By Cmdr. Cameron Aljilani, Undersea Warfare Division (OPNAV N97)
To properly address why the U.S. Navy doesn’t buy diesel submarines, it is worthwhile to review the purpose of the Navy. From the CNO’s “Design for Maintaining Maritime Superiority,” The Navy is a global, forward-deployed force capable of power projection “from the sea floor to space, from deep water to the littorals, and in the information domain.” This strategic guidance is consistent with our 240-year history of enabling sea control and power projection from the sea around the world to further our national interests. There are several characteristics that make nuclear-powered submarines uniquely capable to meet these global requirements. They are: speed, endurance, weapons volume and diversity, sensor capacity, stealth, sustainability, and cost.
Sun Tzu said in “The Art of War,” “Speed is the essence of war. Take advantage of the enemy’s unpreparedness; travel by unexpected routes and strike him where he has taken no precautions.” A submarine’s ability to maneuver at high speeds is the key to repositioning within a theater of interest and for maintaining the initiative in peacetime or wartime engagements. Speed is vital.
Speed gives U.S. submarines the agility to respond to contingencies worldwide. For diesel submarines, the fastest transit posture would be on the surface—an operationally unsatisfactory approach. The best submerged transit speed for a diesel submarine is around 7 knots and depends to some degree on the weather and adversary surface surveillance, which can complicate snorkeling operations to recharge the battery. Today’s Virginia-class submarines can operate at three to four times that speed, sustain that speed indefinitely, and is unaffected by weather or adversary surface surveillance. For perspective, a 2,000-nautical mile (nm) transit from Guam to the South China Sea would take about 12 days for a diesel submarine at 7 knots and about three days for a nuclear submarine operating at 25 knots.
Speed is also essential when intercepting targets, running down an evader, or escaping from a pursuing adversary. Unclassified estimates of high-end Russian or Chinese nuclear submarines and warships suggest they can operate at speeds in excess of 30 knots. By comparison, a Japanese Soryu-class submarine has a maximum speed of 20 knots submerged but can only maintain this speed for a brief sprint, after which it would have to disengage.
Submarine endurance is the ability to transit far from home and then operate unsupported in a mission posture for an extended period. For diesel submarines, endurance “on station” is limited by the fuel capacity that remains after the transit, the duration of the operation, and the transit distance required to reach a place to refuel. Endurance in a stealthy “mission posture” is limited by the need to find an opportunity to securely snorkel to recharge batteries. In addition, diesel submarines need to manage a safety margin of reserve battery capacity or fuel to accommodate unexpected circumstances. Nuclear-powered submarines do not have these operational limitations; long transits are quick, there is no need to create snorkel windows, there is no need for fuel or battery safety margin management, and there is no need to budget fuel for the return transit.
Weapon Volume and Diversity
Because submarines operate alone far forward without logistical support, it is vital that they carry enough ordnance to make the risk involved in getting in and out worth the impact the submarine makes on station. Modern diesel submarines carry from 8 to 24 weapons, almost all of which are launched from the torpedo tubes. Nuclear-powered fast attack submarines, in contrast, carry 36 weapons that are a combination of 12 vertical and 24 horizontal weapons. Virgnia-class submarines equipped with the Virginia Payload Module will be able to carry an additional 28 Tomahawk-sized missiles or an equivalent volume of other payloads for a total ordnance load of 64 torpedoes/missiles. On Los-Angeles-class submarines and the Virginia-class Block I and II submarines, the vertical payload volume is 12 21” tubes. Only on the Block III Virginias and beyond submarines does an 87-inch payload volume exist. Therefore, a nuclear submarine can carry a payload that is about three times as large as a diesel submarine payload, depending on the diesel submarine in the comparison, and can carry a much more flexible range of payloads able to support a wider range of missions.
Sensor Capacity—Space, Weight, Power, and Cooling
In addition to weapons, submarine payloads include sensor systems such as sound navigation and ranging (SONAR), periscopes, and electromagnetic warfare systems. Each of these systems imposes a structural footprint—the SONAR array, the mast arrangement, the processing and display equipment—that involves space and weight, including shock mounting, maintenance access, and repair parts storage. In addition, each of these systems places a demand on electrical power and cooling systems. Diesel submarines are smaller and therefore necessarily must constrain the size of the sensors and support systems they can carry. Large acoustic arrays, for example, are problematic.
