Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!bellcore!faline!thumper!karn From: karn@thumper.bellcore.com (Phil R. Karn) Newsgroups: sci.space.shuttle Subject: Re: Launch Window question Summary: geostationary launch windows Message-ID: <1329@thumper.bellcore.com> Date: 19 Sep 88 23:48:17 GMT References: <15019@ames.arc.nasa.gov> <2234@ssc-vax.UUCP> Organization: Bell Communications Research, Inc Lines: 53 > If the purpose of the mission is to deploy a geosynchronous satellite, > why is there a launch window? What difference does it make when one > launches into earth orbit? There are two main classes of geostationary communications satellite: spin stabilized and 3-axis stabilized. At separation, both types of satellite are generally oriented by the launcher (either expendable or STS) with the proper attitude for kick motor firing, and spun up. The satellite spins, maintains this inertial attitude during the ~5 hour coast up to geostationary altitude, trimming it as necessary with attitude control fuel (usually hydrazine or cold gas). The firing attitude puts the kick motor thrust vector nearly in the plane of the equator and perpendicular to the major axis of the transfer orbit. (I.e., at apogee the satellite's spin is parallel to the earth's surface directly beneath it). For a launch in the northern hemisphere the nozzle is actually pointed slightly north of west so the transfer orbit inclination can be reduced to zero. The operational attitude for a spin-stabilized spacecraft always has the spin axis parallel with the earth's axis. Since the kick motor is also aligned with the spacecraft spin axis, the firing attitude is therefore nearly perpendicular to the normal operational attitude. In order not to starve the solar arrays during this period, the sun must be aligned roughly with the major axis of the orbit. In other words, the apogee and perigee points of the transfer orbit must lie above spots on the earth that are near local noon and midnight (or local midnight and noon). So you generally have two launch windows per day -- one centered around local noon at the orbit injection point and another around local midnight. Since the launch always takes place from a site to the west of the injection point, the local time at the launch site will be a couple of hours before noon or midnight. A second consideration is eclipses -- you want to avoid them during the transfer orbit, if you can help it. Particularly if you have deployable solar arrays, you are working from a limited budget during the transfer orbit phase and you need all the power you can get. So the daytime launch window often closes during the spring and fall seasons, when the transfer orbit apogee would be on the night side of the earth and in the earth's shadow. That's why you see so many nighttime launches -- they put the apogee on the day side of the earth. Three-axis spacecraft are generally operated in a spin-stabilized mode during the orbit transfer and kick motor firing phases, and their deployable arrays are usually folded up against the side of the spacecraft. So for the purposes of setting launch windows, these spacecraft are the same as spinning spacecraft. There are some exceptions, notably payloads riding on IUS (which doesn't spin), and their launch windows are probably based on eclipse considerations more than sun attitude. Phil