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

SEOUL (Reuters) — North Korea test-fired a 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 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.