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Re-Entry through satellite constellations



 
 
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  #1  
Old June 7th 19, 11:24 AM posted to sci.space.policy
Jeff Findley[_6_]
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Default Re-Entry through satellite constellations

In article ,
says...

With plans to launch billions and billions (ok, thousands) of small
satellites in very low Earth orbit, does this have any impact for manned
vehicles performing re-entry and passing through those altitudes?

If you need to land in a fairly small target area, your de-orbit burn
has to be done from a fairly precise location and thus this, right?

Is it just a question of slight change in orit altityde prior to
de-orbit burn to change the time where you pass over ]point where
de=orbit burn has to happen? Would doing so then often cause a conflict
with another satellite since there would be satellites passing near your
re-entry corridor to cover that land all the time?


Not any more than flying thousands of aircraft creates a "conflict" in
the air. Space is big. In fact, it's very big. It's so mind
bogglingly big that it's not a problem. These satellites are all
tracked so it's not a problem to make sure that you're avoiding them.

Or it is mofe likely to remain on schedule, but shift a capsule sideways
enough to avoid collision and then use aerodynamics/parachutes to get
back to target landing zone?

I take it BFR/BFS will have greater cross range capabilities with
powered landings and BFS having some aerodynamics and can more easily
steer todo a re-entry corridor that won't have collision and then steer
back on track to landing target?


Or is that a total non-issue with satellite density not being sufficient
to warrant such manoeuvers ?


Total non issue.

Jeff
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These posts do not reflect the opinions of my family, friends,
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  #2  
Old June 8th 19, 10:05 AM posted to sci.space.policy
Fred J. McCall[_3_]
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Default Re-Entry through satellite constellations

JF Mezei wrote on Sat, 8 Jun 2019
00:48:55 -0400:

On 2019-06-07 06:24, Jeff Findley wrote:

Not any more than flying thousands of aircraft creates a "conflict" in
the air.


In flight, aircraft are assigned non conflicting flight paths, just like
satellites. The difference is that a plane that is following another
one on same path can be told to slow down to maintain its distance.


Spacecraft, too. All the orbital elements are known. Missing them by
a wide margin isn't difficult.


And the bigger difference is that a plane can be told to circle airport
until the plath to ;land is clear, planes on the ground can be held to
create openings for planes to land.

Musk can't just tell its satellites to hold on so some spaceship can
re-enter.


Actually he can. All his satellites have thrusters to allow them to
'dodge' orbital debris and thus they could presumably adjust orbits if
necessary to allow something reentering to get past them.


If a re-entering spaceship slows down or accelerates to avoid one
satellite during de-orbit, it now has to deal with other satellites that
may be in conflict.


Take off your shoes and figure it out. There are only around 50
satellites per orbital plane. Go break out your crayons and figure
out how far apart that puts the satellites.


If you're landing anywhere in the Pacific, it is easier to dodge
satellites below. But if you are tarketing an X on a landing pad, unless
you are given lots of cross range like the Shuttle, you have 2 orbit
opportunities per day. What happens when both have conflicts?


Your premise is bull****. You can come down almost any time and miss
all the satellites.


These contellations are meant to provide coverage 24/7 in every area of
covered continents. And because they are verlo low orbit, each satellite
has small footprint, which means neighouring satelites need to be
nearby. So is re-enty really that easy and without concern?


Do the ****ing math. Given 50 satellites per orbital plan at around
400 km altitude (the lowest ones; higher ones will be further apart).
That's around 7.2 degrees between satellites. The Earth has a radius
of 12,742 kilometers. Add in the altitude of the orbits and you get
around 13,100 kilometers for the radius of the circle and a
circumference of around 82,300 kilometers for a spacing between
satellites in the same orbital plan of around 1650 kilometers. If you
can't hit a 1600 kilometer 'slot' to reenter, you probably have no
business putting things in orbit.

So yes, reentry is really that easy and without concern. You've been
told three times now (once by Jeff and twice by me).


Commercial aircraft have limits on how close they can be to each other
when at same altitude.


Yes, they do. So what?


How close to a Starlink/Oneweb/other satellite would NASA allow a
re-entering spaceship pass as it descends through their altitudes?


Mostly that isn't NASA's say, since they don't own most of the
vehicles.


