LM/LK ascent question

I was thinking about this a few days back, and realized I didn't have a
clue about how a particular aspect of ascending from the surface of the
Moon back into orbit for the docking with the Apollo CSM or Soviet LOK
was handled.
Once landed, the LM or LK could lift off into lunar orbit in pretty much
any orbital inclination due to the low rotational rate of the Moon.
So how was the correct orbital inclination for liftoff determined?
Land on the Moon with your spacecraft facing 5 degrees from its intended
directional bearing in regard to what you can see out of its windows in
a N-S-E-W direction, and unless you adjust something it will ascend into
a orbit that's 5 degrees off of what was intended...which may exceed the
ability of the CSM or LOK to change its orbital inclination to
rendezvous with you.
So, once landed, did you adjust something to get the guidance system
aligned with your intended ascent trajectory in a horizontal bearing
sense to make sure you were heading for the right orbital inclination on
liftoff?

Pat

"Fairly complicated - depending on exactly where the CSM was at the time of
lift-off, considering cases where lift-off time was driven by something
other than CSM orbital location, there were at least three diffeerent
scenarios for rendezvous orbital change and phasing maneuvers. I think the
answer to your basic question is yaw the vehicle on lift-off and you'r
facing whereever you want.

On Feb 7, 7:11*am, Pat Flannery wrote:
I was thinking about this a few days back, and realized I didn't have a
clue about how a particular aspect of ascending from the surface of the
Moon back into orbit for the docking with the Apollo CSM or Soviet LOK
was handled.
Once landed, the LM or LK could lift off into lunar orbit in pretty much
any orbital inclination due to the low rotational rate of the Moon.
So how was the correct orbital inclination for liftoff determined?
Land on the Moon with your spacecraft facing 5 degrees from its intended
directional bearing in regard to what you can see out of its windows in
a N-S-E-W direction, and unless you adjust something it will ascend into
a orbit that's 5 degrees off of what was intended...which may exceed the
ability of the CSM or LOK to change its orbital inclination to
rendezvous with you.
So, once landed, did you adjust something to get the guidance system
aligned with your intended ascent trajectory in a horizontal bearing
sense to make sure you were heading for the right orbital inclination on
liftoff?



The guidance knew where the LM was and the CSM location was uploaded.
Also the LM rendezvous radar provided updates and would be the prime
instrument if MCC comm was lost.

Pat,

It's a little scary that I was thinking about the same thing,
wondering if a roll program would be used to establish the correct
orbital inclination as it was with the Saturn V and the STS.

My question is how was the required amount of roll calculated,
especially for Apollo 11 where there seemed to be a lot of uncertainty
about exactly where on the lunar surface Eagle had landed.

A number of the descents made by LM's to the lunar surface did include
the crew taking manual control to retarget exactly where the vehicle
came down. It seems if the LM was yawed at any point in the descent,
it would no longer be facing in the direction of the initial descent
orbit's inclination. I am presuming that the C/SM would remain in the
same orbital inclination as it was in at the time of LM separation to
simplify the task of descending to assist the LM if something went
sour during either descent or ascent. So how did they calculate and
implement the needed correction to bet back up into a correctly
inclined orbit?

Take care all . . .

John

John wrote:
Pat,

It's a little scary that I was thinking about the same thing,
wondering if a roll program would be used to establish the correct
orbital inclination as it was with the Saturn V and the STS.

My question is how was the required amount of roll calculated,
especially for Apollo 11 where there seemed to be a lot of uncertainty
about exactly where on the lunar surface Eagle had landed.

That's the part I was trying to figure out. Also,landing on a slightly
inclined surface could screw things up when it came time for liftoff.

A number of the descents made by LM's to the lunar surface did include
the crew taking manual control to retarget exactly where the vehicle
came down. It seems if the LM was yawed at any point in the descent,
it would no longer be facing in the direction of the initial descent
orbit's inclination. I am presuming that the C/SM would remain in the
same orbital inclination as it was in at the time of LM separation to
simplify the task of descending to assist the LM if something went
sour during either descent or ascent. So how did they calculate and
implement the needed correction to bet back up into a correctly
inclined orbit?


