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From: kcarroll@utzoo.UUCP (Kieran A. Carroll)
Newsgroups: net.columbia
Subject: Re: Launch Windows
Message-ID: <5188@utzoo.UUCP>
Date: Thu, 7-Mar-85 19:32:57 EST
Article-I.D.: utzoo.5188
Posted: Thu Mar  7 19:32:57 1985
Date-Received: Thu, 7-Mar-85 19:32:57 EST
References: <225@tellab2.UUCP>, <5179@utzoo.UUCP>
Organization: U of Toronto Zoology
Lines: 45

(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!kcarroll