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On aircraft that carry their IR missiles on external stores, like the F/A-18, an IR-missile acquires a lock by having its seeker being directed to the target heat source by whatever means and the missile then locking on the the heat contrast with its own seeker. The missile only gets fired once the seeker has a positive lock.

How would this work in an aircraft that carries its IR-missiles internally?

Would the weapons bay doors have to be open to let the missile acquire a target with its own seeker or would the missile just be given an approximate position where to search and it would then acquire a lock on its own once it's out of the weapons bay?

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    $\begingroup$ "The missile only gets fired once the seeker has a positive lock" - I don't think that is a valid assumption. $\endgroup$ – Mike Brockington Jun 18 at 9:23
  • $\begingroup$ As far as 4-gen fighters and before go I would say its correct. $\endgroup$ – hph304j Jun 19 at 5:16
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If you are referring to the Sidewinder missile, the first iteration AIM-9X did not have a requirement to be launched from internal stores, known as captive carry, but in testing it demonstrated that it could lock on after launch. This was deemed interesting enough that Raytheon got a contract for the Block II variant, which among other things adds a datalink capability so that an aircraft with the right equipment can direct the missile to where the target is located, after which it locks on with the IR sensor. This allows it to be used on the F-35, F-22 and even submarine platforms.$^1$

Block III has the same datalink capability (derived from tech developed for the AMRAAM missile) but also has increased range and an insensitive warhead that reduces possibility of accidental detonation on the ground, among other things.

A big reason why the AIM-9 series continues to be upgraded is because of its IR capability. Some countries have developed jamming technology that can mess up the more modern AIM-120's radar tracking system.


$^1$: Raytheon AIM-9X Block II Missile Completes First Captive Carry Flight, spacewar.com, 2008

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    $\begingroup$ And AFAIK the launchers are moved into the airstream prior to launch, giving the missile time to lock on (though you'd have a several second delay of course, not ideal). $\endgroup$ – jwenting Jun 18 at 3:44

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