By now we've all seen this amazing footage. This is Falcon 9's first stage trying to land on the "Autonomous Spaceport Drone Ship" as seen from an onboard camera (on board to the barge, not the rocket obviously). While this is an amazing showcase of great performance of the rocket, and it demonstrates SpaceX was really really near to nail it, it may not seem so at first sight. I mean, even if it had not exploded, it still hit the ship from the side and at about 45º from vertical, right? That doesn't look like the tests of the grasshoper at all, does it?
Well, let me try to explain why, when I look at the video on top, I see beautiful precision instead of lack of accuracy. Facts first, and I think this is the key, we know from Elon Musk that the cause of the failure was supersonic fins running out of hydraulic fluid. How could that be? They're called supersonic for a reason, and the rocket doesn't seem to be moving near that fast in the video, so what difference can they make anyway? To answer this question we have to move backwards a little bit, to the final seconds of supersonic deceleration of the stage.
We are a few kilometers high, moving faster than sound and straight to the ocean. Luckily we have a Merlin Engine burning at the bottom, providing thrust and decelerating us. But the engine can't do all the work on its own. Meet the supersonic fins. By controlling these fins, one can displace the center of pressure of the rocket, and so guide its trajectory aerodynamically. And at this height, subtle variations in trajectory can lead to huge differences in the final touchdown position.
For illustration purposes, think of a basket ball on the top edge of a roof. Moving one millimeter to one side or the other, will cause the ball to fall various meters away. This is basically the same, except your ball is supersonic, and it isn't on top of a roof but coming straight from space.
Back to our rocket, we can guess that if your fins stop working, you will not deviate a meter or two from your target, but a whole lot more. And even so, the rocket did hit the barge. And in a very spectacular way. Why is that? Because at some point, probably short after the fins failed, the rocket noticed his misalignment and started to compensate with the main engine. We've seen this maneuver before:
Looks familiar? That's pretty much like the lateral displacement we saw in the rapid unscheduled disassembly video, only the former was much more extreme. It was the Falcon 9 rocket, pulling strength from every bit of it to make it to the barge. And it made the whole way back to the barge! even with half its guidance mechanism completely useless!!
It steered to the limit that its structure could handle, and throttled the engine so that it could reach the barge. With all odds against, the Falcon did reach the target! Overcame system failure and got to the target. What we've seen here is not a failure, it's the most epic attempt by a rocket to fulfill his duty.
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