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#40
by
Proponent
on 17 Oct, 2012 09:19
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Those look like some pretty significant difficulties for manned polar missions. If I understand it:
(*) Difficulty of free return. (though I still dont really have any quantitative idea of how difficult, anyone care to speculate?)
Attached to
this post is a paper from the early Apollo era that discusses lunar free-return trajectories, including those of the polar variety.
But NASA dropped free-return trajectories after Apollo 12 and even after Apollo 13 didn't reinstate them. My guess is that they're not really necessary, except maybe for initial flights with a new systems.
(*) Lack of any-time return. You only have an option twice a month (unless your ascent vehicle can also take you all the way home without having to meet anything in lunar orbit)
Or unless you can change planes, as Jorge described for Constellation, or unless you stage at a Lagrange point. The catch is that at minimum delta-V, the transit time between the lunar surface and L1/2 is something like a day or two (and the lander's delta-V obviously needs to be larger than if you're staging in LLO). I would think that if you're going to and from a lunar base, this probably isn't such an issue. For sortie missions, in which a lightly-built lander might need to get the crew to safety when solar flare looms, it might be a bigger deal.
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#41
by
Proponent
on 17 Oct, 2012 09:37
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My understanding has always been that the Earth rotates counterclockwise around the Sun, seen from the Earth's north pole.
Yes, although one usually says that the earth
revolves rather than rotates about the sun.
Therefore -Xε is in the direction of the Earth's motion! This is starting to make sense!
Not following you here. At the moment of the vernal equinox, the direction of the earth's motion is approximately in the -
Yε direction. Over the course of a year, the direction of motion changes, although the
Zε component is always zero. The motion is in the -
Xε direction sometime around the winter solstice. If the earth were moving in the -
Xε direction at the vernal equinox, it would be moving straight away from the sun.
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#42
by
baldusi
on 17 Oct, 2012 13:32
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The hardest part of following BMW is the notation. I'm used to a different one when doing dynamic optimization problems, and it's extremely difficult to switch. Imagine two languages that use the same words but have different meanings in each.
I highly recommend to start reading bit by the annexes, specially the notation one. And only then start reading.
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#43
by
JohnFornaro
on 17 Oct, 2012 14:55
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Thanks again.
My understanding has always been that the Earth rotates counterclockwise around the Sun, seen from the Earth's north pole.
Yes, although one usually says that the earth revolves rather than rotates about the sun.
Right. I knew that. Sloppy language.
Therefore -Yε is in the direction of the Earth's motion! This is starting to make sense!
Not following you here. [/quote]
Right. Went back to the illustration, and corrected my handwritten notes on that page. I pasted the wrong term. Edited my post while I was at it.
Turns out, the Constellation Aries (the ram) is more or less where -
Xε points to at the vernal equinox. So, if you had a star finder linked to this constellation, is that how you'd keep your bearings on your inertial frame of reference?
And it has the words, "Don't Panic!" written in large friendly letters across its cover.
Hah!
Your copy does.
I highly recommend to start reading bit by the annexes [appendices?], specially the notation one. And only then start reading.
Thanks, but Appendix C only reviews vector notation. Chapter 2.3 goes over the Classical Orbital Elements. So I'll just keep re-reading stuff.
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#44
by
Proponent
on 17 Oct, 2012 15:34
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CxP's solution to both problems was a three-burn sequence for both LOI and TEI. LOI-1 placed the Orion/Altair stack in a highly elliptical, low inclination LLO (thus allowing economical free-return). LOI-2 was a plane-change burn at first apogee post-LOI-1, then LOI-3 circularized. For TEI, Orion would do basically the same thing in reverse.
I can see how that works nicely for landings at the poles, but what if you want to land at a latitude of, say, 45
o? With LOI-3, Orion/LSAM circularizes into an orbit inclined at least 45
o to the lunar equator, and the LSAM lands. Let's say we need to abort 7 days later, by which time the moon has rotated by about 90
o. How do we get Orion and LSAM into the same plane, so they can rendezvous? Does Orion (or LSAM?) have the delta-V reserve for a big additional plane change?
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#45
by
Proponent
on 17 Oct, 2012 15:41
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Turns out, the Constellation Ares (the ram) is more or less where -Xε points to at the vernal equinox.
You mean +
Xε, right?
So, if you had a star finder linked to this constellation, is that how you'd keep your bearings on your inertial frame of reference?
In principle, you need to know just the directions to two objects whose co-ordinates are known. Then you can orient yourself. Even though the fundamental reference point, the vernal equinox, is in Ares, there's no particular reason you have to be able to see it.
Consider navigation on Earth, for example. In the western world, we generally take north as the fundamental direction, and measure bearings with respect to north. But if you had a device that pointed, say, east, that would do just as well -- once you know where east is, just turn 90
o to your left and you're facing north.
EDIT: Grammar.
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#46
by
JohnFornaro
on 17 Oct, 2012 20:27
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You mean +Xε, right?
Yes, dammit! Apparently I know less about cutting and pasting than astrology...
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#47
by
Hungry4info3
on 17 Oct, 2012 20:35
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Yes, dammit! Apparently I know less about cutting and pasting than astrology...
Emphasis mine.
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#48
by
JohnFornaro
on 17 Oct, 2012 20:44
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And about starfinders. Ok, so you pick two or more convenient stars, apparently on brightness? Not necessarily a astrological "constellation", but a constellation in that it's more than one star.
What stars do they pick, typically?
Also, I thought I remembered seeing a surplus rad hardened satellite starfinder for sale on Ebay once. Just took a googol and could only find terrestrial ones. What are the starfinders used on satellites called? Can you buy them?
