SpaceX CRS-16 Landing Failure

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.

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Author: John Walker

Founder of Ratburger.org, Autodesk, Inc., and Marinchip Systems. Author of The Hacker's Diet. Creator of www.fourmilab.ch.

8 thoughts on “SpaceX CRS-16 Landing Failure”

  1. It stopped spinning nearly at the last second. Presumably, that despin was all engine/thruster rather than the grid fins regaining function.

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  2. ctlaw:
    It stopped spinning nearly at the last second. Presumably, that despin was all engine/thruster rather than the grid fins regaining function.

    That looks right.  You can see that the grid fins don’t move at all between the time the spin starts until the impact with the water.  Extending the landing legs (which are a lot bigger and heavier than they look when viewed from the top of the first stage where the camera is located) would also reduce the spin purely due to conservation of angular momentum.  I don’t know how the first stage does roll control when only burning the centre engine during the landing burn.  In a three engine burn you can get roll control by gimbaling the outboard engines, but you can’t do that with just the centre engine.  They may just rely on the cold gas thrusters for the brief time between when the grid fins lose authority and touchdown.

    On the second stage, which has a single engine, they do roll control by vectoring the turbopump exhaust nozzle.  It’s possible the centre engine on the first stage also has this capability, but I haven’t been able to confirm whether it does or not.  If so, it would probably be sufficient to stop the roll, combined with extending the landing legs.

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  3. Please excuse my language…

    Son of a B**ch!

    WoW!

    It’s amazing how far we have come from the rudimentary camera footage NASA did in their early years and more importantly how far we have come in the way of reusable devices. Just imagine if the shuttle were designed now, with what advances have been made. WoW, Just WoW!

    (Thanks, John, for this post!)

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  4. Here is a video from Scott Manley with detailed photos of the recovered booster being towed back to port and analysis of components in the interstage section which aren’t usually visible.  You can see damage the interstage suffered upon impact with the water.  One engine nozzle is said to be dented, but I couldn’t see that in the video.

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  5. There’s been a lot of speculation about what caused the spin-down of the first stage in the final seconds before it touched down on the water.  In this video Scott Manley works out the math of angular momentum with the landing legs retracted and extended and concludes that extending the legs could halve the rate of rotation, but that’s it.

    He suggests that the rotation was arrested mostly by the reaction control system which kicked in at the point where the grid fins would, in a normal landing, lose control authority as the vertical velocity decreases.  There may have been some residual rotation at touchdown, which would have been immediately damped when the extended landing legs went into the water.

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