usually several kilograms of high explosive surrounded by metal that fragments on detonation (or in some cases, pre-fragmented metal). The rear part of the missile contains the propulsion system, usually a rocket of some type and the control actuation system or CAS. Dual-thrust solid-fuel rockets are common, but some longer-range missiles use liquid-fuel motors that can "throttle" to extend their range and preserve fuel for energy-intensive final maneuvering. Some solid-fuelled missiles mimic this technique with a second rocket motor which burns during the terminal homing phase. There are missiles in development, such as the MBDA Meteor, that "breathe" air (using a ramjet, similar to a jet engine) in order to extend their range. Modern missiles use "low-smoke" motors – early missiles produced thick smoke trails, which were easily seen by the crew of the target aircraft alerting them to the attack and helping them determine how to evade it. The CAS is typically an electro-mechanical, servo control actuation system, which takes input from the guidance system and manipulates the airfoils or fins at the rear of the missile that guide or steers the weapon to target. Missile range A missile is subject to a minimum range, before which it cannot maneuver effectively. In order to maneuver sufficiently from a poor launch angle at short ranges to hit its target, some missiles use thrust vectoring, which allow the missile to start turning "off the rail", before its motor has accelerated it up to high enough speeds for its small aerodynamic surfaces to be useful. Performance A number of terms frequently crop up in discussions of air-to-air missile performance. Launch success zone The Launch Success Zone is the range within which there is a high (defined) kill probability against a target that remains unaware of its engagement until the final moment. When alerted visually or by a warning system the target attempts a last-ditch-manoeuvre sequence. F-pole A closely related term is the F-Pole. This is the slant range between the launch aircraft and target, at the time of interception. The greater the F-Pole, the greater the confidence that the launch aircraft will achieve air superiority with that missile. A-pole This is the slant range between the launch aircraft and target at the time that the missile begins active guidance or acquires the target with the missile's active seeker. The greater the A-Pole means less time and possibly greater distance that the launch