Explained: Why North Korea Is Developing More Solid Fuel Missiles

North Korea test-fired an intercontinental ballistic missile (ICBM) powered by solid fuel on Monday, South Korean officials said, in what would be the third such test this year to boost capabilities for launching with little preparation.

Seoul did not identify the specific missile, but the nuclear-armed North unveiled its solid-fuel Hwasong-18 ICBM at a military parade in February and previously tested it during launches in April and July.

Last month North Korea also said it had tested solid-fuel engines for an upcoming new type of intermediate-range ballistic missile (IRBM).

Here are some characteristics of solid-fuel technology, and how it can help the North improve its missile systems.

What are some advantages of solid fuel?

Solid-fuel missiles do not need to be fuelled immediately ahead of launch, are often easier and safer to operate, and require less logistical support, making them harder to detect and more survivable than liquid-fuel weapons.

"These capabilities are much more responsive in a time of crisis," said Ankit Panda, a senior fellow at the U.S.-based Carnegie Endowment for International Peace.

What is solid-fuel technology?

Solid propellants are a mixture of fuel and oxidiser. Metallic powders such as aluminium often serve as the fuel, and ammonium perchlorate, which is the salt of perchloric acid and ammonia, is the most common oxidiser.

The fuel and oxidiser are bound together by a hard rubbery material and packed into a metal casing.

When solid propellant burns, oxygen from the ammonium perchlorate combines with aluminium to 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.

Who has that technology?

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

North Korea uses solid fuel in a range of small, shorter-range ballistic missiles.

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.

South Korea has also said it has secured "efficient and advanced" solid-propellant ballistic missile technology, though in much smaller rockets so far.

Solid vs Liquid

Liquid propellants provide greater propulsive thrust and power, but require more complex technology and extra weight.

Solid fuel is dense and burns quite quickly, generating thrust over a short time. Solid fuel can remain in storage for an extended period without degrading or breaking down - a common issue with liquid fuel.

North Korea said the development of its new solid-fuel ICBM, the Hwasong-18, would "radically promote" its nuclear counterattack capability.

After the first launch South Korea's defence ministry sought to downplay the testing, saying the North would need "extra time and effort" to master the technology.

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