Are we talking a minimum of 1m% 100m? 1km? 100km?


Even taking your preposterously large upper number of 100 km for a
'near miss' (the actual 'warning threshold' is about an order of
magnitude smaller than that) there is more than 16 times as much space
between even satellites in the lowest StarLink orbit.

Again, THIS IS A NON ISSUE, SO ****ING DROP IT ALREADY. I once again
did the basic math for you. You know, you could just do it for
yourself and eliminate a lot of stupid questions on your part. Or you
could believe it when someone gives you an answer and stop trying to
push for your stupid questions.


--
"Ignorance is preferable to error, and he is less remote from the
truth who believes nothing than he who believes what is wrong."
-- Thomas Jefferson
  #3  
Old June 8th 19, 02:40 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Re-Entry through satellite constellations

In article ,
says...

On 2019-06-07 06:24, Jeff Findley wrote:

Not any more than flying thousands of aircraft creates a "conflict" in
the air.


In flight, aircraft are assigned non conflicting flight paths, just like
satellites. The difference is that a plane that is following another
one on same path can be told to slow down to maintain its distance.


Exactly how far apart are Starlink satellite? Certainly you can do the
math given their orbital altitude and the number of satellites SpaceX
just launched into a single orbital plane.

And the bigger difference is that a plane can be told to circle airport
until the plath to ;land is clear, planes on the ground can be held to
create openings for planes to land.


Again, look at the distances involved. Starlink satellites will already
be spaced very far apart. This really is a non-issue since they're all
tracked and their orbits are all known.

Musk can't just tell its satellites to hold on so some spaceship can
re-enter.


Because he doesn't have to.

If a re-entering spaceship slows down or accelerates to avoid one
satellite during de-orbit, it now has to deal with other satellites that
may be in conflict.


That's not how this works. The reentry burn will be timed such that it
avoids all tracked objects below the reentering vehicle. This isn't
that hard.

If you're landing anywhere in the Pacific, it is easier to dodge
satellites below. But if you are tarketing an X on a landing pad, unless
you are given lots of cross range like the Shuttle, you have 2 orbit
opportunities per day. What happens when both have conflicts?


There really is no dodging. Just proper timing.

These contellations are meant to provide coverage 24/7 in every area of
covered continents. And because they are verlo low orbit, each satellite
has small footprint, which means neighouring satelites need to be
nearby. So is re-enty really that easy and without concern?


Again, do the math. How far is one satellite away from the other?

Commercial aircraft have limits on how close they can be to each other
when at same altitude.


And how many thousands of aircraft are in the air around the entire
planet at the same time? Now compare that to how many Starlink
satellites are planned. If we can manage aircraft that land and take
off at relatively few locations (which is where ATC is busiest), we can
manage Starlink satellites which are evenly spaced and tracked.

How close to a Starlink/Oneweb/other satellite would NASA allow a
re-entering spaceship pass as it descends through their altitudes?

Are we talking a minimum of 1m% 100m? 1km? 100km?


This isn't really done by distance. It's done by calculating the chance
of debris hitting something like the ISS. See he

https://phys.org/news/2012-10-space-station-debris.html

Distance would vary based on the confidence in the path of the debris.

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.
  #4  
Old June 8th 19, 09:15 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default Re-Entry through satellite constellations

JF Mezei wrote on Sat, 8 Jun 2019
13:52:24 -0400:

On 2019-06-08 05:05, Fred J. McCall wrote:

Do the ****ing math. Given 50 satellites per orbital plan at around
400 km altitude


One orbinal plane at 340km, 50 satellites. Each travels at roughly
27,000kmh covering circumference of roughly 42,000.

Do the math. That is one satellite in that one orbital plane passing
avery 1.9 minutes.


Uh, so? You must have a hell of a time merging with traffic, since
I'm pretty sure there is a lot less than 1.9 minutes between cars in
the traffic stream.


Now, the 340km altitude will be covered by 7500 satellites, at 50 per
orbital plane, this means 150 orbital planes, or roughly one orbital
plane every 2.5°, or about 300km apart. So if you target re-entry to
pass between 2 satellites in one orbital plane, odds are there will be
satellite in the orbital plane 300km away.


So what?


Remember that in order to provide reliable service, there needs to
always be at least one low orbit satellite over any one spot in served
areas.


