What are solid-fuel missiles that North Korea is developing?

North Korea launched intercontinental ballistic missile (solid-fuel missile) on Thursday. Solid-fuel missiles don't need to be fuelled immediately ahead of launch. They require less logistical support, making them harder to detect. Solid propellants are fuel and oxidiser mixture, bound together by rubbery material and packed into metal casing. Metallic powders like aluminium often serve as fuel.