Hwasong-18: Five things to know about North Korea's solid-fuel ICBM

North Korea has announced the successful testing of a new solid-fuel intercontinental ballistic missile (ICBM),  marking a significant breakthrough in Kim Jong Un's banned weapons programme.

It was Kim's latest effort to warn "enemies" of its defence capabilities that  "constantly strike extreme uneasiness and horror into them by taking fatal and offensive counter-actions until they abandon their senseless thinking and reckless acts".

Pyongyang has always criticised US and South Korean joint military drills, calling them rehearsals for a potential invasion of the reclusive Communist country. 

North Korea said that it used a "new type" of missile — named Hwasong-18 —  which aims "to confirm the performance of the high-thrust solid-fuel engines for multi-stage missiles (and) estimate the military feasibility of the new strategic weapon system."

But what exactly is a solid-fuel missile and why does it matter that Pyongyang now has one?

What is a solid-fuel missile?

With this type of missile, the propellant — the fuel that powers it — is made of a solid chemical mixture, which is combusted to create exhaust.

"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, told AFP news agency.

When solid propellant burns, they create generate enormous amounts of energy and temperatures of more than 5,000 degrees Fahrenheit (2,760 degrees Celsius), creating thrust and lifting the missile from the launch pad.

In contrast, liquid-fueled missiles typically require that 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 Center for North Korean Studies at the Sejong Institute.

A solid-fuel missile can be thought of more like a "portable battery" which gives the user much greater flexibility, he said.

Moreover, once a liquid-fuel missile is prepared, "it has to be fired within a short period of time — which is not something you have to worry about with solid-fuel missiles", he added.

Solid-fuel missiles, however, need to be stored, maintained and handled carefully, and if this isn't done, the quality of the missile can degrade over time which could cause it to fail, Panda said.

But they are generally quicker to deploy and launch during a war compared to liquid-fueled missiles. This "makes solid fuel missiles very attractive for a number of military applications", Panda added.

Solid fuel dates back to fireworks developed by the Chinese centuries ago, but made dramatic progress in the mid-20th century, when the US developed more powerful propellants.

The Soviet Union fielded its first solid-fuel ICBM, the RT-2, in the early 1970s, followed by France's development of its S3, also known as SSBS, a medium-range ballistic missile.

China started testing solid-fuel ICBMs in the late 1990s.

So North Korea's definitely got one now?

In state media images, it is possible to see the missile's exhaust plume, which is "consistent with a dirty, smoky solid propellant at work", said Panda.

"Everything looks consistent with a successful solid-fuel ICBM test," he added.

But even if the North Koreans have pulled off one successful test of the new weapons system, it will still "be likely some way from credible deployment in any meaningful numbers", said Joseph Dempsey, a defence researcher at the International Institute for Strategic Studies.

With North Korea's main liquid-fueled ICBMs, which have been tested repeatedly -- but only on a lofted trajectory, which is not how they would be used in a real-life situation -- key questions remain over whether they are effectively operational.

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.

South Korea's defence ministry said they believed the Thursday launch was simply an early test and that developing the Hwasong-18 properly "would need additional time and effort".

"Pyongyang still faces many technical hurdles and resource constraints to actually deploy the missile forces it advertises in state media," Leif-Eric Easley, a professor at Ewha University, told AFP.

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", said the Sejong Institute's Cheong.

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

"The United States does deploy all solid-fuel ICBMs and SLBMs, but Russia and China both still operate large liquid-fueled missiles," said Panda.

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 told AFP.

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 preemptive strike system, which allows Seoul to launch a preemptive attack if there are signs of an imminent North Korean attack.

The Hwasong-18 solid-fuel ICBM would be far harder to detect, which could upend this preemptive strike formula — although Seoul's defence ministry on Friday 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 told AFP.

"The South's existing plan in case of war with the North is to preemptively 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."