Friday, April 17, 2015

If At First You Don't Succeed ...


Click the arrow to watch the Falcon 9 landing attempt. Video source: SpaceX YouTube channel.

SpaceX tried and failed April 14 at its second attempt to land a Falcon 9 booster on its barge autonomous spaceport drone ship.

(Elon Musk says not to call it a barge.)

According to Aviation Week:

Video of the stage descending to the landing ship showed the vehicle approaching quickly but decelerating. However, closer to the platform the Falcon 9 showed an excessive horizontal velocity component that prompted the single engine used for landing to gimbal to correct the flight path angle. Exhaust from the Merlin engine could be seen raising clouds of water from around the platform as the stage maneuvered close to the edge of the landing zone. The control system then commanded vectoring of the engine nozzle to an angle that effectively over-compensated for the previous flight path angle correction. By this time the vehicle was too low to make further corrections and landed at too great a tilt and speed to safely land.

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 In a later tweet that was subsequently withdrawn, Musk then indicated that “the issue was stiction in the biprop throttle valve, resulting in control system phase lag.”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.


Another video of the landing failure posted on Vidme by an anonymous user.

Various media reports this week quoted SpaceX president Gwynne Shotwell as saying the next attempt may occur on land and not at sea. Since SpaceX has only two launch locations, Cape Canaveral and Vandenberg, that means the company would need permission from the U.S. Air Force to land at their base.

If permission were received, the next Cape opportunity might be in June with the next Dragon cargo delivery to the International Space Station. The next Vandenberg launch is in July, the NASA/NOAA Jason-3 ocean surface topography mission. But SpaceX is also scheduled sometime this year to demonstrate an in-flight launch abort for the crew version of Dragon, the V2, at Vandenberg.


The damaged drone ship after its return to Jacksonville seaport. Image source: Imgur.com.

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