JF Mezei wrote on Tue, 21 May 2019
19:28:14 -0400:

On 2019-05-20 22:05, Fred J. McCall wrote:

You understand that the Gateway isn't in an orbit around the Moon,
right? It's in a LaGrange halo orbit.

Was not aware of that. I was under the impression it was in lunar orbit.


I've explained this before. You should have been aware.


How does one get to La Grange from earth orbit? Raise elliptical orbit
to have apogee at La Grange and when you reach this, you circularize
orbit with angular speed around the earth that matches the moon's
angular speed around earth ?

Or just shoot up vertically such that you decelerate to 0 at La Grange
alttude and let Moon pull you forward as it rotates around the earth to
give you a circular orbit (matching moon's rotation so that you remain
between moon and earth) ?


You're a bit confused. There are five LaGrange points, not just one.
You're apparently thinking of L1. The planned orbit is around L2,
which is beyond the Moon. To be specific, the planned orbit would be
what is called a Near Rectilinear Halo Orbit around L2 such that it
would pass very low over the Moon's North Pole, somewhat higher (and
slower) over the Moon's South Pole, then out beyond the Moon to 'halo'
around L2. Now, halo orbits tend to be unstable, so require constant
powered adjustment to maintain.

Here, read this, watch the video, and read the cited papers.

https://space.stackexchange.com/ques...ear-halo-orbit


If the lunar lander ends up traveling from surface to Gateway in "direct
ascent", would that one be straight up ? or also raising orbit?


It could be anything, depending on where on the Moon you land. The
'best' current target for exploration is the South Pole, where we
think there is a lot of water ice.


The direct Ascent makes sense if you have weekly flights from La Grange
to moon orbit, but landers stay on moon for a month. Once the transfer
shuttle has dropped the lander in moon orbit, it can return to La Grange
to pick another lander to deliver to a different moon orbit (so first
lander wouldn't be able to catch a ride on it).


I'm missing something in your thought process here because the
preceding doesn't make sense. The Gateway is going to make its fast
and low north to south orbital segment around the Moon about every six
days. The Transfer Element(s) are going to take Lander(s) from the
NRHO of the Gateway to a low circular orbit about the Moon. From
there the Lander Element separates and lands. The Transfer Element
can't really get back to the Gateway for six days. So let's say it
does that because we want the Lander Element to spend roughly a month
on the surface. So they refuel it, mate with a new Lander, and 6 days
later (12 from the first Lander) it repeats and deploys a second
Lander. Another 12 days later and it deploys a third Lander. This
time instead of returning to the Gateway, it waits around. The first
Lander is 24 days into its mission. Six days later (during the next
Gateway pass) the first Lander flies up and rendezvous with the
Transfer Element, which then transfers back to the Gateway. Repeat
the same schedule an it recovers the other two Landers. You only have
a problem if one Lander wants to stay down N days and another lander
wants to stay down N-12 days, because then you have two Landers trying
to return at the same time.

Keep in mind that Gateway isn't all that big so you're not going to
have dozens of Landers and Transfer Elements available.


But I have to wonder about Gateway.

You not only have to carry the cargo to Gateway, but also the weight of
the stage that gets you there. So if you need 10 trips to fuel a ship at
Gateway, you are also raising 10 stage 2 (and the fuel needed for that
stage 2 to de-orbit).

If you do this from LEO, you have much smaller role for stage2, and you
end up raising to La Grange altitude only the final fully
assembled/fueled ship.


Or you don't do that at all (remember, we're talking about L2 here),
which is why I said there was something like a 20% 'cargo penalty'
when you used the Gateway rather than going directly to the Moon.


Say Gateway is supplied by only the Shuttle and Shuttle is grounded for
2 years. How long before it starts to fall towards Earth or moon?
aka: are frequent thurster adjustments needed or can the object be left
inert at that location for very long time ?


Shuttle couldn't get to the Gateway orbit even if it was still flying.
Gateway is intended to get something like one resupply mission a year.

pretty much limited to Orion duration. It does allow reuse of the
Descent and Transfer Elements, but it seems like all the fuel for them
will have to be lifted from Earth in the near term.

Doesn't the lamnder fiunction like the LEM, by leaving the lander on the
moon and ascending back to Gateway with only an "upper stage"? Or are
the modular section pr NASA graphics expected to remain attached as one
unit ?


I mistyped. That should have read "Ascent and Transfer Elements".


And if there a compelling reason while the lander couldn't leave from
Gateway and drop down to surface?


Other than that you would need (and throw away) a much larger and more
capable Descent Element because now it needs the 'grunt' to get from
NRHO to LLO in addition to what it needs to get from LLO to the
surface.


By how much would the transfer vehicle reduce the mass of the lander
compared to a lander that has the bigger tanks to be able to make the
journey on its own ? Is this some hiuge difference, or is the difference
small enough that both concepts get investigated ?


How many million dollars do you have that you can give me to develop
this answer for you?


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn