13 thoughts on “The Latest SpaceX Video”

1. They might be able to overcome valve stiction with software tweeks. The idea would be that instead of moving the valve a short distance, the software would overshoot and then immediately move it (almost all the way) back. Stiction, as I understand it, interferes with small changes in valve position, not large ones.

   1. If it is stiction. I’m not sure that Guy didn’t just expand Elon’s deleted tweet to Carmack into a full story.

   2. Can folks comment on some causes of stiction? Is it due to thermal extremes? Lubricants not well suited to the environment they are forced to operate within? Manufacturing lot variance? Valve being over/under pressured, or overworked?

2. The cleanest video I’ve seen looks a lot more like a sticking gimbal, not a sticking throttle.

3. It still looks to me like they’re coming in too hot. Seems like a successful landing should look a lot slower and more controlled.

   1. They don’t really have much choice. They can’t throttle deep enough.

      1. What is the thrust on a Draco? Could they add a few to the first stage as well? Cut to one engine and use the Dracos for throttling and perhaps some control authority as well… would the weight penalty of extra, smaller engines and extra piping and so on mean first stage cutoff even sooner? I can see in my mind’s eye engineers scribbling furiously on napkins…

   2. If you look at the video shot from the barge, there is very close to zero vertical speed at touchdown. It’s just not terribly vertical.

4. What are the flames around the base of the rocket after engine shutdown, and before it topples?

   1. Leftovers. You can see similar on the Grasshopper tests.

5. But what about this video?

6. I think it would have made it if was landing over a wide open space instead of on a relatively small barge out in the middle of the ocean. It was coming down straight but at the last second had to translate over 50-100 meters to hit the landing pad. That took energy away from slowing the rocket down to instead moving laterally. Ultimately it was that lateral momentum that proved to much for the system to compensate for and it ran out of time when it hit the barge. I guess another way to look at it would be to improve their aim at hitting the center of the barge at a higher altitude so that the rocket doesn’t have to make those hard steering corrections at the very end. Naturally it would be easier to just make the target bigger rather than make the aiming more precise. I think they’ve demonstrated that the aiming is precise enough as it is to go for a land-based landing site next time.

   1. The lateral momentum looks to me like a side effect of attitude control issues deriving from an engine gimbal problem.

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