No. There needs to be at least one low orbit satellite with LINE OF
SIGHT. This is not the same thing as 'over'.


So if BFS is targeting a landing at the Wall Street heliport in
Manhattan, odds are that there will be a satellite over Manhattan at any
point in time.


No. There will be a satellite with LINE OF SIGHT to Manhattan. Not
the same thing as 'over'.


The problem is that when you land on a target, your de-orbit time needs
to be very precise to hit that target landing zone.


This is really only true for a pure ballistic reentry. Nothing that
reenters under control is pure ballistic.


But if that time
doesn't coincide with a 2 minute gap between satellites, what do you do?
wait for an orbit where the de-orbit time does match a gap between 2
satellites?


Think about it. A satellite will move roughly 7.5 km in ONE SECOND. So
any given satellite is 'in the way' for something like 400
milliseconds, even if you adopt a preposterous 'safety window' of
missing by at least a kilometer.


If youa re targetting "somewhere" in the Pacific ocean, then you have
much more leaway in when you de-orbit and thus can more easily find a
gap through which you can pass.


No. Space is mostly empty regardless of where you want to come down.
Think about it. If you for some reason need to fly directly through
the orbital plane of a group of satellites (already a bad assumption,
as you could simply avoid that) your path to ground will be blocked
for 0.4 seconds every 114 seconds even if you use a huge safety margin
like 1 km. That's like you're blocked for 0.35% of the time. In
other words, your flight path is clear more than 99.6% of the time. If
you DON'T need to fly straight through one of those orbital planes,
your flight path is clear 100% of the time.


Now, add not only Starlink but also Oneweb. Does the math still resuilt
in the "no brainer" you assume?


Yes. And I didn't 'assume' anything. I understood the issue, which
puts me far, far ahead of you.

If you
can't hit a 1600 kilometer 'slot' to reenter, you probably have no
business putting things in orbit.


How did you come up with 1600km?
Circumference at 340km altitude is ~42000km. You have 50 satellites,
that means 840km between satellites. It takes 1.9 minutes for a
satellite at ~27,000kmh to travel 840km.


I apologize. My number is a bit less than twice too large as I
accidentally took a figure for Earth diameter as radius. My error
should have been obvious to you, SINCE I GAVE THE BLOODY NUMBERS I
USED (which you apparently 'cleverly' removed).


Repeat this for the 1 or 2 orbital planes to the west, knowing that
satellites in orbital plane are strategically phased to cover the gaps
between 2 satelites in the orbital plane next to it (to ensure customers
have continued connectivity).


No. You still only have to fly through, at most, ONE orbital plane,
which means you are blocked AT MOST for 400 milliseconds out of each
114 seconds.

Even taking your preposterously large upper number of 100 km for a
'near miss' (the actual 'warning threshold' is about an order of
magnitude smaller than that)


So they are happy with 10km distance from another satellite during
er-entry? Is re-entry considered with same standards for distance
between objects in stable orbits with constant speeds and altitude?


Actually they're 'happy' with much closer passes than that. That 10
km is the 'alert threshold' for DEBRIS. It's that big because debris
aren't under active control and because it's merely a "you might want
to start paying attention to this" alert.

yourself and eliminate a lot of stupid questions on your part.


If you know everything, yet unwilling to explain without insults, whyt
do you even come here?


If you know nothing, yet unwilling to do any work at all to inform
yourself, why do you even come here?

This isn't your personal teaching hospital, bucko. The rest of us
don't exist to answer your stupid questions only to have you then
argue about the answer because reality just isn't what you wanted. You
need to do one of two things. You either need to actually inform
yourself and then argue or else ask questions and say 'thank you' when
someone exerts the effort to answer you. Note that your typical
combination of ask questions and then argue with the answer is right
out.


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn
  #5  
Old June 8th 19, 09:18 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default Re-Entry through satellite constellations

JF Mezei wrote on Sat, 8 Jun 2019
13:55:35 -0400:

On 2019-06-08 09:40, Jeff Findley wrote:

That's not how this works. The reentry burn will be timed such that it
avoids all tracked objects below the reentering vehicle. This isn't
that hard.


Doesn't that preclude precise landing for ballistic objects such as
capsules ...


Most capsules aren't pure ballistic objects. They can generate lift.


