Here's What You Need to Remember: During World War II, submarine and aircraft-dropped torpedoes sank hundreds of merchant ships and warships. Unlike the numerous aerial bombs or cannon shells required to sink large warships, just one or two torpedo hits could and sometimes did suffice to sink huge aircraft carriers and battleships.

In 2019, an annual report released by the Department of Testing & Evaluation revealed that a potentially revolutionary new torpedo-defense system installed on five American aircraft carriers had proven unsatisfactory and would be withdrawn from service.

This article first appeared earlier and is being reposted due to reader interest.

The system combined a towed Torpedo Warning System sensor array designed to detect incoming torpedoes with a quick-acting launcher called the Anti-Torpedo Device System (ATTDS) that could spit out a miniature 220-pound Countermeasure Anti-Torpedo (CAT) measuring only 171 millimeters in diameter. The CAT torpedo was designed to home in on the incoming torpedo and blast it short of its target.

Starting in 2013, the Navy installed the system on five Nimitz-class super-carriers—the George H. W. Bush, Harry Truman, Nimitz, Dwight D. Eisenhower and Theodore Roosevelt. You can see a photo of one being fired from its six-cell launcher here.

But in September 2018 the Navy concluded testing and began removing the systems from the ships. Reportedly, they had failed to demonstrate enough improvement to be operationally viable. The Pentagon had by then invested $760 million in the torpedo-defense program of which ATTDS was a part, though other components of the program may still prove successful.

Details on the ATTD’s deficiencies are vague. While it demonstrated “some capability” at intercepting torpedoes, but its reliability was “uncertain,” and its lethality “untested.” The Navy also failed to test it versus simulated foreign torpedoes, relying on U.S.-built torpedoes instead.

Particularly, earlier DOT&E reports indicated the TWS had a major false positive problem—implying it confused friendly ships and systems with possible torpedo threats. Because such short-range close-in defense weapons depend on super-fast automated systems to respond to incoming threats, the inability of that system to distinguish genuine threats from nearby ships could have posed a major problem and potentially a friendly-fire risk.

The Torpedo Threat

During World War II, submarine and aircraft-dropped torpedoes sank hundreds of merchant ships and warships. Unlike the numerous aerial bombs or cannon shells required to sink large warships, just one or two torpedo hits could and sometimes did suffice to sink huge aircraft carriers and battleships.

The downsides to torpedoes are that they were malfunction-prone and tricky to deliver on target, as warships could undertake evasive maneuvers to avoid them. Thus submarines (which could attack with surprise) and aircraft or fast motorboats (which were too fast to avoid) proved the most effective torpedo-delivery platforms.

Submarines and torpedoes have only become considerably stealthier, faster and deadlier since World War II—but have been rarely used in combat. A notable exception is the Falkland Island War, in which the British submarine Conqueror torpedoed the Argentine cruiser General Belgrano, in one stroke inflicting over half of the war’s Argentinian fatalities. Meanwhile, the Argentine submarine San Luis twice launched torpedo attacks on British ships which went undetected; torpedo malfunctions saved the British vessels.

However, anti-ship missiles inflicted considerable damage in Falkland Island conflict, Iran-Iraq clashes in the Persian Gulf, and the 1971 Indo-Pakistani War. As a result, navies have developed sophisticated multi-layered missile-defense systems for their carriers, cruisers and destroyers: powerful radars to detect incoming threats, long-range missiles to shoot at them from far away, radar jamming and decoys to misdirect them, and short-range missiles and rapid-firing auto-cannons called Close-In Weapon Systems (CIWS) that automatically attempt to blast incoming warheads on their terminal approach.

Submarines and torpedoes, of course, are harder to track at a distance, and existing defensive systems against them are not quite as dense. Helicopters with dipping sonars and land-based patrol planes drop sonar buoys to patrol a wide perimeter searching for submarines which they can then engage with air-dropped homing torpedoes. Sub-hunting frigates and destroyers form a closed perimeter around the carriers and cruisers they are escorting. Carriers also deploy acoustic decoys like the towed SLQ-25 Nixie designed to attract torpedoes to them.

Despite these precautions, diesel and nuclear-powered submarines have repeatedly succeeded in evading detection and “sinking” U.S. carriers during naval exercises. The new generation of Air-Independent Propulsion and/or Lithium-Ion Battery powered submarines are relatively cheap yet remain very quiet and have weeks of underwater endurance. Furthermore, they are just as capable of launching advanced new torpedoes as the U.S. Navy’s pricier nuclear-powered submarines.

Particularly, new wake-homing torpedoes such as the Russian Type 53 and Chinese Yu-9 are designed to track a large vessel's wake rather than its acoustic signature, rendering towed decoy and other countermeasures ineffective.

Given the apparent difficulty of ensuring that submarines never enter torpedo-attack range, it made sense for the Navy to pursue a short-range “hard-kill” defense system designed to blast approaching torpedoes out of the water.

Hard-kill active-protection systems are currently being installed on U.S. armored vehicles, and in the early 2020s the Air Force plans to test laser-based and possibly kinetic hard kill systems to protect aircraft.

Unfortunately, the ATTDS program’s failure indicates that reliable hard-kill protection against torpedoes for surface warships has yet to be realized. However, the problems with the system’s sensors do not strictly implicate its ATT anti-torpedo torpedoes. Reportedly, an ATT on the George H.W. Bush successfully intercepted seven incoming torpedoes in 2013.

The ATT is, in fact, a spinoff of a program to develop cost-efficient high-speed miniature torpedoes called the Common Very Light Weight Torpedo. The Navy’s 2020 budget documents now suggest that the anti-torpedo torpedo may instead show up on U.S. submarines as an “Anti-Torpedo Torpedo Compact Rapid Attack Weapon” system for potential integration into the AN/BYG-1 weapons loading systems used by U.S. submarines, as first reported in detail by The Drive.

Theoretically, the best platform for hunting down a submarine is another submarine. Indeed, carrier task forces often are accompanied by a submarine which quietly prowls the nearby seas for hostile counterparts.

This admittedly remains untested in real-world combat, as World War II submarines—with the notable exception of the sinking of U-864 by the HMS Venturer—lacked the capability to engage each other underwater, and no confirmed submarine clashes have occurred since. However, multiple underwater collisions indicate that submarines are quite capable of stalking their less discrete underwater adversaries.

If a viable submarine CAT can be developed, submarine commanders will get an additional weapon in their toolkit. That would be useful when confronting not only enemy submarines but also small underwater drones or surface warships that don’t merit the deployment of an expensive heavyweight 533-millimeter torpedo. Theoretically, up to four mini-torpedoes could be stored in the space of a single heavyweight torpedo, but devising a system to launch the small weapons remains a technical challenge.

Of course, just like the carrier-based ATTDS, the CRAW anti-torpedo torpedo would also require testing to see if they work as well in practice as they do in theory. Nonetheless, the concept of adding an additional close-range layer of protection to submarines has merit given the increasing capability of modern torpedoes, and the sobering reality that a submarine would be lucky to survive even a single torpedo hit.

Sébastien Roblin holds a Master’s Degree in Conflict Resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring. This article first appeared earlier and is being reposted due to reader interest.

Image: Reuters