I can think of two ways of getting everything lined up to assure it goes
into the proper inclination orbit on ascent:

1.) The guidance gyros are kept spun up during the entire time it is on
the lunar surface, so that it knows where it is and exactly which way it
is facing, although gyro drift might get things a bit off kilter with time.

2.) The gyros are spun up and information put into them from star
observations using some sort of telescope/sextant mounted in the roof of
the ascent stage.

This page states that although the Command Module did have a sextant in
it, primary navigation was via radar observations from Earth with the
data being relayed up to the astronauts:
http://www.irbs.com/lists/navigation/0405/0009.html
I don't know if that applied to the LM though, and suspect they just
kept their guidance system running during the whole stay on the surface.
The Soviet LK moon lander did have something that looked like a
periscopic sextant mounted on it to the left of the pilot, and extending
through the exterior of the pressure hull:
http://www.astronautix.com/graphics/l/lkcolor.jpg
Sometimes a similar device is shown attached to the upward/forward
looking porthole directly above the pilot:
http://www.astronautix.com/graphics/l/lkyanbig.jpg
http://www.astronautix.com/graphics/l/lkextkor.jpg
Although it looks like it has small disc-shaped antennas of some sort
mounted on it also.

Pat

On Feb 8, 3:51*pm, Pat Flannery wrote:
John wrote:

* Pat,
*
* It's a little scary that I was thinking about the same thing,
* wondering if a roll program would be used to establish the correct
* orbital inclination as it was with the Saturn V and the STS.
*
* My question is how was the required amount of roll calculated,
* especially for Apollo 11 where there seemed to be a lot of uncertainty
* about exactly where on the lunar surface Eagle had landed.

That's the part I was trying to figure out. Also,landing on a slightly
inclined surface could screw things up when it came time for liftoff.

* A number of the descents made by LM's to the lunar surface did include
* the crew taking manual control to retarget exactly where the vehicle
* came down. *It seems if the LM was yawed at any point in the descent,
* it would no longer be facing in the direction of the initial descent
* orbit's inclination. *I am presuming that the C/SM would remain in the
* same orbital inclination as it was in at the time of LM separation to
* simplify the task of descending to assist the LM if something went
* sour during either descent or ascent. *So how did they calculate and
* implement the needed correction to bet back up into a correctly
* inclined orbit?

I can think of two ways of getting everything lined up to assure it goes
into the proper inclination orbit on ascent:

1.) The guidance gyros are kept spun up during the entire time it is on
the lunar surface, so that it knows where it is and exactly which way it
is facing, although gyro drift might get things a bit off kilter with time.

2.) The gyros are spun up and information put into them from star
observations using some sort of telescope/sextant mounted in the roof of
the ascent stage.

This page states that although the Command Module did have a sextant in
it, primary navigation was via radar observations from Earth with the
data being relayed up to the astronauts:http://www.irbs.com/lists/navigation/0405/0009.html
I don't know if that applied to the LM though, and suspect they just
kept their guidance system running during the whole stay on the surface.
The Soviet LK moon lander did have something that looked like a
periscopic sextant mounted on it to the left of the pilot, and extending
through the exterior of the pressure hull:http://www.astronautix.com/graphics/l/lkcolor.jpg
Sometimes a similar device is shown attached to the upward/forward
looking porthole directly above the pilot:http://www.astronautix.com/graphics/...l/lkextkor.jpg
Although it looks like it has small disc-shaped antennas of some sort
mounted on it also.

