thoth@tellab2.UUCP (Marcus Hall) (03/05/85)
On some launches which only deployed satellites, I still hear about "launch windows". On a mission that must rendezvous with something already in orbit, there is an obvious launch window. There are also time limits associated with sitting on the pad that I understand, but I sometimes hear about a "launch window" for launching a satellite that is going into geo-synchronous orbit. This doesn't make sense to me, since it seems that launching it an hour later would only result in it reaching its orbit an hour later. Could somebody please enlighten me. Thank you. marcus hall ..!ihnp4!tellab1!tellab2!thoth
zeek@pyramid.UUCP (Jim Zeek) (03/06/85)
Shuttle launches have windows because if their launching a geo-stationary satelite which orbits over the same spot over earth the have to enter orbit at the right spot over earth. jim zeek @ pyramid technology (pyramid!zeek)
henry@utzoo.UUCP (Henry Spencer) (03/07/85)
I'm not an expert on this, but there are at least two reasons why there
are definite launch windows for Clarke ("geostationary") orbit.
First, remember that a satellite in that orbit remains over essentially
the same spot on the equator. This means that you are not only trying
to get the satellite into that orbit, it has to be in the right *place*
in that orbit. There are further complications because most of the
launch sites are not on the equator, which means that plane changes as
well as orbital shape changes are needed.
Second, most current satellites do not deploy most of their solar panels
until they are in the final orbit, because the support structures are
not strong enough to survive orbital maneuvering. This means that the
satellites are very short of power while in transit, and the angle of
the sun with respect to the solar cells is significant. (Satellites are
often not free to turn to get the best angle, due to things like spin
stabilization and the directions rocket motors point.) Passing through
the Earth's shadow can also be a problem for a power-starved satellite.
For these (and probably other) reasons, satellite launches most definitely
do have time constraints as well as position constraints.
--
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henrykarn@petrus.UUCP (03/07/85)
> Shuttle launches have windows because if their launching a geo-stationary > satelite which orbits over the same spot over earth the have to enter orbit > at the right spot over earth. Not exactly. As the earth rotates, it carries the "target point" AND the launch site with it, so you could launch at any time and still reach the desired spot by flying the same trajectory. The reason you have "launch windows" for geostationary missions is because the spacecraft must be pointed in a different direction during the orbit maneuvers and the climb to geostationary altitude, and the resulting sun angles may be unacceptable if the launch were to take place at certain times. Depending on the mission, there may be additional constraints on launch time. In a rendezvous mission you must launch as the launch site passes through the orbital plane of the target. There may be sunlight and visibility requirements at various places (at emergency landing sites for the shuttle, at observation targets for weather and spy satellites, landing sites on the moon, etc) which all combine to produce a set of acceptable launch times. Phil Karn
fisher@dvinci.DEC (03/07/85)
From: DVINCI::FISHER 7-MAR-1985 13:42 To: FISHER Subj: net.columbia > ...why are there launch windows for geosync satellites? >> ...they have to be at the right place at the right time to end up >> in the right place over the earth Well, actually there is more to it. If you end up in the wrong place in geosync orbit, it is pretty cheap to move a satellite, as long as you are not in a real big hurry. You just give it a push and let it drift. In general the OPENING of the launch window is usually defined by when the AOA (Abort Once Around) landing site (usually Edwards AFB) will have enough light. Thus try not to launch earlier than one orbit-time before sunrise at EAFB. Another important consideration is that satellites are typically designed for the conditions they will encounter during their operational life, and not a whole lot more. Ex: In operation, many geosync sats spin, and are in sunlight MOST of the time. Thus they are not designed to tolerate darkness very often, and they are not designed to tolerate direct sunlight when they are not rotating. The launch timing is critical (I think typically the window closeing is based on) the sun angles on the satellite while it is climbing to geosync altitude. Aside: I have always been amazed at how fussy and specialized space vehicles are. And you wonder why it costs $millions to keep a space vehicle like Voyager going once it is up there? Someone has to keep monitering the critter to make sure it does not exceed some weird design limitation! Burns UUCP: ... {decvax|allegra|ucbvax}!decwrl!rhea!dvinci!fisher ARPA: fisher%dvinci.dec@decwrl.ARPA Posted: Thu 7-Mar-1985 13:46 Eastern Standard Time To: rhea::decwrl::"net.columbia"
kcarroll@utzoo.UUCP (Kieran A. Carroll) (03/08/85)
(a virtual line)
Henry Spencer mentions that most spinning satellites are not free
to orient themselves in the best possible position for receiving
solar power through their panels of solar cells while in transfer
orbit; this leads to a desire to keep low-earth-to-geosynchronous
transfer times as short as possible, so as not to run down the
batteries on the spacecraft before geosynchronous orbit is
achieved (at which time, all the solar panels can be deployed,
and power ceases to be a constraint). Quite true. In case anybody's
interested, here's one of the reasons that spacecraft orientation
is constrained during the transfer orbit:
During transfer, the vehicle must be able to sense it's attitude
(ie. its orientation with respect to some inertial reference frame),
so that, when it reaches apogee, it'll know in which direction to point
its apogee kick motor, to circularize its orbit. The most common
way to perform this attitude sensing, for a rotating spacecraft,
is with sun-sensors and earth-sensors. These sensors have a limited
field of view, and so must be pointed roughly in the direction of the
earth or the sun (whichever is applicable) to start with;
after that, provided nothing disastrous happens, they'll stay
locked on their target. Also, one has to be careful not to allow
an earth sensor to face the sun (or the moon, for that matter),
or else it might get confused, and the satellite might lose its
attitude reference. These requirements translate into a set of
constraints as to which direction the satellite's spin axis may point;
generally, pointing perpendicularly to the line joining the satellite
with the sun is not the best orientation. Thus, the solar cells
on the sides of the spacecraft don't face directly towards the sun,
and power-collection is restricted. For some missions, the
spinning solar cells don't even collect enough power to make up
for the housekeeping power drain during transfer orbit; the
extra power is drained from batteries on board the satellite (fully
charged at launch). The satellite must reach geosynchronous orbit,
stop spinning and deploy the rest of its solar cells before the
batteries run down. This places a constraint on the amount
of time the satellite can spend in transfer orbit.