... since they have to time re-entry burn for "traffic" at 340km
altitiude instead of timing it to land next to the recovery ship?


99.6% clear. The 400 ms at most that you can't fly through aren't
going to appreciably affect anything.


And for powered landings, doesn't that require much more fuel/cross
range in order to paliate for doing a re-entry at a sub=obtimal time in
order to avoid the traffic?


Again, 400 ms, at most. It won't matter.


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn
  #6  
Old June 9th 19, 03:50 AM posted to sci.space.policy
Sylvia Else[_3_]
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Posts: 87
Default Re-Entry through satellite constellations

On 9/06/2019 10:08 am, JF Mezei wrote:
On 2019-06-08 16:18, Fred J. McCall wrote:

Most capsules aren't pure ballistic objects. They can generate lift.


At 340km altitude, do they generate any lift? Aren't they pure ballistoc
at that point?


Near enough, but the deorbit burn can be timed/adjusted to put the
spacecraft into a suitable gap between the satellites, while still
allowing the targeting of a specific landing point.


99.6% clear. The 400 ms at most that you can't fly through aren't
going to appreciably affect anything.


So what you are stating is that NASA is perfectly OK with de-orbiting
any any time because the odds of being at the wrong place and wrong time
during that 400ms are so low as to not even bother?


I don't think that was the point being made. Rather that any orbital
modification required to achieve a 400ms change can be dealt with later
during the non-ballistic phase.



If they do "aim" to take the threat seriously and pass between 2
satellites based on their orbital elements, is there confidence that a
de-orbit burn can not only start right on time (easy) but also
decelerate at the exact rate (consider a capsule's mass may not be fixed
depending on what cargo they are returning) such that they will cross
path with Starlink orbit at just the right time?


The craft can measure its deceleration, so it's not just dead-reckoning.

Sylvia.

  #7  
Old June 9th 19, 02:38 PM posted to sci.space.policy
Jeff Findley[_6_]
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Posts: 2,307
Default Re-Entry through satellite constellations

In article ,
says...

On 2019-06-08 09:40, Jeff Findley wrote:

That's not how this works. The reentry burn will be timed such that it
avoids all tracked objects below the reentering vehicle. This isn't
that hard.


Doesn't that preclude precise landing for ballistic objects such as
capsules since they have to time re-entry burn for "traffic" at 340km
altitiude instead of timing it to land next to the recovery ship?


Not really. You need wide open spaces (or ocean) to land a capsule
anyway. All you need to do is to predict when you're going to do the
burn (based on when you have a clear path down). This determines
precisely where you're going to land (splash down). So you preposition
your recovery forces at that location. If you have a backup landing
burn time, you do the same for the backup recovery forces.

Also, capsules do have the capability of producing hypersonic lift.
Apollo needed that lift in order to stay in the upper atmosphere longer
than a purely ballistic capsule would. That hypersonic lift can be used
to produce some crossrange capability. So even a capsule can aim for a
predetermined landing point. Look at the landing accuracies of the
Apollo missions for an idea of how accurate this can be, even with 1960s
technology.

This isn't that hard.

And for powered landings, doesn't that require much more fuel/cross
range in order to paliate for doing a re-entry at a sub=obtimal time in
order to avoid the traffic?


No. Crossrange is achieved mostly via hypersonic lift. That's how the
space shuttle did it (those big delta wings) and it was not a "powered"
landing. Dreamchaser is a lifting body for that same reason. Even
Starship will have hypersonic lift mostly from the body and partly from
its relatively small aerodynamic surfaces. So no additional propellant
necessary.

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.
  #8  
Old June 9th 19, 02:44 PM posted to sci.space.policy
Jeff Findley[_6_]
external usenet poster
 
Posts: 2,307
Default Re-Entry through satellite constellations

In article ,
says...

On 2019-06-08 16:18, Fred J. McCall wrote:

Most capsules aren't pure ballistic objects. They can generate lift.


At 340km altitude, do they generate any lift? Aren't they pure ballistoc
at that point?


Capsules have an offset CG and that combined with their conical (or
biconic) shape produces lift as they pass through the atmosphere.
Obviously there is no atmosphere to speak of at 340 km altitude, but
what's your point? You brought up reentry and landing so the capsule
would necessary have to pass through the atmosphere to do that.