Pat

Pat,

Regarding keeping the gyros spinning, I agree, gyro reccession would
seem to be a source of error, but sextant sightings with an adjustment
to account for the moon's motion over the span of the stay could be
the answer. Although I have no idea how much current it would take to
keep the gyros up to speed, it seems that keeping them spinning may be
an issue for the LM which only had batteries as a source of power. It
may be that the draw was minimal.

w/r

John

John wrote:

Pat,

Regarding keeping the gyros spinning, I agree, gyro reccession would
seem to be a source of error, but sextant sightings with an adjustment
to account for the moon's motion over the span of the stay could be
the answer. Although I have no idea how much current it would take to
keep the gyros up to speed, it seems that keeping them spinning may be
an issue for the LM which only had batteries as a source of power. It
may be that the draw was minimal.

I found out some more on how it was done; between the two astronauts in
the LM sits the Alignment Optical Telescope, running up to a periscopic
head atop the LM ascent stage mounted just behind the rendezvous radar dish.
It's visible in this cutaway:
http://www.fi.edu/pieces/schutte/moonpics/LMconfig.gif
Directly behind it sits the Inertial Measuring Unit:
http://www.fi.edu/pieces/schutte/moonpics/ascentext.gif
The astronauts take star sightings and feed the info into the Inertial
Measuring Unit to get everything aligned as to the LM's position and
orientation on the Moon's surface: http://history.nasa.gov/alsj/aot.htm
So they can indeed update their guidance system to compensate for gyro
drift while on the lunar surface.

Pat

On Feb 8, 3:51*pm, Pat Flannery wrote:
John wrote:

* Pat,
*
* It's a little scary that I was thinking about the same thing,
* wondering if a roll program would be used to establish the correct
* orbital inclination as it was with the Saturn V and the STS.
*
* My question is how was the required amount of roll calculated,
* especially for Apollo 11 where there seemed to be a lot of uncertainty
* about exactly where on the lunar surface Eagle had landed.

That's the part I was trying to figure out. Also,landing on a slightly
inclined surface could screw things up when it came time for liftoff.

* A number of the descents made by LM's to the lunar surface did include
* the crew taking manual control to retarget exactly where the vehicle
* came down. *It seems if the LM was yawed at any point in the descent,
* it would no longer be facing in the direction of the initial descent
* orbit's inclination. *I am presuming that the C/SM would remain in the
* same orbital inclination as it was in at the time of LM separation to
* simplify the task of descending to assist the LM if something went
* sour during either descent or ascent. *So how did they calculate and
* implement the needed correction to bet back up into a correctly
* inclined orbit?

I can think of two ways of getting everything lined up to assure it goes
into the proper inclination orbit on ascent:

1.) The guidance gyros are kept spun up during the entire time it is on
the lunar surface, so that it knows where it is and exactly which way it
is facing, although gyro drift might get things a bit off kilter with time.

2.) The gyros are spun up and information put into them from star
observations using some sort of telescope/sextant mounted in the roof of
the ascent stage.

This page states that although the Command Module did have a sextant in
it, primary navigation was via radar observations from Earth with the
data being relayed up to the astronauts:http://www.irbs.com/lists/navigation/0405/0009.html
I don't know if that applied to the LM though, and suspect they just
kept their guidance system running during the whole stay on the surface.
The Soviet LK moon lander did have something that looked like a
periscopic sextant mounted on it to the left of the pilot, and extending
through the exterior of the pressure hull:http://www.astronautix.com/graphics/l/lkcolor.jpg
Sometimes a similar device is shown attached to the upward/forward
looking porthole directly above the pilot:http://www.astronautix.com/graphics/...l/lkextkor.jpg
Although it looks like it has small disc-shaped antennas of some sort
mounted on it also.

Pat


LM had a periscopic sextant also.

John wrote:
Pat,

It's a little scary that I was thinking about the same thing,
wondering if a roll program would be used to establish the correct
orbital inclination as it was with the Saturn V and the STS.

My question is how was the required amount of roll calculated,
especially for Apollo 11 where there seemed to be a lot of uncertainty
about exactly where on the lunar surface Eagle had landed.

A number of the descents made by LM's to the lunar surface did include
the crew taking manual control to retarget exactly where the vehicle
came down. It seems if the LM was yawed at any point in the descent,
it would no longer be facing in the direction of the initial descent
orbit's inclination. I am presuming that the C/SM would remain in the
same orbital inclination as it was in at the time of LM separation to
simplify the task of descending to assist the LM if something went
sour during either descent or ascent. So how did they calculate and
implement the needed correction to bet back up into a correctly
inclined orbit?