Another problem is maintaining an acceptable temperature range in the
satellite during transfer orbit, but I won't go into that right
now...
There! My spacecraft design class taught me something, after all!
--
Kieran A. Carroll @ U of Toronto Aerospace Institute
{allegra,ihnp4,linus,decvax}!utzoo!kcarrollpeterb@pbear.UUCP (03/08/85)
Its not that they have to be at a particular spot to launch a geo-synchronous sattelite. They caould launch any time and still get to that spot. Rather it is that during the time of the launch window the sattelite would have a clear path through the many thousands of objects on its way to geosynchronous orbit. Peter Barada ima!pbear!peterb
atchison@umn-cs.UUCP (03/09/85)
One of the big factors which effects the launch window is the gyroscopes. At some point in the launch countdown (I don't know exactly when it is), they set the on board gyroscopes. Once these are set, the launch must occur at a specific time within a couple hours (?) or so. The reason for this is because the earth is rotating which continuously changes the gyroscope settings. This is the basis of the launch window. If they cannot launch within this time, they must cycle the countdown back to some point before the time the gyroscopes are set and start over again. This is just one of the many things that, once started, must finish at a predescribed time period. Other factors include fueling. Once they send in the liquid hydrogen/oxygen, they must launch within a particular time period. There are many examples of these types of things. The closer the time to launch, the smaller the launch window becomes as more and more critical (time dependant) things are accomplished. If these time dependancies are not met, the count must be stopped and cycled back to some previous point in time and continued from their. As an interesting example, when the count reaches about T-3seconds, the onboard computer takes over. At some point past this (I don't know the exact time), the launch window is only a few milliseconds long. If they (the computer) must delay the launch longer than a few milliseconds, the launch must be aborted until much later. This happened with the first shuttle launch, the computer stopped the launch at T-3seconds, and it was stopped longer than possible and they had to recycle the launch. In that example, they had to cycle back, I believe, 3 days.
stevel@haddock.UUCP (03/09/85)
There are telemetry and tracking considerations as well as
orbital insertion considerations. They like to deploy satilites
so that they are talking to ground stations just before and just
after burn. Sometimes I have read that the time is set so it can
be optically tracked by telescope.
For the India Palapa B I belive it the burn was to take place
over India shortly after sunrise because of the above
considerations.
Sketchilly yours
Steve Ludlum, decvax!yale-co!ima!stevel, {ihnp4!cbosgd}!ima!stevelfisher@dvinci.DEC (03/12/85)
<> > ...the comsat uses batteries until it can reach geo, stop spinning, and > unfold its solar panels This may be true for some of them (the stop spinning and unfold part), but the ones I know about are either 3-axis stabilized and don't spin at all (like TDRSS) or are spin stabilized and spin during their entire life (like Palapa and Westar). I don't think this invalidates any of the other stuff about sun sensors, etc, since a spinning satellite would still have lots of constraints. For example, stuff that can't spin must either be mounted on the satellite's spin axis, or on a "despun" section, which itself must be attached to the satellite on the spin axis. Not to mention mass being distributed evenly around the cicumfrance, fuel having to drain evenly. AND...it must be pretty hard to change the attitude of a spinning satellite, so you want to do that as little as possible. What a pain! With all that, I wonder why the launch window is so wide! Burns UUCP: ... {decvax|allegra|ucbvax}!decwrl!rhea!dvinci!fisher ARPA: fisher%dvinci.dec@decwrl.ARPA