Re: launch windows

john2375@xxxxxxxxxxx wrote in news:1154991978.129111.291310
@i3g2000cwc.googlegroups.com:

I saw on the CBS News page that the launch window for STS-115 extends
from Aug 27th to Sept. 7th. However if Atlantis doesn't fly then,
there are two launch days in October (23rd and 24th I think) and one in
December - the 23rd -and that's it!
I know basically why they have restricted launch windows, because of
the lighting conditions and the limited ISS rendezvous windows anyhow,
but this webpage said the reason for the limited time is because of
temperature restrictions on the ISS... anyone able to explain that to
me?

Sure. How detailed an explanation you want? :-)

The temperature restrictions are expressed with respect to a parameter
called "solar beta angle". That's the angle between the ISS orbit plane
and the line-of-sight to the sun. This varies throughout the year due to
the Earth's axial tilt with respect to its orbit around the sun (called
the "obliquity of the ecliptic", about 23.4 degrees), and the Earth's
equatorial bulge, which causes the ISS orbit plane to shift westward
about five degrees per day.

Beta angle affects both solar power generation and thermal control. At
higher beta angles, ISS spends less time in the Earth's shadow, and must
fly different attitudes to keep its solar arrays facing the sun and its
radiators edge-on away from the sun. At beta angles above 70 degrees,
there is no orbital night at all. The maximum beta angle is 75 degrees,
the sum of the ISS orbit inclination (51.6 degrees) and the obliquity of
the ecliptic.

Throw a docked orbiter into the mix and you have a third constraint,
shuttle thermal control. The attitude of the orbiter/ISS stack must be
chosen to satisfy ISS power/thermal constraints and shuttle thermal
constraints. It turns out that at beta angles larger than 60 degrees,
there is no attitude that satisfies all three. So the shuttle program has
a launch window cutout when the beta angle will exceed 60 degrees at any
time during docked ops.

These cutouts occur around the solstices, because that's the time of year
when the sun's declination is farthest north (summer) or south (winter)
and so the odds are greatest that the sum of the sun's declination and
the orbit inclination will exceed 60 degrees. The summer cutout affects
night launches and the winter cutout affects day launches. The shuttle
return-to-flight restriction on night launches has effectively mooted the
summer beta cutout, but the combination of the night launch restriction
and the winter beta cutout makes winter launch opportunities few and far-
between.

The 60 degree limit on beta angle during docked ops is a certification
limit. The station program is looking at expanding certification to relax
the limit. This will not be complete any time soon.

Also, they (media, and NASA) said a few weeks ago that STS-116
Discovery was hopeful for the December launch window, however with only
one possible day that doesn't seem very worthwhile to even consider
it..more than likely it'll be pushed into next year - obviously
slipping downstream flights as well..

Different story there. :-) The night launch restrictions are only in
effect until the shuttle program has two consecutive flights with foam
shedding within allowable limits. STS-121 was the first, so if STS-115
flies clean, STS-116 will be able to launch at night, which greatly
expands the launch window.

--
JRF

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check "Organization" (I am not assimilated) and
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Relevant Pages

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