I may have asked this, but I guess never got proper answer.

What is the exact actual meaning when media mention a spacecraft may
skip on the atmosphere and be thrown out in space if it isn't precisely
aimed for re-entry?

Is this just a case of aircraft failing to aim towards atmpsphere dense
enough to slow it down for re-entry and instead entering very elliptical
orbit?

Or are there actual aerodynamics involved where spacecraft would
generate lift in thin atmosphere and gain altitude (and then get ito
elliptical orbit?)

And if the capsule has its het shield oriented to be perfectly
perpendicular to direction of travel, would it still generate lift when
it hits atmosphere?

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On 11-Dec-22 8:33 am, JF Mezei wrote:

I may have asked this, but I guess never got proper answer.

What is the exact actual meaning when media mention a spacecraft may
skip on the atmosphere and be thrown out in space if it isn't precisely
aimed for re-entry?

Is this just a case of aircraft failing to aim towards atmpsphere dense enough to slow it down for re-entry and instead entering very elliptical orbit?

Or are there actual aerodynamics involved where spacecraft would
generate lift in thin atmosphere and gain altitude (and then get ito elliptical orbit?)

And if the capsule has its het shield oriented to be perfectly
perpendicular to direction of travel, would it still generate lift when
it hits atmosphere?

It's not going to bounce off as if the atmosphere were some kind of
trampoline, because there's nowhere to store the energy temporarily in
the atmosphere (unlike a trampoline, which has springs).

But if a spacecraft is returning from the Moon, for example, it's going
very fast. If it hits the atmosphere at too shallow an angle, it may not
slow enough to follow the curve of the Earth. Its direction would
change, but as the Earth curves away, the spacecraft could find itself
back above the atmosphere, and now moving away. Not as fast as it was originally going, but still fast enough to head off into space, not to
return for some time.

Plunging straight into the atmosphere at right angles would obviate
that, but now the problem is that the g forces build up very quickly,
and even if the spacecraft can be designed to withstand that, the humans
inside cannot. There's also the heating that arises when getting into
the denser atmosphere while still going too fast. Even heat shields have
their limits.

Sylvia.

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On 2022-12-10 18:26, Sylvia Else wrote:

It's not going to bounce off as if the atmosphere were some kind of trampoline, because there's nowhere to store the energy temporarily in
the atmosphere (unlike a trampoline, which has springs)

On CNN , former NASA head Jom Brindenstine (sp?) says Orion will do
that. Start to penetrate atmosphere, then roll 180° and rise back up
into space to cool down and then re-enter a second time.


https://youtu.be/jvYU1F6wtk0?t=90


I take it they'll just skim upper atmosphere on first shot, not enough
to "go down" and remain in elliptical orbit and on second fall back will
hit enough atmosphere to "go down" ? So when Brindenstine says it will
go back up, this isn't aerodynamic lift, but just orbital mechanics of
an elliptical orbit?

Assuming equatorial orbit for sake of discussion, If it does the
"aerobraking" say over latititude 0, would re-entry happen at 180°?

Would such an orbit have 2 perigee and apogees? how would such be
called? (can such orbits exist ?)

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On 11-Dec-22 9:00 pm, JF Mezei wrote:

On 2022-12-10 18:26, Sylvia Else wrote:

It's not going to bounce off as if the atmosphere were some kind of
trampoline, because there's nowhere to store the energy temporarily in
the atmosphere (unlike a trampoline, which has springs)

On CNN , former NASA head Jom Brindenstine (sp?) says Orion will do
that. Start to penetrate atmosphere, then roll 180° and rise back up
into space to cool down and then re-enter a second time.

https://youtu.be/jvYU1F6wtk0?t=90

I take it they'll just skim upper atmosphere on first shot, not enough
to "go down" and remain in elliptical orbit and on second fall back will
hit enough atmosphere to "go down" ? So when Brindenstine says it will
go back up, this isn't aerodynamic lift, but just orbital mechanics of
an elliptical orbit?

Assuming equatorial orbit for sake of discussion, If it does the "aerobraking" say over latititude 0, would re-entry happen at 180°?

I assume you intended "longitude". If I understand you, you're asking
where the next interaction with the atmosphere will occur. Provided the
craft gets a reasonable distance outside the atmosphere, it's going to
occur in roughly the same place again.

Would such an orbit have 2 perigee and apogees? how would such be
called? (can such orbits exist ?)

If an orbit takes a craft into the atmosphere, then the perigee is just
a notional lowest point the craft would have reached if the atmosphere
hadn't been there. Whether the craft reaches that lowest point, or
indeed gets even lower, will depend on aerodynamic effects. But either
way the next apogee is being lowered.

