarines with a much larger infrared heat signature. 

In addition, improvements in battery technology have extended the range of AIP diesel submarines. With electro-catalytic fuel cells and the high energy density of Lithium-ion batteries, AIP submarines can operate at a patrol-quiet state or rest on the seabed for several weeks without surfacing. German Type 212 submarines can stay underwater without snorkeling for up to three weeks, traveling 1,500 miles (2,400 kilometers) or more. Furthermore, over the past two decades, the industrial sector has achieved substantial increases in diesel engine efficiency, which could be applied to AIP submarines to further increase range and endurance. The ability of AIP was demonstrated in 2005, when HMS
Gotland, a Swedish AIP submarine, “sank” many U.S. nuclear fast-attack subs, destroyers, frigates, cruisers, and even the USS Ronald Reagan (CVN-76) aircraft carrier in joint exercises. 11

While nuclear power allows the submarine force to operate within the blue-water Navy as a whole, the Navy no longer operates in a unipolar world and must face the emergence of regional conflict. With a shift from global to regional disputes, the Navy has been forced to master littoral combat. With the proliferation of antiaccess/area denial (A2AD) weapons, surface fleets are becoming increasingly vulnerable and need to stay farther from littoral areas. Submarines are among the few weapon systems that can counter the threat and master this domain. 

Innovations to the Virginia class, such as fly-by-wire ship control to provide improved shallow-water ship handling and delivery of special operations forces, echo this littoral focus. However, the Virginia ’s nuclear propulsion system does not reflect the shift to the littoral zone. Because of the need to cool the reactor, nuclear-powered submarines prefer to keep to deep, cold water unless operating on station. 12 In the littorals, such as the South China Sea, diesel submarines could be a versatile asset. With naval bases in Okinawa, Singapore, Subic Bay, and Guam, range and endurance become less of a concern. For naval combat within the first island chain, fighting with a purely nuclear-powered fleet is a waste of assets. Conventional submarines would be of benefit in littoral waters that offset their limited operating endurance. 

In addition, not all undersea missions require nuclear power. In augmenting the current force, there is a wide variety of missions that diesel-electric submarines could fill without the need for the range or endurance of a nuclear submarine. Diesel submarines could be used for coastal defense and antisubmarine warfare, a particular need at a time when Russia boasts it can approach U.S. coasts undetected. Diesel subs could be surged to regional areas of concern in the event that conflict arises in the South China Sea, Korean Sea, Black Sea, Baltic Sea, or Sea of Japan. Diesel submarines operating from forward bases would represent a very cost-effective and stealthy means to conduct the Navy’s sea-control and sea-denial missions. 

If the Navy has no plans to return to diesel-electric technology in flagship submarines, a return may come in the emergence of unmanned underwater vehicles (UUVs). Manned submarines must support crew and act as multimission platforms; UUVs, on the other hand, can be built and optimized for very specific tasks, such as intelligence, surveillance, and reconnaissance (ISR), or mine warfare. The submarine force already is integrating and deploying large diesel-electric UUVs; Boeing and Lockheed Martin’s Orca extra-large unmanned underwater vehicle is capable of operating completely independently of a ship or submarine, unlike smaller UUVs that require a manned ship to be nearby to serve as a host, and can be limited by other factors. 

Nuclear submarines have proven to be one of the most effective platforms in the U.S. Navy since the Nautilus first launched, but this does not mean the Navy has to be restrained to one platform to accomplish numerous missions. Diesel submarines are very capable platforms that would be able to supplement a nuclear-powered fleet to perform specific mission sets. They are cheaper, lower risk, and could be produced faster, allowing the United States to grow the fleet quickly in the current time of need. The nation should reassess its undersea strategy and employ a combination of nuclear and conventional submarines, giving the Navy more submarines for less money with a greater range of capabilities.

1. Hans J. Ohff,  “ Nuclear Versus Diesel-Electric: The Case for Conventional Submarines for the RAN ,” The Strategist , 11 July 2017.

2. David Szondy, “ Rising Tide: Submarines and the Future of Undersea Warfare ,” New Atlas: New Technology & Science News , 5 July 2017.

3. “ AIP vs Nuclear Submarines, Defense Issues, 3 March 2013.

4. Derek Trunkey and Matthew Goldberg, “The Cost-effectiveness of Nuclear Power for Navy Surface Ships,” Congressional Budget Office, Pub. No. 4028 (Washington, DC), 2011, 1-28.

5. “ Navy: Plan to Build New Strategic Sub Requires ‘Unsustainable’ Funding ,” Nuclear Threat Initiative, 8 July 2014.

6. Kyle Mizokami, “ China Is Building the World’s Largest Submarine Factory ,” Popular Mechanics.
24 April 2017.

7. Brian Wang, “ USA Is Not Buying the Cheaper and Deadlier AIP Stealth Submarine Capabilities But the Rest of the World Is ,”, 30 January 2016.

8. Heinrich, Torsten, “ Why the US Needs Conventional Submarines ,” The Diplomat, 17 April 2016.

9. James Holmes, Doug Bandow, and Robert E. Kelly, “ One Way the U.S. Navy Could Take on China: Diesel Submarines ,” The National Interest, 17 March 2017.

10. Jonathan O’Callaghan, “ Death of the Nuclear Submarine? Huge Diesel-Electric Vessel Could Replace Other Subs Thanks to Its Stealth and Efficiency ,” Daily Mail Online, 4 November 2014.

11. Sebastien Roblin, James Holmes, Doug Bandow, and Robert E. Kelly, “ Did Sweden Make America’s Nuclear Submarines Obsolete? The National Interest, 30 December 2016.

12. Vego Milan, “The Right Submarine for Lurking in the Littorals,” U.S. Naval Institute Proceedings, 137, no. 6, June 2010, .

Ensign Walker and Ensign Krusz are future submarine officers. They realize that by writing this article they have committed the highest form of blasphemy.



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