Yesterday, 2018-12-05, SpaceX successfully launched a Dragon spacecraft from Cape Canaveral to deliver more than 2500 kg of cargo to the International Space Station (ISS). The Dragon spacecraft (apart from its disposable “trunk” section) was previously flown on the CRS-10 mission to the ISS in February 2017. The Falcon 9 booster was new, on its first flight. Here is a video of the launch, starting at 15 seconds before liftoff through deployment of the Dragon’s solar panels.
The primary mission was delivery of the Dragon to an orbit to rendezvous with the ISS, and was entirely successful. SpaceX intended to recover the first stage booster for subsequent re-use (it is a “Block 5” model, designed to fly as many as ten times with minimal refurbishment between launches) back at the landing zone at Cape Canaveral. This involves, after separating the second stage, flipping the first stage around, firing three engines in a boost-back burn to cancel its downrange velocity and direct it back toward the Cape, a three engine re-entry burn to reduce its velocity before it enters the dense atmosphere, and a single engine landing burn to touch down.
Everything went well with the landing through the re-entry burn. As the first stage encountered the atmosphere, it began to roll out of control around its long axis. The “grid fins” which extend from the first stage to provide aerodynamic control, were not observed to move as they should to counter the roll moment. As the roll began to go all Kerbal, the feed from the first stage was cut in the SpaceX launch coverage in the video above.
In the post-launch press conference, Hans Koenigsmann, Vice President of Build and Flight Reliability at SpaceX, showed a video which picks up at the moment the feed was cut and continues through the first stage’s landing off the coast of Cape Canaveral. He describes how the safety systems deliberately target a water landing and only shift the landing point to the landing pad (or drone ship) once confident everything is working as intended.
Here is a video taken from the shore which shows the final phase of the first stage’s braking and water landing. Note how the spin was arrested at the last instant before touchdown.
In this video, Everyday Astronaut Tim Dodd explains the first stage recovery sequence and what appears to have gone wrong, based upon tweets from Elon Musk after the landing.
After splashing down, the first stage completed all of its safing procedures, allowing a recovery ship to approach it and tow it back to port. SpaceX has said it will be inspected and, if judged undamaged by the water landing, may be re-flown on a SpaceX in-house mission (but not for a paying customer).
The most likely cause of the accident is failure of the hydraulic pump that powers the grid fins. In the present design, there is only one pump, so there is no redundancy. This may be changed to include a second pump, so a single pump failure can be tolerated.