Generally, missile in-flight guidance is divided into three phases-boost, midcourse, and terminal. These names refer to the different parts or time periods of a trajectory (fig. 9-16).

Boost Phase

The boost phase of missile flight is also known as the launching phase or initial phase. It is during this period that the missile is boosted to flight speed. It lasts until the fuel supply of the booster bums up. For the medium-range (MR) missiles that use a dual-thrust rocket motor (DTRM), the booster propellant grain is consumed and bums out. For extended range (ER) missiles, the separate booster drops off at burnout.

The boost phase is very important to the flight path of the missile. The launcher and missile are aimed in a specific direction by orders from the FCS computer. This aiming establishes the line of sight (trajectory or flight path) the missile must fly along during the initial phase. At the end of boost, the missile must be at a calculated point. Some missiles are guided during boost; others are not.

Midcourse Phase

The second or midcourse phase of guidance is often the longest in both distance and time. During midcourse (or cruise) guidance, the missile makes any corrections necessary to stay on the desired course.

Figure 9-15.-Basic missile guidance system.

Figure 9-16.-Guidance phases of flight.

Guidance information can be supplied to the missile by various means. The object of midcourse guidance is to place the missile near the target.

Terminal Phase

The terminal phase of guidance brings the missile into contact or close proximity with the target. The last phase of guidance must have quick response to ensure a high degree of accuracy. Quite often the guidance system causes the missile to perform what is best described as an "up-and-over" maneuver during the terminal phase. Essentially, the missile flies higher than the target and descends on it at intercept.

We will now discuss the various types of guidance systems and how they direct the missile to the target. The four main categories are (1) command, (2) homing, (3) composite, and (4) self-contained.