99.6% clear. The 400 ms at most that you can't fly through aren't
going to appreciably affect anything.


So what you are stating is that NASA is perfectly OK with de-orbiting
any any time because the odds of being at the wrong place and wrong time
during that 400ms are so low as to not even bother?


That's not what he said. The 400ms in time difference would be between
the "ideal" reentry burn time and one which had to be chosen to "miss" a
Starlink. These things are traveling bloody fast in orbit, so that's
why the time difference is only 400ms.

If they do "aim" to take the threat seriously and pass between 2
satellites based on their orbital elements, is there confidence that a
de-orbit burn can not only start right on time (easy) but also
decelerate at the exact rate (consider a capsule's mass may not be fixed
depending on what cargo they are returning) such that they will cross
path with Starlink orbit at just the right time?


This reads like word salad to me at 9:40 a.m. local time.

We keep telling you this isn't the big problem you make it out to be!
Space is effing big. The max delay for a reentry burn is only 400 ms.
Any difference in trajectory caused by that could be made up by a
capsule or lifting body via hypersonic lift during reentry through the
atmosphere.

Again, look at the landing accuracy of Apollo capsules. This was
achieved partly due to their ability to generate a significant amount of
hypersonic lift.

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.
  #9  
Old June 9th 19, 05:42 PM posted to sci.space.policy
Fred J. McCall[_3_]
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Posts: 10,018
Default Re-Entry through satellite constellations

JF Mezei wrote on Sat, 8 Jun 2019
20:08:27 -0400:

On 2019-06-08 16:18, Fred J. McCall wrote:

Most capsules aren't pure ballistic objects. They can generate lift.


At 340km altitude, do they generate any lift? Aren't they pure ballistoc
at that point?


As usual you totally miss the point. Let me try and explain it to you
in detail even you will grasp. First, remember that any given bit of
space I need to fly through is empty 99.6%. For the other 0.4% of the
time I might have to do a reentry burn that is slightly suboptimal. If
I'm really stupid I might have to use the OMS to avoid a collision.
Now, once I pass that 340 km altitude, I don't carry those conditions
with me what with me being reentering and all. Instead, I continue to
go down and reach the point where the lift of the capsule can be used
to adjust the impact point to precisely where I want it. THIS IS HOW
IT WORKS NOW AND THERE'S NO REASON IT WOULD CHANGE BECAUSE OF
STARLINK.

Get it now?

99.6% clear. The 400 ms at most that you can't fly through aren't
going to appreciably affect anything.


So what you are stating is that NASA is perfectly OK with de-orbiting
any any time because the odds of being at the wrong place and wrong time
during that 400ms are so low as to not even bother?


No, dear boy. I'm stating what I'm stating, not the stupid things you
try to imply. In order to avoid that satellite you MIGHT have to do
your reentry burn either 200 ms early or 200 ms late, worst case. Even
on a pure ballistic reentry, how far does that move your landing
point? It's certainly within the lift capability of the capsule to
correct.


If they do "aim" to take the threat seriously and pass between 2
satellites based on their orbital elements, is there confidence that a
de-orbit burn can not only start right on time (easy) but also
decelerate at the exact rate (consider a capsule's mass may not be fixed
depending on what cargo they are returning) such that they will cross
path with Starlink orbit at just the right time?


Yes. The capsule's mass is always 'fixed' in that it doesn't change
in flight. If you can't predict the reentry precisely then you cannot
predict the landing point precisely and you cannot do spaceflight.
Since we obviously CAN do spaceflight, your 'concern' is obviously not
a real problem.


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn
  #10  
Old June 9th 19, 05:46 PM posted to sci.space.policy
Fred J. McCall[_3_]
external usenet poster
 
Posts: 10,018
Default Re-Entry through satellite constellations

Jeff Findley wrote on Sun, 9 Jun 2019
09:44:06 -0400:


Again, look at the landing accuracy of Apollo capsules. This was
achieved partly due to their ability to generate a significant amount of
hypersonic lift.


Or look at the landing accuracy of Dragon capsules, which do much
better than Apollo did.


--
"The reasonable man adapts himself to the world; the unreasonable
man persists in trying to adapt the world to himself. Therefore,
all progress depends on the unreasonable man."
--George Bernard Shaw
 




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