Cheonan sinking likely caused by bubble jet from explosion

Posted on : 2010-04-17 14:02 KST Modified on : 2019-10-19 20:29 KST
The joint military-civilian investigation team is now looking for the source of the explosion
 April 16.
April 16.

A joint military-civilian investigation team tentatively concluded Friday that the Cheonan was split in half by a water shock wave, or “bubble jet,” resulting from an external explosion. While military authorities had been eying the possibility of an external explosion since early after the incident, observing are noting the emphasis now has been on a bubble jet resulting from an underwater explosion, rather than a direct strike to the hull from an external explosive.

A bubble jet is a powerful shock wave and high-pressure gas bubble that arises when a torpedo or mine explodes underwater underneath a vessel. It generally appears when an explosive detonates beneath the vessel and repeatedly expands and contracts, causing the vessel to rise and fall. The hull of the vessel splits in two as a result of this shock.

In the Cheonan’s case, the team believes that a torpedo or mine detonated underwater beneath the left side of the hull. In a briefing with journalists Friday, Co-chairman of the joint military-civilian investigation team into the sinking of the Cheonan Yoon Duk-yong said, “It appears that it exploded underwater beneath the left-side draft line.” Yoon added, “The explosive force went in the left side and out the right, so the right side is open.” In other words, the explanation is that a powerful water shock wave that appeared from an underwater explosion pushed in through the hull’s bottom plating and rose upward before bursting outward to the right. It is also because of this direction of the force that the right side of the Cheonan’s stern deck appears to be bent outwardly more than the left side. Yoon said that for this reason, “It appears that the explosion occurred on the right side.”

This form of external explosion can be caused by a torpedo or a mine. However, among types of torpedoes, direct hit or straight running torpedoes that explode after direct contact with the hull must be excluded as a possibility, as they do not generate bubble jets. In the case of a direct hit torpedo, a hole would be found in the impacted part of the hull, but no such hole was found on the Cheonan’s stern. Yoon said, “It appears to be a bubble jet, not a torpedo going through the hull and into the boat.” However, Yoon added that it is still too early to conclude whether a mine or torpedo was responsible.

On this question, military officials believe it is more likely to be a bubble jet-type torpedo than a mine. This is because one of the characteristics of a torpedo is that it can find its target more precisely than a mine. Also, while mines mostly explode below the bottom of the vessel, the fact that the explosion occurred on the lower left side below the draft line rather than underneath lends strength to the possibility of a torpedo.

At the time of the incident, Cheonan was hit by an explosive shock to the outside on the left of the vessel while traveling northwest along the southern coast of Baengnyeong Island. Based on these circumstances, some observers are raising the possibility of a torpedo attack by a North Korean submersible. In this scenario, a vessel waiting out to sea from Baengnyeong Island could have aimed at the lower part of the Cheonan. North Korea possesses weapons such as straight running torpedoes with 150 to 300 kg of TNT and sound-sensitive torpedoes with 100 to 300 kg. During a recent National Assembly question session recently, Defense Minister Kim Tae-young speculated that North Korea possesses bubble jet-type torpedoes that approach a vessel and detonate from a distance. The major weakness with this argument, however, is the fact that no North Korean submersible activity was detected at the time of the incident.

For this reason, the possibility of a mine continues to be raised. Mines generally create an enormous water column of 100 meters or so in height due to their explosive force, and in the process draw in mud and other elements from the sea floor, leaving traces within the hull. Also, because of the large explosive force, there are also large indications of damage to the bottom of the vessel. Such indications are known to have not been clearly in evidence on the Cheonan’s stern, but the possibility that a lost mine from the navy or U.S. Forces Korea (USFK) could have generated some limited explosion cannot be ruled out. Recently, so-called “smart mines” have been in development, including capsule torpedo mines in cylinders laid on the sea floor that are launched in response to sound, but it has not been confirmed whether North Korea possesses such mines.

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