Mode decision?

Assuming the Moon/Mars/Beyond initiative actually gets into the hardware
and flying stages, there is an important, basic set of decisions to be
made.

The first of which is, what mode do we use to get back to the Moon?

Do we use LOR? Do we use EOR and land the whole vehicle? Do we use a
combination of EOR/LOR? Do we go into lunar orbit first, or do we
approach like Surveyor and most of the Soviet unmanned landers?

Don't we *need* to go into orbit first if we're going to land outside of
the classic equatorial zone?

What do y'all think?

Doug

In article ,
Doug... wrote:
The first of which is, what mode do we use to get back to the Moon?
Do we use LOR? Do we use EOR and land the whole vehicle? Do we use a
combination of EOR/LOR?

Whether orbital assembly (EOR) figures into the plan depends on how big
the launchers are and how the project trades off convenience today against
growth potential tomorrow. (It is more convenient to launch everything in
one piece, but eventually, as the missions grow, you'll have to start
doing orbital assembly, so an emphasis on long-term growth says you might
as well get started now.)

Assembly has some up-front overhead costs, but at present, so does
all-in-one launch, since we have no suitable heavy launchers. (Remember,
a good bit of the money for the plan is supposed to come from closing down
shuttle operations -- that means no shuttle-derived launchers! In any
case, if you must have a heavy launcher, an EELV cluster or a fresh-start
development which *doesn't* use LC-39 is almost certainly *cheaper* than a
shuttle derivative -- yes, even including development cost -- as soon as
the number of launches goes beyond a handful.)

Whether LOR looks good depends on a number of things, notably surface stay
length and the complexity of the surface operations. The more the mission
is dominated by a heavy lander carrying a substantial crew and a lot of
surface equipment and supplies, the less you save by leaving the return
vehicle in orbit. A contributing factor is that for most landing sites,
a particular lunar orbit will be reachable only twice a month due to the
Moon's rotation, which is a problem if you want the option of doing an
emergency return on short notice.

Apollo's choice of LOR reflects Apollo's nominal mission being a short
surface excursion in the middle of a rather longer mission dominated by
cruise/orbit phases. Even the later Apollos, with mere three-day stays,
paid a price in things like constant orbit adjustments. Most recent
proposals assume that a lightweight return vehicle is taken down to the
lunar surface and launches from there.

Do we go into lunar orbit first, or do we
approach like Surveyor and most of the Soviet unmanned landers?
Don't we *need* to go into orbit first if we're going to land outside of
the classic equatorial zone?

No, not really. The only big advantage of the classic equatorial zone is
its compatibility with free-return trajectories.

A straight-in approach saves a bit of fuel and doesn't really constrain
the landing site much. Where it hurts is timing issues and backup/abort
plans: you only get one try and you'd better be ready. If you run into
even small delays, as Apollos 14 and 16 did, you've lost the landing and
you may even have trouble getting home.

On the other hand, if you haven't left anything important in lunar orbit,
there's no reason to stop in orbit on the way *back*.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

In article , says...
In article ,
Doug... wrote:
The first of which is, what mode do we use to get back to the Moon?
Do we use LOR? Do we use EOR and land the whole vehicle? Do we use a
combination of EOR/LOR?

Whether orbital assembly (EOR) figures into the plan depends on how big
the launchers are and how the project trades off convenience today against
growth potential tomorrow. (It is more convenient to launch everything in
one piece, but eventually, as the missions grow, you'll have to start
doing orbital assembly, so an emphasis on long-term growth says you might
as well get started now.)

Assembly has some up-front overhead costs, but at present, so does
all-in-one launch, since we have no suitable heavy launchers. (Remember,
a good bit of the money for the plan is supposed to come from closing down
shuttle operations -- that means no shuttle-derived launchers! In any
case, if you must have a heavy launcher, an EELV cluster or a fresh-start
development which *doesn't* use LC-39 is almost certainly *cheaper* than a
shuttle derivative -- yes, even including development cost -- as soon as
the number of launches goes beyond a handful.)

OK -- this is one of the things I wanted to get discussed. The mode
decision has a huge impact on the launchers you want to use, which in
turn has a huge impact on launcher development programs. My point is
that if you think you're going to go with a given mode, you need to
takeinto account the development program time for the launcher(s) you
want to use. So we need to get the mode wrung out ASAP, I think.

