Abstract: Singular perturbation is used for synthesis and analysis of a near optimal midcourse guidance law for realistic air-to-air engagement. The proposed midcourse guidance law uses a five-degree-of-freedom (5-DOF) mathematical model of tactical air-toair missile. This paper considers the midcourse guidance law derivation from optimal control viewpoint formulation of boost-sustain motor for air-to-air missile based on singular perturbation techniques. The slightly advance in down range and final velocity and specific energy of the missile was obtained through introducing the coast time interval between the booster and sustainer motors. This means that the sustainer motor ignited only after the introduced fixed coast interval. Therefore, the effect of variable coast time interval on the performance of the missile is studied in this paper.