Hudson Institute

Fighting into the Bastions: Getting Noisier to Sustain the US Undersea Advantage

Senior Fellow and Director, Center for Defense Concepts and Technology
Senior Fellow, Center for Defense Concepts and Technology
The USS Tuscon transits the East China Sea on July 26, 2010. (US Navy)

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Airpower enthusiasts concluded after World War I that “the bomber will always get through.”1 Fast, high-flying aircraft seemed to hold a decisive edge over air defenses and offered commanders the assurance they could attack targets deep inside an opponent’s territory. But airpower advocates’ spirits were quickly dampened during World War II by the advent of interceptor fighters, radar-guided artillery, and electromagnetic warfare (EW), which together demonstrated that bombers could be stopped—often with devastating results. Air forces adapted, adding escort fighters, radar jammers and decoys, and higher-altitude bombers to sustain their ability to strike behind enemy lines. During the Cold War, this move-countermove competition continued. New surface- and air-launched missiles targeted bombers, and countries developed specialized aircraft and weapons to suppress and destroy enemy air defenses or use stealth to avoid detection and targeting altogether.

Today, the US submarine force faces a similar challenge. The US fleet of nuclear-powered attack submarines (SSNs) has long been considered an asymmetric advantage against potential adversaries like the People’s Republic of China (PRC) and Russia, which lacked robust and effective anti-submarine warfare (ASW) capabilities. Relying on this advantage, the US Department of Defense (DoD) has increasingly relied on SSNs to compensate for the impact of improved air defenses on the ability of bombers and strike fighters to execute attacks.2 However, the unfailing ability of US SSNs to reach their targets is eroding as emerging technologies and weapons proliferation combine in new approaches to ASW that could neutralize America’s undersea advantage.

During the last decade, the PRC and Russian Federation began fortifying their undersea defenses in the South and East China Seas and Arctic Ocean, respectively. Leveraging geographic constraints, US adversaries have instrumented these bastions with networks of fixed and mobile acoustic and non-acoustic sensors, complemented by aircraft and ships capable of pouncing on contacts or deploying dense mine barriers.3 Improved adversary defenses could degrade or defeat US undersea operations, preventing US submarines from conducting critical missions such as sinking a Chinese invasion fleet or tracking Russian ballistic missile submarines (SSBNs).

To sustain its offensive undersea advantage, the US Navy will need to take some lessons from air warfare and begin supporting submarines with systems designed to suppress or destroy enemy undersea defenses. This imperative will fundamentally shift the paradigm for US submarine operations from “alone and unafraid” to “it’s all about team.” Moreover, the emergence of new generations of capable long-range active sonars will demand that the US undersea force increasingly rely on jamming and deception to counter enemy sensors, much as their counterparts already do above the water. The US submarine force will need to learn how to use noise, rather than avoid it.

The Navy should pursue four lines of effort, outlined below, to sustain its offensive undersea advantage. Except for features to be incorporated into the next-generation nuclear-powered attack submarine, the SSN(X), the concepts and capabilities proposed in this report leverage mature or maturing technologies and can be fielded within five years. Implementing these recommendations will require the US Navy to refine its use cases for offensive undersea warfare to use available uncrewed vehicle technologies, rather than continue pursuing purpose-built systems that may take a decade or more to be fielded.4

1. Field a Team for Offensive Undersea Operations

Although uncrewed systems such as mines and torpedoes conduct offensive undersea operations, US commanders will want a human operator to verify that rules of engagement (ROE) are met before an attack is initiated. Submarines will therefore remain the centerpiece of US offensive undersea warfare because they can place operators close enough in time and space to undersea attacks to be accountable for ROE. Like today’s stealth bombers and strike fighters, to achieve this proximity submarines will increasingly need teams of systems that deceive, disrupt, or destroy enemy undersea sensors and weapons, as summarized in figure ES-1.

To support future operations, before conflict uncrewed surface vehicles (USV) and uncrewed undersea vehicles (UUVs) would rigorously map the ocean floor and water column to find likely adversary sensors, power and communication networks, mines, and UUV support infrastructure. When offensive undersea missions are needed, US naval forces would prevent SSNs from being accurately targeted by destroying adversary undersea sensors, communications, or weapons using UUVs; by dispersing USV-based mast decoys on the ocean’s surface; or by deploying UUV-based acoustic jammers or decoys near adversary sonar arrays. These concepts fundamentally shift US submarine operations from a hider-finder competition to a competition about sensing and sense-making.

