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If you watch the landing footage of the Orbcomm OG2 mission, where the first stage of the F9 1.1 Full Thrust lands, just after it touches down, shuts down the engine, a triple boom is heard.

Most videos of people watching it, seem to suggest they thought it exploded, but with no fire/explosion visible they get over that fear quickly.

Even Elon Musk initially thought the booms were an explosion.

If you look at the various vudeos, it really looks that way, due to distance and speed of sound hitting the timing just right.

In this video the boom arrives before the landing, so still up high, less to possibly reflect off. But why a triple boom? Shuttle had a double boom, one from the leading edge of the wings, the second from the top of the tail.

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  • $\begingroup$ You mean videos like these: youtube.com/watch?v=FBrfZrlpWaI ? That was shot a long way away from the Cape, which explains why the booms were so late. The booms should arrive at the landing pad before the rocket does, but the SpaceX webcast didn't carry audio of the landing (or it wasn't audible over all the cheering going on). $\endgroup$
    – Hobbes
    Dec 23, 2015 at 21:01
  • $\begingroup$ @Hobbes There are so many, I did not bother linking to any. $\endgroup$
    – geoffc
    Dec 23, 2015 at 21:04

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Current speculation seems to be one boom for the leading edge, one for the hyper-sonic grid fins, and the third for the trailing edge.

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    $\begingroup$ Just to look into it a bit, I decided to take a look at the ratio between those three parts of the stage, and compare it to the time between the three booms. I'm certainly not an expert, and I really don't know how much insight this will provide, but here's what I found. Also, I've uploaded the audio by itself if anybody want's to use it. $\endgroup$
    – zgyork
    Dec 23, 2015 at 23:35
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If I had to guess, the first boom is the from the tip of the flame from the rocket engine, the second boom is the bottom of the rocket and the third is the wake shockwave. Also, I would like to add, this Shockwave is probably unique, in that is a normal shockwave generated at Mach 1. Normally, we don't hear normal shockwaves because they are not aimed at the ground, but in the direction the aircraft is moving.

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