space@mit-mc (03/07/85)
From: rachiele@NADC
Wait a minute. That explains why the satalite needs a launch window, not
the shuttle itself. The shuttle is in LEO, which is not Geo-sync. So the
satalite could be launched from any point, provided the shuttle could acheive
the correct orbit, right? Maybe the calculations would be too difficult to
do on the fly, and have to be done long beforehand?
Jim Rachielespace@mit-mc (03/07/85)
From: Carl Rosene <animal@rice.ARPA>
Launch windows for satellite missions come from at least a couple of things:
1) Accompanying experiments or observations (especially astronomical
observations or tracking) might have requirements for the orbit. (What
portion of the sky is visible during the night etc.)
and
2) Satellite deployments typically will have lighting requirements (So that
you can you are doing). These lighting requirements are met by seeing
that the shuttle's "day" (or "night") sky is over the right portion of
the earth.
The above considerations lead you to launch windows.space@mit-mc (03/09/85)
From: David Smith <dsmith%hplabs.csnet@csnet-relay.arpa> Another consideration: If the satellite gets put up at the wrong longitude, it takes a while to slew it around. So it is best to put it up at the right longitude if possible. That means the transfer orbit needs to have its apogee at the correct longitude, over the equator. This in turn means that the boost into the transfer orbit must occur 180 degrees from the desired longitude, while crossing the equator. So if the shuttle gets off the pad late, it may need to delay release of the satellite for nearly an extra day. David Smith HP Labs ucbvax!hplabs!dsmith dsmith%hp-labs@csnet-relay
al@ames.UUCP (Al Globus) (03/13/85)
> > Wait a minute. That explains why the satalite needs a launch window, not > the shuttle itself. The shuttle is in LEO, which is not Geo-sync. So the > satalite could be launched from any point, provided the shuttle could acheive > the correct orbit, right? Maybe the calculations would be too difficult to > do on the fly, and have to be done long beforehand? > Jim Rachiele I'm not sure where this started, but there are a number of launch constraints that I know of. Some are: To get a satellite from shuttle orbit you must start your burn when the shuttle orbit crosses the plane of the equator. This only happens twice per orbit. This 'node crossing' must occur in the correct place to get the satellite to the right place in geo-sync. It's possible to drift a satellite around in geo-sync, but the initial location must be within line of site of a ground control station for check-out. Most satellites have thermal constraints that require launch from the shuttle on the dark or light side, but not either. There are constraints on the shuttle due to various abort options. E.g., there must be sufficient light at the Dakar runway used for some contigencies. I'm sure there are plenty of other constraints. When it comes down to actually getting a satellite into orbit, it's amazing how complicated things become.
space@mit-mc (03/15/85)
From: WOO IL LEE <lee@su-star> I would like to clear up something that has me confused. Up to now I have assumed that the PAM upper stage starts its burn when the shuttle's orbit crosses the equatorial plane, and that the longitude of this intersection is chosen so that the transfer orbit's perigee is near the satellite's final station. Is this correct? Does the shuttle maneuver so as to establish the proper nodes? Or does PAM have a plane change capability? ( I am sure the IUS does). Puzzled Emilio P. Calius Stanford Univ. ------
karn@petrus.UUCP (03/18/85)
> I would like to clear up something that has me confused. Up to > now I have assumed that the PAM upper stage starts its burn when the > shuttle's orbit crosses the equatorial plane, and that the longitude of > this intersection is chosen so that the transfer orbit's perigee is near > the satellite's final station. > Is this correct? Does the shuttle maneuver so as to establish > the proper nodes? Or does PAM have a plane change capability? ( I am > sure the IUS does). This is essentially correct. The deployment takes place on an equator crossing, and the PAM ignites on the next crossing, half an orbit later. This allows the orbiter to do a separation maneuver at the time of deployment so that it will be some distance away by ignition time. Selecting the orbit on which deployment occurs allows the resulting transfer orbit apogee to occur near the desired parking spot to minimize the amount of near-stationary drifting required. Any engine can do a plane change if it is pointed out of plane. I've seen the transfer orbit elements for a few PAM missions and they do reduce the inclination by about 3 degrees. I'm not sure why because it is more efficient to let the apogee motor take it out instead; perhaps it allows finer balance between the two stages without having to shave out propellant. Phil
henry@utzoo.UUCP (Henry Spencer) (03/19/85)
As Phil mentions, it is generally more efficient to do plane changes
at apogee. I thought people might be interested in why. Basically,
when you're far away from a gravitating body, all orbits look similar
in one sense: velocities are small, and the differences between the
velocity vectors of different orbits are therefore small. So you can
make orbit changes in general, and plane changes in particular, with
relatively modest expenditures of fuel. Of course, you have to haul
the fuel up to the maneuvering altitude, but it turns out that you
still come out ahead in general.
--
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henry