Sylvia.

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Thus spake JF Mezei:

On 2022-12-10 18:26, Sylvia Else wrote:

It's not going to bounce off as if the atmosphere were some kind of
trampoline, because there's nowhere to store the energy temporarily in
the atmosphere (unlike a trampoline, which has springs)

On CNN , former NASA head Jom Brindenstine (sp?) says Orion will do
that. Start to penetrate atmosphere, then roll 180° and rise back up
into space to cool down and then re-enter a second time.

https://youtu.be/jvYU1F6wtk0?t=90

I take it they'll just skim upper atmosphere on first shot, not enough
to "go down" and remain in elliptical orbit and on second fall back will
hit enough atmosphere to "go down" ? So when Brindenstine says it will
go back up, this isn't aerodynamic lift, but just orbital mechanics of
an elliptical orbit?

I think it's aerodynamic lift. All the articles I've found so far (not
many) use the term "lift", and during the re-acquisition after the
first blackout, the track chart shown indicated the path was to descend
to 200000 (ft?) and then increase to about 290000; I'm not sure that
can be accounted for by a tangential path.

Between blackouts, the PAO described Orion as "at skip apogee and
executing a series of roll maneuvers to bleed off energy", which
implies lift.

Also, the first blackout period lasted 4m, suggesting a substantial
amount of plasma generation, and the second seems to have been about 3
1/2 m

<URL:https://scitechdaily.com/nasa-artemis-i-orion-will-attempt-the-first-skip-entry-for-a-human-spacecraft/>
as a NASA chart comparing direct reentry (Apollo) to both a short skip
and a long skip, altitude vs range [from entry interface?], and the
short skip looks like the graph of Orion's path past South America.

The article also has the quote
<quote>
“We extend the range by skipping back up out of the atmosphere where
there is little to no drag on the capsule. With little or no drag, we
extend the range we fly,” said Madsen. “We use our capsule lift to
target how high we skip, and thus how far we skip.”
</quote>
(That's Chris Madsen, Orion GNC subsystem manager)

_The Atlantic_'s article includes Kelly Smith (former Orion engineer)
telling the author that Apollo engineers knew all about the "magic of
the skip entry", but their computers couldn't handle the sensitive
calculation with enough accuracy to risk a crew. <URL:https://www.theatlantic.com/science/archive/2022/12/nasa-orion-spacecraft-returns-from-moon/672433/>

Ooooooo ... there's an IEEE document (4526287) that discusses [per the abstract] skip reentry and hypersonic lift-to-drag ratios on range
capability

/dps

--
As a colleague once told me about an incoming manager,
"He does very well in a suck-up, kick-down culture."
Bill in Vancouver

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JF Mezei asserted that:

On CNN , former NASA head Jom Brindenstine (sp?) says Orion will do

Your search-fu is that bad?

<URL:https://en.wikipedia.org/wiki/Jim_Bridenstine>

-d

--
"Maintaining a really good conspiracy requires far more intelligent application, by a large number of people, than the world can readily
supply."

Sam Plusnet

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On 2022-12-11 19:11, Snidely wrote:

I think it's aerodynamic lift. All the articles I've found so far (not
many) use the term "lift", and during the re-acquisition after the
first blackout, the track chart shown indicated the path was to descend
to 200000 (ft?) and then increase to about 290000; I'm not sure that
can be accounted for by a tangential path.

During discussions after Columbia, I asked specifically if they could
have used the Shuttle aerodynamics to stay at higher levels longer to
more slowly bleed off speed, and the answer was an unequivocal "NO"
because Shuttle not designed to fly.

So quite surprised that a capsule with no wings would be able to "fly"
in such thin atmosphere.

The emphasis on landing site accuracy is interesting. Does this just add
the ability to adjust distance to landing site in straight line versus
direct re-entry where landing site is dictated by time the vehicle
leaves moon orbit and speed of travel?

From accuracy point of view, can the same level of accuracy be reached
with direct entry? Shuttle, Soyuz, Dragon have done very good jobs with direct-re-entry precision. Or is coming from moon much harder because
of the much longer time between the equivaent of de-rbit burn (when you
leave moon orbit) and time to hit atmosphere?


If teh capsule has such lift abilities, does that imply it could also
have cross range (left or ight) capabilities too at that stage?

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On Sunday, JF Mezei queried:

If teh capsule has such lift abilities, does that imply it could also
have cross range (left or ight) capabilities too at that stage?

Yes, as part of choosing alternate landing zones. But the main range
factor is distance.

/dps

--
"I am not given to exaggeration, and when I say a thing I mean it"
_Roughing It_, Mark Twain

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