Video of the landing attempt shot from the ASDS Just Read The Instructions was posted anonymously on vidme. A YouTube version is included below.
While it is perhaps silly to attribute human or even animal characteristics to mechanical hardware which is just doing its job, there is something poignant about how hard the cold gas thrusters at the top of the Falcon 9 first stage appear to be fighting to prevent the entire assembly from toppling over. The first time I saw this, I found myself leaning against the direction of travel in an unconscious effort to do the same thing.
The video is also revealing, and taken along with the original “footage” below, seems to confirm a twitter exchange between Elon Musk and Armadillo Aerospace founder John Carmack. The gimballed center engine appears to be reacting too slowly to the booster’s angle of approach, resulting in a clearly visible over-reaction.
An Aviation Week story summarizes it this way:
“SpaceX founder and chief technology officer Elon Musk tweeted that “excess lateral velocity caused it [the booster] to tip over post landing.” In a later tweet that was subsequently withdrawn, Musk then indicated that “the issue was stiction in the biprop throttle valve, resulting in control system phase lag.” In this statement, Musk was referring to “stiction” — or static friction — in the valve controlling the throttling of the engine. The friction appears to have momentarily slowed the response of the engine, causing the control system to command more of an extreme reaction from the propulsion system than was required. As a result, the control system entered a form of hysteresis, a condition in which the control response lags behind changes in the effect causing it.”
Although the exchange was deleted, the brief explanation appears to both make intuitive sense and conform with the visual evidence. A slowly responding valve in a very different flight regime would also explain why we have not seen the same thing happen with Grasshopper or F9-R testing in McGregor. While the F9-R Dev 1 was lost due to an auto-destruct command triggered by a sensor failure, neither vehicle appeared to have the same control issue seen on the CRS-6 attempt. Results from that incident may be behind the decision to delete the twitter exchange as well.
It was not long thereafter that SpaceX opponents in Congress, all of whom happened to represent districts with close ties to ULA, began citing the purely experimental F-9 Dev-1 loss in Texas as a reason the company should be treated with deep suspicion. Even though the barge landing is clearly experimental as well, Elon Musk likely realized that in addition to being premature, public speculation about a control problem would likely result in the same treatment.
SpaceX has posted upgraded video of Tuesday’s Falcon 9 first stage landing attempt.
As the video clearly shows, the stage came just about as close to making a successful landing as it could, without actually succeeding. Considering that this marked only the second attempt at a barge landing, with the first having gone awry when the reservoir of hydraulic fluid activating the four hypersonic grid fins ran prematurely dry, a deficiency since corrected, there seems little doubt that the SpaceX is on the cusp of succeeding.
The next attempt will be on the NASA CRS-7 launch, currently scheduled for June 19th, but whether the recovery attempt will once again take place onboard the Automated Spaceport Drone Ship, or on dry land at Cape Canaveral’s SLC-13 is a surprisingly open question. Speaking to Defense News in an interview at the 31st Space Symposium in Colorado Springs, SpaceX President Gwynne Shotwell expressed the hope that the next attempt might take place on land.
“Just purely the boat moving, even in a low sea state, it’s hard to imagine that vehicle is going to stay vertical,” Shotwell said. “That vehicle is big and tall, compared to the itty-bity-greater-than-a-football-field-size ship.”
It will be interesting to see which path the company pursues. While there is no doubt that the concrete pad at SLC-13 would make a much more stable target, the Falcon 9 would face a fuel intensive long trek back to the Florida coastline, requiring a very different flight regime from the abbreviated boost back burns which have thus far served to only shorten the booster’s down range distance. Given how close SpaceX came on the CRS-6 flight, and its long term interested in perfecting sea landings for both energy intensive Falcon 9 launches as well as for center core recovery of the Falcon Heavy, expect the CRS-7 landing effort to once again aim for the ASDS.
Either way, the launch, which is scheduled for 1:51 PM EDT, should offer another opportunity for much needed clear photographic analysis or better yet, documentation of a historic space first.
Before then however, SpaceX has two important items on its agenda. The first is the launch of a commercial mission for Turkmenistan, which the company is targeting for April 24th. If that launch goes as planned, SpaceX will achieved a remarkable 10 day turnaround on the same pad, lending credence to assertions that it can meet a demanding manifest. That rapid pace, which has included four missions so far this year, will then face a temporary slowdown as the company prepares for a critical test Pad Abort test of its Commercial Crew capsule in May.