HST Mission Launch Window
AFS
launch window for this mission is 64 mn.
Can anybody explain why is this window so long given that is a "rendez-vouz
type mission" like those to the ISS?
Or am I completely wrong?
Thanks. AFS
Richard Kaszeta
When the HST was deployed, it was deployed from a standard 28.5
degrees inclination orbit (what you get if you launch due east from
KSC). So to rendezvous with the HST it's simply a matter of timing
the launch so that you're in near proximity to the HST. If you are in
essentially the same orbital plane as your target, small adjustments
can be made without using much fuel. With the allowable propellant
use for the shuttle, this works out to giving you approximately an
hour to match orbits.
The ISS is at a much higher inclination (51.6 degrees IIRC), and
out-of-plane orbital adjustments take a *lot* of fuel compared to
in-plane adjustments, requiring a lot more fuel. Hence the operating
reserve of fuel is smaller, and hence the launch window is smaller.
--
Richard W Kaszeta
ri...@kaszeta.org
http://www.kaszeta.org/rich
Jorge R. Frank
news:y6xd6zr...@kenai.me.umn.edu:
> "AFS" <ant...@arquired.es> writes:
>
>> I have read the mission preview at Spaceflight Now Website and read
>> that the launch window for this mission is 64 mn.
>>
>> Can anybody explain why is this window so long given that is a
>> "rendez-vouz type mission" like those to the ISS?
>> Or am I completely wrong?
>
> When the HST was deployed, it was deployed from a standard 28.5
> degrees inclination orbit (what you get if you launch due east from
> KSC). So to rendezvous with the HST it's simply a matter of timing
> the launch so that you're in near proximity to the HST. If you are in
> essentially the same orbital plane as your target, small adjustments
> can be made without using much fuel. With the allowable propellant
> use for the shuttle, this works out to giving you approximately an
> hour to match orbits.
Some small clarifications here (there may be a terminology problem)...
It's not a matter of timing the launch to be in proximity to HST *itself*
(or any other target, including ISS); it's a matter of launching in
proximity to HST's *orbital plane*. Just because HST's orbital plane is at
28,5 degrees inclination doesn't mean it is always in-plane with KSC; HST's
ground track traces an approximate sinusoid between 28.5 degrees
north/south latitude. An orbit with the same inclination as the launch site
latitude will, of course, remain in-plane with the launch site (or close to
it) far longer than an orbit with a higher inclination. But it doesn't
matter quite as much exactly *where* HST is within the plane; the shuttle
can adjust its height (and therefore catchup rate) to catch up with HST.
> The ISS is at a much higher inclination (51.6 degrees IIRC), and
> out-of-plane orbital adjustments take a *lot* of fuel compared to
> in-plane adjustments, requiring a lot more fuel. Hence the operating
> reserve of fuel is smaller, and hence the launch window is smaller.
The propellant in question is not for out-of-plane orbital adjustments
(which are indeed quite expensive) but for yaw steering during ascent. A
high-inclination orbital plane passes through the yaw steering limits much
more quickly than a plane with inclination equal to latitude.
--
JRF
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james oberg
site 'near' the orbital plane of the target? Actually, it's the 'phantom
plane' you shoot for that will precess into the target plane at rendezvous,
but they are so close that only ballisticians worry about the difference.
Because the ground track of the ISS at 52deg crosses the KSC site at a high
angle -- about 45 degrees -- even with the track widened slightly by the
yaw-steering ability (that is, crabbing left or right during second stage to
shift your insertion vector), the launch site passes through that zone
relatively quickly.
Now, since HST's orbital plane is close to the latitude of the launch site,
it crosses the site nearly due west to east, and when you 'fatten it' with
yaw-steering cabability, the launch site as it rotates west to east will
remain within this band for a much longer period, the numbers quoted at the
top of the thread.
Gemini had even more robust yaw steering, and used a target inclination of
about 32 degrees. It had a planar launch window for rendezvous on the order
of 100 minutes, longer than the orbital period of the target.
Jorge R. Frank <jrf...@ibm-pc.borg> wrote in message
news:Xns91A6C7FC...@204.52.135.10...
Jorge R. Frank
news:OGf68.75370$XZ1.3...@typhoon.austin.rr.com:
> Because the ground track of the ISS at 52deg crosses the KSC site at a
> high angle -- about 45 degrees -- even with the track widened slightly
> by the yaw-steering ability (that is, crabbing left or right during
> second stage to shift your insertion vector), the launch site passes
> through that zone relatively quickly.
>
> Now, since HST's orbital plane is close to the latitude of the launch
> site, it crosses the site nearly due west to east, and when you
> 'fatten it' with yaw-steering cabability, the launch site as it
> rotates west to east will remain within this band for a much longer
> period, the numbers quoted at the top of the thread.
Good words, Jim - I guess that's why you're the writer, and I'm not!