FXR to Nixie - US Torpedo Decoys (1943-2019)

Underseas warfare and electronic countermeasures are two of the most classified areas of naval history. Therefore, the following post, which combines both subjects, is far from complete. However, given how little information I have been able to find on the topic (and of that little how much was outright contradictory), I felt it was worth attempting to provide as much of an overview as I could even if I am unable to provide as much detail as I would like. If any reader has any corrections to what follows, please post them.

The towed acoustic decoy was first introduced by the Allies in late 1943 to provide ships with a defense against the new German homing torpedoes. These torpedoes used simple hydrophones to home on the target's acoustic signature and represented a massive increase in effectiveness over the unguided torpedoes that had been the rule since the 1890's. Homing torpedoes were particularly dangerous to the destroyers and other escorts, as they were often nimble enough to dodge unguided weapons.

However, the Allies were well aware of the German efforts and within months of the introduction of homing torpedoes had fielded a simple yet effective countermeasure - the towed acoustic decoy. The theory was simple. If a ship towed a decoy that was louder than itself, than any homing torpedoes fired at it would attack the decoy instead. Although modern electronics have led to vastly more sophisticated torpedoes and decoys, even today this same basic principle remains the most effective countermeasure against torpedo attack.

Expendable noise makers were also investigated as a possible solution, but it was decided that the number needed would have made such a system impractical. However, it appears that the British did field a 3" rocket-launched decoy dubbed Publican in limited numbers beginning in mid-1944 and similar systems continue to remain in use as a supplement to towed decoys.

First-Generation Towed Decoys: FXR, Foxer, and CAT

The first generation of towed decoys were developed from existing gear designed for sweeping acoustic mines (mines designed to detonate upon hearing a ship pass overhead), modified for the higher speeds that escorts would operate at. Many accounts describe them as having been invented by the British under the name "Foxer" and subsequently copied by the Americans and Canadians under the names "FXR" and "CAT" (the latter standing for "Canadian Anti-acoustic Torpedo", while the former appears to have been nothing more than an abbreviated version of "Foxer"). However, the actual story appears to have been considerably more complex.

First, Foxer, FXR, and CAT were not different names for the same system, but rather generic names for three families of systems that evolved over the course of the war. Four separate variants of FXR were produced as well as three variants of CAT and at least two variants of Foxer. Unfortunately, what follows represents merely my best guess at what happened as wartime secrecy means available information on the development of these systems is fragmentary at best.

It seems that the original Foxer employed a pair of perforated barrels designed to resonate as water passed through them, thus producing the loud noise needed to seduce a homing torpedo away from its intended target. This system may have have then been copied by the United States (it is also possible the United States was simply using British-made systems) and was possibly FXR Mk 1 through Mk 3. However, while Foxer consisted of two decoys towed simultaneously, FXR appears to have used only a single decoy at a time (however the reduction from two decoys to one may have been introduced with a later variant).

Although I have been unable to find a precise date, FXR Mk 1 was probably introduced at the same time as Foxer, soon after the homing torpedoes appeared in September 1943. FXR Mk 2 was fielded in January 1944, while FXR Mk 3 was a lighter variant of the Mk 2 intended for smaller and slower ships.

USS Eisner (DE-192), the original caption describes the two upright barrels on the depth charge rails as being "Foxer (FXR)." (source: http://www.usndazzle.com/ship.php?id=805)

This original system was quite cumbersome and led to a simpler and far lighter system (supposedly less than 100 pounds instead of roughly 3000 pounds, but I am unsure if this is an apples to apples comparison) employing two metal rods loosely attached to each other and towed parallel to the ship's course. When the ship reached the appropriate speed, the bars would beat against each other to produce a loud noise.

The improved parallel bar decoy appears to have originated in Canada as CAT, before being replicated by the United States as FXR Mk 4. However, FXR Mk 4 was likely not a direct copy as it was considered superior to CAT. It is also possible that FXR Mk 2 or Mk 3 was a variant of CAT rather than Foxer.

The Canadian CAT, which were closely related to FXR

Overall, FXR Mk 4 was a significant improvement over FXR Mk 3, improving tow speeds from 12-19 knots to 8-25 knots while increasing noise levels from 20db to 25db and producing a more consistent sound. Production of FXR Mk 4 may have begun as early as mid-1944.

