I know it shouldn't, but it still amazes me the way people continue to feed known trolls at this place. But anyway.
Halo 2 rockets would lock on to any vehicle. Halo 3 rockets wouldn't lock on to anything, but it also had the missile pod that would lock on to any vehicle. Halo Reach dropped the missile pod and brought back the lock-on rockets, however they only lock on to air vehicles, not ground based vehicles like Warthogs and tanks. Halo 4 continued Reach's way of doing things. No missile pod, rockets lock on to air vehicles, but not ground.