The Navy faces several challenges to fielding offensive undersea warfare teams. Beyond the cultural shift to using, rather than avoiding noise, the US Navy’s most significant challenge may be how to deploy uncrewed systems. Pre-conflict surveys of adversary bastions like the Barents Sea or South China Sea will likely need to be conducted clandestinely to avoid alerting the opponent that its undersea defenses have been discovered. But payload space on a submarine comes at a premium, and every UUV that takes up a torpedo stow or missile cell displaces a weapon that can be reloaded only at a ship or pier outside of the conflict area. Therefore, submarine-launched medium UUVs (MUUVs) that can be carried only in the torpedo room should be reusable and few. In contrast, small UUVs (SUUVs) that can be carried in external countermeasure launchers or lockers inside the submarine could be expendable and deployed by SSNs without impacting their weapons capacity.

These constraints, and the potential detectability of launching and recovering UUVs in general, point to some ways of aligning uncrewed systems and deployment mechanisms with particular use cases. For example, during offensive operations, deception and jamming will need to happen away from the protected submarines, but the SUUVs well-suited for these functions are relatively slow. An SSN could therefore deploy SUUVs near adversary sensors while transiting to an operating area, where the SUUVs would loiter or sleep and activate at a predetermined time or on an acoustic signal if conditions allow. Uncrewed air systems (UAS) or USVs could also deploy SUUVs and might be preferred for some use cases, such as quickly distributing numerous SUUVs near enemy sonar arrays.

Larger UUVs such as the large displacement uncrewed undersea vehicle (LDUUV) and extra-large uncrewed undersea vehicle (XLUUV) offer greater endurance and payload volume that can be used for long-duration pre-conflict survey missions as well as deployment of smaller vehicles such as SUUVs or mines. These vehicles could be launched from shore, but some are relatively slow. To maximize their time on station, LDUUVs and XLUUVs could be deployed from amphibious transport docks or other ships with suitable well decks or rigging equipment.

2. Use Uncrewed Systems to Free Up SSNs

The need for SSNs to conduct ASW is a significant constraint on their availability for offensive undersea warfare. Uncrewed systems cannot replace SSNs, but they can perform some functions submarines commonly are called upon to deliver, thereby expanding the scale of US undersea operations and freeing SSNs for higher-priority tasks. Two missions where uncrewed systems can be most impactful are ASW sensing and engagement, as summarized in figure ES-2.

Adversary submarines can be detected, tracked, and trailed by uncrewed passive acoustic sensors like the fixed Sound Surveillance System (SOSUS) or relocatable Transformational Reliable Acoustic Path System (TRAPS); by sonobuoys deployed by uncrewed vehicles; and by sonar arrays towed by USVs or UUVs. Uncrewed vehicles can also enable US undersea forces to exploit active sonar to detect and track increasingly quiet PRC and Russian submarines without risking counter-detection of a crewed US ship. For example, in addition to multi-static active coherent (MAC) sonobuoys that could be deployed by MQ-9B Reaper uncrewed aerial vehicles (UAVs), USVs can tow low-frequency active (LFA) sonar arrays that could enable them to trail enemy submarines at long range.5

The current US ASW approach is to track enemy submarines until they can be attacked and sunk by torpedoes launched from submarines or aircraft.6 However, the lightweight Mk-54 torpedoes carried by surface ships and aircraft have a low probability of destroying submarines due to their relatively short range and small warheads. Submarine-launched Mk-48 heavyweight torpedoes have the highest probability of sinking modern enemy submarines, but US SSNs should focus on engaging enemy surface combatants, only tasking SSNs to destroy threat submarines when they pose an imminent threat to US SSBNs or the US homeland. Instead, non-SSN ASW forces should focus on suppressing enemy undersea operations by exploiting the self-defense, speed, and sensing limitations of submarines; these qualities incentivize commanders to break off and leave an area when detected, rather than standing and fighting like a surface combatant or evading attacks like an aircraft. Small torpedoes like the Compact Rapid Attack Weapon (CRAW) or depth bombs launched by uncrewed vehicles can exploit these limitations to destroy and drive submarines away without tying up a US SSN in a time-consuming prosecution.7

Mines are the most proven form of ASW engagement and if detected would substantially constrain enemy submarine operations. Mines like the Hammerhead could be deployed by uncrewed vehicles such as the XLUUV, and the powered Quickstrike could be emplaced in contested areas by UAVs like the MQ-9 without risking crewed platforms.8 New generations of mobile, smart mines could combine USVs with torpedoes or explosives to create systems that could be launched from shore, crewed vessels, or large aircraft and loiter or navigate in designated areas until commanded to activate and attack selected targets based on their visual, infrared, acoustic, or magnetic signatures.9 And like the ASW suppression operations described above, the presence of mines is likely to keep enemy submarines away, regardless of the mines’ estimated lethality.

