141–150 of 205 posts
Earlier quoted context omitted.
Judging from the ensuing explosion there was still fuel left over.
Making it a little puzzling ask to why the engines didn't try harder to lower the vertical velocity on approach. Perhaps there are improvements to be made in the control systems.
As if this weren't impressive enough already, according to a comment made by Scott Manley[1] it sounds like the final maneuver (where it appears the rocket is making a fairly strong lateral correction) is entirely intentional. Not sure if anyone here can confirm or deny the claim, but apparently the trajectory prior to landing is not supposed to be exactly above the barge as one would expect - I understand it may be…
just the fact they can hit a barge after falling back from space impresses me. I wonder if there are any islands they could use instead, surely they had to have looked for one. Would a stationary oil platform work better?
If all this goes well though, the intention is to eventually fly it back and land it on the mainland.
I wonder what happened there. Looks at 4 seconds the guidance system figured it needed to pull an ultra manoeuvre to slow the the horizontal speed(was drifting left) but ended up with a solution putting the rocket upright again at nearly 0m. If it would have left some altitude for stabilizing I think they would have had it. Arm chair rocket science is fun.
According to Elon's tweet that seems to have disappeared by now, they had static friction in bipropellant valves introduce a lag that the control system couldn't handle.
It sounds like the fuel valves which control the engine's throttle took longer to turn up or down the rocket than expected.
The control system that is constantly processing rocket position and turning that into throttle outputs couldn't handle how long it took for decisions to turn into actions. That tends to cause wobbling in most controllers (overshoot, undershoot, etc).
Earlier quoted context omitted.
According to Elon's tweet that seems to have disappeared by now, they had static friction in bipropellant valves introduce a lag that the control system couldn't handle.
Can you or someone else please explain what that means in slightly less technical terms without dumbing it down too much?
Since valves cannot open instantaneously, the control system has to know how long the valve takes to open/close, and start that process a little bit early so the valve is open when it needs to be open. If something goes wrong with the valve, it doesn't open as quickly, and so the computer model thinks it should be open and isn't, which could easily go off into an unstable state.
Was this filmed from a drone? I couldn't find any info.
Was this filmed from a drone? I couldn't find any info.
In the middle of the ocean It seems doubtful... I'm also intrigued by the angle - maybe a helicopter?
Earlier quoted context omitted.
According to Elon's tweet that seems to have disappeared by now, they had static friction in bipropellant valves introduce a lag that the control system couldn't handle.
Can you or someone else please explain what that means in slightly less technical terms without dumbing it down too much?
The "lag" is a delay between your steering signal and the state of the system changing in response. In this case, because of static friction, valves controlling fuel flow reacted with delay (I'm not sure if they're talking about Merlin's valves or attitude control engines though) - so the rocket did not react immediately after being told to do so.
There's a whole math field around that topic, known as "control theory". There are ways to determine the limits of inputs and outputs that will allow your control system to keep everything in the desired state. In this case however, the input lag wasn't accounted for and it pushed the system out of the space of stable states, beyond the ranges where the control unit could keep up with the changes, so it ended up oversteering. Fortunately, in this case it's mostly a software problem, i.e. they could tweak the controller to deal with the lag.
The best way to imagine it is to recall your first experiences with a bike or ice skates - trying to balance yourself on them is exactly the kind of thing the rocket was doing, probably with similar results.
My control theory is a bit rusty nowadays, but I hope this clears up some things.
[0] - if you ever hear about "PID controller", this is exactly where the name comes from - proportional, integral, derivative.
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If it's slow motion, I suggest changing the YouTube setting to 1.25x and watching it at that speed. It seems quite natural actually. Edit: Or maybe 1.5 or 2x! I don't know..
I can't figure it out either. It's really difficult to determine the speed because of the scale involved here.
Earlier quoted context omitted.
Making it a little puzzling ask to why the engines didn't try harder to lower the vertical velocity on approach. Perhaps there are improvements to be made in the control systems.
I have a feeling their engineers may have considered this. This is after all rocket science.
There is a tremendous amount of variables to consider, and the analysis of how to take each into account is non trivial -- if you don't measure some variable precisely enough it might make the whole system less stable, the solution might get worse due to increased computational load, the conditioning might degrade, etc.
So with the number of controls they have and the predictive models available, I'm sure they're actually scrambling to refine their systems right now.