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.