ZS-4 is a black powder rocket motor used by the AIM-9 Sidewinder air-to-air missile. It is a fuel-rich, solid-propellant (black powder) rocket motor which develops high internal pressure and high temperatures to generate thrust. It has a propellant formulation of ammonium perchlorate, aluminum, iron oxide and HTPB, and a nitrocellulose-based igniter. It is used to provide the Sidewinder missile with a high-speed boost at launch and is used to sustain the Sidewinders speed through its entire potential burn time. The motor has a rated burn time of 1.3 seconds and a thrust of 3,250 lbs. It is able to accelerate the missile to a speed of Mach 2.4.