Whether LOR looks good depends on a number of things, notably surface stay
length and the complexity of the surface operations. The more the mission
is dominated by a heavy lander carrying a substantial crew and a lot of
surface equipment and supplies, the less you save by leaving the return
vehicle in orbit. A contributing factor is that for most landing sites,
a particular lunar orbit will be reachable only twice a month due to the
Moon's rotation, which is a problem if you want the option of doing an
emergency return on short notice.

Apollo's choice of LOR reflects Apollo's nominal mission being a short
surface excursion in the middle of a rather longer mission dominated by
cruise/orbit phases. Even the later Apollos, with mere three-day stays,
paid a price in things like constant orbit adjustments. Most recent
proposals assume that a lightweight return vehicle is taken down to the
lunar surface and launches from there.

So -- we can get pretty much anywhere we want at pretty much any time,
as long as we're not worried about free return trjectories? OK... so,
you think NASA (in its current gutless state) will ever commit to a TLI
with no free return capability?

Do we go into lunar orbit first, or do we
approach like Surveyor and most of the Soviet unmanned landers?
Don't we *need* to go into orbit first if we're going to land outside of
the classic equatorial zone?

No, not really. The only big advantage of the classic equatorial zone is
its compatibility with free-return trajectories.

A straight-in approach saves a bit of fuel and doesn't really constrain
the landing site much. Where it hurts is timing issues and backup/abort
plans: you only get one try and you'd better be ready. If you run into
even small delays, as Apollos 14 and 16 did, you've lost the landing and
you may even have trouble getting home.

On the other hand, if you haven't left anything important in lunar orbit,
there's no reason to stop in orbit on the way *back*.

True. Then again, there's a lot to be gained from man-tended orbital
exploration, too, don't you think?

Doug

In article ,
Doug... wrote:
OK -- this is one of the things I wanted to get discussed. The mode
decision has a huge impact on the launchers you want to use...

Mostly the part about whether to do orbital assembly -- specifically, a
decision *not* to do substantial orbital assembly would mean major new
launcher requirements, and the details would become very dependent on the
spacecraft design. (That alone is a strong argument for orbital assembly,
which largely separates spacecraft and launcher issues.)

So -- we can get pretty much anywhere we want at pretty much any time,
as long as we're not worried about free return trjectories? OK... so,
you think NASA (in its current gutless state) will ever commit to a TLI
with no free return capability?

I think it likely that the preferred approach would be the same used for
Apollo: start with free-return trajectories, but gradually move away from
them -- to open up the choice of landing site -- as experience builds up.
Apollo 11 went to the Moon in a free-return trajectory, Apollo 12 started
out in one but moved away from it en route, and Apollo 17 was not in one
at any time. Even the later Apollos stayed relatively *close* to free-
return trajectories for the sake of abort options... but just what "close"
means depends on your performance margins. Many aspects of such missions
get easier if you invest in more generous margins than what Apollo had.

On the other hand, if you haven't left anything important in lunar orbit,
there's no reason to stop in orbit on the way *back*.

True. Then again, there's a lot to be gained from man-tended orbital
exploration, too, don't you think?

Correct, but anything that's designed to shift for itself for a while in
lunar orbit doesn't need to be revisited by the *same mission*. There is
little reason to put people in lunar orbit if all they are going to do is
look at displays and push buttons, which is about all that normally needs
doing for remote-sensing operations; the earlier lunar-orbit assets can
reasonably be unmanned for normal operation. An occasional servicing
visit can be done either by a dedicated mission, or by a landing mission
on the way down.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

(Henry Spencer) wrote:
A contributing factor is that for most landing sites,
a particular lunar orbit will be reachable only twice a month due to the
Moon's rotation, which is a problem if you want the option of doing an
emergency return on short notice.

How does this change if your only return criteria is to hit a reentry
corridor with particularly caring where you come down? (Emergency
recovery forces have three days to deploy, vice the hours which forces
location constraints on LEO emergency deorbits.)

Is the difficulty orbital mechanics, or fuel?

D.
--
Touch-twice life. Eat. Drink. Laugh.

(Henry Spencer) wrote:
Assembly has some up-front overhead costs, but at present, so does
all-in-one launch, since we have no suitable heavy launchers.

The overhead of orbital assembly is more than up front, but continues
throughout the assembly operations phase is the mission is of any
size. You take a performance hit either per launch in the form of the
rendezvous and docking hardware, or across the whole pipeline by using
an OTV.

D.
--
Touch-twice life. Eat. Drink. Laugh.

Doug... wrote:
Assuming the Moon/Mars/Beyond initiative actually gets into the hardware
and flying stages, there is an important, basic set of decisions to be
made.

