North Korea's latest submarine missile test suggests the communist nation has shifted to developing a solid-fuel missile after repeated failures with a liquid-fuel engine, and the short flight distance in the latest test does not mean it's a failure, a U.S. expert said Monday.

The North conducted the submarine-launched ballistic missile test on Saturday in the latest in a series of weapons tests aimed at defying international pressure over its nuclear and missile programs. The launch was seen as a failure because the missile flew only about 30 kilometers.

But John Schilling, an expert on North Korean missile capabilities, said that the missile could have been carrying fuel only for that distance. More importantly, the launch showed that Pyongyang has switched to development of a solid-fuel missile, he said.

"North Korea has revealed images of a submarine-launched ballistic missile test indicating that it has abandoned the liquid-fuel design that has consistently failed in the past and switched to a more robust solid-propellant system that will have a better chance of actually working in an operational environment," Schilling said in an article carried by 38 North.

"The new design is still in the earliest stages of testing, and much work, including development of a full-scale motor, needs to be done. Nevertheless, the simpler design is likely to be less troublesome to develop and could be ready by 2020," he said.

Images the North released after last year's SLBM test showed yellow-orange streak of fire from the missile's tail, a key sign that the missile runs on liquid fuel. But images of the latest test show "an almost incandescent white plume," suggesting it's using solid fuel, Schilling said. 

With regard to the flight distance, the expert said that flying 30 km is not an easy job.

"In order to fly even 30 km, a ballistic missile has to not only launch successfully but accelerate well past the speed of sound. For a single-stage missile, those are the hard parts. Once accomplished, the safe bet is that the missile will continue to accelerate until it runs out of fuel," he said.

"If the missile flew 30 km, there is a good chance it was only carrying fuel for 30 km. To be fair, after four failed test launches, one can understand the test crew not wanting to have a full 10 to 20 tons of rocket fuel falling on their heads, expensive barges or submarines," he said.

Compared with a liquid-fuel missile, a solid-fuel missile has much less range, Schilling said. He added, however, that the reduced range still represents a serious threat.

If equipped with a liquid-fuel engine, the North's SLBM, also known as KN-11, would have flown 1600 km with a 650 kg warhead, but a single-stage solid-propellant version will probably be good for only 900 km, he said.

"This range is still enough to reach all of South Korea and parts of Japan from North Korean territorial waters. If the boat ventures even a little ways into the Sea of Japan, it can reach targets anywhere in Japan," he said.

"And the ability of North Korea's submarine force to reach targets further afield has always depended on the ability of the submarines to reach the open sea, not on the range of the missiles.

If a North Korean submarine can escape the Sea of Japan and come within 1600 km of Guam, or Hawaii, it can almost certainly cover an extra 700 km," he added. (Yonhap)