World War II sonars had a maximum effective range of roughly 2000 yards. Given that wartime submarine torpedoes were rarely effective past about that range, and indeed usually used much closer, that wasn’t a terrible problem. But submarine sonars, and passive hydrophones, being submerged deeper, almost always provided better detection than surface escorts.
The trend in post-war escorts was to use more powerful, larger, lower frequency sonars, which gave better detection ranges. The first in wide use with the US Navy was the SQS-4 series, with a range of 4500-5500 yards. Later the SQS-23 series, a massive sonar housed in a dome at the forefoot of a ship’s bow, could, under ideal circumstances, achieve detection out to as much as 40,000 yards. More typically, the -23 could reliably detect submerged targets from 10,000 to 20,000 yards.
While the lightweight homing torpedoes discussed in the previous post were quite capable, they had one glaring shortcoming. By the time an escort was in range to use them, they were already well within range of heavier submarine launched homing torpedoes. A means of extending the range of surface ASW weapons, both to protect the escort, and to take advantage of increased detection range, was a high priority.
The first, most obvious idea was to use large diameter torpedo tubes to fire heavy homing torpedoes. But the challenges of accurate fire control at extreme ranges meant heavy torpedoes were less than optimal. Worse, any heavy torpedo would simply reach a parity with any submarine torpedo. Something better was needed.
The US Navy had the bright idea to use a rocket to lob the Mk44 Lightweight Torpedo onto a sonar contact. Originally the Rocket Assisted Torpedo (RAT) was hoped to be a lightweight, rather simple system. A parallel program also was started to lob a nuclear depth charge. The minimum safe range of about 5 miles (~10,000 yards) mandated a far more substantial rocket. The programs were merged, with resulting ASROC (Anti-Submarine Rocket) becoming the primary US Navy surface ASW weapon from 1961 well into the 1990s.
An 8-cell “pepperbox” launcher would elevate and train for a simple ballistic, unguided rocket to drop either a Mk 46 torpedo or a nuclear depth bomb onto the enemy submarine. The ASROC maximum range of about 19,000 yards, combined with the range of (then) modern sonars gave escorts a good standoff weapon.
ASROC was also used by a great many allied nations, and continues in widespread use with other navies. Ships equipped with certain guided missile launchers could fire ASROC from them, obviating the need for the 8-round launcher. Alternatively, the 8-round launcher could be modified to fire Harpoon Anti-Ship missiles, and even the Standard Missile.
If the above videos are a bit long, here’s a quick video of loading and firing ASROC from a Greek destroyer.
The nuclear depth bomb variant of ASROC was only live fire tested once.
With the introduction of the Mk41 Vertical Launch System on Aegis cruisers and destroyers, a new version of ASROC, after a surprisingly difficult development, entered service.
But ASROC wasn’t the only way of launching a lightweight torpedo to a distant contact.
The Australians developed their own approach, with a rocket boosted radio controlled glider used to deploy a homing torpedo. Dubbed Ikara, this weapon had the advantage that the water entry point of the torpedo could be updated during the flight of the rocket. Slaved to the sonar fire control system, this meant maneuvers by the target during the time of flight could be countered.
Ikara served with the Australians, the New Zealanders, and with the Royal Navy.
The French Malafon system operated on a similar principle.
On the Soviet side, the SS-N-14 operated on a similar principle, but had a much greater range. The terminal guidance could be made by an helicopter operating from the parent ship.
We will address shipboard helicopters in our final post in this series.