Though SpaceX’s Falcon 9 rocket hasn’t quite mastered the trick of showing us just what a reusable rocket can do, it’s certainly shown us some impressive launch moves. Its latest launch attempt, though, had us looking at the rocket a little closer and wondering, “Just what is that?”
Fortunately, some commenters had the answer, and it’s actually a fairly mundane one for something so other-worldly:
Bonus awesomeness: see how the bottom section of the core is black? That’s soot from the engines firing on the way down, the exhaust plume wrapping round the rocket body. You can see the V-dart shapes where the legs have unfolded. Why the crisp clean line? That’s probably where the RP-1 tank ends and the Lox tank starts; the Lox tank area ices up so the soot doesn’t stick.
The threshold where the atmospheric pressure changes the plume is really abrupt (1:37), all kinds of wild stuff going on here:
Of course, the burn coming off of the rocket couldn’t be accomplished without a lot of fuel, but perhaps a little less than we might think thanks to some clever flight tricks:
The Grasshopper test articles had a lower thrust to weight ratio - Carry more fuel so they can balance weight and thrust better and iron out the software at slower response speeds, with more time to burn.
The returning Falcon 9 v1.1 first stage has thrust way in excess of it’s weight, even on one engine, and must do what SpaceX calls ‘hoverslam’ (and Kerballer’s call Suicide Burn - Come in hot, and blast away at just the right time to reach 0 velocity at 0 altitude (realistically, probably, altitude +~1m or so). This has the benefit of also being the most fuel efficient use of the remaining fuel, besides being really the only practical solution given current technology (and acceptable tradeoffs of weight/lift/cost).
Right after, Musk tweeted that they had “valve stiction” on the main throttle valve. Basically, the throttle was sticky and lagging behind the computer’s commands, causing delayed responses and over-corrections. If that’s true, it easily explains the failure - the computer was pulsing the thrust and gimballing side to side to finesse the landing, but the throttle was producing sluggish responses - and the computer responded with over corrections. Then you get your rocket sliding in with just too much horizontal velocity, and - Boom.