Re: LM question

From: Henry Spencer (henry_at_spsystems.net)
Date: 03/09/05 

Date: Wed, 9 Mar 2005 02:49:04 GMT

In article <p5gs21hik5un8dpnb7td9vpos8lq5fiogo@4ax.com>,
Kelly McDonald <kellymcdonald@<no-spam> wrote:
>>ready to launch an Apollo every two months (every second launch window),
>
>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.

Details of orbital mechanics dictated the time of day. Loosely speaking,
the point in space where the departure burn had to be done was dictated by
the Moon's position, and the parking orbit had to pass through that point.
You could make up for an early or late launch, within limits, by varying
the launch direction and thus the inclination of the parking orbit. The
limits were set mostly by range-safety constraints (the early flight path
had to be over ocean).

For a while, there was a notion that you could widen the windows by
training the crew for a landing at any of several sites, so a scrubbed
launch could be accommodated by retargeting for a site farther west a few
days later. This was never really done, because there just wasn't enough
time for the extra training -- as it was, training ran late enough,
because of problems like the unreliability of the simulators, that the
10 and 11 crews were only barely flight-ready. (There was serious
consideration of slipping 11 to the August launch window to give more
training time.) The one vestige of this concept that survived was the
choice of an eastern site as the first target. And of course, the whole
idea was dropped after the first landing was accomplished, and precision
landing at a preselected point became the order of the day.

Later on, as targets were increasingly chosen on scientific merits and the
system was tuned for maximum performance, there started to be time-of-year
constraints for sites that were only barely reachable. 

-- 
"Think outside the box -- the box isn't our friend."    |   Henry Spencer
                                -- George Herbert       | henry@spsystems.net
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