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  #21  
Old 05-02-2021, 06:31 AM
Startrek (Martin)
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Originally Posted by glend View Post
Well, SN8 and SN9 seemed to suffer the same engine restart issue, and let's ignore the orientation differences for now. After the SN8 hard landing there was talk about low header tank pressure being the reason it started with poor thrust output. Without correct fuel mix it had hardly any thrust, and boom. At the time I believe Elon mentioned the possibility of pressurising the nose tank with helium to push the fuel. The problem with using helium as a tank pressure medium is that it is not easily available on Mars. I have not heard if they change the fuel system for SN9. The issue with the rapid build of Starships is that any required design correction can take a long time to get to the pad, what with so many older designs in the queue. Maybe they did not change anything on SN9 except procedural, system commands, sequence stuff. We may never find out.

One thing I think we can eliminate is fuel/oxidizer starvation, at least there was plenty still in the tanks when it hit the ground. I watched one head on view video where you can see the fuel erupt at the rear as the ship hit the ground, in an orange billowing cloud, but the front section split open and LOX shot forward in a white cloud, not burning at all.There was obviously still a lot of pressure in the LOX tank.

Beyond that, without knowing what data they collect, it is hard to speculate further.
Thanks Glen
Good summary of events, possible cause and outcome
Cheers
Martin
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  #22  
Old 05-02-2021, 02:13 PM
Xeteth (David)
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What puzzled me about today's test was why they left it so late to try to flip, it was basically horizonal when it hit the ground, and traveling much faster than SN8 as it neared the ground.
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I also feel they don’t seem to leave enough head room before flipping back to vertical. I’m no rocket scientist though so there could be a very good reason to leave it late.
My best guess on this is that they're trying to find the 'sweet spot' with regards to performing the flip as late as possible. Since the surface area of Starship is greatest when horizontal they can actually use the aerodynamic pressure to slow Starship down as it hits the denser atmosphere the lower in altitude it gets. By waiting until the last seconds to perform the flip it minimises the amount of fuel required for the landing burn and thus reduces the overall payload of Starship.

I could be completely wrong on this - but it seems to be the case. I do agree however that the approach (particularly through the rapid development cycle) of waiting to the last seconds is perhaps biting off more than they can chew at this time. It would make sense to master the flip first at a higher altitude and then fine tune this as the development cycle continues.

Elon has stated that they will attempt to light all three Raptors for SN10 then turn one off if all three light successfully, thus adding a bit of redundancy. Looking forward to seeing the next flight!
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  #23  
Old 05-02-2021, 04:26 PM
RyanJones
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Originally Posted by Xeteth View Post
My best guess on this is that they're trying to find the 'sweet spot' with regards to performing the flip as late as possible. Since the surface area of Starship is greatest when horizontal they can actually use the aerodynamic pressure to slow Starship down as it hits the denser atmosphere the lower in altitude it gets. By waiting until the last seconds to perform the flip it minimises the amount of fuel required for the landing burn and thus reduces the overall payload of Starship.
I had never considered that but it makes total sense
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  #24  
Old 05-02-2021, 07:20 PM
glend (Glen)
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It might make more sense to find that sweet spot working from altitude down, instead of from the explosion up. Flip at 1500 ft not 50ft.
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  #25  
Old 05-02-2021, 07:34 PM
Startrek (Martin)
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The launch, ascent , apogee , flip and glide during descent work perfectly and no doubt further test flights will achieve this
Why don’t they just do 500m to 750m hops with an attitude adjustment at apogee to resolve the landing issue. Once they nail the landing a few times then go for the complete test ??
Elon did do previous hops to get this far !

Any thoughts ??
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  #26  
Old 05-02-2021, 09:45 PM
glend (Glen)
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Originally Posted by Startrek View Post
The launch, ascent , apogee , flip and glide during descent work perfectly and no doubt further test flights will achieve this
Why don’t they just do 500m to 750m hops with an attitude adjustment at apogee to resolve the landing issue. Once they nail the landing a few times then go for the complete test ??
Elon did do previous hops to get this far !

Any thoughts ??
I believe they need more altitude (than 500-750m) to achieve terminal velocity in the freefall, and to learn the wing configuration adjustments, how much aero drag they get, throttle setting and duration. But yeah, learning to land is pretty important too. Surely they have computer simulations of all this stuff, augmented by real flight data now.
I am puzzled by why they did not destroy the ship when it obviously lost control and was not going to recover. As it was there are pieces everywhere. I watched the RGV flyover video today, and Mary's debris video; there were big pieces everywhere. They had to get heavy machinery out on the mud flats at low tide to drag out the big pieces. They will be picking up pieces for a week.

RFV flyover video here:

https://youtu.be/DMtd-4D4Dlw

Mary's NSF video of the cleanup, start at about 15 minutes into this videro:

https://youtu.be/MJhHaLANaYk
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