Space station future?
Brian Gaff
where the station is just at the end of life as the new vehicles come into
operation. I'd think maybe we need a station for storing stuff etc, and as a
way station, or is the idea still to perhaps go to the moon with a base?
Brian
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Alan Erskine
> OK, it seems to me that with the current situation we will have a position
> where the station is just at the end of life as the new vehicles come into
> operation. I'd think maybe we need a station for storing stuff etc, and as a
> way station, or is the idea still to perhaps go to the moon with a base?
> Brian
>
My preference is the Moon. No need to store anything in LEO, not even
propellant - too expensive.
JF Mezei
> My preference is the Moon. No need to store anything in LEO, not even
> propellant - too expensive.
Forgetting the ISS's actual orbit for a minute.
Building a mars expedition ship will require many heavy lifts to send
the modules, outfit them, and more importantly, load fuel into the ship.
If the Russians are to participate in this and want to use Baikonour
instead of Korou, then 51.6° might be required too. IF so, then using
the ISS as a base from which to assemble the expedition ship makes sense.
You have trained crews, manipulation arms, airlocks, life support,
electric power etc. So this is a great base from whcih you can start to
assemble the ship. And because you have the basics already operating, it
gives you greater flexibility in terms of assembly sequence since you
don't need to start with ECLSS, power etc.
The station also already has docking for both USA and russian systems as
well as berthing to receive cargo. So again, a good "port of call" from
which to do assembly work and interior outfitting/testing.
Once the expedition ship is done, it can detach from ISS and leave to Mars.
However, if assembly is to be done closer to an equatorial orbit, then
ISS becomes useless and assembly sequence will have to be done similar
to that of ISS with early modules, then early power etc etc.
David Spain
> If the Russians are to participate in this and want to use Baikonour
> instead of Korou, then 51.6° might be required too. IF so, then using
> the ISS as a base from which to assemble the expedition ship makes sense.
>
Well that makes some good sense to me, however, at that high an inclination,
it seems problematical for a Mars vehicle to depart for Mars w/o some fancy
LEO maneuvers first to get it back into a trajectory that would intersect
Mars. I suppose you could launch on some type of parabolic trajectory that
would intersect the ecliptic at the right time that Mars shows up, but that
doesn't look a whole lot like the low energy Hohmann transfer orbits I've read
about.
?
Dave
Dr J R Stockton
com>, Wed, 12 Oct 2011 00:11:02, David Spain <nos...@127.0.0.1> posted:
The inclination and precession of the orbit of ISS mean that, about once
a month, it will be heading in the direction that Earth is; and that is
approximately what is wanted. One of those per synodic period will be
near enough the right time for a near-enough Hohmamm orbit to Mars.
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David Spain
> The inclination and precession of the orbit of ISS mean that, about once
> a month, it will be heading in the direction that Earth is; and that is
> approximately what is wanted. One of those per synodic period will be
> near enough the right time for a near-enough Hohmamm orbit to Mars.
OK I'll bite.
Orbital mechanics is not my specialty. If you don't have the time to explain,
I'll settle for a reference link. But here is what I don't understand from
your posting:
I thought precession was a ground-based phenom. because the Earth is rotating
underneath the ISS. But that type of precession has nothing to do with its
orbital inclination which is fixed. 51.6 degrees relative to the Equator, 75
degrees to the Sun vector*. Is there another form of precession I'm missing?
I mean the precession of the Earth's axis is like 50,000 years so I'm sure
we're not talking about that.
For an ideal Hohmann orbit wouldn't you want the Sun vector to be nearly zero
or let's say opposite the 23.4 degree tilt of the Earth's axis? Or better yet,
for a Mars mission, the Sun vector being as close a match to the difference in
solar orbital angles between Earth and Mars?
Who can set me straight?
Thanks,
Dave
Dr J R Stockton
com>, Fri, 14 Oct 2011 14:13:17, David Spain <nos...@127.0.0.1> posted:
>Dr J R Stockton wrote:
>> The inclination and precession of the orbit of ISS mean that, about once
>> a month, it will be heading in the direction that Earth is; and that is
>> approximately what is wanted. One of those per synodic period will be
>> near enough the right time for a near-enough Hohmamm orbit to Mars.
