Boost Phase
The first phase of a ballistic missile trajectory during which it is being powered by its engines. During this phase, which usually lasts 3 to 5 minutes for an ICBM, the missile reaches an altitude of about 200 km whereupon powered flight ends and the missile begins to dispense its reentry vehicles. The other phases of missile flight, including midcourse and terminal, take up to the remainder of an ICBM’s flight time of 25 to 30 minutes. (USSPACECOM)
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