From: tl@none.invalid
JF Mezei wrote:
>Elon said engines performed great. I watched it and when one engine went
>out, there was a big fire in the area inside the skirt and lots of smoke
>out the rocket, but it was short lived. Then another engine shutdown
>with less impact but lots of smoke. I would have expected engine
>shutdown to be "clean".
If you look at the engine bells you can see that just before the first
one shuts down the other bells move to prepare for the shutdown...
Likewise just before the second shuts down the one that will keep on
running is clearly prepared for the other Raptor's upcoming shutdown.
All evidence suggests that they used three Raptors for initial
take-off and then did controlled shutdowns to keep speed and max
altitude under control. And presumably to collect data on shutdown and
restart during flight, that was part of the goal of SN8, collect as
much data as possible.
Also note that the first engine to shut down (IE least running time)
is the first to re-lit after the bellyflop, followed by the second one
to shut down. This proves the Raptors didn't have problems earlier,
they clearly balanced the amount of run-time on them!
>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.
Last time we saw that green exhaust it was from "engine rich
combusion", this can easily result from insufficient fuel and/or
oxidiser (either hot O2 rich exhaust or insufficient cooling), people
were already speculating about fuel/oxidiser issues before Elon
confirmed it.
It may well not be "lack of fuel pressure", it could just as easily be
an effect of the bellyflop, perhaps the fuel sloshed and thus starved
it temporarily... Or something else, SpaceX probably has lots of data
on it now.
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.
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.
>Despote the fireworks to celebrate landing, the concept of free fall on
>your belly and then rotate at end and land vertical was proven, and I
>think this is HUGE. Of course, remains to be seen how the vehicle
>performs with tiles.
>
>Based on the NASAspaceflight guys, the 3 engines aren't enough to launch
>a fully fueled Starship. This was likely the reason they had to scale
>back from 15 to 12.5km because it would have required too much fuel at
>liftoff and not enough power.
All evidence says SpaceX had to quickly shut down one and then later a
second Raptors to avoid vastly overshooting the height target and/or
the speed targets they set themselves, this also suggests (but doesn't
prove) that they probably also ran the Raptors at or near the lowest
possible thrust too.
I consider it far more likely that they decided that <12.5km gave them
more than sufficient safety margin for the bellyflop and there was no
reason to spend more on fuel, especially since there was a decent
chance it would RUD long before it reached that altitude anyway!
>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 VERY unlikely that SN8 had anywhere near the amount of
fuel you seem to think.
>Eventually, starship will need to launch from sea level and get to
>orbital speed so it can test heat shield for re-entry. Extra weight, and
>different aerodynamics.
>
>WOuld it be fair to state that for any launch of Starship from sea
>level, we'll never see an outfitted nosecone cargo or pax) because such
>will only happen with super heavy because Starship doesn't have the
>"oumph" t lift off meaningfully with cargo?
>(thsi question ovviously depends on wheynert vaccum engines could be
>used to help at sea level).
It's been explained to you multiple times before here.
Yes, Starship probably CAN do SSTO from Earth without payload which is
really convenient for their future testing regime but outside testing
they won't be doing that because it makes no sense (on Earth).
A lot depends on the final dry weight, if they get that down it might
even be able to loft small payloads.
However for not that much more (SuperHeavy is much simpler in many
ways) they can greatly increase the payload (1-2 orders of magnitude)
which is why everyone expects the SH+SS combination going to be used
for all non-test orbital Earth launches.
On Mars OTOH Starship is a decent SSTO, much lower energy requirement
and much less dense atmosphere helps. It's almost like someone has
thought this through during planning... Would be inconvenient if they
had to bring SuperHeavies to Mars.
And yes, Elon has stated that the Raptor Vacuum engines CAN run at
ground level, the expansion ratio is higher than the ground engines
but not high enough to cause dangerous instabilities - mostly because
there wouldn't be space for the required engine bells then.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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