How To Prevent Raptors From Destroying Superheavy Pt.2

New CSI Starbase episode just dropped.

Edit: Additional background info from Zack: https://x.com/CSI_Starbase/status/1830325913776726136

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Excellent video & analysis, as always.
I was highly irritated by the erroneous claim that things would settle to the earth-facing side of the tank though. The damn thing's in free-fall, the direction of the gravity vector is entirely irrelevant! The conclusions mostly still work, but it has more to do with jostling and slosh causing the snow to move, not gravity
- The damn thing’s in free-fall, the direction of the gravity vector is entirely irrelevant!
  Ooh, good point. I missed that. I suppose the gravity vector could play a role on reentry, depending on the angle of attack, but you're right, it's probably irrelevant at MECO.
- There was a lot to digest, so I might be wrong, but I was under the impression that he was referring to the boost back phase, so not in free fall. I’m not smart enough to figure out if the gravity vector plays any significant role compared to the one from the engines in that situation :) Neither during the landing burn. There is a huge amount of deceleration which would be a vector straight through the booster? There is a slight tilt to the booster most of the way down, but I don’t know if that could make the debris settle unevenly.
  In between those burns nothing can settle as you say. If Zac was referring to that phase than that was indeed an oversight.
  
  No, even when the engines are firing it's in free-fall. The only forces on the booster or fuel (aside from internal ones like gyroscopic or centrifugal dynamics) are thrust, control thrusters, and depending on the phase of flight drag & aerodynamic control.
  Thrust always points roughly along the length of the booster, and drag always acts against the direction of travel, so the external forces acting on the fuel are almost 100% up or down during all phases of flight. The only exceptions are manoeuvres when the attitude control systems is rotating the vehicle, either by grid-fin or thrusters, so any redistribution of the fuel or snow will be entirely driven by those movements, and their own inertia
  
  I had a feeling I was stretching into territory where I don’t know enough, thank you for the clarification.
  Following that, and I hope you don’t mind me asking instead of following up myself, would I be correct to conclude that the vehicle technically is in free fall while going up? Just about everything to do with spaceflight can be so unintuitive, which is simultaneously what makes it interesting as well as frustrating at times :)
  
  I would describe it as being in free-fall whenever it's not in being held up by any interaction with a solid surface, even indirectly. I'm not sure everyone would agree with my definition, but it's not a term you'll see used much in serious engineering precisely because it is a bit vague.
  For example, an aircraft in flight isn't in free-fall because it's being held up by the air, which is in turn held up by the ground. An aircraft (or spacecraft) which has no wings is being slowed down by air resistance, but not actually held up and is therefore in free-fall.
  An ascending rocket is generating forces which hold it up, rather than transferring forces to something which won't move (like the ground), so I would consider it to be in free-fall
  
  Hmm, yes, normally one would consider an object in free fall when gravity is the only force acting on it (or following Einstein, no force is acting on it). This is what led me to my initial reply about the booster hardly (if ever) being in free fall during its flight.
  But your point was about the relevance of gravity on the settling of the debris in the LOX tank in which case one can ignore most forces on the booster and still observe it as free falling.
  The why is not easy to wrap my head around, so I understand why Zack and his team made the mistake, but I do get that it is the case. Thanks again!

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