Parking Orbit

On Fri, 17 Feb 2006 18:12:56 -0600, in a place far, far away, "Jorge
R. Frank" made the phosphor on my monitor glow
in such a way as to indicate that:

D'oh!. I meant to say 180 degrees in half a lunar month, or 360 a
month. Or any multiple of that, since I don't require a constant angle
with respect to to the system. I want to arrange the precession so
that every time the lauch window opens to a particular lunar orbit,
the moon is in the plane of the parking orbit.

Then I'm afraid you're out of luck. Regression for a 100 nmi equatorial
orbit is nine degrees per solar day (less than 270 degrees per lunar
month), and the effect decreases with both altitude and inclination.

I thought he was referring to a lunar orbit, not an earth orbit.

On Fri, 17 Feb 2006 20:26:20 -0600, in a place far, far away, "Jorge
R. Frank" made the phosphor on my monitor glow
in such a way as to indicate that:

I thought he was referring to a lunar orbit, not an earth orbit.

Ah. If so, mea culpa.

I should add, that I'm guessing that this question arose in the
context of a recent discussion at my blog, which may (or may not)
clarify things:

http://www.transterrestrial.com/archives/006473.html

On Fri, 17 Feb 2006 20:33:47 -0600, in a place far, far away, "Jorge
R. Frank" made the phosphor on my monitor glow
in such a way as to indicate that:

I thought he was referring to a lunar orbit, not an earth orbit.

Ah. If so, mea culpa.

I should add, that I'm guessing that this question arose in the
context of a recent discussion at my blog, which may (or may not)
clarify things:

http://www.transterrestrial.com/archives/006473.html

Context is everything.

If we're talking about a lunar orbit, I agree L1 beats low lunar orbit, not
least because of the instability of LLO.

Right. Mascons are nasty things...

On Fri, 17 Feb 2006 20:40:45 -0600, in a place far, far away, "Jorge
R. Frank" made the phosphor on my monitor glow
in such a way as to indicate that:

I should add that the only reason I interpreted Will's question to be about
LEO rather than LLO was because he specified "reasonably efficent to reach
from KSC". That makes a big difference for LEO, not so much for LLO.

Yes, well, your original interpretation may be correct, though I don't
then really understand the question, given the context. Perhaps he
will clarify shortly.

Rand Simberg wrote:

Yes, well, your original interpretation may be correct, though I don't
then really understand the question, given the context. Perhaps he
will clarify shortly.

I should have given more context earlier. I'm thinking about ways,
based on the hardware NASA plans to build, to do the lunar mission more
efficiently.

The first is a LEO propellant depot. If commercial launchers can get
propellant there for less than NASA's marginal cost on the big booster,
it makes sense to refuel there and take more payload with each launch.

That raise a constraint. Suppose you are trying to reach a specific
site on the lunar surface between the equator and the pole by LOR and
you have two opportunities a lunar month. If your earth parking orbit
lines up for the first window, it may not for the second because of
precession. Based on Jorge's post it looks like you can't tailor the
parking orbit to get it to precess 180 degrees in half a lunar month
but should be able to do 180 degrees in a month. Your launch windows
from the depot would be a lunar month apart.

The same issue arises if you are doing the mission with two launches
and EOR, and your second launch misses the window for some reason.

My next question is, how close to optimum can you make such an orbit
from KSC by trading inclination and altitude? You don't want it to be
higher inclination or higher altitude than necessary.

Will McLean

Rand Simberg wrote:

Yes, well, your original interpretation may be correct, though I don't
then really understand the question, given the context. Perhaps he
will clarify shortly.

I should have given more context earlier. I'm thinking about ways,
based on the hardware NASA plans to build, to do the lunar mission more
efficiently.

The first is a LEO propellant depot. If commercial launchers can get
propellant there for less than NASA's marginal cost on the big booster,
it makes sense to refuel there and take more payload with each launch.

