North Korea may be able to develop an intercontinental ballistic missile powered by solid propellant by 2025 out of the experience of its recent two launches of liquid-propellant ones, an American aerospace engineer has said.

"North Korea may consider the liquid-propellant KN-20 (Hwasong-14) to be strictly an interim system to provide limited deterrence by threatening US west coast targets," John Schilling, an expert in missile and spacecraft propulsion, said in an article posted on Tuesday at 38 North, a US website specializing in North Korea analysis.

He said the Hwasong-14 ICBM may also be aimed at supporting "technology development for advanced warheads and other systems to be incorporated in a new solid-propellant ICBM in perhaps 2025."

North Korea carried out its first successful test-launch of the Hwasong-14 ICBM on July 4, followed by another successful launch of the ICBM at night on July 28. They demonstrated an ability to put the entire US within its range of attack.

"North Korea has an ambitious solid-rocket development program but has so far only been able to build smaller medium-range missiles using solid propellant," Schilling said. "But the greater mobility and responsiveness afforded by solid propellant makes it the clear choice for ICBMs as well."

Drawing on the North's latest missile test last week, he said it demonstrated "a substantially higher level of performance" from the first test in terms of flight range. But it was only because the second missile's payload would probably have been reduced to 300-350 kilograms from the first test's 500 kg, he pointed out.

With a lighter payload, the second ICBM could have reached as far as New York City if it was launched on a maximum-range trajectory, he said. "(But) For more realistic 500-600 kg warheads, the KN-20 would be limited to west coast targets."

"As early as next year, they could begin production and deployment of a proven, reliable (missile) design with trained crews. After that, they might be able to stretch the KN-20 design to reach Washington D.C. with a light warhead," the expert also predicted.

As for a coveted ability to load a thermonuclear warhead to the ICBM, "North Korea has never successfully tested such a device and the sophisticated designs that allow the major nuclear powers to deliver hundreds of kilotons of yield from a package of only a few hundred km weight are probably beyond Pyongyang's reach for now," he noted. (Yonhap)