On Mon, 16 Sep 1996 13:34:43 -0400, Raghu Tumkur <raghu.tum...@omitron.com>
wrote:
>I was wondering what the reason was for the launch window
>being so small on the Mir Docking missions? Is it to try
>and get to the Mir in the most fuel-efficient route?
Here's a long-winded explanation I wrote last summer (updated to today) of
how we design
shuttle launch windows. It should explain your question, and
then some.
There are a number of different constraints on launching a ground-up
rendezvous mission; among them are:
* the orbiter's phasing capability
(depends on the phase angle between the shuttle and target at
launch and the OMS-2 insertion altitude)
* ascent performance margin
(how much MPS propellant the orbiter can spare in order to go
out-of-plane)
* ET impact
(can't drop it too close to any land mass)
For a Mir rendezvous mission, we first protect for ascent performance
margin (APM): we budget 2000 lbs of MPS propellant to steer out-of-plane.
It just so happens that 3.5 minutes on either side of the in-plane time
costs 2000 lbs. The further away you go from the inplane time, the more it
costs, and it gets steeper: for example, it costs 1000 lbs for the first
2.5 minutes, and 1000 lbs for the next 1 minute. But we *HAVE* to protect a
minimum of five minutes.
So the maximum window is 7 minutes, and during the design phase we protect
for at least 5. Of course, on launch day we look at how much APM we have
based on the winds of the day, and if we don't have enough, we will slip
into the window until the performance required is equal to the amount we
have available. In fact, this is what happened for the STS-79 flight: we
had to slip over a minute into the window, so instead of being 7:00 it was
more like 5:46.
I also mentioned ET impact as a constraint, but it doesn't play much of a
part on a high inclination mission. For due east missions (28.45°
inclination), the window opening is based on not dropping the tank on
Kingman Reef (somewhere southeast of Hawaii), and the closing is based on
not dropping the ET more than 25 miles off of the coast of Hawaii. This
leaves a 72-minute window max. For 51.6° missions, we had to design around
French Polynesia, but the open and close times are based on performance.
Depending on the phase angles, we might have two 7-minute launch
opportunities on one day, the second separated from the first by about
360°. These two "panes" (part of the launch "window") overlap, and we get
close to 10 minutes total, from the beginning of pane 1 to the end of pane
2.
-----
Michael R. Grabois | http://ourworld.compuserve.com/homepages/mgrabois
Houston, TX | or...@ix.netcom.com CI$: 74737,2600
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Gravity. It's not just a good idea, it's the law.
