North Korea said on Thursday it has successfully tested the Hwasong-18, its new solid-fuel intercontinental ballistic missile, for the second time. The test on Wednesday unlocks a major breakthrough for North Korea's banned weapons programs, but what exactly is a solid-fuel missile and why does it matter that Pyongyang has one?

The test on Wednesday unlocks a major breakthrough for North Korea's banned weapons programs, but what exactly is a solid-fuel missile and why does it matter that Pyongyang has one?

What is a solid-fuel missile? With this type of missile, the fuel that powers it is made of a solid chemical mixture.

"This propellant is cast into the missile's airframe when the missile is built: imagine a firecracker rocket, ready to go," Ankit Panda, a nuclear policy expert at the Carnegie Endowment for International Peace, said.

In contrast, liquid-fueled missiles typically require that the fuel and an oxidiser be inserted into the missile before they can be fired -- a slower and more cumbersome process.

Why is it better? Preparing a liquid-fuel missile for launch "takes time just like pumping gasoline to your car", said Cheong Seong-chang, director of the Centre for North Korean Studies at the Sejong Institute. And once a liquid-fuel missile is prepared, he added, "it has to be fired within a short period of time." Solid-fuel missiles need to be stored, maintained and handled carefully and if this is not done, the quality of the missile can degrade over time, which could cause it to fail.

They are generally quicker to deploy and have "an advantage in immediate launch", Japan's top government spokesman Hirokazu Matsuno told reporters, saying that this week's test appeared to be of the same solid-fuel ICBM missile that North Korea fired in April.

Are North Korean solid-fuel ICBMs operational now? Images of both the April and July launches show the missile's exhaust plume, which is consistent with a dirty, smoky solid propellant at work.

"The shape of flame is consistent with solid-fuelled missiles -- with the long white smoke coming out of the propellant seconds after lift-off being most demonstrative," Han Kwon-hee of the Missile Strategy Forum said.

The launches look technically similar, he said, indicating the second test "was intended to verify and certify the accuracy and precision of the Hwasong-18".

North Korea's main liquid-fueled ICBMs have been tested repeatedly -- but only on a lofted trajectory, which is not how they would be used in a real-life situation. That leaves key questions about their performance.

But as North Korea's threshold for what is effectively fielded is likely different from others, the new weapon may be considered operational even if other militaries would require more testing.

Who else has solid-fuel missiles? Most militaries first start off with liquid-fuel missile technology but soon strive to acquire solid-fuel missiles, which require more advanced technology.

However, not all advanced militaries exclusively deploy solid-fuel missiles.

The United States deploys solid-fuel ICBMs and SLBMs, but Russia and China both still operate large liquid-fuelled missiles, experts say.

South Korea, for its part, has the technical capacity for solid-fuel missiles, and even has some in its arsenal, "but their range is limited to cover the Korean peninsula", Kim Jong-dae of the Yonsei Institute for North Korean Studies said.

Is this a game-changer? North Korea's leader Kim Jong Un claims the Hwasong-18 solid fuel ICBM will "radically promote" the country's nuclear counterattack capabilities, and experts say it could indeed change the security situation on the peninsula.

South Korea's self-defence plan relies in part on a so-called Kill Chain pre-emptive strike system, which allows Seoul to launch a pre-emptive attack if a North Korean attack is imminent. The Hwasong-18 would be far harder to detect, which could upend this pre-emptive strike formula -- although South Korea's defence ministry has dismissed this fear as "excessive worry".

But if North Korea were to deploy solid-fuel ICBMs, it would "signal a game-changer in possible warfare with them", the Yonsei Institute's Kim said.

"The South's existing plan in case of war with the North is to pre-emptive strike and destroy the North's missile system after confirming signs of launch preparations," he said.

"But there will be no such signs if the North Koreans prepare solid-fuel missiles aiming at the South."