SpaceX Successfully Launches and Lands a Used Rocket For the Second Time (theverge.com)

SpaceX has successfully launched and landed a recycled Falcon 9 rocket for the second time. "The rocket's first stage -- the 14-story-tall core that houses the fuel and the rocket's main engines -- touched down on one of the company's autonomous drone ships in the Atlantic Ocean shortly after taking off from a launchpad at nearby Cape Canaveral, Florida," reports The Verge. From the report: This particular rocket previously flew in January, when it was used to put 10 satellites into orbit for communications company Iridium. The rocket then landed on a drone ship in the Pacific Ocean. SpaceX retrieved the rocket and spent the next few months refurbishing it in preparation for today's launch. This afternoon, it was used to launch Bulgaria's first communications satellite for TV service provider Bulsatcom. The landing wasn't easy, though. Because the rocket had to push BulgariaSat-1 to such a high orbit, the first stage experienced more force and heat during reentry than any other Falcon 9, according to a tweet from SpaceX CEO Elon Musk. Musk even warned that there was a "good chance [the] rocket booster doesn't make it back." Shortly after the landing, though, Musk returned to Twitter to add that the rocket booster used "almost all of the emergency crush core," which helps soften the landing.

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Re:SpaceX

[Gavagai80]

http://www.spacex.com/webcast but the landing isn't quite visible because the feed failed for a few seconds.

This was the most difficult stage landing to date

[catchblue22]

She wasn't centered very well, must have had a hard landing due to a swell or wave.

The landing was hard because the stage had an extremely difficult landing profile, the most difficult one so far. It entered the atmosphere at a ridiculously high speed. The speed at the beginning of the re-entry burn (just before the stage really bites into the atmosphere) was 8600km/hr and 6600km/hr at the end of the burn. Going at 6600km/hr through the upper atmosphere puts you right on the edge of burning up. The final landing burn had to use three engines as opposed to the usual one engine.

In comparison, for the CRS11 landing, the second stage was going at 4500km/hr at the beginning of the re-entry burn, and 3500km/hr at the end of the burn. The landing burn was using only one engine. Because of the slower speed, it was far more easy for the stage to make a nearly perfect landing.

IIRC, on a really dodgy landing like today's the stage actually aims for the side of the ship and not the centre, so that if the landing burn fails, the stage doesn't sink the drone ship. When the landing burn begins, the stage corrects its target towards the centre. If you watch the feed from the ship, you can see from the water disturbance that the stage is over the water on the far side. It must have done a crazy divert to land on the near side of the ship, which explains the fact that the legs used their crush core shock absorption. It was probably 50/50 that this stage would survive.