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Falcon landing



 
 
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  #1  
Old June 25th 17, 03:58 AM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Falcon landing

In article om,
says...

Elon Musk warned today's launch would represent a very difficult landing
for Stage 1 due to higher speeds and stresstes on it during re-entry.

And while it succesfully landed, at least one or two crush cores got,
well, crushed. (indicating hard landing).


Not necessarily. Depends on how far they were crushed. I believe Musk
said they used up almost all of the (length of the) crush cores. But
that would still be within the design parameters, so I wouldn't call it
a "hard landing" necessarily.

Since most of the re=entry is aerodynamic, would speed near sea level be
more of less constant because the stage would have reached terminal
velocity well before?


Pretty much. But this stage started with a higher velocity than normal
due to very limited fuel reserves. So the reentry burn could not be as
large as normal. So, the reentry itself was faster than normal (i.e.
hotter than normal). Also, due to lack of fuel reserves, this was a
three engine landing, which is necessarily harder than a single engine
landing.

Trying to understand if the difficulty today was simply due to shortage
of fuel as it neared "I still love you", or if it still had greater
speed than usual as it neared it.


There was not as much fuel to begin with for reentry and landing. This
is all carefully choreographed. There was no "shortage of fuel" beyond
what was planned.

Jeff
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  #2  
Old June 25th 17, 07:17 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Falcon landing

In article om,
says...

On 2017-06-24 22:58, Jeff Findley wrote:

Pretty much. But this stage started with a higher velocity than normal
due to very limited fuel reserves.


And this explains the "re-entry" being hotter and more stressful than
normal. I get that.

But by the time, the stage has fallen through atmosphere, wouldn't its
vertical speed be comparable to other landings?


hotter than normal). Also, due to lack of fuel reserves, this was a
three engine landing, which is necessarily harder than a single engine
landing.


If you have X fuel left, is there an advantage of running 3 engines for
2 seconds instead of 1 engine for 6 seconds?


Yes. You're neglecting gravity losses. Firing for 6 seconds means the
stage also has gravity acting on it for 6 second too. Only firing for 2
seconds means you're saving those 4 seconds fighting gravity. Hence the
term gravity losses.

Isn't there some fuel wasted during engine start-up? As such, woudln't
starting a single engine and running it longer be more efficient?


I doubt there is much "wasted" during startup. The turbopumps spin up
very fast.

Also, how does re-entry burn get started? Since the stage is sort of
free floating at that point, I take it the fins are not very useful? Do
the nitrogen thrusters turn the stage around such that the engines are
in the front, so when they fire, they decelerate the rocket's orbital
velocity?


Yes, watch the video closely. You can clearly see the nitrogen
thrusters firing in order to slowly turn the stage to get it ready for
reentry.

Or does de-orbit burn happen once the rocket has fallen sufficiently
that the fins are holding it vertical and the firing slows vertical
descent rate?


The fins are deployed by then, but aren't nearly effective enough, hence
the nitrogen thrusters firing.

(from the video, attitude isn't so obvious relative to direction of
travel).


The camera is at the top of the booster looking down which is why you
can see the engines firing for the reentry burn. Since you can see the
earth below, the burn is mostly to get rid of the stage's velocity. The
barge is pore-positioned at the optimal location so there is no need for
a "boost back" burn.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.
  #3  
Old June 25th 17, 07:26 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default Falcon landing

JF Mezei wrote:

On 2017-06-24 22:58, Jeff Findley wrote:

Pretty much. But this stage started with a higher velocity than normal
due to very limited fuel reserves.


And this explains the "re-entry" being hotter and more stressful than
normal. I get that.

But by the time, the stage has fallen through atmosphere, wouldn't its
vertical speed be comparable to other landings?


No.


hotter than normal). Also, due to lack of fuel reserves, this was a
three engine landing, which is necessarily harder than a single engine
landing.


If you have X fuel left, is there an advantage of running 3 engines for
2 seconds instead of 1 engine for 6 seconds?


Yes.


Isn't there some fuel wasted during engine start-up? As such, woudln't
starting a single engine and running it longer be more efficient?


Hardly enough to matter (and in fact they restart engines TWICE on the
way down).


Also, how does re-entry burn get started? Since the stage is sort of
free floating at that point, I take it the fins are not very useful? Do
the nitrogen thrusters turn the stage around such that the engines are
in the front, so when they fire, they decelerate the rocket's orbital
velocity?


Did you watch the video I've posted twice now? The stage is never
'free floating'


Or does de-orbit burn happen once the rocket has fallen sufficiently
that the fins are holding it vertical and the firing slows vertical
descent rate?

(from the video, attitude isn't so obvious relative to direction of travel).


The stage is always pointed essentially 'down'. The OMS thrusters are
used to make sure it is properly oriented for whatever burn of the
main engine(s) is required. You can see them firing in the video.
Also once the 'fans' come out, they, too, help keep the bottom of the
vehicle properly oriented.

Just as a caution, I've noticed that at one point in the video the
vehicle suddenly gains like 18,000 kph of velocity. I suspect the tm
changed its reference point for some reason, but from that point on
18,000 is essentially '0', as you can see on the velocity TM on
landing.


--
"Insisting on perfect safety is for people who don't have the balls to
live in the real world."
-- Mary Shafer, NASA Dryden
  #5  
Old June 26th 17, 12:30 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default Falcon landing

JF Mezei wrote:

On 2017-06-25 14:26, Fred J. McCall wrote:

Did you watch the video I've posted twice now? The stage is never
'free floating'


At MECO, it looks to me like free floating. In fact, the speed starts to
drop off as the stage is free floating and still increasing altitude. At
that point, the engine bells still point to KSC.


Look at the velocity. Do you see it hit zero? Me either. Therefore
never 'free floating'.


Watching again, I see that they do turn the stage slowly so engine bells
go from pointing at KSC to pointing towards europe, and this happens as
the stage starts to gain velocity as it falls. And by the time the
engine fire, the bells are almost aiming down to ocean.

at the same time as its speed starts to increase (aka: starts to
accelerate downwards).

The burn drops speed from 8600kmh down to 6600kmh and after that, the
drag slows it down to 5712 (last telemetry shown).


Pardon? Velocity values are shown all the way to at rest on the barge
on the video I posted. Are you talking about some other video? If
so, you should provide a link to it so the rest of us know what the
**** you're talking about.


The mention the drone as AOS, but when they switch to the drove video,
the telemetry doesn't get updated. Interesting that despite not having
telemetry, they can announce that the landing burn has started.

Since they do not show speed just prior to landing burn on this flight
versus others, I can't learn by myself if the vertical speed was truly
different for this one versus others. Hence my question on whether
terminal velocity would normalize all landing approaches to roughly the
same vertical speed.


Not what you originally asked. And the answer is no.


Just as a caution, I've noticed that at one point in the video the
vehicle suddenly gains like 18,000 kph of velocity.


I did not see that. Maximum speed was roughly 8600kmh before first burn.

What you may have seen is when they switch the display of telemetry from
stage 1 to stage 2.


The video I posted never did that. It's Stage 1 TM all the way, which
is why you see the altitude peak at around 120 km and then start
decreasing as the first stage starts falling back. When the first
stage is resting on the pad it show 18,000 kph and 0 altitude. If the
display had switched to Stage 2 TM (and it's clearly labeled as Stage
1), just how the hell do you think it would suddenly jump 18,000 kph?
Magic?


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn
 




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