Foreign Affairs

Torpedo Sunk South Korean Ship

A preliminary investigation into the March 26 sinking of the South Korean warship Cheonan has found it was caused by a torpedo. That finding has shifted the probe to determine why Cheonan failed to make sonar contact with the torpedo, or the submarine that launched it, and the type of torpedo used.

One disturbing theory being examined is whether North Korea introduced a new kind of torpedo—one unlike any other ever used.

The evolution of submarine warfare has been a cat-and-mouse game in which a technological advantage can turn the hunter into the hunted. The March 26 incident may now give the submarine the advantage.

The South Korean government will soon release the results of an investigation into Cheonan’s loss, conducted with the help of an international team of experts. Broken in half, the ship’s wreckage lay in shallow water, making recovery relatively easy. Thus, it was quickly determined its loss was caused by an explosion external to the ship’s hull.

Accordingly, the focus shifted to whether a mine, torpedo or kamikaze mini-submarine was responsible. Metal remnants found on the seafloor proved helpful to investigators in establishing it was a torpedo.

In examining Cheonan’s hull, of note was the absence of heat exposure or a hole of the sort normally caused by contact explosives. In other words, the lethal blow was delivered by a non-contact explosion occurring underneath the ship, generating a powerful shock wave and high-pressure gas bubble called a “bubble jet.” This bubble jet caused the ship repeatedly to rise and fall, until it snapped in two.

With metal torpedo remnants discovered, the focus shifted to why Cheonan failed to establish sonar contact and the type of torpedo used—the latter being critical to shedding light on where it was manufactured and, ultimately, who launched it.

The shallow waters where Cheonan went down, combined with the noise generated in that busy shipping lane, may have impaired sonar detection.

But there is another possibility. Cheonan was patrolling waters near a disputed border—waters it routinely sailed. Knowing this, Pyongyang could have, on many occasions prior to March 26, positioned a submarine to lie still on the sea floor as patrol ships passed. By doing so, repeated South Korean patrols coming across it in the same location may have simply learned to accept it as wreckage or some other non-threatening contact. Thus, on March 26, that same contact would have been ignored by Cheonan’s crew as a North Korean submarine crew prepared to launch the fatal torpedo.

As to the type of torpedo, there are two possibilities—a "heavy torpedo," with which all North Korean submarines are known to be armed, or a "supercavitation torpedo," a devastatingly effective weapon known to be possessed by countries with interests adverse to the U.S. South Korean sonar men are better trained to identify the acoustics signature of an incoming heavy torpedo, which is easier to detect, than a supercavitation torpedo, which requires special adjustments to the sonar system to do so.

But both torpedoes are deadly—the supercavitating even more so as its design incorporates a law of physics loophole that makes detection and escape by a targeted ship virtually impossible. And, as of today, no defense exists against it. Both types are capable of generating non-contact bubble jet explosions underneath a target.

A heavy torpedo cutting through the water is governed by the laws of physics—its underwater speed limited to between 40-50 mph. China, which manufactures such a weapon—a passive acoustics homing torpedo called the Yu-3—is known to sell them to North Korea.

This "speed limit" on heavy torpedoes in water exists because traveling through a dense medium, friction and turbulence above this rotational speed of the propeller will cause it to rip apart. To go any faster, a torpedo, or any underwater vehicle, would need to cut through the water absent such friction and turbulence.

In the 1960′s, a Soviet scientist sought to convert this disadvantage into an advantage. He hit upon the concept of "cavitation" or forming a partial vacuum within a liquid by a swiftly moving solid body. In a nutshell, supercavitation involves creating a gas bubble within which an underwater vehicle can ride, totally protected from this friction. The concept exists in practice today in the form of a Russian-made supercavitation torpedo—the "Shkval," capable of traveling at speeds of 200 mph. Its only weakness is its very limited eight-mile range.

A torpedo capable of reaching such speeds poses a severe threat to the U.S. Navy.  Even if detected, it provides the target ship, running at speeds of approximately 30 knots, no time to outmaneuver it.

Interestingly, the Shkval already has claimed an earlier, albeit unintended, victim. On August 12, 2000, while operating in the Barents Sea with 118 crewmen aboard, a mysterious explosion sank the Russian submarine Kursk (K-141). As some crewmembers survived the explosion and awaited rescue on the seafloor, Moscow—lacking adequate rescue and recovery assets to save them—repeatedly rejected international offers of help. Delays resulted.

Moscow’s delay ultimately claimed the lives of the few surviving crewmen. We now know the Russian refusal stemmed from concern supercavitation torpedoes onboard Kursk would be discovered. It was probably the premature detonation, after launch, of a Shkval during a training exercise that sank the Kursk. The Russians later went to extreme efforts to ultimately recover Kursk, tow her back and dismantle her.

Our Russian "friends" have sold Shkval torpedoes to the Iranians—as well as submarines from which to launch them. The Russians believed Shkval could not be reversed engineered. They were wrong. Today, Iran manufactures its own supercavitating torpedo, claiming it can reach speeds in excess of Shkval’s.

If a supercavitating torpedo sank Cheonan, it is more likely the North Koreans obtained it from Iran than Russia. Pyongyang has been providing Tehran with assistance on its nuclear weapons program; accordingly, Tehran would feel an obligation to reciprocate in some measure.

Another piece of evidence concerning torpedo type is based on an analysis of acoustics data generated at the time of Cheonan‘s loss. (It would be surprising if this data was not obtained from South Korean underwater listening devices similar to those planted by the U.S. to monitor Soviet submarine activity during the Cold war.)

From this data, it appears an object approached the ship at 40 mph and effected a non-contact explosion eight feet below Cheonan with equivalent power of 450 pounds of TNT. A slower approach speed would suggest Cheonan‘s fate was sealed by a heavy torpedo. Hopefully, the data will also reveal whether the torpedo was launched from a larger submarine—necessary for a heavy torpedo—or a mini-sub, making use of a supercavitation torpedo possible.

There is little doubt Cheonan‘s demise was met with cheers in four world capitals where leaders saw it as a precursor to the demise of American naval dominance. Obviously, rejoicing took place in Pyongyang for pulling off its dastardly deed. In Beijing, cheers went up over what possibly was the first successful kill by its heavy torpedo. And, in both Moscow and Tehran there was recognition, that if a heavy torpedo was used with such success, their supercavitating torpedoes will be unstoppable against the US Navy.

Sign Up