The risky ‘side launch’ that doomed Kim Jong Un’s new warship

But the moment of celebration turned into calamity.

The 5,000-ton destroyer lost its balance as it lurched into the water, toppling over and embarrassing Kim, who seeks to modernize his Soviet-era naval fleet. Four North Korean officials have been detained over the mishap, according to state media, which called it an “unpardonable crime."

What has become clearer in the aftermath is how an unconventional choice of launch method, Kim’s rushed timetable and a top-heavy warship overladen with weapons systems was a recipe for disaster, according to satellite-imagery analysis, naval experts and North Korea’s official statements.

“I haven’t seen a failure like this one," said Mark Cancian, a retired Marine colonel now at the Center for Strategic and International Studies.

‘Side launch’ goes sideways

The 470-feet-long warship—Kim’s second Choe Hyon-class destroyer—had been built at a port in the northeastern city of Chongjin, a major cargo hub near the Russian border. North Korea’s largest steel mill sits nearby. Vessels carrying oil and wheat dot the surrounding waterways.

Days before the May 21 launch, satellite imagery showed the vessel alongside the dock, resting on a sloped ramp designed to slide it side first into the water.

This setup meant North Korea had chosen to attempt a risky “side launch," as opposed to other, more common methods. The country hadn’t before been observed attempting the method with other military vessels, according to 38 North, a website that analyzes North Korea. The movement requires precise calculations predicting the ship’s center of gravity and the launch angle.

With Kim looking on, the warship didn’t descend the rails evenly. Instead, only its stern slid into the water, causing the vessel to topple over into the water with its bow remaining on the dockside, according to experts who viewed the satellite images.

North Korea said the accident occurred because of “inexperienced command and operational carelessness." Naval experts said a miscalculation of how to keep the ship balanced appears likely.

Blue tarps now cover the warship, concealing from the outside world the extent of the damage. The bottom of the warship wasn’t punctured in the accident, North Korea’s state media reported. Pumping out seawater from the submerged part of the vessel takes several days, North Korean state media said. Getting the ship back upright should be completed by next week.

Even when a side launch is successful, the ship violently bobs back and forth before coming to a standstill. This was the case for the 2013 side launch of the USS Milwaukee, a littoral combat ship, in a video from its manufacturer Lockheed Martin. The Milwaukee is also significantly smaller than the capsized North Korean vessel, at about 3,400 tons and 388-feet long.

The U.S. now generally avoids side launches for large warships, since other options are safer and more stable. South Korea’s military said following the North Korean accident that it avoids the method.

Time pressure

Side launches are commonly used for cargo vessels or tankers, as they have flatter hulls and are therefore easier to balance after being dropped into the water, naval experts said. The method is risky for warships, since their hulls are narrow to maximize speed and they have bulky weapons systems mounted atop them.

To North Korea, a side launch is also a less costly option and requires less advanced equipment than the common alternative, a so-called floating dock launch, in which a dock is filled with water.

This approach is considered safer and more controlled than a side launch, and is how the U.S., South Korea and other military powers typically ease their big warships into water, naval experts said.

It is also the method used by the Kim regime just weeks earlier at a different port, in the west coast city of Nampo, for the launching of the first Choe Hyon-class warship. Nampo’s shipyard is larger and more modern.

Kim was on hand for that launch, too. Standing aboard a vessel draped in North Korean flags, he smiled with his daughter as confetti fell from the sky.

The North Korean leader’s demands for speed also likely played a role in the accident, said Neil Watts, a former South African navy captain.

“It’s a combination of lack of technical expertise and the pressure to get the ship launched in a short period of time," said Watts, a former member of the now-disbanded United Nations panel monitoring North Korean sanctions compliance.

The first Choe Hyon-class warship had been built in roughly 400 days, North Korean state media said, with plans to deploy it by early next year. The second began construction about a year ago, according to satellite-imagery analysis of Chongjin port.

That would be a fast turnaround for a country with North Korea’s industrial limitations and inexperience, naval experts said. It typically takes Japan and South Korea about three years to build a frigate or destroyer. Europe takes three to six years and the U.S. often longer.

Moreover, North Korea appeared to have affixed roughly 70 weapons systems atop the destroyer before launch, plus other arms, whereas the U.S., South Korea and others typically mount far fewer and take years to fully equip a military vessel, said Yoon Suk-joon, a retired South Korean navy captain and now senior fellow at the Korea Institute for Military Affairs.

“North Korea got too ambitious," he said.

Write to Dasl Yoon at dasl.yoon@wsj.com