Comment by ChuckMcM

Comment by ChuckMcM a day ago

Will be interesting to hear the postmortem on the second stage. The booster part seemed to work pretty flawlessly with the exception of a non-firing engine on boost back which then did fire during the landing burn.

If the person doing their on-screen graphics is reading this, I wonder if you have considered showing tank LOX/CH4 remaining as a log graph. I believe it decreases logrithmically when being used (well it would if you keep 'thrust' constant) so that would create a linear sweep to the 'fuel level' status.

modeless a day ago

I don't believe they throttle the engines up or down much during the second stage burn. Fuel decreases ~linearly and thrust is relatively constant. Acceleration increases as fuel mass decreases.

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hinkley a day ago

Don’t they throttle back at MaxQ?

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- modeless a day ago
  
  Yes, on the first stage.
  
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  hinkley a day ago
  
  Oh! Second stage. Misread.
  No, second stage has 3 vacuum engines and 3 atmospheric engines, so they'll have to be able to throttle for the cutover.
  
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  modeless 17 hours ago
  
  The engines are able to throttle but they run all of them at 100% throttle for most of the second stage's first burn, I expect. For efficiency. The telemetry showed all of them firing at the same time. The vacuum engines will be used by themselves for on orbit maneuvers and the sea level engines will be used by themselves for landing, but for the first burn to reach orbit they need all the thrust they can get.
  
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ChuckMcM a day ago

I would be surprised if that was the case, my reasoning to that is that computing where a thing is going, when it's under going with changing acceleration AND changing mass, is pretty complicated. Especially if you already have the capability to throttle the engines and keep 'a' constant.
They might, I'm not saying your wrong, I'm just saying that I cannot imagine how you would justify the added complexity of doing it that way.

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- modeless a day ago
  
  The computations are complicated but not that complicated relative to everything else SpaceX is doing. It's much more important to optimize the propellant mass by using it most efficiently than to simplify some computations. And it's probably most efficient to burn the propellant as fast as possible.
  
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- Galxeagle a day ago
  
  Any extra time spent during a burn is wasted fuel. Intuitively, any time before the rocket is in orbit, some part of the rocket thrust is resisting the force of gravity or else it would fall back down to earth. The longer that time is, the more thrust (and thus fuel) was spent negating that force. It's the main reason why the Falcon 9 boosters do a 'hoverslam' on return and land at close to full throttle - any extra time during that burn is less fuel efficient.
  Better fuel efficiency = more payload to orbit = plenty of justification for the extra complexity.
  Admittedly gravity losses are more significant at the beginning when the booster/ship are ascending purely vertically than later in second stage flight which is mostly horizontal, but definitely still a factor.
  
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  hinkley a day ago
  
  Designing a rocket that's strong enough to survive 100% thrust at maxQ also wastes fuel, because you've overbuilt.
  
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  mr_toad 15 hours ago
  
  The rocket is well beyond max Q by the time the second stage engines ignite. There’s no reason not to run them at full throttle at that point.
  
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  floating-io a day ago
  
  I thought the hoverslam was because the Merlin can't throttle down sufficiently to actually hover?
  
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  ChuckMcM 12 hours ago
  
  Yup, apparently Raptors have more dynamic range in their throttling.
  
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