The short answer is that the LM computed the required yaw program using
its IMU attitude and the current LM and CSM state vectors. The long
answer requires some explanation of how the IMU attitude and the state
vectors were maintained valid.

Prior to entering the powered descent braking program (P-63), the LM's
computer was maintaining state vectors for both the LM and the CSM. Once
P-63 was entered, I believe the computer stopped updating the CSM state
vector but the vector was still there with a valid timetag so it could
be predicted forward to the current time if required (and for an abort,
it would be), as long as the CSM performed no maneuvers. The LM state
vector was, of course, continuously propagated throughout all the
powered descent programs (63, 64, and 66) using the Average-G routine.

Snapping the landing site vector was handled differently in the PGNS and
the AGS. After the "Stay at T1" call, the crew would exit P-66 and run
the Landing Confirmation Program (P-68) in the PGNS. This set the lunar
surface flag, transformed the LM state vector to moon-fixed coordinates
and stored it. It would also store the current IMU attitude. For the
AGS, the LMP would set register 413 immediately post-landing which would
set the AGS lunar surface flag and store the LM azimuth for future
attitude alignments (hence the cryptic "413 is in" call that the LMP
would make during the postlanding checklist). After "Stay at T1" the LMP
would set register 414 which overwrote the AGS state vector with the
targeted landing site location.

The LGC and the IMU were typically powered off during lunar stay, but
the LGC's erasable memory was non-volatile so the landing site vector
would be preserved. The IMU could be re-aligned by several different
methods: two AOT star sightings, one AOT star sighting plus gravity
vector (determined by IMU accelerometers), gravity vector plus stored
axis, or two stored axes. The stored axis methods simply assumed the LM
body axes had not moved (in moon-fixed coordinates) since the last
align. The landing site vector can also be refined using the gravity
vector; barring mascons it should always point at the moon center.

MCC used ground stations to update the CSM state vector and uplinked it
to both the CSM and LM. They would also take the downlinked LM vector
and uplink it back to the CSM to provide a starting point for sextant
tracking, which could be used to further refine the landing site vector
(though many CMPs had difficulty tracking the LM with the sextant while
it was on the surface).

MCC had various other sources of data to compute an updated landing site
vector for the LM, including the onboard nav state vectors, the ground
(PFP) state vector, map/crew survey, and the aforementioned gravity
vector methods. They would weigh the data according to their confidence
and uplink an updated LM vector at touchdown+1 hour, which would be used
for a liftoff at T3 if required.

The CSM orbit would not remain constant during lunar surface stay. In
particular, a plane change maneuver was required (for all but Apollo 11)
to rotate the CSM orbital plane over the landing site, due to the slow
rotation of the moon. Mascons also perturbed the CSM orbit. After each
maneuver, MCC would perform a tracking pass and uplink the updated CSM
state vector to both the CSM and the LM.

Two hours prior to liftoff, the LM performed a tracking pass on the CSM
using the rendezvous radar, which would provide an updated CSM state
vector for the LM.

The nominal ascent program (P-12) began with a vertical rise phase
during which the LM would rotate to the proper azimuth to line up with
the CSM orbital plane. Due to the orientation of the LM this was called
a yaw program rather than a roll program. The LM would then pitch within
that plane to ascend into orbit.

I agree! Thanks Jorge.

Did you ever consider posting regular snippets like this to the group?
Or maybe to sci.space.tech? You know something like, topic for this
month is ...?

Throw a bone to us unwashed every now and then so we could learn something?

I mean we all enjoy ribbing Pat now and then, but no offense to Pat it's a
poor substitute for content.

Ok, I know, I know, the *real* answer is buy my book....

You have a bibliography handy? [There you were asked, it's not a violation
of Usenet rules if you were asked!]

;-)

Dave