By Hyonhee Shin
SEOUL (Reuters) -&nbsp;North&nbsp;Korea's Hwasong-18 intercontinental ballistic&nbsp;missile&nbsp;(ICBM), launched for the second time on Wednesday, uses&nbsp;solid-fuel&nbsp;technology, giving it the capability to launch with little preparation.
Here&nbsp;are&nbsp;some&nbsp;characteristics&nbsp;of&nbsp;solid-fuel&nbsp;technology,&nbsp;and&nbsp;how&nbsp;it&nbsp;can&nbsp;help&nbsp;the&nbsp;North&nbsp;improve&nbsp;its&nbsp;missile&nbsp;systems.
WHAT IS&nbsp;SOLID-FUEL&nbsp;TECHNOLOGY?
Solid&nbsp;propellants are a mixture of&nbsp;fuel&nbsp;and&nbsp;oxidiser. Metallic powders such as aluminium often serve as the&nbsp;fuel,&nbsp;and&nbsp;ammonium perchlorate, which is the salt of perchloric acid&nbsp;and&nbsp;ammonia, is the most common oxidiser.
The&nbsp;fuel&nbsp;and&nbsp;oxidiser are bound together by a hard rubbery material&nbsp;and&nbsp;packed into a metal casing.
When&nbsp;solid&nbsp;propellant burns, oxygen from the ammonium perchlorate combines with aluminium to generate enormous amounts of energy&nbsp;and&nbsp;temperatures of more than 5,000 degrees Fahrenheit (2,760 degrees Celsius), creating thrust&nbsp;and&nbsp;lifting the&nbsp;missile&nbsp;from the launch pad.
WHO HAS THAT&nbsp;TECHNOLOGY?
Solid&nbsp;fuel&nbsp;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&nbsp;Korea uses&nbsp;solid&nbsp;fuel&nbsp;in a range of small, shorter-range ballistic&nbsp;missiles.
The Soviet Union fielded&nbsp;its&nbsp;first&nbsp;solid-fuel&nbsp;ICBM, the RT-2, in the early 1970s, followed by France's development of&nbsp;its&nbsp;S3, also known as SSBS, a medium-range ballistic&nbsp;missile.
China started testing&nbsp;solid-fuel&nbsp;ICBMs in the late 1990s.
South Korea has also said it has secured "efficient&nbsp;and&nbsp;advanced"&nbsp;solid-propellant ballistic&nbsp;missile&nbsp;technology, though in much smaller rockets so far.
SOLID&nbsp;VS. LIQUID
Liquid propellants provide greater propulsive thrust&nbsp;and&nbsp;power, but require more complex&nbsp;technology&nbsp;and&nbsp;extra weight.
Solid&nbsp;fuel&nbsp;is dense&nbsp;and&nbsp;burns quite quickly, generating thrust over a short time.&nbsp;Solid&nbsp;fuel&nbsp;can&nbsp;remain in storage for an extended period without degrading or breaking down - a common issue with liquid&nbsp;fuel.
Vann Van Diepen, a former U.S. government weapons expert who now works with the 38&nbsp;North&nbsp;project, said&nbsp;solid-fuel&nbsp;missiles are easier&nbsp;and&nbsp;safer to operate,&nbsp;and&nbsp;require less logistical support, making them harder to detect&nbsp;and&nbsp;more survivable than liquid-fuel&nbsp;weapons.
Ankit Panda, a senior fellow at the U.S.-based Carnegie Endowment for International Peace, said any country that operates large scale,&nbsp;missile-based nuclear forces would seek&nbsp;solid-propellant&nbsp;missiles, which do not need to be&nbsp;fuelled immediately ahead of launch.
"These capabilities are much more responsive in a time of crisis," Panda said.
North&nbsp;Korea said the development of&nbsp;its&nbsp;new&nbsp;solid-fuel&nbsp;ICBM, the Hwasong-18, would "radically promote"&nbsp;its&nbsp;nuclear counterattack capability.
After the first launch South Korea's defence ministry sought to downplay the testing, saying the&nbsp;North&nbsp;would need "extra time&nbsp;and&nbsp;effort" to master the&nbsp;technology. On Thursday, a ministry spokesperson said they were still analysing the latest launch.
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(Additional reporting by Josh Smith; Editing by Lincoln Feast.)

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Explainer-What are solid-fuel missiles, and why is North Korea developing them?