adolph@ssc-vax.UUCP (Mark C. Adolph) (09/18/88)
In article <15019@ames.arc.nasa.gov>, yee@ames.arc.nasa.gov (Peter E. Yee) writes: > > NASA today set a target launch date of Sept. 29 for STS-26, > the next Space Shuttle flight. The opening of the launch window > is approximately 9:59 a.m. EDT. 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? -- -- Mark A. ...uw-beaver!ssc-vax!adolph
henry@utzoo.uucp (Henry Spencer) (09/19/88)
In article <2234@ssc-vax.UUCP> adolph@ssc-vax.UUCP (Mark C. Adolph) writes: >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? A possible candidate for the frequently-asked questions list, Eugene! Sun angles are important during transit to Clarke (geostationary) orbit, because the satellite is not fully unfolded and careful attention to temperature control and solar-array power output is needed. This makes the transfer orbit's exact position in space somewhat critical. If the launch site were on the equator, this wouldn't be a problem. But when the starting orbit is inclined, which shuttle orbits are, the transfer orbit has much the same inclination (it is cheaper to make almost all of the plane change on leaving transfer orbit, not on entering it). This makes the orientation of the starting orbit important. Hence a specific launch window. (The serious satellite folk are welcome to correct this if I've botched anything; I'm at home with a massive head cold and I'm not thinking too well.) -- NASA is into artificial | Henry Spencer at U of Toronto Zoology stupidity. - Jerry Pournelle | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
phil@titan.rice.edu (William LeFebvre) (09/20/88)
In article <2234@ssc-vax.UUCP> adolph@ssc-vax.UUCP (Mark C. Adolph) writes: >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? The launch window is not for the benefit of the payload (at least, not in this case). It is primarily for the benefit of the orbiter, to ensure that all the aborts are safe to take. It is also effected by the scheduled duration of the flight. The following things (at least---there may be others) effect the launch window: sunset at the trans-atlantic abort sites (to insure that they can still see to land if a TAL is necessary), sunrise at the landing site on the scheduled end of mission (EOM) day, sunrise at the landing site on EOM + 1 day and EOM + 2 days (in case the orbiter has to stay up longer than planned, perhaps because of problems on board or on the ground) and probably EOM - 1 day if the launch is scheduled during the spring, the 3 hour crew constraint (the crew is not allowed to sit in the orbiter more than three hours after the window opens for fear of fatigue, boredom, muscle cramps, and certain natural functions---see below). There are also constraints imposed by the payload: even a geosynchronous one. I'm not too clear on the reasons behind the payload constraints, but it may have something to do with launching the payload in daylight (or with the sun in a specific place). I don't understand the 9:59 EDT launch time. The chart I have shows that the window for the 29th doesn't open until 10:07 EDT. Maybe they came out with a different window schedule (the one I have is dated May 24, 1988). "Other natural functions": What's the first thing the crew does after getting into orbit? Look out the windows. What's the second thing? Line up for the bathroom! I'm serious. Think about it.... Something else I heard: the flight deck only holds 4 crew members during ascent and entry. The remainder must ride it out on the middeck. You might think that those on the middeck really lose out because they can't see out during launch. But there's an advantage that makes up for it: they're the first in line for the "waste disposal facility"! William LeFebvre Department of Computer Science Rice University <phil@Rice.edu>
eugene@eos.UUCP (Eugene Miya) (09/20/88)
Well, since I used to work for OTDA (Office of Tracking and Data Acq).
I should point out also that the TDRS is a pretty big satellite, and
we want it in a particular place. In part of this case, we are talking
about a substantial amount of fuel would could better be used for
attitude correction. Another reason for the "window" of this mission.
Another gross generalization from
--eugene miya, NASA Ames Research Center, eugene@aurora.arc.nasa.gov
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"Send mail, avoid follow-ups. If enough, I'll summarize."karn@thumper.bellcore.com (Phil R. Karn) (09/20/88)
> 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
knudsen@ihlpl.ATT.COM (Knudsen) (09/21/88)
In article <1897@kalliope.rice.edu>, phil@titan.rice.edu (William LeFebvre) writes: > hour crew constraint (the crew is not allowed to sit in the orbiter more > than three hours after the window opens for fear of fatigue, boredom, > muscle cramps, and certain natural functions---see below). There are also This whole problem was rather thoroughly documented in _The Right Stuff_, where Alan Shepard received a "go" to go -- while still on the pad. Soiled his spacesuit. Funniest part of the movie, just couldn't resist.
henry@utzoo.uucp (Henry Spencer) (09/21/88)
Read Phil's explanation, not mine. He got it right. (Having one's brain cells working properly helps.) -- NASA is into artificial | Henry Spencer at U of Toronto Zoology stupidity. - Jerry Pournelle | uunet!attcan!utzoo!henry henry@zoo.toronto.edu