Re: LM question

From: Doug... (dvandorn1_at_NOSPAM.mn.rr.com)
Date: 03/12/05 

Date: Sat, 12 Mar 2005 10:35:28 GMT

In article <d0th32$t1c$1@news01.intel.com>, Philip Lantz
<"prl"@ibeam,intel.com> says...
> Henry Spencer wrote:
> >
> > In article <p5gs21hik5un8dpnb7td9vpos8lq5fiogo@4ax.com>,
> > Kelly McDonald <kellymcdonald@<no-spam> wrote:
> > >
> > >Talking about launch windows, what was the criteria involved in
> > >determining them? Were they based on mission rules, or orbital
> > >mechanics? Did they vary based on planned landing location?
> >
> > For the early flights, the main issue was sun angle at the landing site:
> > to have the Sun low on the horizon (so surface obstacles would cast
> > conspicuous shadows) behind the LM (so the pilot could see the LM's shadow
> > on the surface, giving him a sense of scale on terrain with no familiar
> > objects) required landing soon after dawn at the landing site. So the
> > windows came once a month for any particular site.
>
> How long was the launch window for a single site? Was the sun angle
> within the range they were looking for for only a single day?

IIRC, optimum launch windows were on the order of six to eight hours
long for a given site on a given day. (Times varied, of course, from
flight to flight.)

One thing to remember is that Apollo flew a long coast out to the Moon.
By varying the velocity and aiming just a little bit, you could adjust
your translunar injection burn to make up for a launch delay of up to
several hours.

For example, the launches of Apollos 14 and 17 were delayed -- I don't
remember the exact delay times off the top of my head, but IIRC 14 was
delayed on the order of 45 minutes and 17 was delayed something like two
or three hours. In both cases, Houston adjusted the TLI burn and the
translunar trajectory such that the spacecraft entered lunar orbit at
the pre-planned time (within a few seconds, anyway).

On the earlier flights, they pretty much limited a launch window for a
given landing site to a single day out of the month, yes. The thinking
was that if you scrubbed on the planned day, you just did a two-day
recycle and landed at the backup landing site. If you had a problem
that delayed you more than two days, you just emptied all the tanks,
fixed your problem, and waited until the following month to try again
for your prime landing site.

On later flights, when there were no backup landing sites and you had to
land at your prime site or wait until next month, they eased some of the
restrictions and allowed for landings at higher sun angles. Starting
with Apollo 13, they could support a one-day launch recycle with a
combination of a quicker translunar coast and allowing the crew to land
between 12 and 24 hours after the planned time (with sun angles somewhat
higher than planned). They had to add special thermal coatings to the
ascent stages, especially on the longer-stay J missions (Apollos 15-17),
to ensure the LM could easily handle the increased solar heating (and
heating on surfaces that otherwise would be in the shade) resulting from
landing late.

But these were just some of the contingency plans. The mission planners
put together what were called contingency missions for each Apollo
flight. There were contingency missions designed for a variety of
definable situations. If a crew couldn't leave earth orbit, for
example, the plan was to dock with the LM, remove photographic equipment
from it, dump it and then spend six days or so doing earth resources
research from LEO. Another example that almost occurred on Apollo 16
was the situation in which, after the spacecraft entered lunar orbit,
the SPS engine becomes unreliable or unusable. In that case, they would
spend from one to six days in lunar orbit, performing orbital science.
Then they would use the LM's DPS engine to break out of lunar orbit and
head back to earth.

I always found it interesting that the contingency mission for the
situation in which you couldn't dock with your LM just after TLI was to
continue on, CSM-only, to complete a lunar orbital mission. After
Apollo 13, I would have thought they would have been a little gun-shy
about sending a CSM-only spacecraft all the way out to the Moon, with no
possibility of a lifeboat if the CSM went to worms... Granted, after
Apollo 13 they redesigned the power and oxygen systems such that a CSM
could survive a 13-style accident and get home without needing to use
the LM as a lifeboat -- but still, it seems like a CSM-only lunar
orbital mission (except for Apollo 8, which was a political necessity)
was pushing it a little bit close to the edge. 

-- 
"The problem isn't that there are so   |   Doug Van Dorn
many fools; it's that lightning isn't  |   dvandorn1@NOSPAM.mn.rr.com
distributed right."  -Mark Twain
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