Why is North Korea developing solid-fuel missiles?

North Korea tested its latest Hwasong-18 intercontinental ballistic missile (ICBM) on Wednesday (July 12) with “the maximum output” of its solid-fuel engine that can fly far enough to strike targets anywhere in the United States with warheads, analysts say. Kristy Kilburn explains characteristics of solid-fuel technology, and how it can help the North improve its missile systems.