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What happens during the boost phase of a ballistic missile flight?

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rocket and missile system U.S. soldiers firing an FGM-148 Javelin antitank guided missile during training in Grafenwöhr, Ger., 2006.

During the boost phase of a ballistic missile flight, the missile's rocket engine or engines provide the precise amount of thrust required to place the missile on a specific ballistic trajectory.

The guidance system, which typically uses inertial guidance, calculates the necessary velocity, heading, and position to reach a predetermined target. It issues commands to the thrust system to ensure the payload is placed at the correct point in space with the required velocity. During this phase, small course corrections may be made using thrust-vector control—such as gimballing the main engines or using small thrusters—because aerodynamic controls like fins become ineffective as the missile exits the atmosphere.

The boost phase concludes when the engine shuts off, marking the transition to the midcourse phase where the missile follows a purely ballistic path. To maximize range and throw weight, missiles are often multistaged, shedding empty fuel tanks and associated equipment to reduce mass as the flight progresses.1