It doesn't take that many rockets to kill a scout or fighter UFO. A scout has 100 HP, a fighter 150. A sparrowhawk does 30 damage. With two targeting computers installed on the interceptor, there's a good chance for most / all of the missiles to hit, and in the case of the fighter, the ALC finishes off the remaining hitpoints. And yet my stockpile of missiles gets consumed at an alarming rate - if I buy 20 reloads, I find one of my two interceptors out of ammo a week or so later. So no, I don't believe aircraft ammo is conserved in the same way H-Hour described for ground combat ammunition.