Friday, January 9, 2015

SpaceX CRS 5 Scrubbed Due to Faulty Thrust Vector Control Actuator

Yesterday's historic attempt to land the Falcon 9 on the Autonomous Spaceport Drone Ship (ASDS) was scrubbed due to unexpected drift in the Z-axis thrust vector control actuator (a what?) on the second stage Merlin Vacuum engine.

For the non-engineers: the actuator is just a big piston used to steer the Merlin rocket engines.  The specific part in question is probably this one below (details stolen directly from JASC's web page.) It is powerful, very precise and has a built-in function to report exactly how far the piston is pushed in or out.  This is connected directly to the rocket motor (see picture below) and so it tells the computers what direction they are currently pointed.

 Thrust Vector Control Actuator

This actuator was designed to control the thrust vector angle on the SpaceX Falcon 9 first- and second-stage engines.  It is designed for high vibration and shock loads, and has excellent frequency response characteristics.  It also was designed for short-term immersion in salt water, as evidenced by the protective cover on the electro-fueldraulic servo-valve (EFSV) and hard anodic coatings.  Redundancy is accomplished via dual-coils in the EFSV first-stage torque motor.
Embedded within the single-ended piston is a dual-element LVDT, which provides two independent channels of position feedback to the external controller.  A single D38999 Series III connector on the EFSV housing communicates all signals to and from the controller.  An optional electronic control unit (ECU) is available.

Late in the count-down yesterday, the computers reported drift (uncontrolled movement) in the actuator.  Probably, one of the countdown tests was to swivel the second stage engine around.  The computers then measure the signal back from the actuators to insure that engine is pointed exactly where the computers told it to point.  What likely happened was that the engine was slowly moving after the computers instructed it to stop.  That's bad. So, the countdown was halted.

Below is a picture of a first-stage Merlin 1D engine where you can clearly see one of the actuators.  I don't know which one might be Z axis and it might look a little different on the second stage engine, but overall it doesn't look that hard to replace ...

Except that the bad actuator is HERE at the top of the rocket, inside the interstage shroud ...

I'm not sure who gets to crawl inside the interstage to replace it, but I'm glad it's not me.  Hopefully it can be done without lowering the rocket and separating the second stage which seems like a lot more work. (Note: the pic above is actually a V1.0 Falcon so the interstage is a little different, but you get the idea.)

If the all goes wells, the next opportunity to catch the space station -- and possibly land the first stage on the ASDS -- will be Saturday (1/10) at 4:47 AM.

For those who have asked, No the landing will not be seen live.  Apparently, we don't have telescopic TV cameras and video/internet feeds in the middle of the Atlantic.  Plus, it will be really dark at 5 in the morning.


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