However, the loudness of FXR Mk 4 was almost too much of a good thing. Because these early decoys were broadband noisemakers, they also interfered with the sonar on the ship towing them. A proposed solution was FXR Mk 5, which kept the improvements of FXR Mk 4 while reducing sound levels closer to those of FXR Mk 2, but this development was cut short by the end of the war. Instead, the "Harp" modification was introduced, which allowed a ship to turn its decoy on or off by adjusting the tow cable.

The Canadian CAT Mk III, which could also be turned off and on like the FXR Mk 4

Post-War Decoys: Fanfare and Nixie

After the war, towed decoys took a major step forward with the introduction of the Mk 6 Fanfare. A serious drawback to the first-generation systems was that they generated noise mechanically and had a life measured in hours before they beat themselves to pieces (the original Foxer system lasted just 25 hours when being towed), which meant that it was generally only streamed after a ship had detected a nearby submarine. This had obvious limitations, and of the 32 escorts hit by T-5 acoustic torpedoes, only 7 had a towed decoy deployed when hit (and at least one of those, USS Manges (DE-320), was hit after the two FXR decoys fouled each other during evasive maneuvers - incidentally revealing the United States was using a dual-tow setup as late as August 1944).

In order to remedy the short life of the FXR system, Fanfare used a radically different approach and replaced mechanical noisemaking with an electric noisemaker towed inside a streamlined body. Another significant advantage of this new system was that rather than generating broadband noise, it mimicked the sounds of the ship's propellers, improving effectiveness against increasingly sophisticated torpedoes. Fanfare also appears to have been an overall lighter system than FXR and dispensed with the complex system of floats that the older system required to keep maintain the correct depth and position.

A Fanfare decoy

When Fanfare was replaced in the early 1970's, the traditional Navy "Mark" designations had been succeeded by the unified Army/Navy system. Thus, the new decoy was dubbed the AN/SLQ-25 Nixie. Unlike the step from FXR to Fanfare, the move to Nixie seems to have changed little. It remained a lightweight electronic noisemaker and the main improvement appears to have been the introduction of a fiber optic tow cable.

However, Nixie has proved extremely versatile and has received substantial upgrades over the years. The first major upgrade came in 1987 with the AN/SLQ-25A, which was apparently almost a new design and introduced active countermeasures. This allowed the decoy to function against torpedoes that were increasingly being equipped with active as well as passive sonar homing.

The AN/SLQ-25B was a further improved version that, in conjunction with other sensors, provided a torpedo detection capability. The appears to have entered service around 1995, but may have been limited to aircraft carriers. This upgrade also appears to have been related to the LEAD (Launched Expendable Acoustic Decoy) program, which fires noisemakers from the standard Mk 36 decoy launchers and began production in 1997.

A Nixie decoy being deployed from the stern of an Arleigh Burke-class destroyer.

The AN/SLQ-25C was first procured in 2007 and was a general electronics upgrade of the AN/SLQ-25A. It is appears to be the standard version fielded by the United States Navy today and is commonly described as having introduced "new countermeasures modes" and a longer tow cable.

The AN/SLQ-25D was an open architecture upgrade first awarded in 2008. This program then morphed into the later cancelled AN/SLQ-25X effort to develop a Nixie variant compatible with the active torpedo defense system (which fired miniature anti-torpedoes at incoming torpedoes) then being developed. However, the anti-torpedo program was subsequently ended after limited fielding.

Today, modernization of the venerable Nixie continues with the AN/SLQ-25E. Like the AN/SLQ-25C, this is a described as an electronics upgrade and does not appear to be introducing any major new capabilities. This program was first announced in 2017 and was funded in 2019.

While this continued commitment to upgrading Nixie means that the system will remain as the most common towed decoy in the United States Navy for the foreseeable future, the new AN/SLQ-61 LWT (Lightweight Tow) is currently being developed to protect the Littoral Combat Ships. This system is more compact and lighter than Nixie in order to fit aboard the smaller ship, and is also tailored for the shallow coastal waters that LCS is primarily intended to operate in.