The design and operation of a diesel submarine necessitates the limiting or rationing of power and cooling demands, which often means shutting down systems or system components. Nuclear submarines are much less constrained on structure, power, and cooling. They are able to carry large arrays, large processing banks, long towed arrays, robust display consoles, and the necessary parts and logistic support. Nuclear submarine crews do not have to decide whether they want all of the systems operating or just some of the systems to conserve resources for a prolonged mission posture until the next snorkel opportunity. In summary, nuclear submarines have larger and more capable sensors, more flexibility on sensor options, and are able to fully employ those sensors without compromise.
Stealth and Vulnerability
A well-designed diesel submarine that is professionally operated, submerged and running on the battery, and lying in ambush is perhaps the stealthiest and most capable maritime threat today. This particular posture plays directly to the strengths of a diesel submarine. U.S. Navy submarines have other missions to carry out that are not so well suited to diesel submarines as is the above scenario. And even in this ideal ambush mission, there are other phases of the operation that must be performed, and in those phases the weaknesses in stealth and vulnerability of diesel submarines come into play. The submarine must transit to its ambush location, it must periodically recharge while lying in wait, and it must return home. In general, these are not strengths of a diesel submarine.
That said, there are countries whose only submarine mission is local defense. For such countries, there would be no transit and the mission location would be in home waters. This also means that there would be no surface or air threats to the diesel submarine while recharging. These countries should buy diesel submarines because they are a perfect match for their mission and circumstances. The United States is not in this situation. We will not have submarines lying in ambush in our local waters waiting for a threat to arrive. We have global responsibilities and a broad range of missions. We have to transit long distances quickly, and we need the best stealth that can be achieved to support this mission set.
When a submarine is operating far forward, the crew must carefully manage the risk of detection. All submarines are designed to limit their acoustic signature and can further limit their visual and radar vulnerabilities by not operating at periscope depth or, when they are at periscope depth, by minimizing the signature produced through good operational discipline. The signature produced by a diesel submarine snorkeling is much greater than the signature of a prudently exposed periscope.
Even with Air-Independent Propulsion systems, which also depend on consumables with limited onboard supply, non-nuclear submarines remain more operationally constrained in speed and flexibility. For nuclear submarines, the ability to produce essentially unlimited electricity and propulsion while submerged enables limiting mast exposure to the bare minimum needed for sensor effectiveness. When a nuclear submarine accepts detectability risk to gain information or conduct an operation—for example, exposing a periscope or launching a missile—it is a deliberate choice made as a calculated risk in pursuit of an operational gain. It is not a step imposed on the crew due to a limitation in the platform’s performance, as is the case with a diesel submarine forced to snorkel.
Sustainability refers to how effectively the Navy’s support infrastructure is able to provide for the needs of the submarine. This includes food, repair parts, repair equipment and training, crew training, and the availability of fuel and other consumables. As a forward-deployed Navy, our surface and air forces have mature supply lines and a developed expeditionary sustainment capability. Some of our sustainment capability comes from allied and partner-nation support. In a contested environment, this regional support may be unavailable due to political sensitivities or physical destruction of infrastructure.
To compensate for this possibility and to add operational flexibility, Military Sealift Command has a fleet of ships that provide underway replenishment. There is no sustainment infrastructure for diesel submarines in the U.S. Navy; a large percentage of the equipment on diesels would be specialized and unique, and so too would be the support. This entire infrastructure would have to be built up from scratch, requiring investment in a separate independent support network compared to nuclear submarines. Nuclear-powered submarines are largely self-sustaining while forward deployed and, for the work that must be done while forward, the infrastructure already exists and is finely tuned.
