RSF@SU-AI@sri-unix.UUCP (08/04/83)
From: Ross Finlayson <RSF@SU-AI> I'm confused as to exactly why the shuttle needs to be launched within a precise launch window so that a communications satellite can be released over India "at the correct place and time". Since KSC is in a fixed position relative to the intended final position of the satellite (in geostationary orbit), it will also presumably be in a fixed position relative to the point of release (in LEO). So why then can STS-8 be launched only within a certain narrow window at night? Presumably there are other, time-dependent factors involved, but what would these be? Ross.
karn@eagle.UUCP (08/08/83)
You are correct in that both KSC and India are moving together at the same rate and speed, since both are rigidly attached to the earth. Launch windows are set for earth orbit missions (except those performing rendezvous) based on sun position. When the satellite is ejected from the payload bay, it must be pointed in the correct inertial attitude for its PAM firing 45 minutes later. At the same time the sun should be shining as nearly perpendicular as possible to the spin axis in order for the satellite's solar panels to function as well as for the spacecraft's thermal health. In addition, there are limits on the duration of an eclipse that can be tolerated during the transfer ellipse. If the satellite's apogee kick motor is to be fired at the first apogee following the PAM firing, the apogee of the transfer ellipse at geostationary altitude must occur over the equator and preferably near the satellite's operational position. In order for this to happen, the PAM kick has to be at the equator on the opposite side of the earth. If you put all these factors together with the time of year and you get a required orbit plane orientation; specifically, the right ascension of the ascending node (RAAN) is set. The way you control RAAN is by launching at a certain time of day, hence the launch window. Typically with expendable launchers, there are two launch windows each day around local noon and midnight at the injection point which is usually also near perigee. A day launch puts the apogee on the dark side of the earth, while a night launch puts apogee on the day side. Near the spring and fall equinoxes the day window closes because the satellite would be eclipsed excessively near apogee. Of course, with the shuttle you have the option of waiting an arbitrary number of orbits before deploying the satellite, but since the orbit plane stays relatively fixed in space this doesn't buy you much. If you waited long enough (many days) the orbit plane would eventually precess around due to earth oblateness perturbations to the proper orientation, but a night launch is much easier! Phil Karn