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#49
by
JohnFornaro
on 17 Oct, 2012 20:46
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Yes, dammit! Apparently I know less about cutting and pasting than astrology...
Emphasis mine.
Yeah. I've misplaced the sign twice now. And I'm
studying the orbital stuff even as we speak!
When the Moon is in the second house,
And Jupiter aligns with Mars....
La la la
la la-la la...
Age of Aquarius.....
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#50
by
JohnFornaro
on 18 Oct, 2012 13:03
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#51
by
sdsds
on 12 May, 2013 04:45
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Has there ever been any analysis of rendezvous during trans-lunar coast? I'm imagining a concept where the crew vehicle leaves Earth-orbit on a free return trajectory and then is met by another (presumably automated) vehicle along the way to the Moon.
The utility of this would come from the other vehicle providing the propulsion for lunar orbit insertion. That other vehicle could also be the lander, with added descent propulsion system propellant to handle the LOI burn.
For this scheme to work there would need to be a cis-lunar trajectory in which the lander could loiter; perhaps that's a continual free-return figure-eight? The crew vehicle would time its trans-lunar insertion so it met up with lander on one of its Earth passes....
Is this wacky? Does it overly constrain the crew vehicle launch or TLI timing?
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#52
by
RocketmanUS
on 12 May, 2013 06:28
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Has there ever been any analysis of rendezvous during trans-lunar coast? I'm imagining a concept where the crew vehicle leaves Earth-orbit on a free return trajectory and then is met by another (presumably automated) vehicle along the way to the Moon.
The utility of this would come from the other vehicle providing the propulsion for lunar orbit insertion. That other vehicle could also be the lander, with added descent propulsion system propellant to handle the LOI burn.
For this scheme to work there would need to be a cis-lunar trajectory in which the lander could loiter; perhaps that's a continual free-return figure-eight? The crew vehicle would time its trans-lunar insertion so it met up with lander on one of its Earth passes....
Is this wacky? Does it overly constrain the crew vehicle launch or TLI timing?
Read the t/Space PDF. Their tankers were to add propellants to the CEV in elliptical lunar orbit.
exploration.nasa.gov/documents/reports/cer_final/tSpace.pdf
Something like an ACES stage could possible do this if it were developed.
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#53
by
sdsds
on 12 May, 2013 07:32
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Has there ever been any analysis of rendezvous during trans-lunar coast? [...] The utility of this would come from the other vehicle providing the propulsion for lunar orbit insertion.
Read the t/Space PDF. Their tankers were to add propellants to the CEV in elliptical lunar orbit.
exploration.nasa.gov/documents/reports/cer_final/tSpace.pdf
You are right, the t/Space concept is similar. The 50x50000 km lunar orbit it uses is entered with a LOI burn of 310 m/s. Perhaps a rendezvous there makes more sense than a rendezvous during trans-lunar coast.
I certainly admire the t/Space proposal for the boldness of its approach! The concept includes
so many innovative ideas!
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#54
by
QuantumG
on 12 May, 2013 07:49
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Page 15 has a refuel in free-return-trajectory mission mode.
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#55
by
sdsds
on 12 May, 2013 21:02
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Page 15 has a refuel in free-return-trajectory mission mode.
Thanks for pointing that out! That they considered it shows the idea isn't too wacky. The comparison chart on page 17 is quite helpful. Does the note that in this mode the CEV, "Requires synchronized launch with Tanker" indicate they are -- or are not -- envisioning something like a continual figure-eight loiter orbit for the tanker? It could be tough to arrange "launching within a few days of one another," if that's what they mean by "synchronized."
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#56
by
RocketmanUS
on 12 May, 2013 21:06
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Page 15 has a refuel in free-return-trajectory mission mode.
Thanks for pointing that out! That they considered it shows the idea isn't too wacky. The comparison chart on page 17 is quite helpful. Does the note that in this mode the CEV, "Requires synchronized launch with Tanker" indicate they are -- or are not -- envisioning something like a continual figure-eight loiter orbit for the tanker? It could be tough to arrange "launching within a few days of one another," if that's what they mean by "synchronized."
CEV's TLI burn would need to take place at a given time that the CEV and tanker would arrive at the same place in space headed to the moon. Tanker would have had it's TLI burn much earlier do to it's path for the weak stability boundary transfer.
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#57
by
QuantumG
on 12 May, 2013 22:09
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Maybe I missed it, but page 9 says:
This performance is possible by Tankers taking a Weak Stability Boundary
Transfer from LEO to an elliptical lunar orbit. Almost no deltaV is needed
to enter lunar orbit using WSBTs, but the round-trip transit is six months.
How were they avoiding boiloff of LH2?
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#58
by
RocketmanUS
on 12 May, 2013 22:37
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Maybe I missed it, but page 9 says:
This performance is possible by Tankers taking a Weak Stability Boundary
Transfer from LEO to an elliptical lunar orbit. Almost no deltaV is needed
to enter lunar orbit using WSBTs, but the round-trip transit is six months.
How were they avoiding boiloff of LH2?
That was my question I had back in 2005 when I read this.
I think that was one of the reasons this was not chosen.
So the WSBT's would work better for hypergolics.
For return or disposal of the tanker it is better to have the propellant transfer in elliptical orbit. However for the lander for a better performance ( down mass to surface ) it would be better to have the propellant transfer in LLO.
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#59
by
MP99
on 13 May, 2013 07:11
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MethaLox should be OK with a sunshield, once outside LEO.
Cheers, Martin
