Another entry in our occasional series on US rockets and missiles.
In the late 1950s, the US Navy faced the challenge of protecting convoys moving from the US to Europe should the Cold War turn hot. Further, the fast carrier task forces at the heart of the fleet would be required to operate in the northern reaches of the Atlantic and the crowded waters of the Mediterranean. Two main threats faced the fleet. The first was waves of bombers from Soviet Naval Aviation. The second, and historically more dangerous was submarines. The Soviet Navy had a huge fleet of submarines, and they had capitalized on German submarine research seized at the end of the war. The vast majority of the Soviet submarine fleet was modern diesel electric boats, but soon large numbers of fast nuclear powered attack submarines began to join the fleet. Clearly, if war came, the Soviets intended to deny the use of the seas to the NATO powers. Obviously, anti-submarine warfare came to be a prime mission of the US Navy.
As sonars improved from the crude sets of World War II to the then-modern large array SQS-23 series, detection ranges improved. The need for weapons to match this detection range led to one of the most ubiquitous weapons on US Navy ships, ASROC, or Anti-Submarine Rocket.
Torpedoes are faster (usually) than submarines, but not that much faster. If a torpedo has a 10 knot speed advantage over its prey, even an engagement at 5 miles would take 30 minutes for the torpedo to over take its prey. But few torpedoes have that kind of endurance. Further, their small sonars would have virtually no chance of acquiring a maneuvering target. A way to get the torpedo to the target faster was needed. Building on earlier developments, a rocket system to loft a torpedo to the target area was designed- the RUR-5 ASROC.
ASROC isnt’ a guided missile. It is an unguided rocket. Having detected and localized a submarine via sonar, the attacking ship trains out its ASROC launcher on the proper bearing, The rocket booster lofts a lightweight homing torpedo (usually the Mk46) towards the submarine. The booster falls off, a small parachute slows the torpedo, it enters the water, and proceeds to independently attack the submarine.
The ASROC was fired from an eight-cell “pepperbox” launcher. Most installations also had an extra magazine space to carry reloads.
In addition to a torpedo armed ASROC, the challenge of attacking fast nuclear attack submarines led to the development of a variant armed with a nuclear depth bomb. For a long time, the US anticipated that the use of nuclear weapons at sea was almost certain, yet would not necessarily be seen as such an escalation as to lead to a general nuclear exchange. US escorts armed with ASROC typically deployed with two or more live nuclear weapons aboard.
Almost every US destroyer escort, destroyer, frigate and cruiser built from the 1950s through the late 1970s carried the distinctive ASROC pepperbox.* Of those ships that didn’t have the pepperbox, many fired the ASROC from their Terrier or Standard Missile Launchers.
The launcher itself proved pretty adaptable as well. In addition to launching ASROCs, it was later adpated to launch modified Standard missiles as an interim anti-ship missile, and still later to launch Harpoon sea-skimming anti-ship missiles.
Only one live nuclear warhead ASROC was ever test fired.
These two videos show the ASROC system in operation.
More recent warships that use the Mk41 Vertical launch system use a variant of the ASROC developed to be fired from the vertical launch cells. Ship based helicopters provide greater standoff range than VL-ASROC, but VL-ASROC is never grounded for bad weather, and is a relatively cheap weapon. Expect to see ASROC in service for many more years to come.
*the only major class of escorts that comes to mind without ASROC was the FFG-7 Oliver Hazard Perry class of guided missile frigates. Their Mk13 Standard missile launcher could not fire ASROC, but they did have two on board helicopters. Space and budget considerations drove the decision to forego ASROC on the class.