From: tl@none.invalid   
      
   Torbjorn Lindgren   wrote:   
   >JF Mezei   wrote:   
   >>Apparently, the landing engine lacked fuel pressure in header tank I   
   >>would assume this is simple to fix.   
   >   
   >When the second engine relight during descent we see brief flashes of   
   >green in the exhaust from the first. A little while later the second   
   >Raptor one shuts down again, considering the weight vs thrust this was   
   >likely also as planned. Then near the end the exhaust from the   
   >remaining Raptor's exhaust goes solid green.   
      
   Looking at Scott Manleys video with just the relight and shutdown, I   
   now think the plan was indeed not to relight the third engine (Raptor   
   SN42!), it's carefully moved out to give the other Raptors more gimbal   
   range, just like on the shutdowns earlier.   
      
   However, it doesn't look like this is done for the shutdown of the   
   second Raptor and the exhaust of the first Raptor goes green very   
   shortly afterwards (it was just flashes before).   
      
   Combined this suggests the second raptor likely shut down due to fuel   
   starvation which also affected the remaining engine. If it had been an   
   engine failure it might have tried relit SN42 if it had time but not   
   much it can due when fuel starved.   
      
   Other sources think they saw a fuel leak, IIRC Starship use autogenous   
   pressurisation so this could for example be the pipes for this up to   
   the header tank, I would expect this to result in the low fuel   
   pressure Elon mentioned though sloshing is still a possible candidate,   
   people aren't always right about what they think they see.   
      
      
   >It may well be something that only happened because they weren't   
   >flying higher, though 12.5 vs 15km is unlikely to have mattered. If   
   >this was in fact the direct cause I suspect 30+km flight profile would   
   >give the fuel WAY more time to settle down before it's needed for the   
   >landing burn! And the normal profil is obviously far higher than that.   
      
   Based on the rim lines it did have quite a bit of fuel onboard, I   
   suspect the flight profile "only" hit 12.5km due to massive gravity   
   losses caused by the low speeds - which they caused by throttling back   
   and shutting down engines.   
      
   Did it go supersonic, there was no telemetry so it's hard to tell but   
   I know there was some speculations on livestreams that they might have   
   done it this way to avoid that for this test.   
      
   I expect we'll see more detailed third-party analysis later today   
   including estimates on height and speed, this may confirm if that was   
   what they did.   
      
      
   >And while it was clearly moving to fast when it came down it wasn't   
   >moving THAT much too fast, suggesting the plan was likely to land on   
   >one engine.   
      
   Based on the information above this may still be the case, Falcon 9   
   has some profiles where it uses 3 engines and then 1 for the final   
   touchdown. OTOH, if you can land on multiple engines it's more   
   efficient.   
      
      
   >>Assuming this is true, does this mean that going forward, they will need   
   >>to use not only the 3 sea level engines but also additional engines   
   >>(either sea level or vacuum).   
   >   
   >I think it's unlikely that SN8 had anywhere near the amount of   
   >fuel you seem to think.   
      
   As mentioned above it likely had much more fuel than I originally   
   though, I considered the height/speed but forgot about the runtime and   
   how gravity losses can kick in.   
      
   I thought I should acknowledge this mistake.   
      
   --- SoupGate-Win32 v1.05   
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