North Korea is expected to conduct submarine-launched ballistic missile tests more frequently, but the communist nation still faces significant technical challenges and needs at least several more years to deliver an operational system, a U.S. expert said.

The North conducted its latest SLBM test on Saturday, a day after the South and the U.S. announced a decision to deploy the Terminal High Altitude Area Defense missile defense system in South Korea to better defend against the North's nuclear and missile threats.

The latest test, however, ended in failure, with the missile, known as the KN-11, exploding at an altitude of some 10 kilometers after being launched from a submerged 2,000-ton Sinpo-class submarine, according to South Korea's Joint Chiefs of Staff.

"We may learn more in the days ahead if more information becomes available. Nevertheless, it appears that the increased pace of testing of North Korea's Musudan missile is not limited to that program," John Schilling, a rocket expert with the Aerospace Corp., said in comments carried by the website 38 North.

"It is quite possible that there will be more frequent SLBM tests and failures as well. Even with frequent testing, North Korea's SLBM program still faces significant technical challenges and will likely require several years to deliver an operational system," he said.

Noting that the KN-11 is the North's largest solid-propellant missile, the expert said that simply achieving reliable, repeatable performance with large solid rockets is a challenge, as "solid motors are quite unforgiving of small manufacturing errors."

"A design that worked well enough in April, failed this time," Schilling said, referring to an earlier SLBM test in which the missile was successfully ejected from under water and flew about 30 kilometers, which experts said shows the North was making progress.

"It's going to take more than one more test before they can be confident they got the process right, as opposed to just getting lucky," Schilling said.

Other key challenges include achieving precise thrust termination and warhead separation, he said.

"The submerged launch system is going to be tricky. We know North Korea conducted several of its tests from a barge rather than a submarine and apparently damaged the submarine in one attempt," the expert said.

"The latest launches may or may not have been from the sub, we're still trying to find details on that, but if so were carefully choreographed tests from a submarine at rest just under the surface of a safe harbor," he said.

The North carried out its first SLBM test in May last year and conducted at least two more tests during the year. The North's SLBM capability, if fully developed, would pose a serious threat because its mobile nature would make it very difficult to detect preparations for a launch. (Yonhap)