Hehehe.. timing... that's a good word for it.
Truth is, the missiles are traveling at mach 3... and getting there pretty damn fast. Those things burn for about as long as it takes a SAM to get there. At least with a long range missile, you'll have about 2 seconds for every 1 mile of distance (or less). 20,000 feet gives you 8 seconds. Throw reaction time in there... and... well.. you get the picture.
MiniD