That raise a constraint. Suppose you are trying to reach a specific
site on the lunar surface between the equator and the pole by LOR and
you have two opportunities a lunar month. If your earth parking orbit
lines up for the first window, it may not for the second because of
precession. Based on Jorge's post it looks like you can't tailor the
parking orbit to get it to precess 180 degrees in half a lunar month
but should be able to do 180 degrees in a month. Your launch windows
from the depot would be a lunar month apart.

The same issue arises if you are doing the mission with two launches
and EOR, and your second launch misses the window for some reason.

My next question is, how close to optimum can you make such an orbit
from KSC by trading inclination and altitude? You don't want it to be
higher inclination or higher altitude than necessary.

Will McLean

On 18 Feb 2006 06:31:25 -0800, in a place far, far away, "Will McLean"
made the phosphor on my monitor glow in such a
way as to indicate that:

My next question is, how close to optimum can you make such an orbit
from KSC by trading inclination and altitude? You don't want it to be
higher inclination or higher altitude than necessary.

I suspect that the politically incorrect answer is to remove the
constraint that it be operable out of KSC. A lower inclination would
be better, with equatorial being the best.

Will McLean; The first is a LEO propellant depot. If commercial launchers
can get propellant there for less than NASA's marginal cost on the big booster,
it makes sense to refuel there and take more payload with each launch.

And once again, what's the big-ass problem with using the 24/7 LL-1
sweet-spot?
-
Brad Guth

In article .com,
"Brad Guth" wrote:

Will McLean; The first is a LEO propellant depot. If commercial launchers
can get propellant there for less than NASA's marginal cost on the big
booster,
it makes sense to refuel there and take more payload with each launch.

And once again, what's the big-ass problem with using the 24/7 LL-1
sweet-spot?
-
Brad Guth

Getting there.

As to what's the big-ass problem with using the 24/7 LL-1 sweet-spot?

Orval Fairbairn; Getting there.

But all significant missions (other than on behalf of our nuking Earth)
are of those supposedly going far beyond our moon, and the moon itself
is actually offering quite a nifty gravity pull on behalf of getting
loads of stuff into the LL-1 pocket, as well as on behalf of
subsequently transfering such missions past the moon at the greatest
possible exit velocity, whereas all that's necessary via terrestrial
deployments as headed to getting parked within LL-1 is whatever little
energy it takes for barely coasting such tonnage into that sweet-spot.
Therefore, given more days in route, 100t per Saturn-V and perhaps
twice again for a fully LRB first and second stage worth of h2o2/c3h4o
with only the third stage of LOX/LH2 should do that trick at far less
than half if not a fourth the horrific inert mass of what the old
Saturn-V represented.

Again, we're talking about using the least amount of rocket energy per
delivered tonnage into an efficient station-keeping orbit, and since
it's mostly intended for robotics to deal with, there's certainly no
hurry in getting there.

It's actually your NASA/Apollo mindset that's insisting such tonnage
via their old and extremely inert massive Saturn-V was supposedly worth
accomplishing 51t getting entirely past LL-1 and even into orbiting our
moon, and that's along with initially hauling their LES at 4.17t for
the first two stages worth of the launch, and having made considerably
better velocity as per passing that tonnage clean through LL-1 at good
enough speed in just 2.5 days, therefore obviously having taken much
greater energy expenditure than a purely robotic delivery of bulk
rocket fuel to the LL-1 depot would otherwise have demanded.

And once again I'll have to ask, what's the big-ass problem with using
the 24/7 LL-1 sweet-spot?

If serious push came down to shove (such as for a manned mission),
perhaps a 24 hour time from the surface of Earth to reaching LL-1 along
with a butt load of retro-thrusters blazing away seems doable.
Obviously without having significant retrothrust as per velocity
breaking involved, the time from Earth to coasting gradually into home
plate at a few meters/second and thus slowly arriving into LL-1 might
be configured as taking anywhere from 5 to 7.5 days worth. Though
what's the hurry?
-
Brad Guth