The most common argument in favor of diesel submarines is that they cost less to build than nuclear submarines. For perspective, a German Type 212 diesel submarine sold to Norway for delivery in 2019 cost 560 million Euros (~US$623M) while today’s Block III Virginia-class submarines cost ~US$2.7B. Based on these numbers, proponents for diesel submarines argue that the United States could get four diesel submarines for the cost of one Virginia-class submarine. However, this is not an apples-to-apples comparison because the four Type 212 submarines cannot do all the things that the one Virginia-class submarine can do.
A closer but still imperfect comparison would be to analyze Australia’s most recent contract with French shipbuilder DCNS for 12 Shortfin Barracuda Block 1A submarines (to be modified from nuclear to diesel). This contract was $50B Australian dollars (~US$38B) for 12 submarines, which averages to about US$3B per submarine including engineering costs for modification, materials, and infrastructure to build the submarines in Australia. This includes costs for Lockheed Martin’s Submarine Warfare Federated Tactical Systems (SWFTS) advanced fire control and SONAR system, which is used by both the United States and Australia.
When thinking about cost, it is prudent to remember that we are not purchasing submarines—we are purchasing a set of operational capabilities. Imagine a swim-off between the four Type 212 submarines and a Virginia-class submarine in Hawaii. Assume a contingency develops in the Western Pacific that requires a flexible response and a 4,000-nm transit. The Virginia-class submarine would be there in six days, the diesel submarines in more than three weeks. The diesel submarines would be vulnerable to tracking and interdiction in transit due to frequent exposure. When they finally arrive on station, their remaining endurance would be a small fraction of that of the Virginia-class submarine. The four Type 212s would bring twice as many torpedoes as the Virginia-class submarine but would have very little mobility to run down adversaries to administer a torpedo attack.
They would bring no strike weapons. They would have a limited sensor suite. They would not be able to transit in the company of a carrier strike group due to speed constraints. They would not be able to do high speed acoustic searches to clear an area. Any repositioning of the diesels would depend on the plan for refueling them—a risky operation when operating far forward.
All this is not to criticize diesel submarines outright, it is to criticize them as a poor match for the operational environment and requirements facing the U.S. Navy. Type 212 submarines would be excellent for local operations in European littoral waters as part of the German or Norwegian navy. They would be ineffective, however, in supporting the missions of the U.S. Submarine Force due to their slow speed, limited endurance, limited payload size and mix, and their stealth vulnerabilities. It should be clear to see why diesel submarines do not meet the U.S. Navy’s operational requirements.
Another aspect to consider is how the Navy would build diesel submarines. The cost of a single diesel submarine is most appealing when it is available for purchase off an already operating production line, but the United States has not produced a diesel-powered submarine since 1959. Today, U.S. nuclear shipbuilders are designing and producing Virginia-class Blocks III, IV, and V submarines and designing the Columbia-class SSBN, and there is not sufficient industrial capacity to take on additional projects without detracting from the Navy’s current shipbuilding plan. In fact, the Navy would like to accelerate production of fast attack submarines beyond two Virginia-class submarines per year to more quickly reach a total of 66 fast attack submarines but current industrial capacity limitations prevent immediate acceleration. A new design and build effort would detract from existing efforts and would require significant capital investment in the public and private shipyards to produce, design, and maintain diesel submarines and their equipment.
One other argument for the U.S. Navy to have diesel submarines is to be able to train with them. Some argue that the procurement of diesel submarines would enable more realistic training for the U.S. Submarine Force. The U.S. Navy acknowledges the benefit of training with diesel submarines at sea. To accomplish this, the Navy has existing avenues through bilateral and multilateral training exercises with our allies and through the Diesel-Electric Submarine Initiative, which holds Fleet pre-deployment exercises and bilateral tactical development events. While this training is effective, it does not require purchasing diesel submarines.
The U.S. Navy not only has no compelling reason to abandon its nuclear-only Submarine Force policy, it has every incentive to stay the course. In a world beset by powerful competitors, rogue nations, and violent non-state actors, there is no question that the United States has an obligation to ensure the safety and freedom of Americans as well as other less powerful partner states and allies. Our national requirement to operate far forward and quickly respond to crises worldwide requires the speed, stealth, and endurance inherent in our current and future fast attack and ballistic missile submarines.