How Submarines Can Shut Down Trade

In a protracted conflict, the importance of striking at an adversary’s ability to fund and supply its war effort only grows. From the Union blockade that significantly damaged the South’s economy, to the Allied bombing of German factories, to Ukraine’s targeting of the Kerch Bridge, the purpose of each stratagem was to reduce the ability of the enemy to feed its war machine. Submarines possesses a unique capability for doing so by striking at maritime trade routes. 

The primary antiship tool submarines possess is the Mk 48 heavyweight torpedo. While a capable weapon, it has significant disadvantages. Because of the density of water, torpedoes are slow—significantly slower and more limited in range than missiles. That reduces the area an adversary must defend. Conducting an attack using only acoustic data requires complex calculations based on numerous data points from multiple lines of sight to arrive at a usable target solution. This limits a submarine’s maneuverability and takes time. In the world of modern antisubmarine warfare (ASW) sensors, a torpedo attack can be loosely compared to sneaking up behind a sentry to stick a knife in him. This may be a useful thing to do in some situations, but it is extremely risky for the one sneaking up. 

To improve sensor performance and remain undetected, submarines need to reduce their noise output by sailing at slower speeds. An adversary who needs to evade an attack might need only to drive fast to get away, and frequent course changes complicate developing a target solution further—both are tactics used since World War I. While Mk 48s use wire guidance to update the weapon, maintaining a target solution with the precision required restricts maneuver after launch. The need to remain concealed at close range constrains the submarine once the torpedo has been launched, further limiting the area a defender needs to search.

In short, a submarine must accept a high level of risk to close with and perform solution development maneuvers near the enemy to generate an acceptable target solution. Former Chief of Naval Operations Admiral Michael Gilday’s 2022 Navigation Plan includes a call to “Expand Distance: Long-range precision fires across all domains and platforms with greater reach enable naval forces to strike hostile targets while increasing our own survivability.”¹ If submarines are to help sever an adversary’s sea lanes, something with better range than a Mk 48 is needed—a stand-off antiship capability.

The Missile Attack

While actual ranges are confidential, a submarine can hold contact on a target outside of torpedo range, which Jane’s Naval Forces estimates at as much as 50 kilometers (27 nautical miles) for the Mk 48.² A submarine-launched antiship missile that is effective from beyond torpedo range would allow a submarine to take advantage of this longer detection range, improving a submarine’s survivability by increasing distance and time factors. The distance also would allow a submarine to worry less about its own noise emissions, providing options for speed and maneuver without as much risk of either acoustic or radar counterdetection. The submarine would be free to hit and run, putting distance between itself and the launch point before the enemy could bring ASW assets to bear.

A medium-range missile with some self-targeting capability would allow the conduct of an attack with a poor target solution compared to that required for a torpedo. If the prey were inside the weapon’s targeting basket, the missile would find it. This would provide the submarine commanding officer an option for attack when the tactical situation does not allow for obtaining the high-fidelity solution torpedoes need.

From the adversary’s perspective, a submarine-launched antiship missile would reduce the effectiveness of the traditional defense of maneuvering frequently and driving fast. The missile’s range would make it difficult for a convoy or its escorts to sprint through the submarine’s field of fire. Active homing on the missile would mitigate the value of zig-zag maneuvers, improving the submarine’s ability to develop an acceptable target solution.

With every new potential axis of attack, an adversary must monitor a larger area to defend its sea lanes, draining resources. An over-the-horizon midrange missile would impair the enemy’s ability to detect a submerged shooter and generate effective counterfire and likely would require airborne assets to be readily available. All this would complicate its operations.

Despite the advantages, there are some drawbacks. The biggest is targeting imprecision. At the ranges in question, the firing solution may be broadly acceptable, but obtaining unambiguous target classification within a group of ships would not be feasible. Future development should provide the missile with the ability to distinguish between potential targets and decide which to attack. The current state of artificial intelligence suggests this is a near-term possibility. Given that the purpose of this capability would be denial of maritime trade across a given region, however, the present limitations of target discrimination should be acceptable.

Another disadvantage is that modern countermeasures can defeat a missile more easily than a torpedo. The Navy’s current submarine-launched antiship missile, the UGM-84 Harpoon, is old, and countermeasures have come a long way since its introduction. It is also true that a torpedo can do significantly more damage than a missile. However, these objections ignore the manifest difficulties associated with operating inside torpedo range, and a missile hit is still more effective than no hit at all. What is more, firing a “spread” of missiles—similar to the once-common tactic of multiple-torpedo spreads—would help overwhelm these countermeasures.

The final objection is that it would be more difficult to obtain a battle damage assessment. This may offend modern expectations for complete situational awareness of the battlefield, but it is an unavoidable trade-off for an independent hunter using midrange fires.

New Missile Capabilities

The effectiveness of the envisioned missile must be evaluated against four requirements. First, targeting needs to be done using the submarine’s onboard sensors. While being able to fire using off-hull targeting would be an advantage, a lack of this capability should not delay deployment of these weapons. In a war in the western Pacific, a submarine likely would be hunting independently in emission-control or communication-denied environments. In addition, operating at periscope depth for extended periods increases the risk of detection, reduces maneuverability, and is complicated by adverse weather, and the submarine force does not regularly practice this type of fleet integration. Engineering such a capability almost certainly would result in delays and increased cost as well.

