content: I don't get it. Engineering usually proceeds by building a new function on top of an old function that is already perfected. You cannot build a tall building on a weak foundation. For missile defense, that means the rocket engine aiming system has to work before the target tracking system. The math is easy. New designs have a very high failure rate. Suppose the enemy launch detectors have a 90% chance of failing because they are experimental. And so does the engine rocket engine aiming system. And so does the target tracking system. If you try to test them all at once, the probability of success is 0.1 * 0.1* 0.1 = 0.1%. You'd have to launch several thousand test missiles to get any feedback about what the problems are. Nobody can afford to launch 5000 missiles to perfect a new defense system. So instead you rig the machine so that everything is guaranteed to succeed except the one system you are currently working on. You only need a few dozen launches for each phase of development. This you can afford.