Missiles in action: AIM-9 Sidewinder



 This air-to-air missile mercilessly seeks out its prey – there’s little chance of escape!

Named after a venomous snake that is sensitive to infrared and so can sense the heat of its prey, the deadly Sidewinder missile does much the same.

 First tested in 1953, the Sidewinder is a heat-seeking, short-range air-to-air missile used by fi ghter aircraft. Once launched, it will fl y towards a hot target – usually the engines of an aircraft or another missile.

 The key to the system is hidden in the nose of the missile. The seeker consists of an array of sensors that react to infrared light; similar in principle to the CCD sensor in a digital camera but simpler in that it only judges its surroundings as ‘very hot’ or ‘not very hot’. In other words it can ‘see’ heat.

 The sensors, plus its assembly of mirrors and lenses, spin offcentre so that they can scan a wide vista and also work out where the heat is in relation to the missile. For instance, if the target is over to the right, the sensors will detect more infrared when they are aimed in that direction. The sensors feed information to the guidance control system that, in turn, move the fi ns at the back of the missile to steer the Sidewinder towards the target. Or rather, aim it at a point slightly ahead of the target to ensure that it doesn’t end up chasing it and never catching it. This is called proportional navigation and effectively anticipates where the target will be at the point of impact.

 In fact, the Sidewinder doesn’t actually impact with its target, but is designed to explode just before it hits it, to ensure maximum damage. Lasers positioned behind the forward fi ns emit light, and when the missile is close to the target, the light bounces off it and back to sensors on the missile, telling the systems to trigger the warhead.

 The Sidewinder is launched from an aircraft and is initially propelled by a rocket motor that hurls it forward at a speed of Mach 2.5 (about 3,060km/h). Once the fuel has been used, the missile glides the rest of the way to its target. 

The warhead

The front mid-section of the Sidewinder is packed with explosives. Like the rest of the missile, though, this 9kg warhead is highly sophisticated. It consists of a high explosive wrapped with around 200 titanium rods, plus an initiator explosive.

 When the missile is within range of its target, the low-power initiator is activated. This in turn ignites explosive pellets, which then cause the main charge to explode. This blasts the titanium rods apart into thousands of fragments, which hit the target at high speed, causing cataclysmic damage.

 A safety device in the missile means that the warhead cannot be activated unless the missile has been accelerating at 20g for fi ve seconds, therefore ensuring it is at least 2.4km (1.5mi) away from the launching aircraft. 



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