3. Make SSNs More Effective

The US Navy and Congress are funding efforts to increase the delivery rate of new SSNs and reduce the US submarine maintenance backlog.10 These efforts should be sustained, but an equal effort should be made to improve the capabilities of current and programmed SSNs so that they are better able to fight their way into and within enemy bastions. Three priorities stand out in this regard, summarized in figure ES-3.

First, new capabilities should be incorporated into SSNs that improve their ability to understand and respond to threats. As noted above, the relatively slow speed and lack of self-defenses on a submarine require commanders to evade nearly all attacks—including those that would have ultimately missed the submarine. Improved decision-support tools would enable US SSN commanders to avoid or break off offensive operations only when necessary, in the same way the Aegis Combat System and threat warning systems assess incoming missile attacks against surface combatants or aircraft, respectively. To build on sustained improvements to organic submarine sensors, the Navy should also improve SSNs’ ability to receive offboard sensor data while traveling at speed and depth using optical communications or radiofrequency-to-acoustic gateway buoys.

A second priority should be improved defenses. For decades US submarines have relied on their acoustic superiority and countermeasures to defeat attacks. With improving adversary ASW capabilities—particularly in the bastions—and as Russian SSNs approach acoustic parity with US Virginia-class SSNs, US submarines will need to incorporate defenses that allow them to stand and fight. The CRAW should be incorporated into each SSBN and SSN as quickly as possible to provide both a shortrange rapid response capability and an anti-torpedo torpedo for self-defense, just as SM-6 interceptors protect surface combatants from missile attacks. To complement CRAW against air threats, the US undersea force should also quickly field on US SSNs surface-to-air missiles that can engage rotary- and fixedwing maritime patrol aircraft (MPA).11

Third, SSNs should incorporate new systems that better enable them to complete offensive kill chains. The Navy is already investing in longer-range torpedoes and Maritime Strike Tomahawks that would allow SSNs to engage enemy vessels from outside the densest undersea defenses.12 However, an SSN cannot evade its launch area fast enough to avoid enemy ASW aircraft, which could be rapidly deployed to where the noise of a torpedo ejection or the infrared (IR) signature of a missile launch was detected. The US Navy could reduce this vulnerability and enable sustained offensive operations by fielding encapsulated missiles and torpedoes; these would allow SSNs to displace a weapon launch from the submarine in space and time.13

To identify potential targets and help inform SSN commanders if nearby threats could endanger the submarine after a launch, the Navy should field submarine-launched UAS (SLUAS) with longer range and endurance than the current Blackwing small SLUAS.14 And because larger SLUAS and MUUVs could take up torpedo stows, the Navy will need to field smaller submarine- launched weapons to increase both the number of offensive engagements SSNs can conduct and their time on station. For example, multiple CRAWs could be carried in a single longrange torpedo housing, which could transport the CRAWs to an attack area and deploy them on an acoustic or wire-guided command.15

4. Develop SSN(X) and Manage Its Costs

The US Navy plans on replacing today’s Virginia-class SSNs during the mid-2030s with SSN(X), which is intended to provide greater reliability, speed, and torpedo capacity than Virginia.16 However, as noted above, active sonars are becoming more prevalent in ASW as potential adversaries adopt a “reconnaissance- strike” approach to defeating US submarines rather than the “hunter-killer” approach typified by US Cold War ASW operations. Like modern bombers and fighters, US submarines will need to incorporate features that reduce their active sonar returns, as well as payload volume and apertures for deploying UUVs to deceive, degrade, or defeat active undersea sensors.

Another potential concern with SSN(X) is cost. The Congressional Budget Office estimates that each SSN(X) will likely cost around $6.2 billion (in FY2022 dollars), which is nearly twice as much as a Virginia-class Block V SSN and three-quarters as much as the Columbia-class SSBN.17 SSNs are currently procured at a rate of two per year; if SSN(X) costs twice as much as Virginia, the Navy may be unable to sustain this rate— much less ramp up to a higher rate of three to four SSN(X) per year once Columbia-class construction ebbs and more construction capacity becomes available. Consequently, the Navy should critically evaluate the desired attributes for SSN(X) and consider appropriate changes that can reduce costs. For example, investments in improved sound silencing could be weighed against those in active sensor countermeasures and longer-range weapons that could provide equivalent improvements in survivability.

The Operational Imperative

Submarines are the US military’s crown jewel and are relied upon to deliver decisive effects in campaigns against adversaries like the PRC and Russia. However, unless it begins to use noise and field teams for offensive operations, the US submarine force could be denied or simply rendered ineffective in waters where future confrontations are most likely to occur. To sustain its undersea advantage, the Navy should adopt proven approaches from aviation for suppressing, defeating, and circumventing adversary defenses. Unless the US undersea force embraces these new concepts and capabilities, it risks becoming marginalized when it is needed most.