Second, once launched, the missile must have some ability to acquire and target a ship on its own. Given the nature of a submerged attack, as range from the target increases, it becomes more difficult to produce a high-fidelity solution. Ascertaining, with reasonable confidence, a target’s classification, bearing, and ballpark range would be a more realistic expectation.

Third, the missile needs to be launched from a torpedo tube, not a vertical-launch system. An open, vertically oriented hatch limits maneuverability and restricts options before, during, and following an attack. These limitations are even greater with the large Virginia Payload Tube or Virginia Payload Module hatches on newer boats. On the other hand, a missile launched from a torpedo tube allows a submarine to quickly adopt a firing posture and shoot and then maneuver as desired.

The last requirement is that the missile must be able to launch from below periscope depth. Often, the acoustic environment is such that a contact held while deep is difficult to hold while shallow. A submarine would have to take time to come shallow, degrading maneuverability and acoustic situational awareness, and launch the weapon—off old target data. To reposition or regain the target would require the submarine again to go deep. This unnecessarily complicates the commander’s situational awareness and maneuver options.

Why Not Long Range?

A long-range antiship missile may have a role to play but would not be the best tool to enable the submarine as a sea-denial weapon. Its usefulness is predicated on two things. First, these long-range fires assume the ability of off-hull sensors to produce quality fire-control solutions. Second, the submarine must be in near-constant communication with a unit or network. This negates many of the submarine’s inherent advantages by forcing it to live at periscope depth, adversely affecting stealth and maneuverability. 

An effective Chinese antiaccess/area-denial system and attacks on U.S. C5ISR capabilities would pose real challenges to long-range missile employment, especially if China were to keep its merchant shipping tight against its coast. Even ignoring China’s ability to disrupt communications and networks, it must be understood that maintaining constant communication with only a small mast out of the water is complicated in higher seas. Last, as with medium-range missiles, a network-integrated antiship capability would be difficult to implement for a submarine force that does not regularly operate in such a manner. In a talk at the Naval Postgraduate School regarding Aegis combat system development, Vice Admiral Jon Hill emphasized that the integration of combat systems is a complex engineering problem that requires expertise, time, and money.³ Given these constraints, the design trade-offs between a 100- to 200-nm medium-range missile and a long-range missile that travels 1,000 nm, mean more is lost than gained by producing the latter.

In short, employing a long-range capability would reduce submarines to missile platforms in a fleet-support role. While this might work for a guided-missile submarine, such use does not take account of the advantages and limitations inherent in attack submarines. As was discovered in World War II, a submarine untethered from the fleet has the unique ability to penetrate enemy waters to find and strike at an adversary’s internal sea lanes.⁴

Getting Underway

In 1960, Captain Eli T. Reich, who as a World War II submarine commander had experienced the infamous deficiencies of Mk 14 torpedoes, uncovered serious flaws with the celebrated “3Ts” (Terrier, Talos, and Tartar) surface-to-air missile systems by conducting tests at sea.⁵ Thankfully, the Mk 48 has undergone more rigorous testing than the “3Ts” or Mk 14s. However, the Navy may be making a mistake by not considering its limitations when used as an antitrade weapon during a protracted great power conflict.

Submarine-launched UGM-84 Harpoons can be deployed immediately on Los Angeles–class fast-attack submarines. The submarine force should push to integrate the Harpoon on the Virginia class as well, and, noting that a submarine-launched variant was recently in production for foreign sales, consider buying more Harpoons.⁶ While some older submarine-launched Harpoons are being refurbished, the numbers are too low considering the large-scale trade interdiction a war in the western Pacific would require.⁷ And, despite the objections to a long-range missile, it is worth exploring reintroduction of the tube-launched UGM-109B Maritime Strike Tomahawk as a cheaper option.⁸ Simultaneously, development of a new, more capable weapon or adaptation of an existing one—such as the Kongsberg NSM-SL being developed for the Polish Navy—should be considered.⁹

Most important, the submarine force needs to develop maritime trade–denial tactics and train submarine crews in them. The current concept of operations for submarine antisurface warfare remains a must-read for anyone discussing submarine operations in a great power conflict, but it has been some time since it was last updated. The submarine force should formally study the tactical requirements for and risks of severing sea lanes and lines of communication. Scenario-based exercises should pit submarines against surface action groups and convoys that reflect the battle the submarine service would be fighting in a maritime trade–denial role. These should incorporate physical exercise torpedoes and consider potential adversary capabilities. It is essential to move quickly from broad concepts to specific tactics, refining them with practice and experimentation.

In the potential wars for which the Navy is preparing, submarines are the most capable assets for cutting maritime trade routes and isolating an adversary. However, with the tools currently deployed and trained on, the submarine force is accepting significant risk and is more likely to follow in the steps of Germany’s failed submarine campaigns than its own victorious history. Thankfully, there is an available capability that can be used to circumvent many of the risks incurred during a torpedo attack. However, that capability will be significantly less effective if it is not provided in substantial numbers, with effective tactics developed before the shooting starts.