While the Polaris ballistic missile was being developed in the mid-1950s, many submarine officers must have wondered whether the great bulk of U.S. subs—World War II fleet submarines and their immediate successors—would be able to contribute to the new strategic bombardment mission. Some of them pointed out that a shorter-range nuclear attack missile could fit the standard 21-inch torpedo tube.
The idea of a short-range submarine missile attracted little interest, but it did lead to a widely deployed weapon intended to help solve an urgent new problem. By 1957 the U.S. Navy had realized how revolutionary nuclear-powered submarines could be, thanks largely to tactical trials conducted with the prototype USS Nautilus (SSN-571). With a sustained speed of about 23 knots, the Nautilus could easily evade surface antisubmarine forces. She was not quite fast enough to turn on her pursuers, but it was soon clear that much faster nuclear attack submarines, which became the Skipjack class, could fulfill her promise. It was also obvious that the Soviets would soon have nuclear attack submarines of their own. How could they be countered?
The main antidote to fast conventional submarines was the homing torpedo, which by about 1957 seemed reliable and effective, but homing speed was limited by the noise of water flowing over the sonar dome in the torpedo’s nose. It was generally accepted that a torpedo should be 50 percent faster than its target. The nominal speed of a fast diesel submarine was 20 knots (most were somewhat slower), and 30 knots seemed about as fast as a torpedo could go without being deafened by flow noise. The nominal speed of a nuclear submarine was 30 knots, and in the 1950s it seemed unlikely that any torpedo could reach 45 knots without being deafened. The nuclear threat was so serious that in 1956 the Navy held a conference at Nobska Point on Cape Cod, Massachusetts, to consider the problem.
One Nobska Study conclusion was that submarines were the best possible sound platforms; all future U.S. attack subs would be designed for the antisubmarine mission. That finding led directly to the design of the Thresher (SSN-593), in effect the prototype of later U.S. nuclear submarines through the Los Angeles class. It’s why all of these boats have big spherical bow sonars operating at low frequency, their torpedo tubes shoved aft to clear them. The big spheres were conceived for a class of relatively slow, specialized nuclear ASW submarines, only one of which—the Tullibee (SSN-597)—was built. But dealing with fast Soviet submarines would require fast U.S. boats, hence the Thresher, or Permit, class.
These submarines were substantially quieter than their predecessors, both to avoid detection and, at least as important, to make the most of their big sonars. In effect the Navy had concluded that although a torpedo rushing through the water might be unable to hear its target, a big sonar tens of miles away mounted on board a very quiet submarine could detect and track that target. The question was what the submarine could do about what it heard.
Enter the submarine force’s short-range nuclear bombardment missile. The new submarines would detect and track targets passively. They were equipped with digital fire-control computers specifically to convert a series of target bearings (passive sonar data) into a target solution. The solution would not, of course, be precise. The powerful sonar sphere would send out a single ping to obtain an accurate range. The ping, of course, would also alert the target, which would probably begin to run, resulting in the solution obtained by all that laborious listening no longer being accurate.
A lot would therefore depend on how quickly the submarine could exploit what it heard. A torpedo would be far too slow. Enter the midget Polaris that the submarine force wanted. Conceived as an inertially guided ballistic missile, the weapon was tens of times as fast as a torpedo. It entered development as Subroc—SUBmarine ROCket (UUM-44).
Subroc was designed to be launched cold from a torpedo tube, ignite once well outside the launching submarine, surface and travel through the air to the vicinity of the target submarine before reentering the water. If the target failed to detect the single targeting ping, it could hardly miss the sound of ignition. How far it could run depended on how long it took Subroc to reach it. Effective range was probably set by the convergence zone, which in the Atlantic is a ring 5 miles wide and 35 miles in diameter surrounding a sonar (the 5 miles was the imprecision the single ping was intended to resolve). It might take a Subroc a minute or two to reach the convergence zone, and a few seconds more for it to sink to explosion depth. Every minute’s delay would give a target submarine moving at 30 knots half a mile to run in any direction it chose.
Subroc had to make up for that uncertainty. In 1957, when the missile was conceived, there was only one solution: a good-sized nuclear warhead, which would explode far enough from the launching submarine not to damage it. Subroc became operational in 1965, when homing torpedoes were clearly not the solution to the emerging Soviet nuclear-submarine threat.
The pocket Polaris idea that initially had bred Subroc lived on in a provision for using it against a surface target by air-bursting the warhead.
Ironically, while Subroc was conceived as a weapon for use by the mass of fleet submarines, it was not suited to them because the boats could not possibly be fitted with the big spherical bow sonar the weapon required. However, there was no question, at least initially, of rapidly building enough big nuclear attack submarines. The solution, which had been proposed at Nobska, was a nuclear-tipped torpedo targeted by passive arrays on the submarine. At relatively short range (perhaps 15,000 yards), passive arrays could track the target well enough for the submarine to guide the torpedo all the way to the target. If the torpedo was quiet enough, the target would be unaware of its approach until it was quite close. The reason for the nuclear warhead was because the torpedo would not be able to home by itself; it could be guided close to its target, but not into contact with it.
The result was Astor (Anti-Submarine Torpedo: Mk. 45), the development of which began in 1957. The associated passive sonar was PUFFS (Passive Underwater Fire Control Feasibility System: BQG-4), which used three arrays spaced along the submarine’s hull. It had uses other than targeting the Mk. 45.
There was one rub. These weapons were fielded at about the same time the U.S. government began to seek ways of creating firebreaks between non-nuclear and nuclear warfare. It did not want to rely on nuclear weapons at the outset of a war, so it sought alternatives. Fortunately, it turned out that the torpedo homing problem could be solved, the results being the Mk. 48 heavyweight and Mk. 46 lightweight torpedoes.
Both Subroc and its little brother Astor were retired in the 1970s.
Subroc (UUM-44) Submarine-Launched Ballistic Missile
Length: 21 feet
Diameter: 21 inches
Weight: 4,000 pounds
Range: 35 miles
Speed: Mach 1+
Warhead: W55 nuclear (1KT yield)
Manufacturer: Goodyear Aerospace