How does a Sidewinder Missile Work?

The Sidewinder missile is the first truly successful air-to-air missile, and as such it has been widely produced, with more than 110,000 produced so far for over 28 countries, as well as widely imitated. The Sidewinder originates in the United States, and is named after the sidewinder snake, as the trajectory of early versions was reminiscent of the zig-zagging of this species. Similar to snakes, it is also heat-seeking, and can be quite deadly, with more than 270 verified kills since its development in 1956.

The Sidewinder is a supersonic missile, with a speed of Mach 2.5. A typical design, the AIM-9L, has a length of 2.85 m (9.4 ft), a 1-18 km range, depending on weather conditions, and carries a 9.4 kg blast-fragmenting payload. Unit cost is approximately $84,000 US Dollars (USD) as of 2007. It is manufactured by one of three companies: Raytheon Corporation, Ford Aerospace, or Loral Corp. When the Sidewinder was first under development in the early 1950s, the goal was to produce a reliable and effective missile with the “electronic complexity of a table model radio and the mechanical complexity of a washing machine”. This goal was quickly accomplished, and its extremely wide adoption is a testimony to its simplicity and effectiveness.

The Sidewinder uses an infrared (IR) detector based on lead sulfide. When lead sulfide is exposed to heat energy, it reduces the compound’s electrical resistance, an example of photoconductivity. This decrease in resistance can be measured as linked to an action, for instance changing the trajectory of a missile in mid-flight.

In the nose of a Sidewinder, a reflecting mirror points forward, reflecting heat energy from a distant target to the IR detector. The Sidewinder must be roughly pointed in the direction of the target; otherwise it will see nothing and just plow straight ahead. If the target is in its sights, it can measure what angle the heat source is from the direction of its travel. The pitch and yaw of the missile is modified based on the degree of the angle.

The Sidewinder also anticipates the flight path of the target by using a mechanically-based tracking system which “remembers” past measurements and projects them forward, causing the missile to fly a course called proportional pursuit. This allows the missile to “lead” the target, much as a quarterback throws a ball where he thinks a receiver will be by the time the ball arrives, rather than throwing it directly from the start. This is much more efficient than direct pursuit, where the missile simply flies in the current direction of the target.