The first of which is, what mode do we use to get back to the Moon?

No, that's the second decision to be made.

The *first* is; What are we planning to do? Long duration stays?
Single location? Multiple locations? Revisits? etc.. etc.. Once that
basic architecture is laid out, one then decides whether it's better
(by whatever criteria, cost, speed, etc..) to use an existing
launcher, or to develop a new launcher, or to improve an existing
launcher.

Apollo jumped straight to the mode decision because the basic decision
had already been forced upon NASA by Kennedy, and the timeline forced
the use of the hardware already largely under study and/or
development. They didn't have the luxury of a blank hardware sheet
and fully developed and coherent plan because there wasn't time.

D.
--
Touch-twice life. Eat. Drink. Laugh.

In article , derekl1963
@nospamyahoo.com says...
Doug... wrote:
Assuming the Moon/Mars/Beyond initiative actually gets into the hardware
and flying stages, there is an important, basic set of decisions to be
made.

The first of which is, what mode do we use to get back to the Moon?

No, that's the second decision to be made.

The *first* is; What are we planning to do? Long duration stays?
Single location? Multiple locations? Revisits? etc.. etc.. Once that
basic architecture is laid out, one then decides whether it's better
(by whatever criteria, cost, speed, etc..) to use an existing
launcher, or to develop a new launcher, or to improve an existing
launcher.

Apollo jumped straight to the mode decision because the basic decision
had already been forced upon NASA by Kennedy, and the timeline forced
the use of the hardware already largely under study and/or
development. They didn't have the luxury of a blank hardware sheet
and fully developed and coherent plan because there wasn't time.

Well, lessee -- some of the factors that will define the mission have
already been discussed. Things like using lunar resources and building
a permanently manned lunar base have been talked about, I know. And
wouldn't those require landing near the poles and using the water ice
that we think is bound up in the regolith there?

Like I say, I know we have at least *some* of the info we need to start
wrangling this decision. And we're talking about a 10-year program,
right? That's not that much longer than Apollo had.

Doug

In article ,
Derek Lyons wrote:
A contributing factor is that for most landing sites,
a particular lunar orbit will be reachable only twice a month due to the
Moon's rotation, which is a problem if you want the option of doing an
emergency return on short notice.

How does this change if your only return criteria is to hit a reentry
corridor with particularly caring where you come down?

Not at all. The problem is that, to a first approximation, a given site
on the Moon has a launch window to a particular lunar orbit only as the
Moon's rotation carries the site through the orbit's plane. But the Moon
rotates only once a month! Except in a couple of special cases(*), you
only get two launch windows a month (assuming orbit inclination equals or
exceeds latitude, because otherwise you don't get *any*), and typically
they aren't even equally spaced.

(* An equatorial site is always in the plane of an equatorial orbit, of
course, and likewise a polar site is always in the plane of a polar
orbit.)

There is a further complication that any particular lunar orbit, unless
it's near-equatorial, has only two *Earth* launch windows a month --
roughly speaking, only when Earth passes through the orbit's plane, and
the Earth goes around the Moon :-) only once a month.

(The plane of a lunar orbit is essentially fixed in space. The Moon does
not have enough of an equatorial bulge to precess orbits at a significant
rate the way Earth does.)

Hitting a particular spot on Earth is very much a detail by comparison.
Unless you are in a big hurry and want the fastest possible trajectory,
that can usually be arranged from almost any departure, given some
aerodynamic maneuverability during reentry.

Is the difficulty orbital mechanics, or fuel?

Yes. :-) Almost any orbital-mechanics problem becomes trivial given
arbitrary amounts of fuel.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Doug... wrote:
Well, lessee -- some of the factors that will define the mission have
already been discussed.

Things like using lunar resources and building a permanently
manned lunar base have been talked about, I know.

'Discussed' != 'Defined'. Without definition, the best that mode
discussion can do is provide background information to feed back into
the mission definition process.

Like I say, I know we have at least *some* of the info we need to start
wrangling this decision.

Yes, and it can be stated in six words. "let's go back to the moon".
Not even remotely enough information to make a mode decision with any
certainty.

we're talking about a 10-year program, right? That's not that much longer
than Apollo had.

Apollo had a mission and a goal, the current incarnation doesn't
really. Also, the current incarnation has ten years to the first
flight, not ten years to end of program, a considerable difference.

D.
--
Touch-twice life. Eat. Drink. Laugh.