English: Gyro-controlled tailfin of the Sidewinder, also known as a "rolleron". The device is essentially a rudder that is used to keep the missile from rolling. This was needed because the early Sidewinders used a spinning seeker that calculated the angle to the target through timing based on the known rotational rate of the seeker. If the airframe was also rolling, the timing was thrown off and the missile would not guide properly. A similar design, the Falcon, included a gryoscopic system linked to the controls to address this, but this added considerable complexity, Sidewinder's passive system was far simpler and achieved the same end. Each of the fins at the rear of the missile is equipped with a rolleron. This consists of a metal flap with a metal disk embedded in it with just the top of the disk exposed. The disk is edged with small teeth designed to catch the wind. The two small metal fingers guide the airflow onto the teeth, which spins the disk to a high speed and causes it to build up considerable gyroscopic force. If the missile begins the spin, the disks produce a force on the flap, causing it to rotate around the hinge line at the front. This pushes the flap into the airflow, producing a countering aerodynamic force that stops the spinning.