The U.S. Navy has ordered the Anti-Torpedo Torpedo Defense System (ATTDS) removed from the five CVN (nuclear-powered aircraft carriers) it had been installed on for testing and development. The problem was that ATTDS did not perform reliably and while the performance was slowly improving further testing revealed additional problems that made it unlikely that the system would eventually become effective enough to justify installing on all major warships. It was another case of developer assurances that initial deficiencies would be fixed were overly optimistic when put to the test when examined closely.
ATTDS proved unable to reliably detect incoming torpedoes, especially when the defending ship (like a CVN) was surrounded by escorts. The interceptor torpedo that was designed to destroy the incoming torpedo was not only undependable but given that there was little knowledge of how enemy (Russian or Chinese) torpedoes actually operated there was no way to realistically test the interceptor torpedo. Even against modified American torpedoes used as “enemy torpedoes” during tests, ATTDS performed poorly. The actual decision to cancel ATTDS was made in late 2018 and the ATTDS systems won’t be removed from all five CVNs until 2023.
Back in 2013, the U.S. Navy installed ATTDS, its promising new torpedo warning and countermeasures system, on an aircraft carrier (USS Bush, CVN 77) for testing, data collection, and fine-tuning. At that point, ATTDS was called TWS/CAT (Torpedo Warning System/Countermeasures Anti-Torpedo) and the plan was for it to enter service on destroyers, cruisers, carriers, and large amphibious ships by 2015. That never happened because of the technology not performing as effectively as required. Worse there were new problems discovered during the “fine tuning” that ultimately indicated ATTDS was unlikely to achieve sufficient effectiveness to justify installing it on any ships. The ATTDS concepts were sound but the existing ATTDS technology was not capable enough. The unreliability and ineffectiveness of ATTDS was supposed to be secret but since the tests were often conducted on a CVN at sea with its escorts it was impossible to hide the fact that ATTDS didn’t work. The ATTDS developer, or any competitor, would have made some fundamental improvements in the detection and destruction technologies that successful use of ATTDS depended on. None of those breakthroughs were forthcoming in the near future so the Navy cut its losses.
ATTDS (TWS/CAT) consists of a towed sonar array (TWS) that can detect and identify torpedoes so that anti-torpedo torpedoes that intercept the incoming torpedo (CAT) can be launched to destroy the approaching torpedo. While operating on the USS Bush TWS/CAT was tested against various types of torpedo attack under very realistic conditions (at sea when moving with the CVN escort ships nearby). The tests also provided an opportunity to train sonar operators and collect acoustic data so that the system software can be improved.
The 2015 deadline for deploying TWS/CAT was missed mainly because the CAT anti-torpedo torpedo was not working reliably. The TWS was also encountering a lot of false torpedo detection situations. Then new intel on Russian and Chinese wake homing torpedoes indicated that these did not provide a similar “sound signature” that the current American test attack torpedo was generating. This would further delay readiness of the detection portion of ATTDS as well as the accuracy of the interceptor torpedo. There were no quick fixes for either of these problems and Department of Defense support for the project was halted until a developer could create a more promising prototype.
The concept behind ATTDS has been kicking around in the West for decades. Until recently the main problem was sensors the ship could use to detect an approaching wake homing torpedo and then guide a small anti-torpedo torpedo to intercept. That detection problem appeared to have been solved until the question of what exactly an enemy wake homer would sound like to ATTDS detection systems was raised. That also created problems for the Countermeasures Anti-Torpedo Torpedo, which proved unreliable even when used against the American wake homing torpedo during tests. Few details of CAT have been released other than that it is a small 165mm (6.5 inch) torpedo launched from a storage container. This launch container has been seen installed in the rear of a warship and launching a 165mm torpedo.
The main reason for TWS/CAT was to provide a defense against torpedoes that home on the wake of a surface ship. Most wake homers look like standard 533mm (21 inch) torpedoes but they are launched in the general direction of the target and then stalk it. For that reason, it was always known that while CAT may have worked in theory it would be much less effective in practice. Details of how CAT operates and what the problems are will not be revealed because that would give users (like Russia, China, Iran and North Korea) of wake-homing torpedoes help in modifying their wake homing torpedoes to defeat CAT. Apparently Chinese and Russian wake homing torpedoes had already developed new characteristics that were generally known but would require the development of new test attack torpedoes to provide realistic testing for ATTDS.
This all began towards the end of World War II when "smart torpedoes" first appeared. These weapons had sensors that homed in on the sound of surface ships. The Germans pioneered this approach in their acoustic and wake homing designs. The first such acoustic homing torpedoes followed the sound of the target until the magnetic fuze detected that the torpedo was underneath the ship and detonated the warhead. The acoustic homing torpedoes saw use before the war ended. The even deadlier wake homing torpedoes were perfected and put into service (by Russia) in the 1960s and upgraded regularly ever since. The “wake homing” torpedoes detected the wake of a ship and followed the wake to where the ship currently was and detonated. Other nations also developed wake homing designs so the United States has them available to test TWS/CAT. Allied nations also provided information indicating that ATTDS needed far better detection and interception capabilities to be reliably useful. Russia and China have been exporting wake homing torpedoes so there was a lot more information out there about how they performed and that meant it was easier to obtain detailed information on how these wake homers actually operated. In other words, a closer examination of the potential threat from actual enemy weapons indicated that the problems ATTDS already have would be much worse against the real wake homing torpedo threats.