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Starship test



 
 
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  #1  
Old December 9th 20, 11:30 PM posted to sci.space.policy
Sylvia Else[_3_]
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Posts: 87
Default Starship test

Looks as if it went very well apart from the final landing, where it
came down too fast.

Here's hoping SN9 will nail it.

Sylvia.
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  #2  
Old December 10th 20, 04:40 AM posted to sci.space.policy
Torbjorn Lindgren
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Posts: 15
Default Starship test

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.
  #3  
Old December 10th 20, 10:02 AM posted to sci.space.policy
Niklas Holsti
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Posts: 168
Default Starship test

On 2020-12-10 2:18, JF Mezei wrote:

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.


Another likely explanation I've seen is that they wanted to avoid
high-altitude winds at the transition from nose-up to belly-flop.

I don't think they would need to fully fuel the Starship just to reach
15 km altitude at low speed.
  #4  
Old December 10th 20, 01:58 PM posted to sci.space.policy
Torbjorn Lindgren
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Posts: 15
Default Starship test

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.
  #5  
Old December 10th 20, 08:44 PM posted to sci.space.policy
snidely
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Posts: 1,303
Default Starship test

Torbjorn Lindgren explained :
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


rime?

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.


The NSF crew wasn't expecting supersonic even before flight, and
speculation is the apogee target was lowered to avoid issues with winds
aloft.

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.


The NSF crew caught the WB-57 in the area on Tuesday, but sadly the
plane had issues the next morning and didn't fly on Wednesday.

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.



In the NSF streams, it sure looked like the craft was crabbing out to
sea before the third engine shutdown for apogee. That's got to take a
lot of fuel.

/dps

--
The presence of this syntax results from the fact that SQLite is really
a Tcl extension that has escaped into the wild.
http://www.sqlite.org/lang_expr.html
  #6  
Old December 11th 20, 12:34 AM posted to sci.space.policy
Alain Fournier[_3_]
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Posts: 548
Default Starship test

On Dec/9/2020 at 19:18, JF Mezei wrote :

Despote the fireworks to celebrate landing,


I like your phrase. Yes the flight was worthy of celebrations.


Alain Fournier
 




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