>
>OK I'll bite.
>
>Orbital mechanics is not my specialty. If you don't have the time to
>explain, I'll settle for a reference link. But here is what I don't
>understand from your posting:
>
>I thought precession was a ground-based phenom. because the Earth is
>rotating underneath the ISS. But that type of precession has nothing to
>do with its orbital inclination which is fixed. 51.6 degrees relative
>to the Equator, 75 degrees to the Sun vector*. Is there another form of
>precession I'm missing?
>I mean the precession of the Earth's axis is like 50,000 years so I'm
>sure we're not talking about that.
Go to
<http://www.heavens-above.com/PassSummary.aspx?satid=25544&lat=0&lng=0&l
oc=Home&alt=0&tz=UCT>
which is set for location 0 N 0 E & UTC. Select "All".
Look at the times of the highest pass before Noon each day : I see, for
October 15-24
04:15 04:52 03:54 02:56 03:33 02:35 03:13 02:14 01:16 01:53
That's a shift of something like three hours in nine days.
But the interval is 9/360 of a year, and the effect on the time of the
sun's apparent motion around the Earth will be 24 * 9/360 hours = 0.6
hours.
The difference shows that dominant effect is that the plane of the orbit
of ISS must be shifting by something like three hours-worth in nine
days, or once round in 72 days, which is about what I said.
Alternatively, set location to London UK, use "Visible Only", and note
that a period of evening visibility begins on Oct 17. Step to the next
such period, starts on Dec 17. Then 13 Feb, 9 Apr, 3 Jun, 4 Aug, 7 Oct,
9 Dec ... - average, 6 cycles per year.
See also <http://en.wikipedia.org/wiki/Nodal_precession>, which has an
equation. For a rough estimate, set e = 0, r = a, cos i = 0.5, and the
ratio of precessional to orbital frequencies is then -0.75 * J2; and J2
is given as 1.08e-3. The ratio is therefore roughly 1 to 1300; and, as
ISS does 16.5 orbits per day or 100 in 6 days, the precessional period
must be about 6*13 = 65 days - very satisfactory agreement.
>For an ideal Hohmann orbit wouldn't you want the Sun vector to be
>nearly zero or let's say opposite the 23.4 degree tilt of the Earth's
>axis? Or better yet, for a Mars mission, the Sun vector being as close
>a match to the difference in solar orbital angles between Earth and
>Mars?
coplanar, is that the orbital velocity of ISS should reinforce the
Earth's orbital velocity. They must therefore be in essentially the
same direction, which is the direction you want for starting your
Hohmann transfer. That must happen on average once per month,
alternately for the north part of ISS's orbit and the South part.
See also <http://en.wikipedia.org/wiki/Sun-synchronous_orbit>, where
cos i is much smaller and precession is annual.
<http://en.wikipedia.org/wiki/Orbital_perturbation_analysis_%28spacecraf
t%29#Perturbation_of_the_orbital_plane_2>.
--
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David Spain
> See also <http://en.wikipedia.org/wiki/Nodal_precession>, which has an
> equation. For a rough estimate, set e = 0, r = a, cos i = 0.5, and the
> ratio of precessional to orbital frequencies is then -0.75 * J2; and J2
> is given as 1.08e-3. The ratio is therefore roughly 1 to 1300; and, as
> ISS does 16.5 orbits per day or 100 in 6 days, the precessional period
> must be about 6*13 = 65 days - very satisfactory agreement.
spheroid with equatorial bulge which causes the nodal precession.
The day job is consuming my time heavily next week, but when I get some time
back I'll run some numbers.
Thanks John.
Dave
David Spain
> OK, got it. The key here is that the Earth is not a perfect sphere but a
> spheroid with equatorial bulge which causes the nodal precession.
>
gravitational field of a rotating oblate spheroid.
Dave
kevinposada
David Spain;1180224 Wrote:
> David Spain wrote:-
> a
> spheroid with equatorial bulge which causes the nodal precession.
> Or to put it a little more accurately, the ISS is orbiting within the
> gravitational field of a rotating oblate spheroid.
>
> Dave
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David Spain
>> Or to put it a little more accurately, the ISS is orbiting within the
>> gravitational field of a rotating oblate spheroid.
>>
>> Dave
>
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